摘要:

THE ROYAL SOCIETY OF CHEMISTRY Journal of Materials Chemistry Scientific Editor Staff Editor Professor Anthony R. West Mrs. Janet M. Leader Department of Chemistry The Royal Society of Chemistry University of Aberdeen Thomas Graham House Meston Walk Science Park Aberdeen AB9 2UE, UK Cambridge CB4 4WF, UK Assistant Editor: Mrs. F. J. O'Carroll Editorial Secretary: Miss D. J. Halls Graphics Designer: Ms. C. Taylor- Reid Materials Chemistry Editorial Board Anthony R. West (Aberdeen) (Chairman) Peter G. Bruce (St. Andrews) H. Monty Frey (Reading) C. Richard A. Catlow (London) John W. Goodby (Hull) David A. Dunmur (Sheffield) Brian J. Tighe (Aston) Jean Etourneau (Bordeaux) Allan E. Underhill (Bangor) Wendy R. Flavell (UMIST) John D.Wright (Canterbury) ~~~~~~~~ ~ ~ ~ ~ ~ ~~ ~ ~ ~ ~ ~ ~ ~~~~~ Internationa I Advisory Ed itoria I Board M. A. Alario-Franco (Madrid, Spain) A. J. Leadbetter (Daresbury, UK) K. Bechgaard (Copenhagen, Denmark) J. S. Miller (Wilmington, DE, USA) J. D. Birchall (Runcorn, UK) P. S. Nicholson (Hamilton, Canada) D. Bloor (Durham, UK) M. Nygren (Stockholm, Sweden) A. K. Cheetham (Santa Barbara, USA) V. Percec (Cleveland, OH, USA) E. Chiellini (Pisa, Italy) N. Plate (Moscow, Russia) P. Day (London, UK) C. N. R. Rao (Bangalore, India) B. Dunn (Los Angeles, USA) M. Ratner (Evanston, IL, USA) W. J. Feast (Durham, UK) J. Rouxel (Nantes, France) A. Fukuda (Tokyo, Japan) R. Roy (University Park, PA, USA) D. Gatteschi (Florence, Italy) J.L. Serrano (Zaragoza, Spain) J. B. Goodenough (Austin, TX, USA) J. N. Sherwood (Glasgow, UK) A. C. Griffin (Hattiesburg, USA) J. Simon (Paris, France) S-i. Hirano (Nagoya, Japan) J. F. Stoddart (Birmingham, UK) P. Hodge (Manchester, UK) S. Takahashi (Osaka, Japan) H. lnokuchi (Okazaki, Japan) G. J. T. Tiddy (Bebington and Salford, W. Jeitschko (Munster, Germany) UK) 0. Kahn (Orsay, France) Yu. D. Tretyakov (Moscow, Russia) M. Lahav (Rehovot, Israel) J. W. White (Canberra, Australia) R. Xu (Changchun, China) Journal of Materials Chemistry (ISSN 9959-9428) is published monthly by The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK. All orders accompanied with payment should be sent directly to The Royal Society of Chemistry, Turpin Distribution Services Ltd., Blackhorse Road, Letchworth, Herts SG6 1 HN, UK.NB Turpin Distribution Services Ltd., distributors, is wholly owned by The Royal Society of Chemistry. 1994 Annual subscription rate EC (inc. UK) f381.00, USA $718.00, Canada €431 .OO (plus GST), Rest of World f410.00. Customers should make payments by cheque in sterling payable on a UK clearing bank or in US dollars payable on a US clearing bank. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003. USA POSTMASTER: send address changes to Journal of Materials Chemistrx Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003. Second Class postage paid at Jamaica, NY 11 431.All other dispatches outside the UK by Bulk Airmail within Europe, Accelerated Surface Post outside Europe. PRINTED IN THE UK. @ The Royal Society of Chemistry, 1994. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form, or by any means, electronic, mechanical, photographic, recording, or otherwise, without the prior permission of the publishers. Professor A. R. West, Scientific Editor Mrs. J. M. Leader, Staff Editor Tel.: Aberdeen (0224) 27291 8 Tel.: Cambridge (0223) 420066 Fax: (0224) 272938 E-Mail (INTERNET): Telex: 73458 UNIABN G RSCl @RSC.ORG Fax: (0223) 420247 or 423623 Telex: 818293 ROYAL G Advertisement sales: Tel. + 44 (0)71-287 3091; Fax +44 (0)71-494 11 34 INFORMATION FOR AUTHORS The Royal Society of Chemistry welcomes submission of manuscripts intended for pub- lication in two forms, Articles and Materials Chemistry Communications.These should describe original work of high quality dealing with the synthesis, structures, properties and applications of materials, particularly those associated with advanced technology. Articles Full papers contain original scientific work that has not been published previously. How- ever, work that has appeared in print in a short form such as a Materials Chemistry Com- munication is normally acceptable. Four copies of Articles including a top copy with figures etc. should be sent to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB44WF.UK. Materials Chemistry Communications Materials Chemistry Communications contain novel scientific work in short form and of such importance that rapid publication is war-ranted. The total length is rigorously restric- ted to two pages of the double-column A4 format. The manuscript will be returned for reduction if this length is exceeded. For a Communication consisting entirely of text and ten references, with no figures, equations or tables, this corresponds to approximately 1600 words plus an abstract of up to 40 words. Submission of a Materials Chemistry Com- munication can be made either to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK, or via a member of the Inter- national Advisory Editorial Board.In the latter case, the top copy of the manuscript includ- ing any figures etc., together with the name of the person to whom the Communication is being submitted, should be sent simul-taneously to the Editor at the Cambridge address. Authors may wish to contact the Board member to ensure that he is available to arrange review of the manuscript within reasonable time. In order to avoid delay in publication, proofs of Communications are not sent to authors unless this is specifically requested. Full details of the form of manuscripts for Articles and Materials Chemistry Communi- cations, conditions for acceptance etc. are given in issue number one of Journal of Materials Chemistry published in January of each year, or may be obtained from the Staff Editor. There is no page charge for papers published, in Journal of Materials Chemistry. Fifty reprints are supplied free of charge. Any author who is publishing in Journal of Materials Chemistry is entitled to a free copy of the issue in which the paper appears.

ISSN:0959-9428    

DOI:10.1039/JM99404FX001

出版商:RSC    

年代:1994

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99404BX003

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( l),5-8 Phase Formation and Electrical Properties in the System BaO-Li,O-TiO, Leticia M. Torres-Martinez," Claudia Suckut,b Ricardo Jirnenezbgc and Anthony R. WesP a UANL, Facultad de Quimica, Nuewo Leon, Mexico University of Aberdeen, Department of Chemistry, Meston Walk, Aberdeen, UK A69 2UE CSIC, C. Serrano 7 75dpdo, 28006 Madrid, Spain A survey of compound formation and phase equilibrium in the system BaO-Li,O-TiO, for compositions containing >50% TiO, has been made at subsolidus temperatures, ca. 11 50-1 200 "C. Four ternary phases were encountered, all of which have variable composition. These are phase A, centred on Ba3Li2Ti8020, phase B centred on BaLiPTi6014, phase C based on the line Ba2Til,,Li4x0,, and phase E, a hollandite-like phase Ba~Li~~+4~~Ti~~-~-~0,6.Phase A is new; phase B is the phase responsible for high Li+ ion conductivity reported by Zheng et a/. (Solid State lonics, 1989, 35, 235); phase C is related to one reported by Tillmanns and Wendt (Z.Kristallogr., 1976, 144 16) whose formula coincided with x=O.75; phase E was reported by Sucket et a/. (J. Mater. Chem., 1992, 2, 993). In addition, an extensive area of ternary solid solution based on Ba4Ti13030 is reported. Mechanisms of solid-solution formation are discussed; the two principal mechanisms appear to be Ti4+ +4Li+ and 2Ti4+ +2Li' +3Ba2+. Electrical conductivity data are reported; phases A, C and D appear to be very modest electronic conductors. Few studies on the formation of barium lithium titanate phases have been reported, in spite of the great importance of various barium titanates as dielectrics.The synthesis and crystal structure of Ba,Ti,~,,Li,O,, has been reported;' its structure is somewhat unusual in containing partial occupancy of both Ti and Li positions. A new Li' ion conductor has been reported,, its approximate composition was given but nothing is known of its structure. A new solid-solution phase with a hollandite structure has been rep~rted.~ Its formula is Ba3,Li(,, +4y)Ti8 -2x it appears to be a modest conductor of Li+ ions. We have investigated phase formation and equilibria in the system BaO-Li,0-Ti02 for compositions containing >50% TiO,. These results, with preliminary X-ray characterisation of the phases and electrical property data, are reported here.Experimental Starting materials were Li,CO,, BaCO, and TiO,, all reagent grade. The carbonates were dried at 300°C and the TiO, at 900"C, after which they were stored in a desiccator. Samples were weighed out in 5-20 g batches, mixed into a paste with acetone using an agate mortar and pestle, dried and fired in Pt crucibles, initially at 700-900°C for a few hours to drive off CO,, followed by 1150-1200°C for 2-10 days, with intermediate regrindings, to complete reaction. Reaction con- ditions were found by trial and error to be those necessary to achieve equilibrium, i.e. in which no change in the nature of the reaction product(s) occurred on heating at either higher temperatures or for longer times.Loss of lithia by volatilis- ation was not a problem under the conditions used, as shown by (a) weight-loss checks, (b)the self-consistency of the phase diagram results and (c) constancy in the nature of the final products after appropriate heating schedules. Lithia loss did become significant, however, if samples were melted. Reaction products were analysed and identified by X-ray powder diffraction using either a Philips Hagg Guinier camera for routine identification or a Stoe StadiP, psd-based diffractometer, both with Cu-Ka, radiation. For accurate d-spacings, KC1 was added as internal standard. Melting temperatures were determined approximately from visual observation of pelleted samples, heated isothermally at various temperatures for 5-10 min.Conductivities were measured on sintered, pelleted samples, using electrodes made from fired gold paste. Measurements were made over the range 30 mHz to 64 kHz with a Solartron 1250/1286 Impedance System and over the range 100Hz to 10MHz with a Hewlett Packard 4192 Impedance Analyser. Results and Discussion The results of a selection of heating experiments on ~180 compositions have been summarised in tabular form.? These were used to construct the phase diagram shown in Fig. 1. Four ternary phases were encountered, labelled A, B, C and E, together with an extensive area of solid solutions based on Ba4Til,0,,, labelled D and a small area of solid soiutions based on BaTi,O,,. The diagram in Fig.1 refers to tempera- tures of ca. 1150-1200 "C; these are subsolidus temperatures TiOs 10'Ago Fig. 1 Phase diagram BaTiO,-Li,TiO,-TiO, at 1150-1200"C. Single-phase regions are labelled A-E and BT,. 7 Supplementary data available (SUP 56976, 5 pages); details from Editorial Office but nevertheless, are close to melting for most regions of the phase diagram. Results are now summarised for each of the phases encountered. Phase A This is a new phase which forms over an area of compositions (Fig. 1). We know very little about this phase; its 'ideal' composition may be Ba3Li2Ti,020, since this is a simple formula that lies within the solid solution area at the composi- tion marked in Fig. 1. The phase melts at 1240+20"; melting is probably incongruent, to Ba4Ti13030 and liquid.Unindexed X-ray powder diffraction data are given in Table 1. Conductivity data for phase A are summarised in Fig. 2. A single semicircle was seen in the complex impedance data, without any evidence of low-frequency electrode polarisation phenomena. The conductivity appears to be electronic, there- fore. Actual conductivities are low, e.g. 1.6 x lop7R-' cm-' at 400 "C with an activation energy of 0.84 eV. Phase C Tillmanns and Wendt' reported the synthesis and crystal structure of the phase Ba2Tig.25Li3022. This appears to be the same phase as the one we have labelled C, since its X-ray powder diffraction dat? can be indexedo on an orthorhopbic unit cell [a=5.8040(4) A, b=9.9281(6) A, c= 14.0141(9)A for composition 13.5% BaO, 18.5% Li,O, 68% TiO,] which is very simitar to that rFported for Ba2Ti9.25Li3022[a= 5.8081(5)A, b =9.931( 1)A, c = 14.025( 1) A].Our results indicate that phase C occupies an area of compositions on the phase diagram which appears not, how- ever, to include the composition Ba,Ti,,,,Li,022. This is shown on expanded scale in Fig. 3. One edge of the solid solution area may be described by the general formula Ba2Ti,o-xLi4x022: 0.95 <x < 1.15. The composition of Ba,Ti,~,,Li,O,, may also be represented by this formula, with x=O.75 and is indicated by Tf, Fig. 3; it lies significantly outside our experimental range of values. Table 1 X-Ray powder diffraction data for phase A: Ba,Li,Ti,O,, dobs/A I dobslA I 9.02 14 14 1.8866 13 5.3764 47 1.8756 16 4.5022 57 1.7620 19 4.1416 35 1.7184 15 3.6606 61 1.6938 18 3.6252 27 1.6746 11 3.4300 13 1.6237 26 3.2886 28 1.6005 12 3.1 193 21 1.5767 13 3.0712 93 1.5688 14 2.9931 93 1.5438 13 2.9437 100 1.5365 20 2.8371 89 1.5148 12 2.8127 42 1.4993 15 2.6868 37 1.4903 22 2.6524 44 1.4182 28 2.5054 31 1.4134 23 2.3666 29 1.3993 13 2.3033 15 1.3927 14 2.2495 24 1.3555 14 2.23 16 29 1.2554 13 2.0708 52 1.2505 11 2.0562 52 1.2051 14 1.9800 30 1.1917 13 1.9644 22 1.1877 15 1.9465 67 1.1833 15 1.9004 16 J.MATER. CHEM., 1994, VOL. 4 ~O~WT Fig. 2 Conductivity data 12.5:12575 \\ / X/ Y "4 22.5:12.5:65 15 17.5 20 12.5:22.5:65 mol% Li20 Fig.3 Location of phase C solid solutions: 0, single phase; (3, mixture The crystal structure determination of Ba2Ti,.,,Li,O2, showed a partial occupancy of two of the Ti sites, together with partial occupancy of Li sites;' the existence of a solid- solution range with variable Li :Ti ratio, Fig. 3, may be readily understood, in terms of varying occupancies of Li and Ti sites with the replacement mechanism 4Li+ eTi4+(mechanism I). While such a mechanism may appear unusual at first sight, typic$ Ti-0 and ki-0 bond distances are comparable, ca. 1.96 A and ca. 2.10 A, respectively, for octahedral coordination of the metal, and therefore, there are no size constraints to prevent such a solid-solution mechanism. A similar solid solution occurs in the phase Li,TiO, which, in its high- temperature form with a cation disordered rock-salt structure, can accommodate both an excess and a deficiency of Li+ ion^.^,^ The structure determination' of Ba,Tig~2,Li302, showed J.MATER. CHEM., 1994, VOL. 4 four Ti sites, with occupancy factors as follows: Ti( l),4c, n = 1; Ti(2), 8d, n=0.86; Ti(3), 4c, n=0.88; Ti(4), 4c, n= 1. The three Li sites [Li(l), 4c; Li(2), 4c; Li(3), 4a] were all given occupancy n = %. The solid solution Ba2Ti,,-,Li4,O2, can be accounted for by varying the occupancy of Ti(2), Ti(3) sites and the Li sites. Thus, at the high x limit, 1.15, and assuming the Ti vacancies were equally distributed over Ti(2) and Ti(3) sites, the occu- pancy factor would be n =0.81 with an average Li occupancy factor of n=0.77. Conversely, at the lower x limit, x=O.95 these occupancies would be n=0.84 for Ti and n=0.63 for Li.A full structure analysis would, of course, be necessary to confirm these details. The composition of Ba2Ti,,,,Li,022 reported by Tillmanns and Wendt' was, in fact, obtained by refinement of X-ray diffraction data on a single crystal extracted from a melt whose starting composition, Ti, was rather different from the quoted final composition, Tf, Fig. 3. Given the difficulty in determining accurate Li' site occupancies due to the low X-ray scattering power of Li, and the additional possibility of some partial occupancy of Li on the Ti sites, it is conceivable that the crystal studied in ref.1 could have a somewhat higher Li:Ti ratio to that reported. There is not necessarily an inconsistency therefore between the literature formula and the experimental compositions for phase C reported here. In order to account for the existence of a solid-solution area off the join Ba2Til,~xLi4x022, Fig. 3, an additional solid- solution mechanism is required. One possibility (mechanism 11) is that the same mechanism operates as in the hollandite phase, E, namely: 3Ba+2Li e2Ti. This would have the effect of creating barium vacancies at the same time as the Li, Ti site occupancies vary. A second possibility (mechanism 111)is to simply leave out BaO from the structure, in which case both Ba vacancies and 0 vacancies would be generated.At present, we have no evidence in favour of either mechanism. The directions taken by these two mechanisms are indicated in Fig. 3; as an illustrative example, both are shown starting from the com- position with x = 1, i.e. Ba2Ti9Li402,. Perhaps the Ba vacancy mechanism is more likely since this would not involve disrup- tion of the lattice of TiO, octahedra. Conductivity data are shown in Fig. 2. Conductivities are low, as for phase A, and appear also to be electronic. Phase B This phase appears to be the one responsible for the high lithium ion conductivity.2 It also forms an area on the phase diagram, Fig. 1, and its 'ideal' composition may correspond to the formula BaLi,Ti,014 shown; the phase melts at 1270 20 "C, probably incongruently. One limiting edge of the solid-solution area corresponds to the replacement mech- anism 4Li +Ti, giving the general formula BaLi, +4xTi6-xo14, 0.08~~50.16,Fig.4. In order to account for the area of B solid solutions, similar considerations apply to those used for phase C, above. Unindexed X-ray powder diffraction data are given in Table 2; these agree with the data shown as a 'stick diagram' in ref. 2. Conductivity data for phase B are shown in Fig. 2 for one composition. These are comparable to the literature values; a change in slope occurs at ca. 300°C with activation energies in the low- and high-temperature regions of ca. 0.52 eV and ca.0.43 eV, respectively. Conductivities were measured for several compositions within the phase B solid- solution area. All gave comparable conductivities within a factor of 4 to 5 and no obvious trends were apparent. 5590 25:5:70 9 13 17v 21 525:70 mol% Li20 Fig. 4 Location of phase B solid solutions: 0, mixture; 0,single phase Table 2 X-Ray powder diffraction data for phase B: BaLi,ll'i,O,, dabs/A I 1 ~~~ ~ 7.2856 41 2.9748 19 5.6467 21 2.8947 19 5.0739 22 2.7883 78 4.6656 66 2.7347 51 4.5677 16 2.6675 35 4.3259 36 2.2850 14 3.7702 100 2.0722 54 3.5672 14 2.0182 99 3.4950 23 1.9624 23 3.2076 24 1.7735 18 3.1856 15 1.6920 11 3.0970 23 1.5875 11 3.0698 17 1.4456 50 3.0020 18 1.4380 14 Phase D This phase was originally thought to be a new phase but is, in fact, an extensive solid solution based on the stoichiometric phase Ba4Ti13030.6 It occupies an area on the phase diagram, Fig.1,shown on expanded scale in Fig. 5. The crystal structure Ti02 Ti,+ 4 Li v V V V BaTiO3 Ii2Ti03 Fig. 5 Location of Ba,TiI3O3,,solid solutions of Ba,Ti,O,, has been reported. It contains an array of TiO, octahedra in which oxide ions are in cubic close packing. All atom sites appeared to be fully occupied. In order to account for an extensive area of Ba,Ti,,O,, solid solutions, two solid solution mechanisms are necessary. One possibility is shown as direction A in Fig. 4 and involves the mechanism: 2Ti4+ g 2Li' + 3Ba2+. This presupposes a partial substitution of Li onto Ti sites, together with creation of interstitial Ba2+ ions.The second is shown by direction B and involves the mechanism Ti4+ $4Li+ discussed above in connection with phase C. A grid is superposed on the solid solution area, Fig. 5 which allows the Ba4Ti,03, solid solu- tions to be described in terms of the compositional variables x and y. Conductivity data are shown for one phase D composition in Fig. 2; conductivities are again very low. Phase E This phase, a modest conductor of Li+ ions with the hollandite structure, has been described previously.2 It is a solid-solution phase of general formula, Ba,,Li,,, + 4y)Ti(8 in which -,,-,,)016 two principal mechanisms operate, as described above, i.e.2Ti g2Li + 3Ba and Ti 4Li. Its location is shown in Fig. 1. Subsolidus Compatibility Relations The subsolidus phase relations in the system BaTiO,-Li,TiO,-TiO, are shown in Fig. 1. These refer to temperatures of 1150-12OO0C, which are within ca. 50°C of J. MATER. CHEM., 1994, VOL. 4 melting temperatures; at lower temperatures, samples did not usually reach equilibrium within timescales of a few days. The phase diagram contains the four ternary phases, A, B, C, E described above, together with a significant area of Ba,Til,030 ternary solid solutions, D; a small area of BaTi,O,, ternary solid solutions appears to form also. The remaining areas of the phase diagram are divided into a number of two- and three-phase regions; some are marked by 2 and 3, respectively, in Fig.1. Coexisting phases include the ternary solid solutions indicated above and various binary phases, notably BaTiO,, BaTi,O,, Li,Ti,O, and Li,TiO,; the latter forms a very extensive solid solution range at 1150-1200 "C in the Li,O-TiO, diagram (ref.4) and these solid solutions coexist with various of the ternary barium lithium titanate phases. We thank the British Council for supporting the Aberdeen-Mexico exchange programme, the SERC for finan- cial support (to A.R.W.) and DGICSA, SEP C91-03-19-001-365 and C91-01-001-856 (LMTM). References 1 E. Tillmanns and I. Wendt, 2.Kristallogr., 1976, 144, 16. 2 W. J. Zheng, R. Okuyama, T. Esaka and H. Iwahara, Solid State ionics, 1989,35,235. 3 C. Suckut, R. A. Howie, A. R. West and L. M. Torres-Martinez, J. Muter. Chem., 1992,2,993. 4 G. Izquierdoand A. R. West, Muter. Rex Bull., 1980, 15, 1655. 5 A. R. West, J. Muter. Sci. Lett., 1981, 16, 2023. 6 E. Tillmanns, Inorg. Nucl. Chem. Lett., 1971,7, 1169. Paper 3/04518H; Received 28th July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400005

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

Cumulative Author Index 1994 Akirnoto H., 61 Ali-Adib Z., 1 Aliev A. E., 35 Arnold Jr. F. E., 105 Auroux A,, 125 Azuma K., 139 Baba A,, 51 Bach S., 133 Bachir S., 139 Davies A., 113 del Arc0 M., 47 Dennison S., 41 Diamond D., 145 Ellis A. M., 13 Fleming R. J., 87 Fujirnoto T., 61 Gil-Llambias F-J., 47 Glomrn B., 55 Jung K., 161 Kassabov S., 153 Kennedy B. J., 87 King T., 1 Klissurski D., 153 Kossanyi J., 139 KouyatC D., 139 Kuwano J., 9 Lahti P. M., 161 Neat R. J., 113 Nicol I., 29 Nomura R., 51 Ohta K., 61 Pennington M., 13 Pereira-Ramos J-P., 133 Perez-Jimenez C., 145 Povey I. M., 13 Raynor J. B., 13 Baffier N., 133 Beveridge M., 119 Bond S. E., 23 Britt S., 161 Goodby J. W., 71 Harris F. W., 105 Harris K. D. M., 35 Harris S. J., 145 Landee C., 161 Lefebvre F., 125 Le Goff P., 133 Liu-Cai F.X., 125 Richards B. C., 81 Rives V., 47 Robertson M. I., 29, 119 Ronfard-Haret J-C., 139 Carlino S., 99 Carrazan S. R. G., 47 Cervini R., 87 Cheng S. Z. D., 105 Coles G. S. V., 23 Conroy M., 1 Cook S. L., 81 Davidson I. M. T., 13 Hirose N., 9 Hitchman M. L., 81 Hobson R. J., 113 Hodge P., 1 Hudson M. J., 99, 113 Imanishi N., 19 Imayoshi K., 19 Jirnenez R., 5 Loubser G., 71 Macklin W. J., 113 Malet P., 47 Matsuda H., 51 McGhee L., 29, 119 McMeekin S. G., 29, 119 Mills G. P., 13 Murray K. S., 87 Ross A., 119 Russell D. K., 13 Saydam S., 13 Shamlian S. H., 81 Shen D., 105 Sheridan P., 161 Shimokawatoko T., 51 Smart S. P., 35 Snetivy D., 55 Styring P., 71 Suckut C., 5 Takano M., 19 Takeda Y., 19 Torres-Martinez L.M., 5 Uzunova E., 153 Vancso G. J., 55 Wessels P. L., 71 West A. R., 5 West D., 1 Williams G., 23 Winfield J. M., 29, 119 Workman A. D., 13 Yamamoto I., 61 Yamamoto O., 19 Yang H., 55 Zhang W-r., 161 i Confemnce Diary 1994 March 13-16 March 13-18 March 28-30 April 4-9 April 5-8 April 11-13 April 11-14 April 24-28 April 25-29 June 13-16 June 14-17 June 20-22 June 22-24 June 29-4 July July 3-8 July 3-8 July 4-8 July 4-8 Third European Federation of Corrosion Workshop on Microbial Corrosion Estoril, Portugal CBsar Sequeira, Instituto Superior TBcnico, Av. Rovisco Pais, 1096 Lisboa Codex, Portugal or A. K. Tiller, Corrosion Centre, 23 Grosvenor Gardens, Kingston upon Thames KT2 5BE, UK or D.Thierry, Swedish Corrosion Insitute, Roslagsviigen 101, Hus 25, S-10405 Stockholm, Sweden 1994 ACS National Spring Meeting San Diego, USA ACS International Activities Office,1155 16th St. NW, Washington DC 20036, USA The British Liquid Crystal Society -Annual 9th Conference Hull, England Dr M Hird or Professor J W Goodby, School of Chemistry, The University of Hull, Hull HU6 7RX,UK Fax: 44-482-466410 8th High Temperature Materials Conference (HTMC VIII) Vienna, Austria HTMC VIII, Gesellschaft Osterreichischer Chemiker, Nibelungengasse 1l/6, A-1010 Wien, Austria 8th High Temperature Materials Conference (HTMC VIII) Vienna, Austria Professor K L Komarek, Institut fur Anorganische Chemie der Universitat Wien, WtihringerstraRe 42, A-1090, Wien, Austria Tel: +43(222)-345-424; Fax: +43(222)-310-4597 Microscopy of Composite Materials I1 oxford, UK The Royal Microscopical Society, 37/38 St Clements, Oxford OX4 lAJ, UK Deformation, Yield and Fracture of Polymers Cambridge, UK Mrs Debbie Schorer, Conference Department (C406), The Institute of Materials, 1Carlton House Terrace, London SWlY 5DB, United Kingdom Tel 071-839-4071; Fax: 071-839-3576 The American Ceramic Society Annual Meeting Indianapolis, USA Meetings Secretary, The American Ceramic Society Inc., 757 Brooksedge Plaza Drive, Westendle, Ohio 43081-6136, USA Tel: 614-890-4700; Fax: 614-899-6109 International Conference on Metallurgical Coatings and Thin Films (ICMCTF-94) San Diego, USA ICMCTF-94, Dale C.McIntyre, Sandia National Laboratories -New Mexico Advanced Materials Laboratory, 1001 University Boulevard SE, Suite 100, Albuquerque, NM 87106, USA Science and Technology of Pigment Dispersion Luzern, Switzerland Dr A V Patsis, Institute for Materials Science, State University of New York, New Platz.NY 12561, USA Fax: 914-255-0978 Workshop on Polymer Blends and Alloys Luzern, Switzerland Dr A V Patsis, Institute for Materials Science, State University of New York, New Paltz, NY12561, USA Fax: 914-255-0978 16th International Conference on Advances in the Stabilization and Controlled Degradation of Polymers. Luzern, Switzerland Dr A V Patsis, Institute for Materials Science, State University of New York, New Paltz, NY 12561, USA Fax: 914-255-0978 TMS 1994 Electronic Materials Conference Colorado, USA Tim Sands, Department of Materials Science and Mineral Engineering, Hearst Mining Building, University of California, Berkeley, CA 94720, USA Tel: 510-642-2347; Fax: 510-642-9164 8th CIMTEC: Forum on New Materials and World Ceramics Congress Florence, Italy 8th CIMTEC, PO Box 174,48018 Faenza, Italy Tel: +546-22461, + 546-664143; Fax: +546-66-3362 15th International Liquid Crystal Conference Budapest, Hungary Professor L~os Bata, Research Inst for Solid State Physics of the Hungarian Academy of Sciences, Liquid Crystal Department, H-1525 Budapest, PO Box 49, Hungary Tel: 36-1-169-9499; Fax:36-1-169-5380 First European Conference on Synchrotron Radiation in Materials Science Chester, UK Professor G N Greaves, SERC Daresbury Laboratory, Warrington WA4 4AD, UK Tel: +44(0)925-603335; Fax: +44(0)925-603 174 First Euroconference -Ceramic Oxygen Ion Conductors and Their Technological Applications Lake Windermere, UK Ms M Peacock, Conference Department (C4351, The Institute of Materials, 1Carlton House Terrace, London SWlY 5DB Tel: +44 (0)71 235 1391; Fax: +44 (0)71823 1638 20th International Conference on Organic Coatings Science & Technology Athens, Greece Dr A V Patsis, Institute for Materials Science, State University of New York, New Paltz, NY 12561, USA Fax: 94-255-0978 11 July 6-8 July 11-12 July 11-15 July 19-22 July 24-29 July 25-29 July 25-29 August 1-5 August 2-6 August 21-26 August 28-September 2 September 5-7 September 5-9 September 5-9 September 6-9 September 11-14 September 11-14 September 11- 17 September 25-30 October 2-6 Grenoble, France M Cyrot, CNRS, 25 Avenue des Martyrs, 38042 Grenoble, Cedex, France Silicon-Containing Polymers Canterbury, UK Dr R G Jones, Centre for Materials Research, Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH, UK Tel: +44 (227) 764-000 ext.3544; Fax: +44 (227) 475-475 Meeting of the Tetrapyrrole Discussion Group on Chemistry and Biochemistry of Tetrapyrroles London, UK Ray Bonnett or Martin Warren, Queen Mary & Westfield College, Mile End Road, London El 4NS Fax:071-975-5500 36th International Symposium on Macromolecules: MACROAKRON '94 Akron,Ohio, USA Macroakron '94, Cathy Manus-Gray, Symposium Coordinator, Institute of Polymer Science, The University of Akron, Akron,OH 44325-3909, USA International Conference on Excitonic Processes in Condensed Matters Darwin,Australia Dr J Singh, Faculty of Science, Northern Territory University, PO Box 40146, Casuarina, NT 0811, Australia 30th International Conference on Coordination Chemistry Kyoto, Japan Professor H Ohtaki, Laboratories of Coordination Chemistry, Institute for Molecular Science, Myodaiji-cho, Okazaki 444, Japan 35th Microsymposium on Macromolecules Prague, Czech Republic 35th Microsymposium, PMM Secretariat, do Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 162 06 Prague 6, Czech Republic International Conference on Synthetic Metals Seoul, Korea Dr C Y Kim KIST, PO Box 131, Cheongryang Seoul 130-650, Korea Fax: 82-2-965-3852 2nd International Conference on f-Elements Helsinki, Finland Professor L Niinisto, ICFE-2, Conference Chairman, Helsinki University of Technology, Department of Chemical Engineering, Kemistintie 1, FIN-02150 Espoo, Finland Fax: +358-0-462-373 Fourth Asian Conference on Solid State Ionics Kuala Lumpur Secretary, Fourth Asian Conference on Solid State Ionics, do Department of Physics, Faculty of Physical and Applied Sciences, Universiti Kebangsaan Malaysia, 43600 Bangi, Malaysia 1994 ACS National Autumn Meeting Wahington DC, USA ACS International Activities Office, 1155 16th St.NW, Washington DC 20036, USA ECM 16, European Crystallographic Meeting Dresden, Germany Professor P Paufler, Fachbereich Physik, Teknische Universitaet Dresden, Mommsenstrasse 13, D-0-8027 Dresden, Germany Tel: 3378; Fax: 37-51-463-7109 Electroceramics IV,International Conference on Electronic Ceramics & Applications Aachen, Germany Professor Dr Rainer Waser, Institut fir Werkstoffe der Elektrotechnik, RWTH Aachen, D-52056 Aachen, Germany European ESR Meeting on Recent Advances and Applications to Organic and Bioorganic Materials Pans, France Dr Bernard Catoire, GARPE, do ITF-Lyon, BP 60, F-69132 Ecully, France Tel: 78 33 34 55; Fax: 78 43 39 66 6th International Symposium.Scientific Bases for the Preparation of Heterogeneous Catalysts Louvain-la-Neuve, Belgium Dr G.Poncelet, Unit6 de Catalyse et Chimie des Matkriaux Divisks, Place Croix du Sud, 2 boite 17, 1348 Louvain-la-Neuve, Belgium International Conference on Liquid Crystal Polymers Beijing, China Professor Xibai Qiu, Chinese Chemical Society, PO Box 2709, Beijing 100080, China Ceramic Processing Science and Technology Friedrichshafen (Bodensee), Federal Republic of Germany Deutsche Keramische Gesellschaft e.V., Frankfurter StraBe 196, D 5000 Koln 90, Federal Republic of Germany 11th European Conference on Biomateriale Pisa, Italy Professor Paolo Giusti, 1lth European Conference on Biomaterials, Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali Via Diotisalvi, 2-56126, Pisa Italy 1st Euroconference on Solid State Ionics Ionian Sea, Greece Professor Dr W Weppner, Christian Albrechts University, Chair for Sensors and Solid State Ionics, Kaiserstr 2, D-24098 Kiel, Germany International Conference on Molecular Electronics and Biocomputing Goa, India Dr Ratna S Phadke, Scientific Secretary for ISMEBC '94, Chemical Physics Group, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400 005, INDIA Tel: +91 (22)-215-2971; Fax: +91(22)-215-2110 66th Annual Meeting of the Society of Rheology Philadelphia, PA, USA Norman Wagner, Dept.Chemical Eng., University of Delaware, Newark, DE 19716 Tel: (302) 831-8079; Fax: (302) 831-10 ... 111 October 10-12 3rd International Symposium on Structural and Functional Gradient Materials Lausanne, Switzerland FGM '94, Swiss Federal Institute of Technology of Lausanne, Materials Department, LMM, CH-1015 Lausanne, Switzerland Tel: (+41) 21 693 29 15/50; Fax: (+41) 21 693 46 64 October 24-25 International Polypropylene Conference London, UK Ms M Peacock, Conference Department (C446), The Institute of Materials, 1Carlton House Terrace, London SWlY 5DB, UK December 19-22 1994 International Conference on Electronic Materials (ICEM'W) & 1994 IUMRSInternational Conference in Asia (IUMRS-ICA) Hsinchu, Taiwan C/o Materials Research Laboratories, ITRI, Conference Department, IUMRS-ICEhVICA'94, Bldg 77, 195 Chung-hsing Rd, Sec.4, Chutung, Hsinchu, 3105, Taiwan, ROC. Tel: +886-35-820064, 886-35-916801; Fax: 886-35-820247, 886-35-820262; E-mail: 740366@MRL.ITRI.ORG.TW Conference Diary 1995 August 19-25 Clays and Clay Materials Science Leuven, Belgium Professor P Grobet, Secretary Euroclay '95, Centrum voor Oppervlaktechemie en Katalyse, K U Leuven, K Mercierlaan 92, B-3001 Heverlee, Belgium December 10th International Conference on Solid State Ionic8 Singapore B V R Chowdari, Department of Physics, National University of Singapore, Singapore -0511 iv Journal of Materials Chemistry Information for Authors Journal of Materials Chemistry is a monthly journal for the publication of original research papers (articles), feature articles and communications focusing on the chemistry of novel materials.There is no page charge for papers published in Journal of Materials Chemistry.Scope of the Journal Chemistry of materials, particularly those associated with areas of advanced technology: the modelling of materials, their synthesis and structural characterisation, physicochemical aspects of their fabrication, properties and applications. Materials Inorganics: ceramics; layered materials; microporous solids and zeolites; silicates and synthetic minerals; biogenic minerals. Organics: organometallic precursors for thin filmdceramics; novel molecular solids and synthetic polymers with materials applications; polymer composites; biopolymers; biocompatible and biodegradable polymers; liquid crystals (both lyotropic and thermotropic); Langmuir-Blodgett films.Properties and Applications Electrical properties: semi-, metallic and super-conductivity; ionic conductivity; mixed ionidelectronic conductivity; ferro-pyro-and piezo-electricity; electroceramics; dielectrics. Optical properties: luminescence, phosphorescence, laser action; non-linear optical effects; photoconductivity; photo- and electro-chromism, resists, glasses, amorphous semiconductors; optical modulation and switching. Magnetic properties: ferro-, ferri- and antiferro-magnetism, spin glass behaviour, organic magnetism, magnetic bubbles and information storage. Chemical properties: ion exchange, molecular separation, catalytic action, sensor action, topochemical control of reactions. Structural properties: structural ceramics, refractories; hard materials; protective coatings; composites, adhesives, prosthetic applications.Thermodynamic properties and phase behaviour Articles Full papers contain original scientific work that has not been published previously. However, work that has appeared in print in a short form such as a Materials Chemistry Communication or Chemical Communication is normally acceptable. But note that the Society strongly discourages the fragmentation of a substantial body of work into a number of short publications. Papers should be typewritten in double spacing on one side only of the paper. Four copies of text, illustrations (full colour copies for coloured figuredplates), tables and any other matter should be sent to: The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK Feature Articles Feature Articles are published by invitation of the Materials Chemistry Editorial Board.Materials Chemistry Communications Materials Chemistry Communications contain novel scientific work in short form and of such importance that rapid publication is desirable. Authors should briefly indicate in a covering letter the reasons why they feel that publication of their work as a Communication is justified. The total length is rigorously restricted to two printed A4 pages. The manuscript will be returned for reduction if this length is exceeded. For a Communication consisting of text and ten references, with no figures, equations or tables, this corresponds to approximately 1,600 words plus an abstract of up to 40 words.Submission of a Materials Chemistry Communication can be made either to The Editor, Journal of Materials Chemistry, The Royal Society of Chemistry, Thomas Graham House, Science Park, Milton Road, Cambridge CB4 4WF, UK or via a member of the International Advisory Editorial Board. In the latter case, the top copy of the manuscript including any figures etc., together with the name of the person to whom the Communication is being submitted, should be sent simultaneously to The Editor at the Cambridge address. Authors may wish to contact the Board member to ensure that he is available to arrange review of the manuscript within reasonable time.Administration Receipt of a paper will be acknowledged, and the paper will be given a reference number which authors are asked to quote on all their subsequent correspondence. If no such acknowledgement has been received after a reasonable period of time authors should check with the Editorial Office as to whether the paper or the acknowledgement has gone astray. Editorial Policy. Every paper (except Communications) will be submitted to at least two referees, by whose advice the Materials Editorial Board will be guided as to its acceptability. Full details are given in Refereeing Procedure and Policy, J. Mater. Chem., 1994, Issue 1. Papers that are accepted must not be published elsewhere except by permission of the Royal Society of Chemistry.Submission of a manuscript will be regarded as an undertaking that the same material is not being considered for publication by another journal. Conditions governing acceptance are available from the Editorial Office. Copyright. The whole of the literary matter (including tables, figures, diagrams and photographs) in Journal of Materials Chemistry is subject to copyright and may not be reproduced without permission from The Royal Society of Chemistry and such other owner of the copyright as may be indicated. Reprints. Fifty reprints of each paper are supplied free of charge on request. Additional reprints can be purchased if ordered at the time of publication. Details are sent to authors with the proofs.Notes on the Preparation of Papers 1. Manuscripts must by typed in double-line spacing, single sided on A4 paper, with margins at top, bottom and left-hand side of at least 4 cm. 2. The first page should be set out as follows: (i)Name and address of the author to whom the proofs and correspondence should be sent. (ii) Title of the paper, with capitals for the first letter of each noun and adjective only. (iii) Authors’ names, including one forename for each author. (iv) The address where the work was carried out; if this is different from the current address of any author wishing to deal with correspondence a footnote indicating the present address of this author should be included. (v) Abstract, preceded and followed by a horizontal line, and typed in double-line spacing. 3.Suitable headings and sub-headings should be used in the main text as appropriate (except for Communications in which no headings are used). References should be numbered serially in the text by means of superscript arabic numerals. 4. Bibliographic references (not footnotes) should follow the main text and should have the following format: 1 R.M. Barrer and R.J.B. Craven, J. Chem. Soc., Faraday Trans. 1,1987, 83,779. 2 R.M. Barrer and R.J.B. Craven, in New Developments in Zeolite Science and Technology, ed. Y. Murakame, A. Iijima and J.W. Ward, Kodansha, Tokyo, 1986, p.521. Journal titles should be abbreviated according to the Chemical Abstracts Service Source Index (CASSI).5. Tables should be typed on separate sheets at the end of the manuscript. Crystallographic Papers Crystallographic work will be assessed mainly for its relevance to materials. Papers reporting only the results of crystal structure determination may be accepted for publication provided they are for materials with potentially interesting properties, Crystallographic work that forms parts of a wider study, including synthesis or property measurements, should not normally be submitted for publication separately from the results of that study. Papers containing new crystal data should normally make explicit mention of this in the title. The description of a crystallographic structure determination should be as brief as possible; in particular, it is not the policy of the journal to publish lengthy data tables.For publication as part of their papers authors should include: a table of final fractional atomic coordinates (but without anisotropic temperature factors); a table of key bond lengths and angles; and a conventional line drawing of the structure. Additional data (any calculated coordinates; full list of bond lengths and angles; thermal parameters; structure factors; least-square planes) should be submitted as supplementary material for use by the referees. Apart from the structure factors this material will be deposited at the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ (for molecules with ‘organic’ carbon) or the Fachinformationszentrum Karlsruhe, D-7514 Eggenstein- Diagrams should be accompanied by a separately typed set of captions.Extensive identifying lettering should be placed in the captions rather than on the figures. Original artwork should be supplied wherever possible. Colour photographs will be accepted subject to approval by the referees. Bulk information (such as primary kinetic data, computer programs and output, etc) which accompanies papers published in Journal of Materials Chemistry may be deposited, free of charge, with the Society’s Supplementary Publications Scheme, either at the request of the author and with the approval of the referees or on the recommendation of the referees with the approval of the author. Details are available from the Editorial Office.Nomenclature Current IUPAC nomenclature and symbolism should be used. Attention is drawn to the following publications in which the rules themselves and guidance on their use are given: Nomenclature of Inorganic Chemistry, Blackwell Scientific Publications, Oxford, 1990. Nomenclature of Organic Chemistry, Sections A,B,C,D,E,F and H, Pergamon Press, Oxford, 1979 edn. Biochemical Nomenclature and Related Documents, The Biochemical Society, London, 1978. Compendium of Chemical Technology: IUPAC Recommendations, Blackwell Scientific Publications, Oxford, 1987. Units and Symbols The recommendations of IUPAC should be followed. Their basis is the Systkme Internationale d’Unit6s (SI).A detailed treatment is given in the so-called Green Book: Quantities, Units and Symbols in Physical Chemistry, Blackwell Scientific Publications, Oxford, 1993 edn. Leopoldshafen 2 (otherwise). Any request to Cambridge or the Fachinformationszentrum Karlsruhe for deposited material should be accompanied by the full literature citation for the paper concerned. Full details regarding presentation of crystallographic work can be obtained from the Editorial Office. Powder Data Powder X-ray diffraction data may be published, preferably in tabular form, but should be restricted to studies of new materials; also, for cases where the materials are new but have similar powder data to other, well characterized, materials, such data will not usually be included in the journal.However, for the purposes of refereeing, a full data set in tabular form should be submitted as supplementary material, simultaneously with the paper; this material will subsequently be deposited with the Society’s Supplementary Publications Scheme (details available from the Editorial Office). Diagrams showing diffraction patterns of reaction products will not normally be included in the journal, unless they have some distinct feature of particular relevance to the discussion. JOURNALS OF THE ROYAL SOCIETY OF CHEMISTRY Refereeing Procedure and Policy (1994) 1.0 Contributions to Dalton, Perkin and Faraday Transactions, J. Mater. Chem., The Analyst, J. Anal. At. Spectrom. and J. Chem.Research 1.1 Introduction This document summarises the procedure used for assessing papers submitted to the four Transactions, J. Mater. Chem., The Analyst, J. Anal. At. Spectrom., and J. Chem. Research, and provides guidelines for referees engaged in this assessment. 1.2 Subject Matter Papers are submitted to the various journals according to subject matter. If it is felt that a paper would be published more appropriately in an RSC journal other than the one suggested by the author, the referee should inform the Editor. The topics covered by the various journals are as follows. Dalton Transactions (Inorganic Chemistry). All aspects of the chemistry of inorganic and organometallic compounds, including bioinorganic chemistry and solid-state inorganic chemistry; the applications of physicochemical techniques to the study of their structures, properties and reactions, including kinetics and mechanism; new or improved experimental techniques and syntheses.Faraday Transactions (Physical Chemistry and Chemical Physics). Gas-phase kinetics and dynamics; molecular beam kinetics and spectroscopy, photochemistry and photophysics; energy transfer and relaxation processes: laser-induced chemistry; spectroscopies of molecules, molecular and gas- phase complexes: quantum chemistry and molecular structure, statistical mechanics of gaseous molecules and complexes; spectroscopies, statistical mechanics and quantum theory of the condensed phase, computational chemistry and molecular dynamics; colloid and interface science, surface science, physisorption and chromatographic science, chemisorption and heterogeneous catalysis, zeolites and non-exchange phenomena; electrode processes, liquids and solutions; solid-state chemistry (microstructures and dynamics); reactions in condensed phases; physical chemistry of macromolecules and polymers; materials science; thermodynamics; biophysical chemistry and radiation chemistry. Perkin Transactions I (Organic Chemistry).All aspects of organic and bio-organic chemistry. These include synthetic organic chemistry of all types, organometallic chemistry, chemistry and biosynthesis of natural products, the relationship between molecular structure and biological activity, the chemistry of polymers and biological macromolecules, and medicinal and agricultural chemistry where there is originality in the science.Perkin Transactions 2 (Physical Organic Chemistry). Physicochemical aspects of organic, organometallic, and bio- organic chemistry, including kinetic, mechanistic, structural, spectroscopic and theoretical studies. Such topics include structure-activity relationships and physical aspects of biological processes and of the study of polymers and biological macromolecules. Journal of Materials Chemistry. The chemistry of materials, particularly those associated with advanced technology; modelling of materials; synthesis and structural characteris- ation; physicochemical aspects of fabrication; chemical, structural, electrical, magnetic and optical properties; applic- ations.The Analyst (Analytical Science). Theory and practice of all aspects of analytical chemistry, fundamental and applied, inorganic and organic, including chemical, physical and biological methods. Journal of Analytical Atomic Spectrometry. The development and analytical application of atomic spectrometric techniques, including ICP MS. Journal of Chemical Research. All areas of chemistry. The format of this journal (one- or two-page printed synopsis in Part S, plus microform version of authors’ full text typescript in Part M) makes it particularly suitable for papers containing lengthy experimental sections or extensive data tabulations. 1.3 Procedure Each manuscript is considered independently by two referees.The referees’ reports constitute recommendations to the appropriate Editorial Board, which is empowered to take final action on manuscripts submitted. The Editor, acting for the Editorial Board, is responsible for all administrative and executive actions, and is empowered to accept or reject papers. It is the Editor’s duty to see that, as far as possible, agreement is reached between authors and referees; although the referees may need to be consulted again concerning an author’s reply to comments, further refereeing will be avoided as far as possible. 1.3.1 Adjudication of disagreements. If there is a notable discrepancy between the reports of the two referees, or if the difference between authors and referees cannot be resolved readily, a third referee may be appointed as adjudicator. In extreme cases, differences may be reported to the appropriate Editorial Board for resolution.When a paper is recommended for rejection by referees, the Editor will inform the authors and return the top copy of the manuscript. Authors have the right to appeal to the Editorial Board if they regard a decision to reject as unfair. The Editor may refer to the Editorial Boards any papers which have been recommended for acceptance by the referees, but about which the Editor is doubtful. 1.3.2 Anonymity. The anonymity of referees is strictly preserved, and reports should be couched in terms which do not disclose the identity of the writer. A referee should never communicate directly with an author, unless and until such action has been sanctioned by the Society, through the Editor.1.3.3 Conjidentiality. A referee should treat a paper received for assessment as confidential material. Information acquired by a referee from such a paper is not available for citation until the paper is published. REFEREEING PROCEDURE AND POLICY (1 994) 1.4 Policy The primary criterion for acceptance of a contribution for publication is that it should advance scientific knowledge significantly. Papers that do not contain new experimental results may be considered for publication only if they either reinterpret or summarise known facts or results in a manner presenting an advance in chemical knowledge.Papers in interdisciplinary areas are acceptable if the chemical content is considered satisfactory. Papers reporting results regarded as routine or trivial are not acceptable in the absence of other, desirable attributes. Although short papers are acceptable, the Society strongly discourages the fragmentation of a substantial body of work into a number of short publications; such fragmentation is likely to be grounds for rejection. The length of an article should be commensurate with its scientific content; however, authors are allowed every latitude (consistent with reasonable brevity) in the form in which their work is presented. Figures and flow-charts can often save space as well as clarify complicated arguments, and should not be excised unless they are unhelpful or really extrava- gant.If a paper as a whole is judged suitable for the Journal, minor criticisms should not be unduly emphasised. It is the responsibility of the Editor to ensure the use of reasonably brief phraseology, and to assist the author to present his work in the most appropriate format. However, referees should not hesitate to recommend rejection of papers which appear incurably badly com-posed. It should be clearly understood that referees’ reports are made in confidence to the Editor, at whose discretion comments will be transmitted to the author. To assist the Editor, referees are requested to indicate which comments are designed only for consideration, as distinct from those which, in the referee’s view, require specific action or an adequate answer before the paper is accepted.Referees may ask for sight of supporting data not submitted for publication, or for sight of a previous paper which has been submitted but not yet published. Such requests must be made to the Editor, not directly to the author. 1.4.1 Authentication of new compounds. Referees are asked to assess, as a whole, the evidence in support of the homogeneity and structure of all new compounds. No hard and fast rules can be laid down to cover all types of compounds, but the Society’s policy is that evidence for the unequivocal identification of new compounds should wherever possible include good elemental analytical data; for example, an accurate mass measurement of a molecular ion does not provide evidence of purity of a compound and must be accompanied by independent evidence of homogeneity.Low-resolution mass spectrometry must be treated with even more reserve in the absence of firm evidence to distinguish between alternative molecular formulae. Where elemental analytical data are not available, appropriate evidence which is convincing to an expert in the field may be acceptable. Spectroscopic information necessary to the assignment of structure should normally be given. Just how complete this information should be must depend upon the circumstances; the structure of a compound obtained from an unusual reaction or isolated from a natural source needs much stronger supporting evidence than one derived by a standard reaction from a precursor of undisputed structure.Referees are reminded of the need to be exacting in their standards but at the same time flexible in their admission of evidence. It remains the Society’s policy to accept work only of high quality and to permit no lowering of standards. 1.5 Titles and Summaries Referees should comment on titles and summaries with the following points in mind. Titles of papers are used out of context by several organizations for current awareness purposes. To enable such systems to serve chemists adequately, titles must be written around a sufficient number of scientific words carefully chosen to cover the important aspects of the paper. Summaries should preferably be self-contained, so that they can be understood without reference to the main text.1.6 Speed of Refereeing The Editorial Boards are anxious to maintain and to reduce further if possible the publication times now being achieved. In this connection, referees should submit their reports with the minimum of delay, or return manuscripts immediately to the Editor if long delay seems inevitable. 1.7 Suggestions of Alternative Referees The Editor welcomes suggestions of alternative referees competent to deal with particular subject areas. Such suggestions are particularly helpful in cases where referees consider themselves ill-equipped (in terms of specialist knowledge) to deal with a specific paper, and in highly specialized or new areas of research where only a limited number of experts may be available. If, in such a case, the alternative and the original referee work in the same institution, the manuscript may be passed on directly after informing the Editor.1.8 Short Papers and Letters ‘Short Papers’ are published in J. Chem. Research. They are intended for the description of essentially complete pieces of work which can be described in two printed pages or less. They are NOT preliminary communications, nor in any way an alternative to Chemical Communications, for which there are additional criteria of novelty and urgency. The quality of material contained in a short paper should be the same as that in a full paper. Investigations arising out of some larger project but not prosecuted to the same degree are particularly appropriate for this format.A short paper should not normally exceed in length about 8 pages of typescript, including figures, tables, etc. It should comprise a one-sentence abstract and discussion, but adequate experimental details are required. As a consequence of its length, it appears in full in Part S with no microform version in Part M. ‘Letters’, published only in Dalton Transactions, are a medium for the expression of scientific opinions and views normally concerning material published in that journal; it is intended that contributions in this format should be published rapidly. The letters section is for scientific discussion, and is not intended to compete with media for the publication of more general matters such as Chemistry in Britain.Only rarely should a Letter exceed one printed column in length (about 1-2 pages of typescript). Where a letter is polemical in nature, and if it is accepted, a reply will be solicited from other parties implicated, for consideration for publication alongside the original letter. 1.9 Relationship with Communications Journals In cases where a preliminary report of the work described has appeared (for example in Chemical Communications), referees should alert the editor to any excessive and unnecessary repetition of material; this can arise in connection with communications journals in which the restrictions on length ... Vlll REFEREEING PROCEDURE AND POLICY (1994) and the reporting of experimental data are less severe than those of Chemical Communications.Furthermore, the acceptability of the full paper must be judged on the basis of the significance of the additional information provided, as well as on the criteria outlined in the foregoing sections. 2.0 Contributions to Chemical Communic-ations Chemical Communications is intended as a forum for preliminary accounts of original and significant work, in any area of chemistry that is likely to prove of wide general appeal or exceptional specialist interest. Such preliminary reports should be followed up in most cases by full papers in other journals, providing detailed accounts of the work. It is Society policy that only a fraction of research work warrants publication in Chemical Communications, and strict refereeing standards should be applied.The benefit to the reader from the rapid publication of a particular piece of work before it appears as a full paper must be balanced against the desirability of avoiding duplicate publication. The needs of the reader, not the author, must be considered, and priority in publication should not be allowed to determine acceptability. Acceptance should be recommended only if, in the opinion of the referee, the content of the paper is of such urgency that rapid publication will be advantageous to the progress of chemical research. The length of Communications is strictly limited; only in exceptional circumstances should it exceed one printed page (two-and-a-half to three A4 pages of typescript) and referees should be particularly critical of manuscripts longer than this.Communications do not contain extensive spectroscopic or other experimental data, but referees may ask for sight of such data before reaching a decision. The refereeing procedure for Communications is the same as that for full papers, except that rapidity of reporting is crucial in order to maintain rapid publication. 3.0 Communications submitted to Analytical Proceedings and J. Anal. At. Spectrom. Criteria for acceptance of communications submitted to Analytical Proceedings and J. Anal. At. Spectrom. are similar to those for contributions to Chemical Communications, except that they should be concerned specifically with analytical chemistry.A decision whether or not to publish rests with the Editor, who will obtain advice from at least one referee. 4.0 Communications submitted to Perkin, Dalton or Faraday Transactions or J. Mater. Chem. Criteria for acceptance of Communications submitted to Perkin, Dalton or Faraday Transactions or J. Mater. Chem. are similar to those for contributions to Chemical Communications, except that the work will be of more specialist interest. For Perkin and Dalton Communications inclusion of key experi- mental data is expected. Assessment is carried out by a small nucleus of referees, consisting largely of members of the appropriate Editorial Boards. 5.0 Contributions to Mendeleev Communic- ations Mendeleev Communications, published jointly by the Royal Society of Chemistry and the Russian Academy of Sciences, is a sister publication to Chemical Communications, containing preliminary reports of the same type, in any area of chemistry.The majority of contributions are from Russian authors. Assessment involves two stages of refereeing. Manuscripts submitted to the Moscow Editorial Office are refereed initially by a Russian scientist. If found acceptable they are then reviewed by Western scientists chosen by the Royal Society of Chemistry. Manuscripts submitted to the UK Editorial Office undergo this two-stage refereeing process in reverse. 6.0 X-Ray Crystallographic Work 6.1 All papers containing crystallographic determinations will be refereed by two referees, one a structural chemist.If the editor considers it advisable, the paper may also be sent to a specialist crystallographer for comment. Referees will not normally be expected to check values of structural parameters for publication (e.g. bond lengths and angles against atomic co- ordinates; this will be done after publication by the appropriate crystallographic data centre), but should still pay attention to the quality of the experimental crystallographic work. However their primary concern should be such new chemistry as is involved in the structure. 6.2 Papers will often contain the information in their titles that an X-ray structure determination has been carried out. However, this is not obligatory, especially if the X-ray determination forms only a minor part.Summaries should normally contain this information. 6.3 A structure referred to in a Communiciition will normally be fully refined. The Communication can then be considered to fulfil the archival function, and the structure determination may not require further detailed refereeing when presented as part of a full paper. In the full paper, the author’s purpose will then be served by a simple reference back to the original communication. However, if the crystallography is discussed again at any length in the full paper, the data should be re-presented to the referees in full, and re-published if considered necessary. 6.4 There may be other cases when an author wishes to publish a full paper in which the result of a crystal structure determination is discussed, but in which details or extensive discussion are considered unnecessary.The crystallographer may even be omitted as a co-author (for example when the determination is carried out by a commercial company). If the author is able to show the referees that this procedure is appropriate, it will be allowed provided that it does not lead to unnecessary fragmentation. However, the author must provide, as supplementary information, sufficient data relating to the crystal structure determination to allow a referee to make sure that the point made is correct, and co-ordinates rtc. will be deposited. The brief published description of the determination should be supplemented by appropriate reference to ‘unpub- lis hed work’. ix INSTRUCTIONS FOR AUTHORS (1994) APPENDIX IUPAC Publications on Nomenclature and Symbolism 1.O Compilations 1.1 Nomenclature of Organic Chemistry, a 550-page hardcover volume published in 1979, available from Pergamon, Oxford.Section A: Hydrocarbons Section B: Fundamental heterocyclic systems Section C: Characteristic groups containing carbon, hy- drogen, oxygen, nitrogen, halogen, sulfur, selenium and tellurium Section D: Organic compounds containing elements not exclusively those referred to in the title of Section C Section E: Stereochemistry Section F: General principles for the naming of natural products and related compounds Section H: Isotopically modified compounds 1.2 A Guide to IUPAC Nomenclature of Organic Compounds, a 182-page hardcover volume published in 1993, available from Blackwell Scientific Publications, Oxford, to be used in conjunction with item 1.1.1.3 Nomenclature of Inorganic Chemistry, a 278-page hardcover volume published in 1990, available from Blackwell Scientific Publications, Oxford. Chapter 1: Chapter 2: Chapter 3: Chapter 4: Chapter 5: Chapter 6: Chapter 7: Chapter 8: Chapter 9: Chapter 10: General aims, functions and methods Grammar Elements, atoms and groups Formulae Names based on stoichiometry Neutral molecular compounds Names for ions, substituent groups and radicals, and salts Oxoacids and derived anions Co-ordination compounds Boron hydrides and related compounds 1.4 Biochemical Nomenclature and Related Documents, a 348-page softcover manual published in 1992 by Portland Press Ltd.for IUBMB, and available from the publisher (59 Portland Place, London WIN 3AJ, UK). The contents are as follows: Nomenclature of organic chemistry. Section E: Stereo- chemistry (1974) Nomenclature of organic chemistry. Section F: Natural products and related compounds (1976) Isotopically modified compounds Recommendations for the presentation of thermodynamic and related data in biology (1985) Citation of bibliographic references in biochemical journals ( 197I) Nomenclature and symbolism for amino acids and peptides (1983) Abbreviated nomenclature of synthetic polypeptides or polymerized amino acids (1 971) Abbreviations and symbols for the description of the conformation of polypeptide chains (1969) Nomenclature of peptide hormones (1974) Nomenclature of glycoproteins, glycopeptides and peptidoglycans ( 1985) Nomenclature of initiation, elongation and termination factors for translation in eukaryotes (I 988) Nomenclature of multiple forms of enzymes (1976) Symbolism and terminology in enzyme kinetics ( 1981) Nomenclature for multienzymes (1989) Abbreviations and symbols for nucleic acids, poly- nucleotides and their constituents (1970) Abbreviations and symbols for the description of the conformations of polynucleotide chains (I 982) Nomenclature for incompletely specified bases in nucleic acid sequences (1 984) Carbohydrate nomenclature.Part I (1 969) Nomenclature of cyclitols (1 973) Numbering of atoms in myo-inositol(1988) Conformational nomenclature for five- and six-membered ring forms of monosaccharides and their derivatives (1980) Nomenclature of unsaturated monosaccharides (1980) Nomenclature of branched-chain monosaccharides (1 980) Abbreviated terminology of oligosaccharide chains (1980) Polysaccharide nomenclature (1980) Symbols for specifying the conformation of polysaccharide chains (1981) Nomenclature of lipids (1976) Nomenclature of steroids (1989) Nomenclature of quinones with isoprenoid side chains (1973) Nomenclature of carotenoids (1970) and amendments (1974) Nomenclature of tocopherols and related compounds (1981) Nomenclature of vitamin D (1981) Nomenclature of retinoids (1981) Prenol nomenclature (1986) Nomenclature of phosphorus-containing compounds of biochemical importance (1976) Nomenclature and symbols for folk acids and related compounds (1 986) Nomenclature for vitamins B-6 and related compounds (1 973) Nomenclature of corrinoids (1973) Nomenclature of tetrapyrroles (1986) 1.5 Compendium of Analytical Nomenclature, a 280-page hardcover volume published in 1987, available from Blackwell Scientific Publications, Oxford.The contents are as follows: Presentation of the Results of Chemical Analysis Solution Thermodynamics (activity coefficients, equilibria, PH)Recommendations for Terminology to be used with Precision Balances Recommendations for Nomenclature of Thermal Analysis Recommendations for Nomenclature of Titrimetric Analysis Electrochemical Analysis Analytical Separation Processes (precipitation, liquid- liquid distribution, zone melting and fractional crystallis- ation, chromatography, ion exchange) Spectrochemical Analysis (radiation sources, general atomic emission spectroscopy, flame spectroscopy, X-ray emission spectroscopy, molecular methods) Recommendations for Nomenclature of Mass Spec-trometry Recommendations for Nomenclature of Radiochemical Methods Surface Analysis (including photoelectron spectroscopy) X INSTRUCTIONS FOR AUTHORS ( 1994) 1.6 Compendium of Macromolecular Nomenclature, a 172-page hardcover volume published in 199 1, available from Blackwell Scientific Publications, Oxford.The contents are as follows: Basic Definitions of Terms Relating to Polymers Stereochemical Definitions and Notations Relating to Polymers Definitions of Terms Relating to Individual Macromolecules, their Assemblies, and Dilute Polymer Solutions Definitions of Terms Relating to Crystalline Polymers Nomenclature of Regular Single-strand Organic Polymers Nomenclature for Regular Single-strand and Quasi-single- strand Inorganic and Coordination Polymers Source-based Nomenclature for Copolymers A Classification of Linear Single-strand Polymers Use of Abbreviations for Names of Polymeric Substances 1.7 Compendium of Chemical Terminology: IUPAC Recommendations, a 456-page volume published in 1987, available in hardcover and softcover from Blackwell Scientific Publications, Oxford.I .8 Quantities, Units and Symbols in Physical Chemistry, a 166-page softcover volume published in 1993 by Blackwell Scientific Publications, Oxford. 2.0 Documents not included in the compil- ations 2.1 Nomenclature of Elements and Compounds Boron Compounds Nomenclature of inorganic boron compounds (Pure Appl. Chem., 1972,30,681). Delta Convention Nomenclature for cyclic organic compounds with contiguous formal double bonds (Pure Appl. Chem., 1988,60, 1395). Elements Recommendations for the names of elements of atomic number greater than 100 (Pure Appl. Chem., 1979,51, 381). Enzymes Enzyme Nomenclature (1992), published by Academic Press in hardcover and softcover editions.Heterocyclic Compounds Revision of the extended Hantzsch-Widman system of nomenclature for heteromonocycles (Pure Appl. Chem., 1983, 55,409). Hydrogen Names for hydrogen atoms, ions and groups, and for reactions involving them (Pure Appl. Chem., 1988,60, 1115). Isotopically Modzjied Compounds Nomenclature of inorganic chemistry. Part 11. 1. Isotopically modified compounds (Pure Appf. Chem., 198 1,53,1887). tam bda Con uen t ion Treatment of variable valence in organic nomenclature (Pure Appl. Chem., 1984, 56, 769). Nitrogen Hydrides Nomenclature of hydrides of nitrogen and derived cations, anions and ligands (Pure Appl. Chem., 1982,54,2545).Numerical Terms Extension of Rules A-1.1 and A-2.5 concerning numerical terms used in organic chemical nomenclature (Pure Appl. Chem., 1986,58, 1693). Polyanions Nomenclature of polyanions (Pure Appl. Chem., 1987,59,1529). Polymers Nomenclature of regular double-strand (ladder and spiro) organic polymers (Pure Appl. Chem., 1993,65, 1561). Radicals and Ions Revised nomenclature for radicals, ions, radical ions and related species (Pure Appl. Chem., 1993, 65, 1357). Zeolites Chemical nomenclature and formulation of compositions of synthetic and natural zeolites (Pure Appl. Chem., 1979, 51, 1091). 2.2 Terminology, Symbols and Units, and Presentation of Results General Glossary of terms used in physical organic chemistry (Pure Appl.Chem., 1983,55, 1281). Glossary of atmospheric chemistry terms (Pure Appl. Chem., 1990, 62, 2 167). English-derived abbreviations for experimental techniques in surface science and chemical spectroscopy (Pure Appl. Chem., 1991,63, 887). Analytical Recommendations for publication of papers 011 a new analytical method based on ion exchange or ion-exchange chromatography (Pure Appl. Chem., 1980,52,2555). Recommendations for presentation of data on compleximetric indicators, 1. General (Pure Appl. Chern., 1979,51, 1357). Recommendations for publishing manuscripts on ion-selective electrodes (Pure Appl. Chem., 1981, 53, 1907). Recommendations on use of the term amplification reactions (Pure Appl. Chem., 1982,54,2553). Recommendations for the usage of selective, selectivity and related terms in analytical chemistry (Pure Appl.Chem., 1983, 55, 553). Nomenclature for automated and mechanised analysis (Pure Appl. Chem., 1989,61, 1657). Nomenclature for sampling in analytical chemistry (Pure Appl. Chem., 1990,62, 1193). Nomenclature for chromatography (Pure Appl. Chem., 1993, 65, 8 19). Biotechnology Glossary for chemists of terms used in biotechnology (Pure Appl. Chem., 1992,64,143). Selection of terms, symbols and units related to microbial processes (Pure Appl. Chem., 1992,64, 1047). Clinical Physicochemical quantities and units in clinical chemistry with special emphasis on activities and activity coefficients (Pure Appl. Chem., 1984,56, 567). Quantities and units in clinical chemistry (Pure Appl.Chem., 1979,51,2451). Quantities and units in clinical chemistry: nebulizer and flame properties in flame emission and absorption spectrometry (Pure Appf. Chem., 1986,58,1737). List of quantities in clinical chemistry (Pure Appl. Chem., 1979, 51,2481). Proposals for the description and measurement of carry-over effects in clinical chemistry (Pure Appl. Chem., 1991.63, 301). Quantities and units for metabolic processes as a function of time (Pure Appl. Chem., 1992,64, 1569). Glossary for chemists of terms used in toxicology (Pure Appl. Chem., 1993,65, 2003). Colloids and Surface Chemistry Definitions, terminology and symbols in colloid and surface chemistry. I (Pure Appl. Chem., 1972, 31, 577).[I, Hetero- geneous catalysis (Pure Appf. Chem., 1976, 46, 71). Part 1.14: Light scattering (provisional) (Pure Appl. Chem., 1983, 55, 93 1). Reporting experimental pressure-area data with film balances (Pure Appl. Chem., 1985,57,621). xi INSTRUCTIONS FOR AUTHORS (1994) Reporting physisorption data for gas/solid systems with special reference to the determination of surface area and porosity (Pure Appl. Chem., 1985,57,603). Reporting data on adsorption from solution at the solid/ solution interface (Pure Appl. Chem., 1986,58,967). Manual on catalyst characterization (Pure Appl. Chem., 1991, 63, 1227). Electrochemistry Nomenclature for transfer phenomena in electrolytic systems (Pure Appl. Chem., 1981,53, 1827).Electrode reaction orders, transfer coefficients and rate constants-amplification of definitions and recommendations for publication of parameters (Pure Appl. Chem., 1980,52,233). Classification and nomenclature of electroanalytical techniques (Pure Appl. Chem., 1976,45, 8I). Recommendations for sign conventions and plotting of electrochemical data (Pure Appl. Chem., 1976,45, 13 1). Electrochemical nomenclature (PureAppl. Chem., 1974,37,499). Recommendations on reporting electrode potentials in non- aqueous solvents (Pure Appl. Chem., 1984,56,461). Definition of pH scales, standard reference values, measurement of pH and related terminology (Pure Appl. Chem., 1985, 57, 531). Interphases in systems of conducting phases (PureAppl. Chem., 1986,58, 437). The absolute electrode potential: an explanatory note (Pure Appl.Chem., 1986,58,955). Electrochemical corrosion nomenclature (Pure Appl. Chem., 1989, 61, 19). Terminology in semiconductor electrochemistry and photo- electrochemical energy conversion (Pure Appl. Chem., 1991,63, 569). Nomenclature, symbols, definitions and measurements for electrified interfaces in aqueous dispersions of solids (PureAppl. Chem., 1991,63, 895). Nomenclature, symbols and definitions in electrochemical engineering (Pure Appl. Chem., 1993,65, 1009). Kinetics Symbolism and terminology in chemical kinetics (provisional) (Pure Appl. Chem., 198 1,53,753). Photochemistrj. Recommended standards for reporting photochemical data (Pure Appl.Chem., 1984,56,939). Glossary of terms used in photochemistry (Pure Appl. Chem., 1988.60, 1055). Quantum Chemistry Expression of results in quantum chemistry (Pure Appl. Chem., 1978,50, 75). Reactions Nomenclature for organic chemical transformations (Pure Appl. Chem., 1989,61, 725). System for symbolic representation of reaction mechanisms (Pure Appl. Chem., 1989,61,23). Detailed linear representation of reaction mechanisms (Pure Appl. Chem., 1989,61, 57). RheoIogicaI Properties Selected definitions, terminology and symbols for rheological properties (Pure Appl. Chem., 1979,51, 1215). Spectroscopy Recommendations for publication of papers on methods of molecular absorption spectrophotometry in solution (Pure Appl. Chem., 1978,50, 237).Recommendations for the presentation of infrared absorption spectra in data collections. A, Condensed phases (Pure Appl. Chem., 1978,50,231). Definition and symbolism of molecular force constants (Pure Appl. Chem., 1978,50, 1709). Nomenclature and conventions for reporting Mossbauer spectroscopic data (Pure Appl. Chem., 1976,45,211). Recommendations for the presentation of NMR data for publication in chemical journals. A, Proton spectra (Pure Appl. Chem., 1972,29,625). B, Spectra from nuclei other than protons (Pure Appl. Chem., 1976,45,217). Presentation of Raman spectra in data collections (Pure Appl. Chem., 1981,53, 1879). Names, symbols, definitions and units of quantities in optical spectroscopy (Pure Appl. Chem., 1985,57, 105). A descriptive classification of the electron spectroscopies (Pure Appl. Chem., 1987,59, 1343). Presentation of molecular parameter values for IR and Raman intensity (Pure Appl. Chem., 1988,60, 1385). Recommendations for EPR/ESR nomenclature and conven- tions for presenting experimental data in publications (Pure Appl. Chem., 1989,61,2195). Nomenclature, symbols, units and their usage in spectro- chemical analysis. VII. Molecular absorption spectroscopy, UV and visibla (Pure Appl. Chem., 1988, 60, 1449); VIII. Nomenclature system for X-ray spectroscopy (Pure Appl. Chern., 1991,63,735); X. Preparation of materials for analytical atomic spectroscopy (Pure Appl. Chem., 1988, 60,1461); XII. Terms related to electrothermal atomization (PureAppl. Chem., 1992, 64, 253); XIII. Terms related to chemical vapour generation (Pure Appl. Chem., 1992,64,261). Recommendations for nomenclature and symbolism for mass spectroscopy (PureAppl. Chem., 1991,63, 1541). Thermodynamics A guide to procedures for the publication of thermodynamic data (PureAppl. Chem., 1972,39, 395). Assignment and presentation of uncertainties of the numerical results of thermodynamic measurements (Pure Appl. Chem., 1981,53, 1805). Notation for states and processes; significance of the word ‘standard’ in chemical thermodynamics and remarks on commonly tabulated forms of thermodynamic functions (Pure Appl. Chem., 1982,54, 1239). xii

ISSN:0959-9428    

DOI:10.1039/JM99404BP007

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( l), 9-12 Ion-exchange Properties of NASICON-type Phosphates with the Frameworks [TI,(PO,),I and [Ti, m7Alon3(P04)3] Naohiro Hiroset and Jun Kuwano" Department of Industrial Chemistry, Faculty of Engineering, Science University of Tokyo, 7 -3 Kagurazaka, Shinjuku-ku, Tokyo, 762 Japan Ion-exchange properties in aqueous A+ solution (A+=Li+, Na', K+, Ag', Rb+, NH:) are investigated for tne title materials at room temperature. In LiTi,(PO,), and Al-containing Li,.3Til.,AIo.,(P0,)3, Li ' is exchanged rapidly and selectively by Na' and Ag'. The exchange rate of Lil,3Til.7A10.3(P04)3 is faster than that of LiTi,(PO,),; this is probably related to a larger diffusion coefficient of Li' in the former due to its high Li' conductivity. Large ions such as K+ are not readily accepted by the anion frameworks.Accordingly, Lil,,Ti,,7Alo.3(P0,)3 has excellent Na+ selectivity with fast exchange rate to extract even a very small amount of Na' in reagent-grade KOH. The Na+-exchanged product of Lil,3Til.7A10.3(P04)3 can exchange Na' for Ag selectively. The reverse exchange also occurs easily. This material can + be used to recover Ag' in waste solutions. Some kinds of inorganic ion exchangers show excellent selec- tivity for a particular ion and high stability towards heat, chemical reagents and radioactivity compared with organic polymer resins. This stems from their rigid anion frameworks, which consist of covalently bonded 02-and multivalent cations; they behave like ion sieves. This 'ion-sieve effect' can be useful for collecting ions from sea water and waste solutions. The rate-determining step in the exchange is generally con- sidered to be the diffusion of the exchanging ions in the solid.',2 Fast-ion-conducting oxides, such as fi/fi"-A1203 or NASICONs, have rigid anion frameworks and exhibit high conductivities due to the large diffusion coefficients of the monovalent cations.This suggests that these materials should exhibit selective and fast-ion exchange even at room tempera- ture. Ono has reported, that the protonated NASICON-type compound, HZr,( PO,),, shows a selectivity for monovalent cations in aqueous solution in the order: Li', Na+>> Rb' >K+ >NH:; similar results were also described by Alberti.' However, so far most of the exchange properties of fast-ion conductors have been extensively studied only in molten salts6 in spite of the practical importance of ion exchange in aqueous solution.We have previously investigated the ion-exchange properties of Li+-conducting NASTCONs, LiTi2( PO,), [gbulk lo-, Scm-l (Aono et aL7+*)to Scm-' (Ando et al.9"0) at 25 "C] and its Al-containing form, Lil.3Til.7Alo.3(PO,), (Gbu1k% lop3 S cm-' at 25 0C)7-10 in aqueous solution, and have reported"*12 the rapid exchange rates of these materials and good selectivity of their frameworks for monovalent cations in the order: Ag' >Na' >Li+>> Kf , Rb+, NH;. Some of our previous have been recently confirmed by Mizuhara et al., who reported', exchange properties of LiTi,Zr2-x(P0,)3, and also by Hosono et a!., who reported', exchange properties of bulk, microporous LiTi,( PO,), pre-pared as a glass ceramic. This paper describes further ion-exchange properties of the title phosphates in aqueous solution at room temperature.The excellent selectivity of these materials towards a particular ion leads to possible applications. Experimental Samples of LiTi,( PO,), and Lil.3Til,7Alo,3( PO,), were pre- pared by conventional solid-state reactions. The starting mate- t Present address: Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen, AB9 2UE, Scotland, UK. rials, Li2C0,, Ti02, A1203, and NH4H2P04 were mixed in hexane, dried and heated at 650 "C for 2 h to drive off' gases.The intermediates were ground thoroughly and calcined at 900 "Cfor 15 h. The products were reground and used in the following experiments. The average particle size was fcyund to be ca. 0.5-2 pm by SEM analysis. Ion-exchange treatment was carried out in 100ml of 0.25 moll-' aqueous solutions of alkali-metal chlorides and NH,Cl at 25 "C for 4days using 1 g amounts of sample. For the Ag' exchange, nitrate solution was used to avoid the precipitation of AgC1. The solutions were stirred continuously to disperse the exchanger particles and to enhance diffusion at the solid/liquid interface. Some experiments were carried out for different exchange times and with different initial c oncen-trations of the solutions. After the exchange treatment, the products were collected by filtration, washed with distilled water and dried under vacuum overnight at room temperature.The phases present in the samples before and after exchange attempts were identified by powder X-ray diffraction analysis (XRD) (Rigaku, RAD-IC; Kcr radiation with Ni filter). The Ag' concentration in some of the Ag+ exchange solutions was measured with an Ag' selective electrode (Toa Denpa; AG-125, 95% response time: 5-10 s); this allowed the exchange behaviour with exchange time to be studied Results and Discussion Exchange properties were also studied for the Na' and Ag' exchanged products of Lil.3Til,7Alo,3( PO,),. These results are summarized in Table 1 together with our previous results."*12 As reported ca.99% of Li + in LiTi:,( PO,), and Lil~3Til~7Alo,3( PO,), was exchanged by Na+ and Ag'. Fig. 1 and 2 show the variations of XRD patterns during the exchange. The end-member compound, LiTi2( PO,),, exhibited complete exchange in 28 days for Na' and in 4days for Ag'. The exchanged products were identified to be NaTi,( PO,), and AgTi,(PO,), using JCPDS cards (No. 33-1296, 35-737). In contrast, very rapid exchange was observed for Li1.3Ti1.7A10,3(PO,),. The exchange was almost completed in 30 min for Na' and in only 3 min for Ag'. The difference of exchange rate between Lil~,Ti,~7Al,~,(P04)3 and LiTiz(PO,), is probably due to the large diffusion coefficient of Li in the former, as expected from its high Li' conductivity 7-10 As evidently seen from these figures, the exchanged products form as separate phases, by a process of phase sepdration, J.MATER. CHEM., 1994, VOL. 4 Table 1 Extent of A+ exchange of MTi,(PO,),, Li,,3Til,7Alo,,(P04)3and the exchanged products of Li,~,Til~7Alo.,(P0,)3with 0.25 mol I-' A+ aqueous solution at 25 "Cfor 4 days" Li+ compound ionic radius/pm? 90 -LjTi2( PO4), NaTi,( PO,), N AgTi2( N KTiA PO413 N -Li1.3T1.7A10.3(Na +-exchanged product N Ag+-exchanged product N "C =complete exchange; P =partial exchange; N =no accompanied the exchange. A ? 0 10 20 30 Na+ Ag + K+ Rb' NH: 116 129 152 166 166 Ph Cb N N N -P' N N N -NN N N -NN N N Cb Cb N N N -C' N N N -NPC N N exchange.bPhase separation accompanied the exchange. 'Solid-solution formation B 0 A T A 40 10 20 30 40 2Oldegrees Fig. 1 X-Ray diffraction patterns of the exchanged products (triangles) during Na+ exchange of (A) LiTi,( PO,), and (€3) Li,,3Til,7Alo.3(PO,), (circles).A: (a) before exchange, (b)4, (c) 8, (d)28 days; B: (a) before exchange, (b)10, (c) 30 min rather than forming a range of partially exchanged solid solution. For Li1.,Til .7A10.3 (PO,),, excellent selectivity was observed in addition to the very rapid exchange. Fig. 3 shows the XRD patterns before and after treatment with 300ml of 3 mol 1-' KOH for 3 h. Surprisingly, the pattern after the treatment corresponded exactly to that of the Na+-exchanged product phase. It is clear that this compound had exchanged Li' for Na', which is contained in commercial KOH reagents as a small impurity (min.1 wt.%). From a thermodynamic view- point Kf exchange is expected to take place, but no K+-exchanged product was seen in the XRD pattern (Fig. 3). We also confirmed the absence of K+ in this product by atomic absorption spectrometry. This suggests that the K+ exchange is not preferred for kinetic reasons. As mentioned earlier, the rate-determining step in an exchange reaction can safely be assumed to be the diffusion of the exchanging ions in the solid. The diffusion coefficient is expressed by the Arrhenius equation using a pre-exponential factor, Do, and the Boltzmann constant, k,: D =Do exp(-AE/k, T) where AE is the activation energy of diffusion.Since large ions such as K+ require large activation energies to expand the relatively small framework of [Til.7Alo,3( PO,),], their diffusion coefficients in the framework should be very small. Thus, the framework acts as a sieve to K'. In NaTi,(P04)3, Na' was partially and selectively exchanged by Ag' leading to the formation of solid-solution Ag,Na, -xTi2( PO4), after a 4 day exchange treatment (Table 1).12 As expected, the Na+-exchanged product of Lil,3Til.7Alo.3(PO,), also exchanged Na' for Ag' and resulted in a fully exchanged product in 4 days. Fig. 4 shows the extent of Ag' exchange with time in Ag' exchange solutions for Lil~3Ti~~7Alo~3( and the Na +-exchanged PO,), product. The Na+-exchanged product exhibited a fast Ag+- exchange rate although not comparable to the very rapid Ag 'exchange of Lil.3Til,7Alo~3( PO,),.The reverse exchange, in which Ag+ in the Ag+-exchanged product of Li1.3Ti1.7A10.3( PO,), was partially exchanged by Na+ is shown in Table 1. When the Agf-exchanged product was treated with an NaNO, solution with a higher concen- tration of 1.5 moll-' the reverse exchange was almost com- pleted (Fig. 5). J. MATER. CHEM., 1994, VOL. 4 0 I 0 D I I 0 0 0 10 I 1 219/degrees Fig. 2 X-Ray diffraction patterns of the exchanged products (squares) during Ag+ exchange of (A) LiTi2(P04), and (B) Li1.3Ti1.7A103(P04)3 (circles).A: (a) before exchange, (b)1, (c)4 days; B: (a) before exchange, (b) 3, (c) 10 min 0 0 0 A A 10 20 30 40 2Bldegrees Fig.3 X-Ray diffraction patterns of (a) Li~,~Ti~,,Al~.,(PO~)~,(b)the Na+-exchanged product (triangles) after treatment with 3 mol I-' KOH containing a minimum of 1 wt.% Na+ and (c) KTi2(P04), (for comparison) A solid solution, Nal.,Til,7Alo.3( PO,),, which is probably the same composition as the Na+-exchanged product of Li1.3Ti1.7A10.3(PO,),, has been easily prepared by solid-state reaction at 1100"C using inexpensive Na salts. Moreover, the two compounds showed almost the same exchange behav- 3 80-v -awC 2 60-0X a 2 40-).0 c c2 20-01 tlmin Fig.4 The extent of Ag+ exchange uersus time in 0.034 mol 1-' AgNO, solution for Lil.3Til.7Alo,3( PO,), (circles) and the Na+- exchanged product (triangles).For each sample, 0.6 g was used. iour." This inorganic exchanger based on the [Til.7Alo.,(PO,),] framework is stable towards heat and chemical reagents such as strong acids (apart from phosphoric acid and acids with H20212315)and bases (see Fig. 3) and has a relatively large ion-exchange capacity (ca. 3.2 mequiv. g- '). Thus, this selective and rapid exchange (Table 1 and Fig. 4) for Ag' can be used to recover Ag' in waste solutitrns, e.g. those produced by plating industries and photographic processes. Conclusions (i) Very rapid and selective exchange of Na' and Ag+ for Li' takes place in the solid solution, Lil.,Til,,AlO., (PO,),; the exchange rate is faster than that of the end member, 10 20 30 40 2Bldegrees Fig.5 X-Ray diffraction patterns of the exchanged products of Lil,3Til,7Alo,3(PO& with sequential treatments: (a) Na+-exchanged product+) Agt-exchanged product+(c) treatment of (b) with 1.5 moll-' NaNO, LiTi,(PO,),.This is probably due to the higher Li+ conduc- tivity in the former. (ii) Large ions such as K+ have large activation energies for the replacement of Li+ in the small framework of these materials. The K+ is therefore sieved kinetically by the frameworks. This sieve effect is clearly seen by the preferential exchange of Na+ in the treatment of Lil.3Til.7Alo.3( PO,), with aqueous solution of a commercial KOH reagent which contains a small amount of Na' as an impurity.(iii) The solid solution, Lil.3Til.7Alo.3( PO,)3, may be exchanged sequentially by Na' and then Ag+. The Na/Ag exchange is reversible, but the Li/Na(Ag) exchange is not, under the conditions studied here. The Na+-exchanged prod- J. MATER. CHEM., 1994, VOL. 4 PO,),, i.e. Nal.3Til.7Alo.3(uct of Lil.3Til,7Alo,3( PO,),, can be used to recover Ag' in waste solutions. The research was supported in part by a grant from the Japan Private School Promotion Foundation. We are grateful to Professor A.R. West for useful suggestions. References 1 A. Clearfield, Chem. Rev., 1988,88, 125. 2 W. A. England, J. B. Goodenough and P. J. Wiseman, J. Solid State Chem., 1983,49, 289. 3 J. B. Goodenough, H. Y-P. Hong and J. A. Kafalas, Muter. Rex Bull., 1976,11,203; K.D. Kreuer, H. Kohier and J. Maier, in High Conductivity Solid Ionic Conductors, ed. T. Takahashi., World Scientific, Singapore, 1989, p. 242; G. Collin and J. P. Boilot, in Super Ionic Solids and Solid Electrolytes, ed. A. L. Laskar and S. Chandra, Academic Press, London, 989, p. 227. 4 A. Ono, J. Muter. Sci., 1984, 19,2691. 5 M. A. Alberti, in Inorganic Ion Exchange Materials, ed. A. Clearfield, CRC Press, Boca Raton, 1982, p. 75. 6 Y-F. Y. Yao and J. T. Kummer, J. Inorg. Nucl. Chem., 1967, 29, 2453; H. Y-P. Hong, Muter. Res. Bull., 1976, 11, 173; F. d'Yvoire, M. Pintard-Screpel, E. Bretey and M. de la Rochere, Solid Stute lonics, 1983,9 & 10,851; A. Mbandza, E. Bordes and P. Courtine, Muter. Res. Bull., 1985, 20, 251; J.C. Couturier, J. Angenault and M. Quarton, Muter. Res. Bull., 1991,26, 1009. 7 H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka and G. Adachi, J. Electrochem. Soc., 1990, 137, 1023. 8 H. Aono, E. Sugimoto, Y. Sadaoka, N. Imanaka and G. Adachi, Chem. Lett., 1990, 1825. 9 Y. Ando, N. Hirose, J. Kuwano, M. Kato and H. Otsuka, in Ceramics Today- Tomorrow 's Ceramics, ed. P. Vincenzini, Elsevier, Amsterdam, 1991, p. 2245. 10 Y. Ando, N. Hirose, J. Kuwano, M. Kato and H. Otsuka, Phosphorus Res. Bull., 1991, 1,239. 11 N. Hirose, Y. Ando, J. Kuwano and M. Kato, in Ceramics Today- Tomorrow's Ceramics, ed. P. Vincenzini, Elsevier, Amsterdam, 1991, p. 2695. 12 N. Hirose, Y. Ando, J. Kuwano and M. Kato, in New Developments in Ion Exchange, ed. M. Abe, T. Kataoka and T. Suzuki, Kodansha/Elsevier, Tokyo, 199 1, p. 99. 13 Y. Mizuhara, K. Hachimura, T. Ishihara and N. Kubota, Chem. Lett., 1992, 1271. 14 H. Hosono, K. Imai and Y. Abe, J. Electrochem SOC., 1993, 140, L7. 15 N. Hirose and J. Kuwano, unpublished data. 16 R. D. Shannon, Acta Crystallogr., Sect. A, 1976,32, 751. Paper 3/04360F; Received 23rd July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400009

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( l), 13-1 7 Mechanisms of Pyrolysis of Organometallic Deposition Precursors lain M. T. Davidson, Andrew M. Ellis, Graham P. Mills, Mark Pennington, Ian M. Povey, J. Barrie Raynor, Douglas K.Russell,*t Sinan Saydam and Andrew D. Workman Department of Chemistry, University of Leicester, Leicester, UK LEI 7RH The gas-phase pyrolysis mechanisms of a number of potential transition-metal deposition precursors have been investigated using the techniques of: (i)infrared laser powered homogeneous pyrolysis coupled with product identifi- cation by FTIR, NMR and GC-MS; (ii) stirred Flow Reactor kinetic measurements; (iii) EPR spectroscopy of matrix- isolated free radicals. Preliminary results are presented for: (a) MeMn(CO), and AcMn(CO),, both alone and in the presence of Me,SiH; (b)C,H,Mn(CO), and MeC,H,Mn(CO),; (c) C,H,Fe(CO),; and (d) (C,H&Fe, all of which provide clear evidence that purely homogeneous pathways can be very different from those of surface-catalysed decomposition.In contrast to the long-established use of main-group organometallic compounds,' the potential of volatile organo- transition metal compounds as precursors for the deposition of metals, metal oxides, metal silicides, and other compounds has come to be recognised only recently. Until the mid 1980s, deposition of materials containing metals such as W, Ta, Ti or Mo was achieved using volatile fluorides or chlorides. Halides are not always volatile or convenient, however, and frequently generate films heavily contaminated with the hal- ogen, as well as hazardous by-products.Although binary transition-metal carbonyls usually produce very clean films, these compounds bring their own problems of handling and toxicity. For these reasons, attention has turned to simple organic derivatives of carbonyls. For example, Nouhi and Stirn2 have shown that tricarbonyl(methylcyclopentadieny1)-manganese MeC,H,Mn(C0)3 (MCMT), has potential as a manganese precursor, and Pain et aL3 have discussed the advantages of pentacarbonyl(methy1)manganese(MMP) as a dopant source and in the production of magnetic materials such as manganese tellurides. Although some of these potential precursors have been the subject of some investigations of an empirical nature, rather little is known of the fundamental mechanisms involved in their thermal decomposition. As has been trenchantly demonstrated in main group systems, infor- mation of this sort can often lead to the rational design of new precursors of desirable physical, chemical and economic properties.We have shown that the adaptation of methods well estab- lished in physical chemistry can provide unique insights into the mechanisms of metal-organic chemical vapour deposition (MOCVD) and molecular beam epitaxy (MBE) processes. For example, the technique of infrared laser-powered homo- geneous pyrolysis (IR LPHP) has been put to very effective use in the elucidation of the pyrolysis mechanisms of main group precursors such as Me3A1,4 Et3Ga,5-7 Bu:Ga and Bu; Gag and Et,Zn.' The technique of determination of kinetic parameters using the stirred flow reactor (SFR) has proved invaluable in the study of the chemistry of organosilicon compounds." The EPR detection of free radicals produced in the pyrolysis of organic compounds," trapped by matrix isolation (MI), has been known for many years, and has recently been adapted to the study of organometallic com- pounds in our laboratory.In the present work, we describe the application of these techniques to a number of potential organo-transition-metal MOCVD precursors, and present some preliminary conclusions. t Present address: Department of Chemistry, University of Auckland, Private Bag 92019, Auckland, New Zealand. Experimental The experimental methods employed in the present investi- gations have been described in detail elsewhere, and hence only brief summaries of the more significant aspects are presented here.Except where noted below, analytical investi- gations (FTIR, NMR, GC-MS, elemental analysis, and EPR) were conducted using commercial instrumentation, in con- junction with comparison with authentic samples. In most of the studies described here, all three techniques have been applied. Infrared Laser-powered Homogeneous Pyrolysis The majority of qualitative studies were carried out using the technique of IR LPHP. The technical details and adkantages of this method have been described in an extensive recent review by one of the present authors,', and a wide range of A1 and Ga systems have been investigated with its use.13 Static pyrolysis is carried out in a cylindrical Pyrex cell (length 10 cm, diameter 3.8 cm) fitted with ZnSe windows. In compari- son with cheaper materials such as NaCl, ZnSe has ;i higher optical transmission at the IR laser wavelength of 1Opm.In addition, it has greater mechanical strength and is not hygro- scopic, a point of considerable importance in the investigation of moisture-sensitive materials. The cell is filled with up to 10Torr of the vapour of the material under study, Together with 10Torr of SF, (for compounds of moderate volatility, liquid or solid can be condensed into the cell; this does not alter the basic features of the IR LPHP method, but does introduce evaporation as a possible rate-limiting process).The contents of the cell are then exposed to the output of a free- running CW CO, IR laser at power levels of a few watts. The SF, strongly absorbs the laser radiation, which is then con- verted via rapid inter- and intra-molecular relaxation into heat. The low thermal conductivity of SF, ensures lhat this produces a highly inhomogeneous temperature profile, in which the centre of the cell may be heated up to 1500 K, but where the cell walls remain at room temperature. This has a number of advantages. The first is that pyrolysis is initiated unambiguously in the gas phase, eliminating the complications frequently introduced by competing surface reaction. The second is that the primary products of pyrolysis are ejected into cool regions of the cell, where they are protected from further reaction.In favourable cases, these products may be collected as less volatile liquids or even solids. On the other hand, the temperature of the pyrolysis is neither uniform nor easily measured, so that comparisons with conventional methods of pyrolysis must be made with care. Such indications as are available through studies of systems with well known kinetic parameters (e.g.CH3C0,CH3) suggest that the overall cell reaction is almost entirely that at the maximum tempera- ture. Reaction is monitored in the first instance through FTIR spectroscopy, with further product identification by NMR, GC-MS, or elemental analysis. Stirred Flow Reactor Kinetic Measurements The SFR consists of a spherical quartz vessel of volume 10cm3, at the centre of which is either a smaller perforated bulb or a simple jet inlet, which provides rapid mixing and thermal equilibration of the reactant.The reactor is housed in a conventional furnace capable of providing temperatures up to the softening temperature of the quartz. Rather than a continuous flow, reactant diluted in a carrier gas of N, or He is admitted in a pulse or batch mode; this method confers the benefit of economy as well as technical advantages. Reaction in the vessel competes with the sweeping out of reagents and products, so that a controllable proportion of conversion may be achieved. Unreacted starting material and products may be analysed directly via GC-MS, or accumulated in cold traps for subsequent investigation.The output of the flame ionis- ation or thermal conductivity detector of the GC may also be fed directly into a data-capture station for kinetic or other analysis. This kind of technique has been used extensively in the investigation of pyrolysis mechanisms of organosilicon compounds such as Me3Si-SiMe,.l4 Matrix Isolation EPR Spectroscopy Two approaches have been utilised for the matrix isolation of free radicals in pyrolysis reactions. In the first, reagents are pumped through a conventional resistively heated hot-wall quartz tube by means of a mercury diffusion and rotary pump, at pressures of much less than 1Torr. Pyrolysis products are condensed onto a finger cooled to 77 K by liquid nitrogen; the whole cold finger assembly is removable for the examin- ation of EPR or other spectra.Spectra were stored digitally on computer for subsequent manipulation. Radicals may be trapped in a matrix of unreacted starting material, or of a suitable host such as adamantane; the latter usually provides more easily interpreted isotropic EPR spectra, but does add another variable to the experimental arrangement. Matrices sufficient to produce very strong EPR spectra can be con- densed in 5-30 min, depending on flow rates and pressures, Recently, a second approach has been used, in which the conventional heater is replaced by a laser pyrolysis cell of the sort described above. This approach permits the differentiation of radicals produced in homogeneous and surface processes, and hence additional insights into the mechanisms of depos-ition and other reactions.Many of the EPR spectra obtained arose from two or more species; these could be distinguished by judicious variation of experimental conditions (temperature or pressure), followed by computer subtraction. The two techniques have been used in the identification of many radicals in main-group organometallic pyrolyses, for example Me radicals from Me,Ga or Et radicals from Et,In.15 Results We have applied the techniques described above to the study of a number of organo-transition-metal systems of current or potential use in MOCVD or MBE applications. In addition, we have made considerable use of the armoury of techniques widely employed in mechanistic investigations: isotope substi- tution, intermediate trapping, etc.Each of the systems will be J. MATER. CHEM., 1994, VOL. 4 described in turn, and some of the trends discernible at this stage will be discussed in the next section. MeMn(CO), and AcMn(CO), As mentioned above, pentacarbonylmethylmanganese (MMP) has been utilised as a source for Mn doping in CdMnTe epilayer~,~but little is known of its mechanism of decompo- sition. Furthermore, co-pyrolysis of MMP with organosilicons containing Si-H bonds, such as Me,SiH, has potential for the deposition of MnSi layers in applications such as intercon- nects. Finally, the mode of decomposition of acetyl manganese pentacarbonyl (AMP) is of considerable theoretical interest. We have therefore undertaken investigations of the pyrolysis of both MMP and AMP and their deuteriated derivatives, both alone and in the presence of Me,SiH, using the techniques described in the previous section.In addition, three of us have collaborated in spectroscopic investigations of the gas phase and crystal structure of MMP, using IR laser spectroscopy16 and neutron diffra~tion.’~ MMP was prepared by minor modifications of the literature method [conversion of Mn,(CO)lo to NaMn(CO),, followed by reaction with CH,I], and deuteriated MMP similarly, using CD,T in the last stage.’* AMP and deuterated AMP were prepared similarly, using CH,COCl or CD,COCl.SFR pyrolysis yielded methane and Mn,(CO),, as the only products detectable by GC-MS, in accord with the results of earlier photolysis experiments.” Initially, kinetic studies exhi- bited non-Arrhenius behaviour over the temperature range 15O-26O0C, a clear indication of the unpredictable effects of surface reaction. Subsequent studies with a clean vessel yielded first-order kinetic behaviour, with linear Arrhenius behaviour over the range 190-230 “C corresponding to an activation energy of 214(9) kJ mol-’ for the production of CH,. Co-pyrolysis of CH,Mn(C0)5 and CD,Mn(CO), using IR LPHP yielded CH,, CH,D, CD,H, and CD, as isotopic variants of methane; the lack of CH,D, is strong support evidence for the involvement of Me radicals.Using conven- tional MI, very strong EPR spectra identical to those assigned to Mn(CO), radicals by Symons and Sweany20 were obtained, as shown in Fig. 1; identical spectra were also produced by pyrolysis of Mn,(CO),,. No features arising from Me radicals were detected; that this absence was not simply an artefact of the design of the pyrolysis apparatus was confirmed by the observation of very strong Me radical spectra generated in the pyrolysis of Me,Ga.” I 1I I I 31 50 3275 3400 3525 3650 HIG Fig. 1 EPR spectrum of matrix-isolated Mn(CO), radicals produced in the hot-wall pyrolysis of MeMn(CO), J. MATER. CHEM., 1994, VOL. 4 The addition of Me,SiH produced both quantitative and qualitative changes in the reaction.IR LPHP of MMP-Me,SiH mixtures resulted in methane and Me,SiMn(CO), as the sole products; isotope labelling verified that the methane arose from the Mn-Me and the Si-H units. In kinetic SFR studies, addition of Me,SiH substantially increased the rate of pyrolysis, the activation energy falling to 187(13) kJ mol-’; paradoxically, the rate law was zero order in the pressure of the silane, although it remained first order in MMP. In addition, minor quantities of HMn(CO), and Me,Si were detectable using GC-MS. Co-pyrolysis of MMP with Me,SiH, was carried out in the hope of producing as an intermediate Me,Si(Mn(CO),} ’; in the event, only traces of this potential 2: 1 Mn-Si precursor of were detected, the major product being Me,SiHMn(CO),.AMP produced similar results, the initial step in pyrolysis alone being extrusion of CO to yield MMP, which then decomposed as above. In the MI/EPR study, AMP produced Mn(CO), radicals at a significantly lower temperature than MMP (95 ’C as opposed to 180 “C), consistent with the lower laser power required in the LPHP investigations; again, no CH,CO or CH, radicals were detected. Co-pyrolysis of AMP with Me,SiH produced a result significantly different from a previous investigation; in the present study, the only products were Me,SiMn(CO),, CO and CH,, whereas Me,SiMn(CO), and CH,CHO were produced in toluene solution.” C,H,Mn(CO), and MeCSH,Mn(CO), Tricarbonyl(cyclopentadieny1)manganese (CMP) and its methyl derivative (MCMT), frequently known as cymantrene, were among the earliest organo-transition-metal derivatives to be explored for use as MOCVD precursors.’ However, there are problems with their use, principally the incorporation of carbon, and it is therefore of interest to investigate their mode of decomposition.An SFR study of MCMT revealed that the major products of pyrolysis were CO and free methylcyclopentadiene. The kinetic data afforded a good straight-line plot over the tem- perature range 250-390 “C, with an activation energy of 208( 14) kJ mol-’. The hot-wall MI EPR spectra of both sets of pyrolysis products were complex, with at least two contribu- ting species. However, the relative proportions of these could be altered by simply varying the temperature; computer subtraction produced the spectra shown in Fig.2 in the case of CMT. Fig. 2(a) is clearly identifiable as arising from the cyclopentadienyl free radical,2’ but the highly anisotropic spectrum of Fig.2(b) has not yet been identified. MCMT produces a complex spectrum identifiable as arising from the methylcyclopentadienyl radical, overlapped by a contribution almost identical to that of Fig. 2(b).The anisotropic spectrum almost certainly does not contain the organic moiety, there- fore, and its marked shift in g-value from the free-spin value strongly suggests that it contains a metal, presumably Mn. In addition to cyclopentadienes, IR LPHP at laser powers of 1.5-2.0 W for both CMT and MCMT produced hydro- carbons not observed in the SFR system, principally ethyne.This observation was unexpected; although ethyne is often a thermodynamic ‘sink’ in many hydrocarbon reactions, its generation usually requires very much higher temperatures than those employed in the LPHP experiment (estimated at 450-500 K). It would appear that reaction of the cyclopen- tadienyl unit in situ may be responsible. If we assume that the origin of the observed cyclopentadiene is hydrogen abstraction from the parent compound by a released cyclopentadienyl radical, as suggested by the EPR observations, then the resultant Mn-C5H, unit may rearrange in an electrocyclic fashion to yield ethyne and perhaps the species responsible 3300 3350 3400 3450 35 3300 3350 3400 3450 3500 HIG Fig.2 EPR spectra of products of hot-wall pyrolysis of C,H,Mn(CO),: (a) cyclopentadienyl, (b) unidentified anisotropic species, possibly MnC for the anisotropic EPR signal.This hypothesis was tested by co-pyrolysing CMT with deuteriated MMP. The temperature required for release of a CD, radical from deuteriated MMP is lower than that required to initiate reaction of CMT, and CD, radicals are relatively good hydrogen abstracters; this system would therefore produce the conjectural Mn-C,H, centre via a different route. The co-pyrolysis did indeed produce ethyne at the lower temperature, as well as a quantity of CD,H, providing substantial support for this hypothesis. Butadienyltricarbonyliron To date, butadienyltricarbonyliron (BDFECO) has not found use as a MOCVD precursor.However, its mode of pq,rolysis is of fundamental interest, since it is a prototypical q-bonded diene compound of the kind suggested as intermediates in heterogeneous catalytic processes23. There has been one reported study of conventional pyrolysis of BDFEC0,’4 in which the major hydrocarbon products were free butadiene together with its oligomers and polymers (largely vinylcyclo- hexene). SFR experiments resulted in similar products, with a preliminary value for the activation energy of 136(6)kJ mo1-’, close to the estimated Fe-CO bond strength. A GC-MS trace of the results of pyrolysis at 500 K is shown in Fig. 3(a). IR LPHP was carried out at laser powers ranging between 0.5 and 1.5 W, corresponding to similar pyrolysis temperatures of 450-550 K.This resulted in a deposit analysed as approximately FeC,H, (presumably J. MATER. CHEM., 1994, VOL. 4 3 Q)0 \ 13 i4 1 2 3 4 5 6 timdmin Fig. 3 GC-MS traces of products of pyrolysis of C,H,Fe(CO), from (a)the stirred flow reactor, and (b) IR laser powered homogeneous pyrolysis; (1) starting material, (2) benzene, (3) butadiene, (4) toluene; unidentified peaks arise from higher aromatics iron plus butadiene polymers) and free CO. In addition, a range of hydrocarbon products was identified by combined use of FTIR, 'H NMR, and GC-MS, with molar ratios as follows: butadiene ( l.O), benzene (OS), but-1-ene (0.3), cis-but-2-ene (0.3), trans-but-2-ene (0.2), and ethene (O.l), together with traces of toluene and higher aromatics.A GC-MS trace of the results of pyrolysis at 500 K is given in Fig. 3(b)for comparison (note that relative proportions cannot be obtained directly from such spectra, because of differing sensitivities). It is very evident from the more complex spectrum of Fig. 3(b)that homogeneous pyrolysis results in a much greater range of hydrocarbon products than the conventional approach, underlining yet again the dominance of wall pro- cesses in the latter, even under conditions designed to encour- age homogeneous reaction. The mode of production of the additional products is currently under investigation through the use of selectively deuteriated and substituted butadiene complexes.25To date, no EPR spectra of the pyrolysis products of BDFECO have been obtained.Ferrocene Like BDFECO, ferrocene has found little use as an MOCVD precursor; its vapour pressure is rather low, and pyrolysis requires rather high temperatures. However, its mode of decomposition is of interest as a non-carbonyl compound, and as a paradigm for other cyclopentadienyl compounds. To date, only IR LPHP investigations have been conducted. Because of the low vapour pressure of the compound, solid ferrocene was sublimed into the pyrolysis cell, and experiments conducted with the cell maintained at a temperature of 100-150°C by means of resistive heating tape. IR LPHP required rather high laser powers of 8-10 W, corresponding to temperatures of 700-750 K. FTIR spectroscopy [see Fig.4(a)] indicated the production of ethene, ethyne and benzene, as well as free cyclopentadiene; 'H NMR spec-troscopy further revealed the production of considerable quantities of naphthalene [Fig. 4(b)], an observation con-firmed by GC-MS. Discussion and Conclusions The results presented above provide a range of examples of the way in which considerable insight into the mechanisms of 2 1 1 I I 1 I I I 1400 1200 1000 800 wavenumber/cm-' 33 I I I I I 1 7.60 7.20 6.80 6.40 6.00 5.60 5.20 6 Fig. 4 Products of IR laser-powered homogeneous pyrolysis of ferro-cene: (a) FTIR spectra before (lower) and after (upper), (1) cyclopen-tadiene, (2) benzene, (3) ethyne, unidentified peaks =starting material of SF,; (b) partial 'H NMR spectrum, (1) naphthalene, (2) benzene, (3) cyclopentadiene, (4) ethene, unidentified =residual protonated solvent pyrolysis of organo-transition-metal complexes can be obtained by a combination of experimental techniques.The differences found in the SFR and IR LPHP experiments confirm the widely held view that under conventional pyrolysis conditions (which would include MOCVD, and with even more force, MBE) reaction is likely to be dominated by surface processes. Nonetheless, there may be significant contri- butions from homogeneous reactions, which may well play a part in producing deposits of undesirable characteristics; this kind of process is thought to be the origin of the carbon found in films deposited from mixtures containing Al-Me precursors, for example.4J3 Although firm conclusions about the mechanisms of individ-ual complexes must await the results of further investigations, we may make some general observations at this stage. It is to be anticipated, of course, that the initial step in the pyrolysis of all the complexes investigated here will normally be homol- ysis of the weakest bond in the system, unless a lower energy intramolecular route is available.In the case of MMP, this implies breakage of the Me-Mn link, a contention supported by the EPR and IR LPHP observations; for AMP, expulsion of CO is the initial step, followed by a similar pathway. The kinetic results in the presence of Me,SiH suggest that isomeris- ation of MMP may be rate-determining, the isomer perhaps being the unsaturated acyltetracarbonyl proposed to account for the ready CO exchange exhibited by this compound, and tentatively identified in our study of its high-resolution IR spectrum.16 Our failure to detect organic radicals in either system is puzzling; it may be that Me radicals are lost at the wall in the predominantly heterogeneous reaction, and that J.MATER. CHEM., 1994, VOL. 4 Me (or even CH3CO) radicals may be observed using IR LPHP combined with MI/EPR. The primary loss of the organic moiety will in this case result in a deposited metal film largely free of deleterious carbon, in keeping with obser- ~ation.~For the more strongly q-bonded systems, loss of CO (where available) will be the more facile pathway, resulting in coordinatively unsaturated species of the sort widely observed in low-temperature photolyses of such systems;26 for ferrocene, initiation of reaction presumably requires the even higher energy loss of the q5-cyclopentadienyl unit, in keeping with the higher temperatures required.The subsequent reactions of resultant coordinatively unsaturated species are largely unknown, despite the very considerable interest in such inter- mediates through their role in catalysis.23 Our results would suggest that the q-bonded units may undergo in situ rearrange-ment, resulting in products more normally associated with photolytic processes involving the free moiety. In the case of BDFECO, for example, the major products may result from an intramolecular hydrogen transfer in a bisbutadiene complex ( butenes). Alternatively, an in situ electrocyclic rearrangement of butadiene in the mono-complex may result in the ethene and ethyne observed in the photolysis of free b~tadiene,~’ with the latter undergoing the familiar and facile Fe-catalysed cyclisation to benzene.For CMT and MCMT, electrocyclic rearrangement of the native compound would result in ethyne together with a manganese-carbon unit; for the hydrogen abstraction product C,H,Mn(CO),, this may result in manga- nese carbide itself. Catalysis of the cyclisation of ethyne by Mn is less facile, so that benzene is not observed in this case; manganese carbide may be the origin of the anisotropic EPR signal of Fig.2. The observation of naphthalene in the pyrol- ysis of ferrocene strongly suggests a rearrangement involving both cyclopentadienyl units, a hypothesis under investigation with the use of substituted ferrocene~.~’ As a general obser- vation, it would appear that strongly q-bonded complexes are less likely to be useful MOCV D precursors as a consequence of extensive carbon incorporation resulting from ligand rearrangement processes. On the other hand, a-bonded organo-transition-metal carbonyls have more promise, as has already been demonstrated. We thank the SERC for their extensive support for this work, including equipment grants (to D.K.R., J.B.R. and I.M.T.D.), post-doctoral fellowships (to M.P., I.M.P.and A.D.W.), and studentships (to G.P.M.). We also acknowledge support from the Government of Turkey through a studentship to S.S. References G. B. Stringfellow, Organometallic Vapor Phase Epitaxy-Theory and Practice, Academic Press, San Diego, 1989, and references therein. 2 A. Nouhi and R. J. Stirn, Appl. Phys. Lett., 1987,51, 2251. 3 G. N. Pain, N. Bharatula, G. I. Christiansz, M. H. Kibel, M. S. Kwietniak, C. Sandford, R. S. Dickson, R. S. Rowe, K. McGregor, G. B. Deacon, B. 0. West, S. R. Glanvill, D. G. Hay, C. J. Roussow and A. W. Stevenson, J. Cryst. GroMth, 1990, 101, 208; G. N. Pain, G. I. Christiansz, R. S. Drckinson, G. B. Deacon, B. 0. West, K. McGregor and R. 5. Rowe, Polyhedron, 1990,9,921. 4 G.A. Atiya, A. S. Grady, S. A. Jackson, N. Parker and D. K. Russell, J. Organomet. Chem., 1989,378,307. 5 A. S. Grady, A. L. Mapplebeck, D. K. Russell and M. G. Taylorson, J. Chem. SOC., Chem. Commun., 1990,929. 6 A. S. Grady, R. D. Markwell and D. K. Russell, J. Chcm. SOC., Chem. Commun., 1991,14. 7 A. S. Grady, R. E. Linney, R. D. Markwell and D. K. Russell, J. Muter. Chem., 1993,3,483. 8 A. S. Grady, R. E. Linney, R. D. Markwell, G. 1’. Mills, D. K. Russell, P. J. Williams and A. C. Jones, J. Muter. Chem., 1992, 2, 539. 9 R. E. Linney and D. K. Russell, J. Muter. Chem., 1993,3, 587. 10 A. C. Baldwin, I. M. T. Davidson and A. V. Howard, .I. Chem. SOC., Faraday Trans. I, 1975,71,972;I. M. T. Davidson, (3. Eaton and K. J. Hughes, J. Organomet. Chem., 1988,347,17.11 K. Mach, I. Novakova, V. Hanus and J. B. Raynor, Tetrahedron, 1989, 45, 843. 12 D. K. Russell, Chem. SOC.Rev., 1990, 19,407. 13 D. K. Russell, Coord. Chem. Rev., 1992, 112, 131. 14 I. M. T. Davidson and A. V. Howard, J. Chem. SOC., Faraday Trans. I, 1975,71,69. 15 G. P. Mills, J. B. Raynor, D. K. Russell and A. D. norkman, unpublished results. 16 J. Gang, M. Pennington, D. K. Russell, P. B. Davies, G. M. Hansford and N. A. Martin, J. Chem. Phys., 1992,97,3885. 17 M. Pennington, D. K. Russell, D. R. Russell and S. Saydam, work in progress. 18 R. B. King, in Organometallic Synthesis, ed. J. J. Eisch and R. B. King, Academic Press, San Diego, 1965, vol. 1. 19 T. E. Gismondi and M. D. Rausch, J. Organomet. Chem, 1985, 284, 59. 20 M. C. R. Symons and R. L. Sweany, Organometallics, 1982,1,834. 21 R. W. Wegman, Organometallics, 1986,5,707. 22 M. Kira, M. Watanabe and H. Sakurai, J. Am. Chem. Soc., 1980, 102,5202. 23 A. J. Pearson, in Comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press, Oxford, 1982, vol. 8, ch. 58; A. J. Deeming, in comprehensive Organometallic Chemistry, ed. G. Wilkinson, F. G. A. Stone and E. W. Abel, Pergamon Press, Oxford, 1982, vol. 4, ch. 31.3. 24 D. L. S. Brown, J. A. Connor, M. L. Leung, M. I. Paz-4ndrade and H. A. Skinner, J. Organomet. Chem., 1976,110,79. 25 D. K. Russell and S. Saydam, unpublished results. 26 S. P. Church, M. Poliakoff, J. A. Timney and J. J. Turner, J. Am. Chem. SOC.,1981,103,7515. 27 I. Haller and R. Srinivasan, J. Am. Chem. Soc., 1966, 138, 3694; W. J. Leigh, Chem. Rev., 1993,93,487, and references therein. Paper 3/04314B; Received 21st July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400013

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( 11, 19-22 Preparation and Characterization of Sr,-,La,FeO,(O <x 61) Yasuo Takeda,*” Kaori Imayoshi: Nobuyuki Imanishi,” Osamu Yamamoto” and Mikio Takanob a Department of Chemistry, Faculty of Engineering, Mie University, Tsu Mie-ken 514, Japan Institute for Chemical Research, Kyoto University, Uji Kyoto-fu 611, Japan Solid-solution Sr,-,La,FeO, crystallizing in the K,NiF, structure has been obtained for 0 bx b1. In the Sr-rich region, the K2NiF, structure was stable only at low temperature. Heat treatment under an oxygen pressure of 100-500 atm was carried out to form oxygen stoichiometric samples. The structure was refined by the Rietveld method assuming space group /4/mmm. The FeO, octahedron in Sr,FeO, showed almost cubic symmetry, the La substitution causing an increase in the Fe-0 (apical) length without modifying the a-axis length.All the samples showed semiconducting behaviour and positive Seebeck coefficients. An antiferromagnetic transition at 60 K was observed for Sr,FeO,, becoming obscure with increasing La content. The room-temperature Mossbauer spectra showed a symmetric singlet due to Fe4+ and a doublet due to Fe3+, except for Sr,FeO, which showed a quadrupole doublet. Of several oxides containing iron in the unusual valence state Fe4+, perovskite oxides such as SrFeO, and CaFeO, have been extensively studied. ‘9, The octahedrally coordinated Fe4+ ions in SrFe0, are in a high-spin tZg3a*l configuration, where the orbital doubly degenerate narrow a* band of e-orbital parentage is one-quarter filled.The behaviour of the a: electron in CaFeO, can be characterized by a dispro-portionation of the Fe4+ ion into Fe3+ and Fe5+, i.e.: 2Fe4+( t;,a*’) (high-temperature phase) Fe3+(t:get)+ Fe5+(t;,e;) (low-temperature phase) In the Sr-Fe4+-0 system, Sr2Fe04, having the K2NiF, structure, has beehnown to exist. However, only a limited number of studies have been reported on this oxide, because it is difficult to prepare the oxygen stoichiometric samples. Gallagher et al. prepared an oxygen-deficient sample Sr,FeO,,, and identified Fe3+ and Fe4+ ions in the Mossbauer ~pectrum.~Recently, Dann et a!. prepared oxygen stoichio- metric Sr,FeO, and examined the crystal structure by an X-ray Rietveld method., They reported that Fe4+ ions are in a slightly distorted octahedral environment, (4 x 1.93 A)+(2 x 1.95 A).In this study, we prepared almost stoichiometric Sr,-xLa,FeO,(Obx < 1) and refined the crystal structure by X-ray Rietveld analysis. The electrical conductivity, Seebeck effect, magnetic susceptibility, and Mossbauer effect were measured. Experimental Mixtures of Sr(N03)2, La203 and Fe metal powder were dissolved in nitric acid. Before use, La203 was dried at 1000“C for 1 h. The solutions containing these metallic ions at various ratios were dried until the nitrates completely decomposed. The solid thus formed was calcined at 600°C for 15 h and subsequently ground, pelletized, and heated at various tem- peratures depending upon the metallic composition in an O2 gas flow.To attain oxygen stoichiometry, these samples were annealed at 400-700°C under 100-500 atm of oxygen pressure. X-Ray diffraction (XRD) patterns of powdered samples were obtained on Rigaku RAD-RC (12KW) using monochro- mated Cu-Ka radiation. For Rietveld analysis, intensity data were collected at each 0.02” step for 2 or 3 s over a 20 range of 10-100”.5 The average oxidation state of iron was deter- mined by iodometry. The Seebeck coefficient was measured in the temperature range 300-700 K and a temperature differ- ence within a sample of 3-20 K. The magnetic susceptibility was measured by using a SQUID magnetometer (Quantum Design, MPMS,) in a field of 1000G. The Mossbauer effect of 57Fe was performed at room temperature.The velocity was calibrated by using pure iron metal as the standard material. Results and Discussion The phases identified by XRD measurements are summarized in Fig. 1 as a function of synthesis temperature and composi- tion for treatments under 1 atm of oxygen. In an Sr rich region, K2NiF, structure is stable only at low temperatures. For example, Sr,Fe04 -decomposes to Sr,Fe20, -z3-6 and SrO above 800°C. In air or a more reducing atmosphere the decomposition temperature decreases. However, the reaction rate is slow at these low temperatures, especially for A cO.4. The triangles in Fig. 1 show the coexistence of Sr, -,La ,FeO, and La203 or SrCO, after heating for more than 100 h with 15001 :0 00 0 1200 1 0 0’0 0 p 1100 ,,*o ,I‘ V V1000 L V I700tI , , , , , 0.0 0.2 0.4 0.6 0.8 1.0 Fig.1 Formation diagram for the Sr,-,La,FeO, system (0,gas flow): 0, Sr, -,La,FeO,; c7, Sr, -.La,FeO, and La,O,; 0, Sr, -,La,FeO, and Sr, -.La,Fe,O,; 0,Sr,Fe,O, and SrCO, J. MATER. CHEM., 1994, VOL. 4 a few intermittent grindings. Since the specimens thus obtained were oxygen deficient, especially in the Sr-rich region, annealing under more than 150atm of oxygen was carried out at 450 "C for 70 h. As Gallagher et al. also reported,, the as-prepared oxygen-deficient samples underwent decompo- sition in a few weeks even if they were isolated from moisture, while oxygen stoichiometric samples remained stable for more than a few months.In Table 1 are listed the synthesis conditions, lattice param- eters and oxygen contents for the specimen used for the Rietveld analysis. The samples prepared at low temperatures (e.g. 8OOOC) did not give any significant R-factor because of a non-homogeneous mixing of Sr and La ions in the structure. The sample for x =0.1 was finally annealed under 350 atm of oxygen at 1000°C for 6 h to have a well mixed state of Sr and La. Final reliability factors achieved were less than 13, 10, 3 and 5 for Rwp, R,, RE and RI, respectively, except for x=O.l-0.3, for which 20, 17, 3 and 8 were the minimum values obtained. The tetragonal lattice parameters a and c obtained by Rietveld analysis are plotted in Fig. 2 against the La content.The c-axis increases with increasing La content and shows a maximum at x=O.8, while the a axis shows no significant change, the tetragonality ratio c/a showing a maximum at x x0.8. Fe4+ in SrFeO, has an electronic configuration, t;, e:, which would tend to induce a Jahn-Teller distortion of the FeO, octahedron. However, the eg electron in SrFe0, is delocalized and SrFeO, exhibits metallic conductivity and cubic symmetry, with no sign of structural distortion down to 4.2 K. Sr,FeO,, however, has an anisotropic K,NiF, struc-ture which makes us expect the onset of a Jahn-Teller distortion. The structure of Sr,-,La,FeO, was refined by assuming space group I4/rnrnrn. The Sr, La, and O(2) ions are located at 4e sites with coordinates (O,O, z), the Fe atoms at (O,O, 0) in sites 2a, and the O(1) atoms at ( 3 ,0, 0), (0, 3 ,0) in sites 4c.Sr and La were assumed to be randomly mixed at 4e sites. Fig. 3 shows observed, calculated, and difference plots for Sr2-,LaXFeO4 (x=O.9). Table 2 lists the refinement results. The Fe-0 bond length is plotted us. x in Fig. 4. The coordination of Fe4+ in Sr,Fe04 is an almost regular octa- hedron, without any indication of the Jahn-Teller effect. This agrees with the result of Dann et a!., However, the FeO, 12.8 a. 12.7 0 0 05-12.6 02 f? CTI a a,.s 12.5 c -0 0 12.4 3.9 3.8 0.0 0.2 0.4 0.6 0.8 1.0 x in Sr2.&afe0, Fig. 2 Variation of the lattice parameters in Sr, -,La,FeO,: ,c-axis; 0,a-axis octahedron becomes elongated along the c-axis as x is increased.The degree of elongation, Fe--O(2)/Fe-O( l), increases from 1.00 (x=O) to 1.11 (x=l). Substituted La3+ ions which are smaller and more positively charged than Sr2+ would shorten the La(Sr)-0(2) distance along the c axis. Moreover, the excess electrons injected into the [FeO,] -x sheets would occupy the di orbit directed toward the O(2) ions. The Fe- O(2) distance is extended consequently without modifying the a-axis length. The room-temperature resistivity increases with La content from 1.2 x lo2ncm-' (x=O) to 4.5 x lo3i2 cm-' (x=0.4). Fig. 5 shows the Seebeck coefficient for x =0.0-0.6 from room temperature to 400°C, around which the oxygen content Table 1 Synthetic conditions, lattice parameters and oxygen contents in the system Sr,-,La,FeO, lattice parameter (hexagonal) synthetic conditions X a/A CIA oxygen content T/"C P(0Z )/atm time/h 0.0 3.8582(1) 12.3977(4) 3.997( 9) 750 1 120 450 450 70 0.1 3.8554(2) 12.4458( 6) 3.959( 5) 800 1 150 1000 350 6 0.2 3.8537( 2) 12.482(1) 4.01(1) 800 1 72 450 150 70 0.3 3.8512(3) 12.570(1) 4.002( 5) 850 1 60 450 150 70 0.4 3.8495(2) 12.6154(9) 4.005( 2) 900 1 60 450 150 70 0.5 3.8518(2) 12.6568(9) 4.OO2(6) 1000 1 60 450 150 70 0.6 3.8534(2) 12.6974( 6) 3.983(6) 1100 1 60 450 150 70 0.7 3.84981(7) 12.7394( 3) 4.002(6) 1200 1 40 450 150 70 0.8 0.9 1.o 3.85251(5) 3.86366(4) 3.87339(4) 12.7523( 2) 12.7385(2) 12.7171 (2) 4.02 1(4) 4.001(4) 3.996(7) 1200 1300 1300 1 1 1 40 40 40 J.MATER. CHEM., 1994, VOL. 4 21 The magnetic behaviour of compositions x =0.0, (1.1, 0.3, and 0.5 is shown in Fig. 6. Sr2Fe0, shows an antiferromagnetic49 I transition at ca. 60 K. At higher temperatures, the susceptibil-ity obeys the Curie-Weiss law. Calculation of the effective magnetic moment from the linear portion of 1/x-T gives a value of 10.OpB,which is much higher than the spin-only value of 4.9 for high-spin Fe4+.The substitution of only 5% of La in Sr weakens the peak of antiferromagnetic ordering, which finally disappears in Sr,,,Lao.2Fe04.Above 100 K, the susceptibility of Sr2-,La,Fe04(x =0.1,0.3 and 0.5) also obeys the Curie-Weiss law; however, the effective magnetic moments 20 30 40 50 60 70 80 90 100 are somewhat smaller than that of Sr2Fe0,, viz.5.9 pB!5.5 pB 28ldegrees and 5.2 pB,respectively, which reflects the mixed state of high-spin Fe4+ (d4) and Fe3+ (d5). Fig. 3 Observed, calculated and difference plots of Sr,-,La,FeO, The room-temperature Mossbauer spectra are shown in (X =0.9) Fig. 7. The spectrum of Sr,FeO, shows two lines of dmost equal intensity at -0.22 mm s-l and +0.22 mm s-'. Irz 1966, Gallagher et al. reported a Mossbauer spectrum of ouygen-begins to decrease. The positive values suggesting p-type deficient Sr2Fe0,.,3 showing a pair of peaks of different conduction increase with increasing x except for Sr2Fe04.The intensities at -0.24 mm s-l (weak) and +0.24 mm s-l electronic state of Sr2Fe04leading to the strong temperature (strong).Gallagher et al. attributed these peaks to tetravalent dependence of the Seebeck coefficient seems to be strongly (Fe4+) and trivalent (Fe3+) states, respectively. However, affected by La substitution. because our samples are almost oxygen stoichiometric with Table 2 Rietveld refinement results for Sr,-,La,FeO, ~~~~~~ fractional coordinates and thermal parameters Sr, La Fe x in Sr,-,La,FeO, Z B/A2 Z 0.0 0.3572(5) 0.5(2) 0.5(2) 0.5(2) 0.157(3) 0.5(2) 0.1 0.3561(7) 0.3(2) 0.3(5) 0.3(7) 0.161(4) 0.3(7) 0.2 0.356(1) 0.3(3) 0.3(7) 0.6( 11) 0.160(6) 0 6(11) 0.3 0.356(1) 0.1(3) 0.9(8) 0.4( 12) 0.155(7) 0 4( 12) 0.4 0.356( 1) 0.1(3) 0.7(7) 0.4( 11) 0.157(6) 04(11) 0.5 0.356(1) 0.1(3) 0.9(7) 0.4( 12) 0.160(6) 0.4(12) 0.6 0.3570(7) 0.3(2) 0.8(5) 0.5(8) 0.163(4) 0.5(8) 0.7 0.3576(4) 0.3(1) 0.3(1) 0.3( 1) 0.165( 3) 0 3(1) 0.8 0.3578 (4) 0.3(1) 0.3( 1) 0.3(1) 0.166(2) 0 3(1)0.9 0.3584(5) 0.1( 1) 0.1(1) 0.1(1) 0.166(3) 0 1(1) 1.o 0.3588(5) 0.1(1) 0.3(3) 0.7(7) 0.169(3) 0 7(7) 2.21 T 100 -I \\ -I80 I I \I52.1 -L 8 3 I t I Ia 1c II.-in .-60--0 u I I a ELL a I0 I -I I102.0 Y8 40-I a ..---..I a,a ", ... -..-..-.._.._.._.._..__._._..0.6I UI 20 --.....<1.9 0.4 1 ----_ 0.2 I I I -----10.0I1 I I I I 11 0 0.0 0.2 0.4 0.6 0.8 1.0 x in Sr,.&a>eO4 Fig.4 Variation of Fe-0 bond distances in the Sr2-,La,Fe0, Fig. 5 Temperature dependence of the Seebeck coefficient of the system: 0, Fe-O( l)//a-axis; 0,Fe-O(2)//c-axis Sr2-,La,Fe0, system, for different values of x as marked 3.0r 0.0 -----____0.1-.----.-0.3-.-..-,--..-.-0.5 I I I I400 0.00 100 200 300 T/K Fig. 6 Magnetic susceptibility for Sr2-,La,Fe04 for x=O.O (-), 0.1 (---), 0.3 (-.-) and 0.5 (--.--), at 1000 G Fig. 7 Mossbauer spectra for Sr,-,La,FeO, at room temperature for different values of x as marked oxygen deficiencies of < 1%, we have considered the double peaks at k0.22 mm s-' to be a quadrupole doublet: the Fe4+ ions in Sr,FeO, have an isomer shift of zero and a quadrupole J. MATER. CHEM., 1994, VOL.4 splitting of 0.44 mm s-'. The isomer shift is similar to those of SrFeO, (0.054mm s-')~and CaFeO, (0.073 mm s-').~The quadrupole interaction is essentially absent for x =0.2 and 0.4 in Fig. 7, as it is for SrFeO, and CaFeO,. The gap in electric and magnetic properties between x=O and x->O.l as seen in Fig. 4 and 5 seems to have the same root as the disappearance of the quadrupole interaction. One possibility is that the excess electrons injected on chemical reduction jump among several Fe ions located near the substituting La3+ ions. When La3+ content is increased to x >0.4, another quadrupole doublet due to Fe3+ ions becomes identifiable. Conclusions In summary, we have synthesized a solid solution of Sr,~,La,FeO,(O <x <1) and studied their structural, electrical and magnetic properties.A Rietveld analysis of the powder XRD data showed that the almost cubic Fe4+0, octahedron in Sr2Fe0, became elongated along the c axis with increasing x, especially for x>0.4. The measurements of Seebeck coefficient, magnetic susceptibility and Mossbauer effect sug-gested the existence of a gap in the electric states between Sr2Fe04 and the La-substituted ones. Further details are under study. This study was financially supported by 'Grant-in-Aid for Science of High T, Superconductivity' given by Ministry of Education, Science and Culture of Japan. All computations for structural analysis were carried out at the Mie University Information Processing Center.References J. B. MacChesney, R. C. Sherwood and J. F. Potter, J. Chem. Phys., 1965, 43, 1907; Y. Takeda, K. Kanno, T. Takada, 0. Yamamoto, M. Takano, N. Nakanishi and Y. Bando, J. Solid State Chem., 1986, 63, 237; A. E. Bocquet, A. Fujimori, T. Mizokawa, T. Saitoh, H. Namatame, S. Suga, N. Kimizuka, Y. Takeda and M. Takano, Phys. Rev. B, 1992,45, 1561. For examples, F. Kanamaru, H. Miyamoto, M. Mimura, M. Koizumi, M. Shimada and S. Kume, Muter. Res. Bull., 1970, 5, 257; Y. Takeda, S. Naka, M. Takano, T. Shinjo, T. Takada and M. Shimada, Mater. Res. Bull., 1978, 13, 61; M. Takano, S. Nasu, T. Abe, K. Yamamoto, S. Endo, Y. Takeda and J. B. Goodenough, Phys. Rev. Lett., 1991,67, 3267. P. K. Gallagher, J. B. MacChesny and D. N. Buchanan, J. Chem. Phys., 1966,45,2466. S. E. Dann, M. T. Weller and D. B. Currie, J. Solid State Chem., 1991,237. F. Izumi, J. Miner. Soc. Jpn., 1985, 17, 37. S. E. Dann, M. T. Weller and D. B. Currie, J. Solid State Chem., 1992,97, 179. P. K. Gallagher, J. B. MacChesney and D. N. E. Buchanan, J. Chem. Phys., 1964,41,2429. M. Takano, N. Nakanishi, Y. Takeda, S. Naka and T. Takada, Mater. Res. Bull., 1977, 12,923. Paper 3/04301K; Receitied 21st July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400019

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( l),23-27 Metal Stannates and their Role as Potential Gas-sensing Elements Gary S. V. Coles,* Stephanie E. Bond and Geraint Williams Department of Electrical and Electronic Engineering, University of Wales, Swansea, UK SA2 8PP A selective gas sensor, sensitive to the presence of carbon monoxide in preference to the lower hydrocarbons, can be fabricated from a mixture of bismuth oxide and tin dioxide when sintered at 800°C. At temperatures above ca. 650°C a solid-state reaction takes place in which bismuth stannate (Bi,Sn,O,) is formed and in the above sensor all of the Bi203 is converted to the stannate. This material is one of a group of mixed oxide stannates which possess a pyrochlore structure and have the general formulae M2Sn207. Several of these materials can be produced by heating an intimate mixture of lanthanum metal oxides (M203where M =La, Nd, Sm, Gd, Yb, Dy, Tm and Ho) and tin dioxide at temperatures of 1500 "C.Sensors were produced containing these materials in an attempt to reproduce the behaviour of the original device and further understand the chemical, physical and topographical features responsible for conferring selectivity.However, none of the new sensors produce results consistent with those observed for the tin-bismuth system. It has subsequently been shown that when SnO, is subjected to heat treatment at 1500 "C, it can exhibit both resistance increases and decreases upon exposure to the same gas, depending on the operating conditions. In order to reduce these interfering effects the sintering temperature was lowered to 1350"C and sensors fabricated from pure tin dioxide fired at this temperature respond in a conventional manner to all reducing gases tested.A series of sensors produced from some of the SnOJM,O, materials listed above exhibit a general trend of increasing carbon monoxide and hydrogen sensitivity with decreasing M3+ ionic radius. Tin dioxide sensors, operated at elevated temperatures, respond via conductance modulations to a wide range of reducing and oxidising gases. Considerable effort has been directed towards improving the specificity of these devices. A common practice is to incorporate various additives with the SnO, powder prior to sensor fabrication.' For example, the addition of small quantities (typically ca.1 wt.% of the sensing material) of palladium or platinum can increase sensitivity towards reducing gases such as hydrogen and the lower hydrocarbons signifi~antly.,.~ Previous work by this group4 has established that a sensor element fabricated from a tin dioxide-bismuth oxide mixture, when heated to a temperature of SOO'C, responds selectively to carbon monoxide in the presence of methane. High-temperature treatment of the oxide mixture results in a solid-state reaction between the tin and bismuth oxides resulting in the formation of bismuth stannate: 2Sn0, +Bi203 +Bi,Sn,O, (1) X-Ray powder diffraction studies carried out by Roth' estab- lished that this compound possesses a pyrochlore structure.Furthermore, it appears that in addition to Bi, many other metal oxides with the general formula M203 react, when heated together in an intimate mixture with tin dioxide, to form M,Sn20, stannates possessing a pyrochlore structure.6 Metal oxides from the lanthanide series of the periodic table constitute the majority of these and can be substituted in place of bismuth oxide, thus providing a means of examining whether formation of pyrochlore-type stannates induce improved CO selectivity over CH, via an identical mechanistic process. Lanthanide metal stannates with the general formula M2Sn,0, belong to a family of compounds possessing a cubic pyrochlore structure. This has space group Fd3m (Oh7),com-prising eight M2Sn2060' formulae per unit cell., The structure consists of corner-sharing Sn06 octahedra surrounding hexag- onal vacancies, these containing M,O' tetrahedra.Fig. 1 (a) shows the 'pyrochlore unit' consisting of four octahedra sharing corners around a vacancy,8 while Fig. l(b) gives a representation of the octahedral arrangement within the pyr- ochlore frame~ork.~ All Sn-0 bond lengths are identical and are generally of the order of 0.20 nm. Two types of oxygen atom are attached to the M atom: six oxygen atoms associated with the octahedra (M-0 bond lengths are ca. 0.26 nm) plus octahedra Sn06 tetrahedra M&' 00'4BO OM Fig. 1 Diagrammatic representation of the octahedra and tetrahedra constituting the pyrochlore structure. The pyrochlore unit consisting of four corner-sharing SnO, octahedra is shown in (a), while (b)indi-cates the association of these units along with M,O' tetrahedr,i in the pyrochlore framework.two additional oxygen atoms (M-0' bond lengths are cu. 0.23 nrn)., Room-temperature X-ray diffraction was used for structural examination and to determine the approximate temperatures at which the respective lanthanide stannates form with a view towards possibly incorporating lower sintering temperatures in the sensor fabrication process. High-temperature X-ray diffraction was employed to further study the solid-state reaction between the oxides of tin and bismuth and to gain some insight into the mechanism by which the selective response of a sensor element composed of a sintered mixture of the two oxides arises.Experimental A Guinier-De Wolff X-ray diffraction camera manufactured by Enraf Nonius Delft employing Cu-Ka radiation, was used to generate diffraction patterns from powdered crystalline samples at ambient temperature. High-temperature X-ray diffraction apparatus comprising a Siemens D500 diffractometer instrument with 6-26 geometry was used to take X-ray diffraction patterns at temperatures up to 1000°C. The basic principles of the corresponding room-temperature technique were employed. In brief, the sample was contained in a high-temperature environmental cell subsequently mounted on an X-ray diffractometer. The diffractometer was controlled by a PDP-11 computer employing Siemens DIFFRAC 500 software.Temperature- resolved X-ray patterns were produced upon transferring data files to a PC spreadsheet. Two types of sensor were tested during the course of these studies. The first type was fabricated from an aqueous paste containing the Sn02-M203 mixture which had previously been subjected to high-temperature treatment. This was applied across the contact array of an alumina substrate supplied by Rosemount Engineering Ltd., allowed to dry and then sintered for a further 2 h at 1000°C. Alternative sensors employed tin dioxide-based pressed pellets as the active material and were prepared in the following manner. First, a disc of 13 mm diameter and 1 mm thickness was produced by pressing 0.4 g of the desired powder in a stainless-steel die under a pressure of 10 tons for 15 min.The discs were then sintered at the appropriate temperature (usually 1500 "C, ensuring complete solid-state reaction of the SnO, and M203 additive) for 2 h in air. A square section of the disc (ca. 2x2mm) was cut out and attached across the parallel contact pads of an alumina substrate by means of a conducting Pt paste. The complete pressed pellet-substrate assembly was then annealed for 1h at 750°C in order to attain the necessary mechanical stability of the Pt adhesive. Full details of the procedures adopted for blending gas mixtures and determining sensor response are given el~ewhere.~.~ Results and Discussion Room-temperature X-Ray Diffraction Analysis Table 1 gives a list of stannates prepared by heating an intimate stoichiometric mixture of tin dioxide and M203 powders.Initial attempts to form the stannates involved the use of a sintering temperature of 1000°C. However, with the J. MATER. CHEM., 1994, VOL. 4 exception of Bi,Sn,O, none of the systems studied showed any evidence of solid-state reaction when analysed by X-ray powder diffraction. At higher temperatures some stannates are clearly formed more readily than others, for example La,Sn,O, and Sc,Sn,O, (although this latter stannate deviates from a cubic pyrochlore structure). All the materials prepared differ considerably from bismuth stannate in that they require a significantly higher temperature, usually in excess of 1300 "C, to mediate the solid-state reaction involved.Reducing-gas Response of M"'-Doped Tin Dioxide Sensors Initial studies were performed on mixed oxides which had been presintered at 1500°C for 1 h. However, owing to the anomalous sensing behaviour exhibited by undoped tin diox- ide subjected to heat treatment at this temperature," no comparisons between the results given by these materials and the Sn0,-Bi,O, system could be made. The pretreatment temperature was therefore decreased to 1350°C and the duration of the exposure increased to 2 h. A cross-section of M203 additives was chosen from the series givtn in Table 1, covering a wide range of ionic radii from 1.016 A for La3+ to 0.85 A for Lu3+. It has been suggested6 that the size of the M3+ ion present in the M,Sn,O, lattice influences strongly the extent of distortion inherent in the cubic pyrochlore structure, where the greatest distortion of the SnO, octahedra is caused by the smallest M3+ ion.It may be, therefore, that the stannates of Lu, Yb and Y, which possess the smallest M3+ radii of the lanthanide metals studied are most closely related structurally to Bi,Sn,O,. Mixtures of tin dioxide and 15 wt.% M,O, (where M =Lu, Yb, Y, Gd, Sm, Nd or La) were wet ground and subjected to thermal treatment. X-Ray diffractograms of the powders formed confirmed that for each sample the majority of the M203 additive had been converted to the stannate. The gas- sensing properties of these materials were then evaluated after sensor fabrication. A summary of the results obtained for this series of sensors tested at two different operating temperatures in 1 vol.% concentrations of CO, CH4 and H, in dry air is given in Table2.Sensor response is represented by the ratio Ro:Rgaswhere Ro is the resistance in clean dry air and Rgasis the resistance in a reducing gas-air mixture. Therefore, the greater the RO:Rgasratio, the higher the sensitivity of the device to the specified gas. The absence of any results for the Sn0,-Lu203 material is explained by its highly conductive nature (sensor resistance at room temperature = 1.5 R) and therefore its unsuitability as a gas-sensor element. Interestingly, the onset of high conductivity matched the Table 1 Stannates prepared by heating an intimate stoichiometric mixture of tin dioxide and M,03 powders.Sintering times/h are given in parentheses compounds constituting reaction mixture Bi,03-2Sn0, La,O3-2SnO, Sc2O,-2SnO, Yb,O3-2SnO, Tm,0,-2Sn02 Sm,O3-2SnO, Nd,03-2Sn0, Gd,03-2Sn0, Er2O,-2SnO, Lu20,-2Sn0, Y20,-2Sn0, Dy,03-2Sn0, Ho,O,-2Sn02 Eu,03-2Sn0, temperature of stannate emergence /"c 650 (1) 1100 (24) 1200 (24) 1300 (24) 1200 (24) 1350 (2) 1350 (2) 1350 (2) <1420 (2) 1350 (2) 1350 (2) < 1500 (2) > 1500 (2) < 1500 (2) temperature at which the stannate is fully formed/"C 800 (1) 1300 (24) 1200 (24) 1420 (24) 1500 (24) 1420 (2) 1420 (2) 1420 (2) 1420 (2) 1500 (2) 1500 (2) 1500 (2) > 1500 (2) 1500 (2) literature values of stannate formation temperatures/"C 1250 ( 1)5 1550 (1)5 1400 (24)6 1400 (24)6 1400 (24)6 1500 (24)6 1550 (1)' 1400 ( 12)6 1400 (24)6 1400 ( 24)6 1400 (24)6 1500 ( 18)6 1500 ( 18)6 1400 (24)6 J.MATER. CHEM., 1994, VOL. 4 Table 2 Response of Sn02-M203 (15 wt.%) thick films, presintered at 1350°C for 2 h in air prior to sensor fabrication, to 1 vol.% reducing gas concentrations in dry air. operating temperature =300°C dry air dopant resistance RO/G Ro/Rco Ro/R,,, R0/RH2 none 1.8 x lo6 4.2 3.0 29 Yb203 9.5 x lo8 4.5 2.9 33 y2°3 3.4 x lo8 2.8 1.4 20 Gd203 4.2 x lo8 2.6 2.0 9.1 Sm203 8.4 x 10' 1.5 1.3 4.2 Nd203 1.9 x 109 1.5 1.1 6.2 7.8 x 10' 2.4 1.6 8.3 none" 6.0 x 104 31 12.2 134 Bi203" 1.5 x 107 9.8 1.1 19.4 operating temperature =430°C M3+ ion@ dopant radius/A ROIRCO ROIRCH.4 R0/RH2 none -2.4 4.7 29 Yb203 0.858 4.4 4.0 7.7 y2°3 0.893 5.9 3.2 14.3 Gd203 0.938 1.5 1.7 2.7 Sm203 0.964 1.7 1.7 2.1 Nd203 0.995 1.2 1.3 2.1 1.016 1.8 2.3 2.6 none" -1.o 13.3 2.4 Bi203" 0.960 2.3 1.o 5.0 "Sensors were pre-sintered at a temperature of 800 "C.emergence of the stannate since the conductance of Sn0,-Lu203 mixtures sintered at temperatures of 1300 "C or less were similar to pure tin dioxide. However, a striking feature of the remaining sensors are their high resistances, often several orders of magnitude greater than the undoped SnO, subjected to pretreatment at the same temperature. However, this is not wholly unexpected given the analogy with the highly resistive Sn02-Bi,Sn,07 sensor formed upon high-temperature sintering of the corresponding tin-bismuth oxide mixture.' The results listed in Table 2 indicate a general trend of decreasing CO and H, sensitivity and a greater degree of selectivity at the expense of methane response, especially at the lower temperature employed as the M3+ dopant ionic radius is increased.However, there is no real evidence of selectivity over the whole temperature range as is the case with the bismuth oxide system, since CH4 response increases for the majority of devices tested as the operating temperature is raised to 430°C. The lack of a positive result from these studies may be due to three possible causes.(i) It may be that the pyrochlore structure of Bi,Sn207 is not the important factor in determining selectivity to CO and H, in preference to methane. The action of the bismuth stannate may be to change the nature of the oxygen species, such as 0-or O,, present on the tin dioxide surface, thus controlling the types of reaction occurring at the semiconductor/gas interface. (ii) It is also possible that the family of stannates studied are not sufficiently similar to Bi,Sn207 and are therefore unable to moderate reactions in the same manner when incorporated in tin dioxide sensors. The stannates studied here possess cubic pyrochlore structures, though these become more distorted as the M3+ ionic radius of the M,Sn,07 material decreases., However, there appears to be some conflict in the literature concerning the validity of this previous statement.The findings of Vandenborre and HUSSO~,~ who used IR and Raman techniques to analyse some of the structural characteristics of M2Sn207 compounds, suggest that only lanthanum stannate from the series M=La, Sm, Gd, Yb and Lu exhibits some distortion of the SnO, octahedral network. (iii) The re-grinding step required after thermal pretreatment in order to produce an aqueous paste of the material may destroy possible effects caused by any solid-state diffusion of stannate to the surface of tin dioxide grains. This process is probably mfluen- tial if the action of Bi2Sn,07 is that of a molecular sieve on the surface of SnO, grains when incorporated into a tin dioxide-based sensor.In order to overcome possible problems caused by re-grinding the sensing material after the high-temperature sinter- ing step, active sensor elements were prepared from pressed pellets of the SnO,-M,O, mixtures as described in the Experimental. A summary of the results obtained for these devices is shown in Table 3. Sensor materials prepared by this method behave differently in several ways compared to the polycrystalline Sn0,-M,O, thick-film sensors described above. First, in order to attain maximum gas sensitivity, the sensor operating temperatures for pressed-pellet sensors are usually higher than previously observed. This is presumably owing to a lack of contact between the disc section and substrate surface.Consequently, heat transfer must occur across a narrow gap caused by protrusion of the Pt adhesive contact areas. Subsequent thermocouple measurements on the pressed-pellet-type sensors showed that the temperature of the disc section surface was 100-130°C lower than the sub- strate surface temperature. Secondly, it appears that sensitivity to CO and CH4 is greatly reduced while response to hydrogen remains very substantial. There seems also to be some degree of selectivity conferred upon addition of the lanthanide metal oxides, though the effect is not as startling as that observed for Bi,03-doped tin dioxide. Additives such as Yb, Sm, Gd and La appear to yield sensors exhibiting CO selectivity in the presence of methane, though the magnitude of the observed response is not sufficiently large to make any definite conclusions.In comparison to undoped SnO,, the hydrogen respmse of the lanthanide metal oxide-doped pressed pellets is greatly enhanced. Changes in conductance of up to 3 orders of magnitude are observed upon exposure of these types of sensor to 1 vol.% concentrations of H, in air. Fig. 2 shows a comparison of the performance of the two different types of sensor studied here, where the active material in both cases is composed of a Sn0,-Sm,O, (15 wt.%) mixture sinttx-ed at high temperature. The difference in the profiles of conductance uersus [H,] plots for the pressed pellet and thick-film sensors may arise from a change in the following physical properties.(i) Porosity: this is likely to be the most influential controlling factor of diffusion of reducing gas through the sensing material surface. The porosity of pressed pellets is expected to be greatly decreased when compared with a film of srntered Table 3 Response of various pressed-pellet-type sensors fabricated from Sn02-based mixtures to 1 vol.% inclusions of CO, CH4 or H2 in dry air. All pressed pellets tested were subjected to heat treatment at 1500°C for 2 h unless otherwise indicated substrate temperature dopant 1°C RoIRco none 515 1.26 1.10 8.9 Yb203 y2°3 Gd203 460 485 445 1.14 1.72 1.24 1.o 1.46 1.02 36 1500 98 Sm203 Nd203 La203 none" 495 475 435 480 1.59 1.31 1.52 1.37 1.03 1.22 1.06 1.61 260 120 68 10.8 Bi203" 400 2.09 1.o 20 "Heat treatment at 800°C for 2 h.J. MATER. CHEM., 1994, VOL. 4 0.01r IA 0.001 I ,1 UJ 100 1000 10000 53 1000r I 100 -(b) i10 -/x 1°1 /-* / u0.1100 1000 10000 [gas1(PPm) Fig. 2 Response characteristic obtained for sensor elements composed of presintered Sn0,-Sm,O, ( 15 wt.%) in (a)thick-film and (b)pressed-pellet form. Sensor operating temperatures are 310 and 495"C, respectively. x =CO; 0=methane; * =hydrogen polycrystalline SnO,, thus favouring the detection of the lightest gas, namely hydrogen. (ii) Geometry: it has been shown by several researchers" that a change in the thickness of the tin dioxide film can modify response to a given gas significantly and therefore affect sensor specificity.In the present case it should be noted that the thickness of a section of pressed disc is usually significantly greater than the depth of a sintered Sn02 film. High-temperature X-Ray Diffraction Analysis From the results presented above, the Sn02-Bi203 system appears unique in conferring CO selectivity in the presence of methane over the whole operating-temperature range of the sensor. Experiments have shown that a sensor composed entirely of Bi,Sn,O, does not respond to reducing gases when operated at elevated temperatures. Conversely, undoped tin dioxide is an excellent sensing material (see Table 2) which responds to all reducing gases tested. Further study of the system involved X-ray diffraction analysis of the sensor material when sintered at high temperature, which represents an integral part of the fabrication procedure. Fig.3 shows a temperature profile of the reaction initiated when a 2:l mixture of SnO, and Bi203 is heated. The temperature was raised from 200 to 800°C at a rate of 8°C min-l, patterns being obtained at 100"C intervals. The reac-720 700 680 660 640 620 600 dULtemp.room Fig. 3 X-Ray diffraction pattern of a 2Sn0,-Bi,03 mixture with increasing temperature/"C and after cooling to room temperature tion mixture was subsequently cooled at 25 "Cmin-' to room temperature. The complete temperature profile gives details of product formation at various temperatures, thus producing constructive information concerning mechanisms inherent within the reaction. The relevant peaks of an X-ray diffraction profile of the Bi,03 +2Sn0, system at room temperature constituted a reference guide for Fig.3. All profiles were taken with the machine set in the range 28= 18-38". Bi203 peaks existed at 28~19.8,21.8, 24.7, 28, 30.2, 32.5, 33.9, 35.1, 35.4, 37.0 and 37.6", whilst characteristic tin dioxide peaks appeared at approximate 28 values of 26.5 and 37.9'. Solid-state reaction commences at a temperature of ca. 650°C as shown by the appearance of the product Bi,Sn,O,. Also evident at 750°C (28=31.5") is a transitional peak corresponding to the 6 form of Bi203, thereafter absent at 800°C due to its reaction with SnO, for stannate generation.Its pattern indicated a cub& pyrochlore structure with lattice parameters a =b =c =10.7 A. Conclusion of the stannate for-mation process was indicated by the complete absence of reactants after a dwelling period of 25-35 min at 800 "C. Comparison of the room-temperature stannate peak and that present at 800°C showed a peak shift to the right upon cooling. Low intensity reflections present in the low-temperature pattern also indicated distortion of the structure upon cooling. A temperature-programmed X-ray diffraction profile of Bi203,shown in Fig. 4, comprises a temperature range set at 600-800 "C, with patterns recorded using 20 "C increments. The room-temperature pattern taken before heating is shown at the foot of the diagram. The set of patterns shows the transition of monoclinic a-Bi203 to the cubic S-Bi203form occurring within ca.740-760 "C, indicating characteristic peaks for both forms. These results concur with the findings of Levin and Roth12 who determined that the transformation takes place at a temperature of 730 f5 "C. The characteristic a-Bi2O3 peak was revealed at 28=ca. 32.5", the distinguishing 6-Bi203 peak appearing at 28~31.4".A shift in the Bi203 peak from 28~~27.3"(at room temperature) to 28~27.0"(at J. MATER. CHEM., 1994, VOL. 4 iBi203 I room temp. 850 800 750 700 650 600 500 400 300 200 Bi2Sn207 I/i,/,II1,I,I,I,IIlllll,l,l,~ll,//l~ll11­18 20 22 24 26 28 30 32 34 36 38 2Bldegrees Fig.4 Observed changes in the X-ray diffraction pattern of Bi,O, upon heating in the 600-800 "C region 800 "C)was detected. a-Bi203 and d-Bi203 peaks also showed corresponding deviation to the left upon increasing tempera- tures, indicating structural distortion. Conclusions Of the tin dioxide-metal stannate mixtures studied, the Sn02-Bi,Sn207 system appears to be unique in several ways. First, stannate formation via the solid-state reaction of the oxides of tin and bismuth proceeds at relatively low tempera- tures compared with systems incorporating lanthanum group metals. In addition, a sensor composed of a sintered mixture of Sn02 and Bi203 (15-18 wt.%), responds selectively to CO containing atmospheres in the presence of CH4.Gas sensors produced from mixtures of tin dioxide with other pyrochlore- forming M203compounds did not display the same character- istics. A general trend of increased reducing gas sensitivity, most notably to CO and H2, with decreasing M3+ ionic radius was observed for thick-film sensors prepared from Sn0,-M203 powders which had been subjected to heat treatment at 1350°C. However, none of the devices tested exhibited similar selectivity traits to the SnOz-Bi203, i.e. they did not exhibit negligible methane sensitivity over the whole operating temperature range. Sensors fabricated from pressed pellets of the presintered materials displayed greatly reduced CO and CH4 sensitivities. However, upon exposure to hydro- gen, large changes in sensor resistance of up to 3 orders of magnitude were observed.Temperature-programmed X-ray diffraction profiles of heated Bi203 and a Bi,03-2Sn02 mixture have revealed the transitional d-Bi203 phase present above 760 "C, whilst observing Bi203 heated within the temperature range 600-800°C. In addition, this appears upon heating the Bi203-2SnOz mixture to 800 "C, disappearing above 750 "C owing to its reaction with Sn02. Formation of bismuth stannate, Bi,Sn,O,, initially occurs at ca. 650 "C, the product peaks indicating a cubic pyrochlore structureo with correspond- ing lattice parameters of a=b=c= 10.7 A. Further infor- mation supplied by profiles of both Bi203 and Bi203-2Sn02, shows increased distortion of the structure (denoted by the appearance of low-intensity reflections and peak shifts) upon cooling. References 1 S. R. Morrison, Sens. Actuators, 1987, 12,425. 2 K. Ihokura, New Mater. New Processes, 1981,1,43. 3 N. Yamazoe, Y. Kurokawa and T. Seiyama,Sens. Actuators, 1983, 4,283. 4G. S. V. Coles, K. J. Gallagher and J. Watson, Sens. Actuators, 1985,7,89. 5 R. S. Roth, J. Res. Natl. Bur. Stand. (U.S.), 1956,56, 17. 6 F. Brisse and 0.Knop, Can. J. Chem., 1968,46,859. 7 M. T. Vandenborre and E. Husson, J. Solid State Chem., 1983, 50, 362. 8 H. Nyman, S. Andersson, B. G. Hyde and M. O'Keeffe, I. Solid State Chem., 1978,26, 123. 9 G. S. V. Coles, G. Williams and B. Smith, Sens. Actuators, R, 1991, 3, 7. 10 G.S. V. Coles and G. Williams, J. Mater. Chem., 1992,2,23. 11 D. E. Williams, in Solid State Gas Sensors, ed. P. T. Moseley and B. C. Tofield, Adam Hilger, Bristol, 1987, p. 71. 12 E. M. Levin and R. S. Roth, J. Res. Natl. Bur. Stand. (U.S. /,1964, 68(A), 189. Paper 3/04264B; Received 20th Julj,,1993

ISSN:0959-9428    

DOI:10.1039/JM9940400023

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( l), 29-34 Chemomechanical Polishing of Gallium Arsenide and Cadmium Telluride to Subnanometre Surface Finish Evaluation of the Action and Effectiveness of Hydrogen Peroxide, Sodium Hypochlorite and Dibromine as Reagents Laurence McGhee; Scott G. McMeekin,bt Irene Nicol," Max I. Robertsonb and John M. Winfield"* a Department of Chemistry, University of Glasgow, Glasgow, UK G12 8QQ Logitech Ltd., Old Kilpatrick, Dunbartonshire, UK G60 5EU Aqueous hydrogen peroxide and sodium hypochlorite in the pH range 7-9 are more effective chemomechanical polishing reagents for gallium arsenide than is dibromine in methanol. Sodium hypochlorite is an acceptable alternative to hydrogen peroxide for gallium arsenide: it also produces good-quality surface finishes on cadmium telluride over the same pH range.The results of dip-etch and polishing reactions, studied using [825r]-labelled dibromine, atomic absorption spectroscopy (AAS) and pH or concentration variation, are used to propose a model for chemomechanical polishing of these materials. Solution etching and chemomechanical polishing of group 14, 13-15 and 12-16 semiconductors are technologically import- ant processes. They are related, since in both cases topochem- ical reactions are potentially involved. In a chemomechanical polishing process an etching reaction, in which material is removed from the semiconductor surface by dissolution, is moderated by mechanical means. Etching and polishing reac- tions have both received considerable attenti~nl-~ and the general principles involved are well known. They include the formulation of etchant reagents as mixtures of oxidants and complexing agents.However, the investigations have been made largely on an empirical basis and many of the events postulated to occur on a semiconductor surface rest on unproven assumptions or have been inadequately described. Increasingly, this lack of detailed knowledge is a barrier to the production of the high-quality surface finishes required for device manufacture. The current requirement of a chemo- mechanical polishing process is that it should be capable of achieving rapidly a subnanometre finish. This means that the variation between high and low points over a short distance on the surface, as measured by a stylus instrument, should be 1 nm or less.Many semiconductor materials are not robust, hence the mechanical forces experienced by the sample should be kept to a minimum. There is, therefore, a requirement for new and more selective reagents. As a basis for developing new reagents we have made a detailed investigation of the behaviour of dibromine (Br,), hydrogen peroxide( H,O,) and sodium hypochlorite(NaOC1) on gallium arsenide(GaAs) and cadmium telluride(CdTe). The use of Br,, usually in MeOH, for polishing GaAs'-' and CdTe'*6 has been widespread for many years and H202 has received some attenti~n.'-~'~-'~ However, these reagents have never been compared under similar conditions. The hypo- chlorite anion is the central member of the series of reagents represented by -0-0-, -0-X and X-X, where X represents a halogen, but it has been used far less Our approach has been to combine chemomechanical-polishing experiments, carried out with careful control of reagent concentration and pH where appropriate, with physicochemical studies of dip-etch reactions.A preliminary account of part of this work has appeared'* and is part of a t Present address: Department of Electronic and Electrical Engin-eering, University of Glasgow, Glasgow, UK G12 SQQ. general investigation of polishing and etching of electronic, optoelectronic and optical rnaterial~.'~ Experimental General Methods The acidities of the solutions used in the dip-etch and polishing reactions were determined using a standard pH electrode (Russell pH Ltd) with an EIL 7050 meter. A Perkin-Elmer Lambda 9 spectrometer was used to monitor the decomposition of hypochlorite solutions as the pH was varied.The removal of substrate material to polishing or etching solutions was monitored using a Perkin-Elmer 1100B atomic absorption spectrophotometer in conjunction with an MHS-10 hydride generator to detect soluble As, and an HGA 700 graphite furnace to detect soluble Te. Soluble Cd was determined using the flame method. No attempt was made to determine soluble Ga using AAS since its sensitivity to detection is considerably lower than those of the other three elements. Spectrosol standard solutions were employed throughout and standards were matrix matched to the ana- lytes.Sample solutions from etches or polishes required no pretreatment other than dilution. Replicate analyses were performed in all cases. Data presented are specimen results from series of experiments having reproducible outcomes. Reflectance IR spectra of etched wafer surfaces were obtained using a Nicolet IRIS dedicated FTIR microscope. In selected cases, wafer surfaces were mapped by taking 10 random sampling points over a 5 mm x 4 mm area. In dip-etch experi- ments, wafers were weighed before and after a reaction to correlate mass changes with soluble material produced. Polishing experiments were performed using Logitech CP4000 (reagents Br,-MeOH and H202-NH3) or Logitech PM2A polishing equipment, in the latter case with the addition of a PP5 vacuum jig.CS2Br]-labelled dibromine, used to determine the Br, interaction with a wafer surface, was produced from [*'Br]-labelled ammonium bromide by reaction with H2S04 in ~acuo.'~Neutron irradiations, "Br(n, y)82Br, were carried out in the Scottish Universities Reactor Research Centre, East Kilbride, with a neutron flux of 3.6 x 10'' neutron cm- s-' for 3 h. The activity of the CS2Br]-labelled ammonium bromide was ca. 100 mCi. Labelled solids were transferred to Glasgow ca. 24 h after irradiation to ensure complete decay of short-lived components. The specific activity of the [82BrJ-bromine labelled Br, was determined by counting a measured aliquot of Br, in AnalaR chloroform (1cm3).CS2Br] count rates were determined using a sodium iodide scintillation counter with a scaler ratemeter (Ecko and NE). The usual background corrections were made and radiochemical purity was deter- mined by half-life measurement (tl,, =35.3 h). Preparation of Reagents The etchant solutions were prepared from analytical grade materials and were freshly prepared prior to each experiment, since all decomposed to some extent at room temperature. In order to make meaningful comparisons in polishing and dip-etching experiments, it was necessary to determine pre- cisely the extent of decomposition of a reagent. The peroxide reagent was prepared by dropwise addition of aqueous NH, (Pronalys, M & B, 33% wt./wt.) to aliquots (60.0 cm3) of H,O, (AnalaR, BDH, 100 vol.33% wt./wt.) until the pH reached 6.54, 7.4 or 8.0. The solutions were stored in plastic bottles and aliquots taken for analysis every few hours. Each aliquot was diluted x 100, acidified with 0.1 mol dm-3 H2S04 (20 cm3) and titrated against standard 0.020 mol dm-, KMnO, solution. The higher the pH the faster the decompo- sition. At pH 8.0, the oxidising ability was 25% of its original value after 40 h, at pH 7.4 it was 75% after 140 h and at pH 6.54 it was 78% after 160 h. All solutions were sealed during use since minute quantities of impurity catalysed the decomposition at irreproducible rates. Bromine-methanol mixtures (100cm3) were prepared from Br, and MeOH (AnalaR BDH) and the concentration determined by iodi- metric titration using 0.100 rnol dm-3 [S,O,]’-(Volucon M & B).Aqueous sodium hypochlorite (12% wt./wt., Spectrosol, BDH) was brought to the desired pH by dropwise addition of glacial acetic acid (Pronalys, M & B). Aliquots (5 cm3) of Br,-MeOH were pipetted into aqueous KI (1 g in 100 cm3) and the liberated iodine titrated against 0.100 mol dm-3 [S2O3I2-. Approximately 33% of the oxidising power of Br,-MeOH was lost over 24 h. After 6 days, the oxidising power decreased by ca. 50%. This behaviour was observed for Br,-MeOH solutions in the concentration range 0.1-0.4 mol drn-,. Available chlorine in NaOCl solutions (1:100 vol./vol. in water) was determined by iodimetry with 0.100mol dm-, [S2O3I2-over the pH range 5.4-8.1 and over 21 h.The conversion of NaOCl to HOCl was monitored using electronic spectroscopy. For [OCl] -A,,, =292 nm and for HOCl A,,, = 236 nm.17 The rates at which [OCll- was converted to HOCl and underwent decomposition were pH dependent. Solutions whose pH initially was 67 had a pH of 4.5 after 20 h. Their electronic spectra showed no evidence for [OCl] -. Solutions prepared at higher pH were stable for longer periods but all trace of [OCll-in the spectra had disappeared after 24 h. Provided the solutions were prepared immediately prior to etching, no significant change in pH occurred during the 0.5 h duration of an experiment as shown by control experiments. Dichlorine was evolved during decomposition of the solutions and iodimetric titration revealed a decrease in oxidising power after 0.5 h.Dip-Etch Experiments Wafer sections (2.5 cm x 2.0 cm) were etched in solutions (60.0 cm3) which were stirred continuously. The wafer sections were supported in a Pyrex glass holder inside a Pyrex reaction vessel. Etch times were 0.5 h or 10 min for GaAs, and 10 min for CdTe. Material lost to the solution was determined by mass change and by determination of soluble As, Cd or Te by atomic absorption spectroscopy. In separate experiments, the effects of concentration variation in Br,-MeOH, pH J. MATER. CHEM., 1994, VOL. 4 change in H,02 and both effects on etching with OCI--HOCl were determined. Reactions under anhydrous conditions were also undertaken using dry Br, (P,O, in vacuo) in dry dichloro- methane (molecular sieve type 3A).These experiments used a double-limbed Pyrex vacuum vessel incorporating a stop-cock to isolate one limb from the other (40cm3) which was attached to a Pyrex vacuum line ( lop4Torr). Concentrations ranged from 0.662 to 10.85 mol dm-3. Polishing Experiments The polishing experiments employed single-crystal GaAs ( 100) and polycrystalline CdTe wafers (2.5 cm x 2.0 cm). Typical conditions for H20,-NH, and Br,-MeOH polishes were a plate speed of 70rpm, a load of 400 gem-' and a reagent feed rate of 700 cm3 h-l. Polishing with H,O,-NH, was investigated over the pH range 6-8.5 since dip-etch experi- ments showed this to be the optimum. Polishing with Br,-MeOH was carried out over the concentration range 0.1-0.4 mol drn-,.The conditions used for [OCl] --HOCI polishing were a plate speed of 30 rpm and flow rate of 200 cm3 h-’ under minimal load. In all cases a velvet-napped polishing pad was used. Solutions were in the pH range 6.4-12.3 and polishing reactions were performed usually over 0.5 h. In each case the surface was examined by a Zeiss Normaski microscope, and surface roughness of the higher quality finishes was measured over a 5mm range using a Rank Taylor Hobson Talystep stylus instrument (2 pm radius stylus). Data were compared using the parameter R,, the arithmetic mean of the departures of the roughness profile from the mean line. The polishing of polycrystalline CdTe revealed excessive grain-boundary phases, and, consequently poorer finishes resulted.Hence, R, values for CdTe were significantly higher than for GaAs. In all cases the R, value was the result of several traces obtained over a 5 mm trace from different regions of a wafer. Radiotracer Experiments CS2Br]-Labelled dibromine was used to determine the adsorp- tion of Br, on a wafer surface and to follow the outcome of the adsorption process. Etching using radioactive bromine was carried out in the double-limbed vessel under vacuum as described for the anhydrous dip-etches (vide supra);each limb could be fitted into the well scintillation counter enabling rs2Br] count rates from solid and vapour to be determined separately at regular intervals. The count rate from vapour above the wafer was negligible compared with those from the wafer itself and could be ignored without serious error.Results Dip-Etching of GaAs and CdTe Single crystal (100) wafers of gallium arsenide were etched by both aqueous hydrogen peroxide and aqueous sodium hypo- chlorite solutions at room temperature over 0.5 h periods but in both cases the extent of reaction was highly pH dependent. Mass decreases using H,O, varied uniformly from 1.4% at pH 8.1 to 33.2% at pH 8.85 and using NaOCl they varied from 1.0% at pH 7.5 to 16.4% at pH 13.0. In both cases, mass decreases were <1% at lower pH values and reactions were very rapid at high pH; with NaOCl at pH 13.0, a black corroded surface resulted. For comparison, GaAs etching with Br,-MeOH (0.44 mol dm-3) produced mass decreases in the range 3.8-4.8% after 10 min.In contrast, polycrystalline cad- mium telluride wafers did not appear to be etched by H,O, or NaOCl under these conditions but etching with Br, in MeOH occurred rapidly. Mass decreases determined after J. MATER. CHEM., 1994, VOL. 4 10 min etches ranged from 1.8 to 24.9% for Br, concentrations in the range 0.12-0.83 mol dm-3. The effects of reagent concentration and/or pH were deter- mined by analyses of As or Cd and Te in the solutions after a predetermined time and are shown in Fig. 1-3. The data are presented as soluble As, Cd or Te, these being determined from the product of the concentration of the element and the volume of solution.For a given concentration of reagent, the quantity of As removed from GaAs by Br,-MeOH after 0.5 h was significantly greater than that removed by NaOCl (Fig. 1) and etching by the former reagent was detected readily after 10 min even below 0.1 mol dmP3. In all cases the relationship 160 I401 120 20/00.0 0.5 1.o 1.5 2.0 concentration/mol dm-3 Fig. 1 Dependence of soluble As on reactant concentration in dip-etch reactions of GaAs using Br,-MeOH for 0.5 h (A); Br,-MeOH for 10min (A); NaOCl for 0.5 h (0) 30I I 25 tI 15 0=tu, 10 y 02 4 6 8 10 12 14 PH Fig.2 Dependence of soluble As on pH in dip-etch reactions of GaAs for 0.5 h using H,O,-NH, (0)or NaOCl (A) 0.6Oe71 between soluble As and reagent concentration appeared to be linear to a good approximation.Dilution of NaOCl solutions inevitably led to a pH decrease (from 13.0 to 11.3 for the data in Fig. 1).Changes in dip-etch characteristics for GaAs arising from protonation of [OCl] -and deprotonation of H,O, were investigated by determi- nation of soluble As, using solutions prepared as described in the experimental section, with the results shown in Fig. 2. For both etchants these data provided a more sensitive test of the extent of reaction than was possible from simple weighing. In particular, the data indicated that little or no reaction occurred below pH 7.5; above this value the reaction was substantial particularly for the reagent H20,-NH, (Fig. 2). For H202-NH, the relationship between soluble As and pH was exponential; the scatter in the experimental data arises from the combination of data from two sets of reactions.The corresponding relationship for NaOCl etching was lcss well defined, owing to experimental difficulties in obtaining stable pH values in the pH region 7.5-9.5 with relatively small volumes of solution. The lack of a well defined relationship possibly reflects the complexity of the reagent, since [OCll-, HOCl and their decomposition products are all present.17 The most notable feature was the indication of a plateau between pH 9.5 and 11.0, suggesting a buffering effect; a similar plateau was found in mass decrease measurements. Combined, these observations suggest that the plateau was a real effect rather than an experimental artifact.The variation of Cd and Te removal from polycryuitalline CdTe wafers with Br, in MeOH is shown in Fig. 3. At all concentrations both elements were detected in solution after 10min. The data were not sufficient to define precise relation- ships but except for the lowest concentration of Br2 used, more Cd than Te was lost from a wafer. Although this could have been a reflection of wafer quality, it suggests that the Br,-MeOH reaction is not straightforward. Examination of GaAs wafers after etching with H,O,-NH, or Br,-MeOH by FTIR microscopy indicated a distinct difference in the distribution of hydroxy group density between the two surfaces. Bands assignable to surface terminal hydroxy groups were present in both cases but were better resolved after treatment with H,O,-NH,.Mapping this surface using the relatively sharp 1670 cm-I band, indicated a contoured surface in which peaks and valleys were well defined. The surface of GaAs after etching with Br,-MeOH was dmost featureless implying a lack of specificity in the etching reaction. In previous work it has been assumed that the products from etching of GaAs or CdTe by Br, in MeOH are the corresponding bromides, which for Gal", As"' and Te" will be hydrolytically unstable. It was therefore of interest to examine the behaviour of Br, towards these materials under rigidly anhydrous conditions. Etching of GaAs wafers by Br2-CH,C1, solutions in V~CUOled to small mass increases rather than the decreases observed with MeOH solution when trace water was not excluded.However, there was no obvious pattern discernible and labelling of Br, with the radioactive 0.3 q, led to the immediate detection of ["Br] activity from the solid. Thereafter there was a slow, continuous increase in the 0.2 C8,Br] solid count rate with a concomitant decrease in C8,Br] count rate for the vapour. This is illustrated in Fig. 4(a)for a 0.1 reaction between 82BrBr (2.3 mmol) and GaAs (1 rnmol). 0.0 0.0 0.2 0.4 0.6 concentration/mol dm-3 0.8 1.o Saturation of the GaAs surface by Br, was never achieved and the reaction could be accelerated in its later stages by addition of more ',BrBr. The uptake of bromine by GaAs determined for the specific count rate of 82BrBr and the solid tracer C8,Br] was adopted as a more sensitive probe.2 0.4 Exposure of a GaAs wafer to ',BrBr at room temperature Fig. 3 Dependence of soluble Cd (0)and Te (A)on Br, concentration count rate after 14 h was 3.2 mg atom Br(mmo1 GaAs)-'. for 0.5 h dip-etching of CdTe by Br,-MeOH Uptakes determined in a second experiment using 1.61 rnmol J. MATER. CHEM., 1994, VOL. 4 Polishing of GaAs and CdTe /< // ,' I 1 I I5 01 I I I5 0 50 100 150 200 250 300 350 0 0.5 0 I@) 0.211 -0.1 * A & L I I I I I Fig. 4 Reactions between 82BrBr and GaAs or CdTe; (a)increase in solid Cs2Br] count rate (0)and decrease in count rate of 82BrBr(0) with time; (b)comparison between Cs2Br] count rates from GaAs and CdTe under identical conditions.(0)GaAs; (A)CdTe; (*) indicates that volatile and weakly adsorbed material were removed at this time Br, were 0.75 and 2.1 mg atom Br (mmol GaAs)-' after 5.5 and 22 h respectively. After extensive exposure to Br, vapour, a viscous yellow oil was formed on the surface of GaAs and colourless needles on the vessel wall. Their physical appear- ance suggested that they may have been AsBr, and GaBr, respectively. Although the exposure of CdTe to ',BrBr resulted in an immediate uptake of radioactivity by the solid, the solid count rate showed no increase thereafter. Addition of further 82BrBr or the use of a larger quantity initially, had no effect. The behaviour of CdTe and GaAs wafer fragments (0.2 mmol) towards "BrBr vapour (0.05 mmol) at room temperature is compared in Fig.4(b). After 2 h, the count rate from GaAs was approximately seven times greater than that from CdTe. All volatile and weakly adsorbed materials were removed by pumping at this point. The CS2Br] count rate from CdTe was unaffected while that from GaAs decreased to 58% of its former value. There is a strong implication therefore that one of the products (possibly AsBr,) from the reaction between GaAs and Br, is volatile while the other (possibly GaBr,) is more persistent. The effect of an organic solvent on the reaction between 82BrBr and GaAs was examined by allowing the vapour to diffuse into a counting cell containing a GaAs wafer immersed in 1,2-dihydroxyethane. Methanol was unsuitable for this purpose owing to its volatility and C,H,(OH), has been used as an alternative solvent to MeOH in GaAs polishing. The behaviour observed was similar to that in the absence of a solvent but the uptake of CS2Br] activity appeared to be slower.Thus the solvent did not enhance the extent to which reaction occurred but rather exerted a moderating effect in limiting the concentration of Br, that diffused to the GaAs surface. Single-crystal GaAs( 100) wafers (20 x 25 mm) were polished very satisfactorily using aqueous H202 in the pH range 6.0-8.5. In all cases subnanometre surface finishes were obtained, R, being in the range 0.2-0.9 nm. Surfaces viewed through a Normaski optical microscope were featureless. The quantity of material removed during 5 min periods was con- stant at a given pH value over a 1.5 h polishing experiment but increased exponentially with increasing pH over the range 6.0-8.5 (see Fig.1of ref. 14). The relationship determined was log S=O.8 pH -5.26 (1) where pH refers to the H202 solution and S is the stock (material) removed (pm) by a 0.5 h polish, sample loading 400 g cmP2 and H202 solution flow rate 700 cm3 h-l. It is noteworthy that exponential relations with pH have been found for both polishing and dip-etch processes [Fig. 2(b)], the latter being displaced from the former by ca. 1 pH unit. Even after 5 min polishing at pH >8.5 the surface finish was less satisfactory, owing to excessive decomposition of the etchant and the formation of rough areas.Polishing for periods>0.5 h in the pH range 8.0-8.5 led to a similar situation. It is apparent therefore that careful pH control for this reagent is crucial. Dibromine in MeOH was a less satisfactory reagent since even with a low concentration (0.1% vol./vol.), 'orange peel' roughness7 was apparent; at higher concentrations this was visible to the naked eye (cf. Plate 1 in ref. 14). Stock removed during a 0.5 h polish increased linearly from 5 to 183 pm as the Br2 concentration was increased from 0.5-2.0% vol./vol. Surface roughness increased also from R, =2-9 nm. Determinations of total As collected over 0.5 h periods during polishing with Br,-MeOH and aqueous H202 gave compar- able results.For Br, concentrations of 0.1, 0.2, 0.5, 1.0 and 2.0% vol./vol. Values for As were 14, 93, 114, 247 and 322 mg respectively, and for H20, at pH 7.0, 7.5 and 8.0 the As values were 13, 45 and 257mg. Together with the stock removal measurements made, these data indicate that the overall rates of polishing GaAs with 1.0% vol./vol. Br, in MeOH or with 30% wt./vol. H202at pH 8.0 are comparable. The quality of the surface finish is therefore not related directly to the overall polishing rate. Although it was possible to polish CdTe using aqueous H202, at pH values somewhat lower than those used for GaAs, the process was slow and therefore aqueous NaOCl was investigated as an alternative. This reagent polished GaAs satisfactorily within 0.5 h over the pH range 8.0-8.5, R, ca.1 nm, the optimum pH being 8.3. Polycrystalline CdTe was polished to R, ca. 4nm over a similar pH range, optimally at pH 8.8. In both cases stock removal was small, as were the quantities of As and Cd detected in the polishing fluids (Table 1). The trend in the As data was for removal of As to Table 1 Total soluble arsenic and cadmium detected in polishing fluids collected after 0.5 h polishing of GaAs and CdTe with aqueous NaOCl-CH,C02H PH arsenic/mg cadmium/mg 6.4 5.0 7.2 7.2 1.5 7.6 4.3 0.4 8.0 3.9 2.0 8.3 11.2 1.5 8.8 1.2 9.2 12.5 0.8 10.6 0.4 12.3 18.1 0.3 J. MATER. CHEM., 1994, VOL. 4 be greater at high pH; the value of 18mg determined at pH 12.3 corresponded to 3 pm stock removal.In comparison, stock removal from GaAs by H,02 at pH=8.5 under other- wise identical polishing conditions, was 150 pm. The quantities of Cd detected in solution were very small at all pH values (Table 1)and Te was undetectable indicating that these values were less than 0.5 mg. Discussion and Conclusions In this work it has been demonstrated that aqueous H,O, in the pH range 6.0-8.5 is superior to the more widely used Br, in MeOH for chemomechanical polishing of GaAs wafers. Aqueous NaOCl in the pH range 8.0-8.5 is an acceptable alternative and this reagent will also polish polycrystalline CdTe satisfactorily; the quality of the surface finishes on CdTe obtained in this work is a reflection of the polycrystalline nature of the samples used.Chemomechanical polishing of semiconducting materials is often carried out with the addition of abrasives such as a-alumina, presumably to achieve shorter polishing times. This is unnecessary for GaAs and CdTe and thus the problem of sub-surface damage associated with undue mechanical action is minimised.'' Some insight has also been gained into the reasons for the differences in polishing behaviour by reference to the etching reactions of the reagents. Such studies have been made in the past usually by contact gauge or optical microscopy measure- ments or by mass changes. The advantage of our approach which involves investigation of both the wafer and the etchant solution is that a greater understanding of the physicochemical processes occurring is obtained.Our rationalisation of the observations made on the reactions of Br, and H,O, and NaOCl with GaAs and CdTe follows and is summarised in Scheme 1. However, before proceeding, some general com- ments are warranted. First, in accordance with previous we assume that in the case of GaAs oxidation occurs to give Ga"' and As"' compounds as the final products. Second, the outcome of the reaction involving GaAs is depen- dent on kinetic factors which relate to the removal of products from the surface rather than the thermodynamics of the redox reaction. The reaction between aqueous H202 and GaAs almost certainly leads to the formation of an oxohydroxy layer on the surface, as judged by FTIR analysis, which must be removed to enable further reaction to occur.The importance of pH in affecting dissolution of the oxidized surface has been recognised previ~usly.'~'~ The key observation made here is the exponential dependence, on pH of both dip-etching and chemomechanical polishing processes which may be a result of the higher solubility of the species that comprise the layer as the pH increases. In the latter case the layer is removed largely by mechanical wiping, since in the lower part of the effective pH range for polishing, 6.0-8.5, As removal under 1 redox Br, t GaAs H202+ GaAs I 1 adsorbed Br layer oxohydroxy layer I 2complexation GaBr, AsBr, Ga and As oxohydroxides 3 removal of products soluble Ga and As oxohydroxides Scheme 1 Reactions of GaAs with Br,-MeOH and H,0,-NH3 dip-etching conditions is ineffective (Fig.2). If during the mechanical wiping process, the sparingly soluble material forms a passivating layer, an element of selectivity becomes possible. Support for this suggestion is provided by the observation of varying hydroxy group density on a GaAs surface that has been etched using H202-NH,. The passivating layer concept was first described in the context of polishing of silica'' and we have recently identified chemically the constituents of the layer formed when silica is polished by an hydrogendifluoride reagent.I5 Chemomechanical polishing of GaAs by aqueous NaOCl has many similarities with its polishing by H,O,; the process is pH dependent and good surface quality results.Whilst surface passivation of polycrystalline CdTe occurs with aque- ous H202 or NaOC1, polishing with NaOCl is feasible despite the more tenacious character of the surface layer (see Table 1). The passivating layers are most likely to be oxidic in nature with the added possibility of chloro species when using NaOCl. Dichlorine, formed by the reaction of [OCll- or HOCl with C1- anion, inevitably present in commercial hypochlorite solutions,20 could also be one of the redox-active species involved. Chlorine-36 radiotracer experiments to test this possibility are currently in progress. Cg2Br]-radiotracer experiments indicate that bromination of a CdTe surface passivates it towards further reaction with Br, but the dissolution process in MeOH does not confer sufficient selectivity for Br,-MeOH to be a good polishing reagent towards CdTe.One of the so far unexplained features of the etching of CdTe by Br,-MeOH is the higher Cd level detected in solution for the reaction. Recent studies"*22 of CdTe etching by aqueous Br, in the presence of HBr indicate that the reaction is first order with respect to Br,, it has a low activation energy,' indicative of a diffusion controlled reaction' and that the redeposition of Te observed during the reaction occurs uia the production of an undefined soluble Te species.22 Clearly the reactions occurring in the CdTe-Br, system are complex and require further investigation.Adsorption of bromine on GaAs is followed by a redox reaction which continues as long as Br, is present. There appears to be no mechanism for controlling the reaction in this case and the surface finish obtained from chemomechan- ical polishing with this reagent suffers as a result. The authors thank DTI, SERC and Logitech Ltd for financial support of this work, partly through the DTI Nanotechnology Initiative. The staff of the Scottish Universities Research Reactor Centre, East Kilbride and Dr. D. James, Nicolet Instruments Ltd, Warwick are thanked for assistance with neutron irradiations and FTIR microscopy respectively. References 1 B. Tuck, J. Mater. Sci., 1975, 10,321. 2 D. J. Stirland and B. W. Straughan, Thin Solid Films, 1976, 31, 139.3 W. Kern, RCA Rev., 1978,39,278. 4 C. S. Fuller and H. W. Allison J. Electrochem. SOC., 1962,109,880. 5 M. V. Sullivan and G. A. Kolb, J. Electrochem. SOC., 1963, 110, 585. 6 S. L. Reidinger, D. W. Snyder, E. I. KO and P. J. Sides, Muter. Sci. Eng., 1992, B15, L9. 7 J. C. Dyment and G. A. Rozgonyi, J. Electrochem. Soc., 1971, 118,1346. 8 Y. Mori and N. Watanabe, J. Electrochem. SOC., 1978,125, 1510. 9 E. Haroutiounian, J. P. Sandino, P. ClCchet, D. Lamouche and J. R. Martin, J. Electrochem. SOC., 1984,131,27. 10 J. J. Kelly and A. C. Reynders, Appl. Surface Sci., 1987,29, 149. 11 I. Barycka and I. Zubel, J. Muter. Sci., 1987,22, 1299. 12 S. H. Jones and D. K. Walker, J. Electrochem. Soc., 1990, 137, 1653. 34 J. MATER. CHEM., 1994, VOL. 4 13 V. L. Rideout, J. Electrochem. SOC.,1972,119,1778;H. Hartnagel 18 D. F. Weirauch, J. Electrochem. SOC.,1985, 132, 250. and B. L. Weiss, J. Muter. Sci., 1973,8, 1061. 19 L. M. Cook, J. Non-Cryst. Solids, 1990, 120, 152; N. J. Brown, 14 S. G. McMeekin, M. Robertson, L. McGhee and J. M. Winfield, Precision Engineering, 1987,9, 129. J. Muter. Chem., 1992,2, 367. 20 J. W. Mellor, Comprehensive Treatise on Inorganic and Theoretical 15 D. S. Boyle, J. A. Chudek, G. Hunter, D. James, M. I. Littlewood, Chemistry, Longmans, Green and Co, London, 1946, vol. 2, L. McGhee, M. 1. Robertson and J. M. Winfield, J. Muter. Chem., p. 245; G. Peintler, I. Nagyphl and I. R. Epstein, J. Phys. Chem., 1993, 3, 903; M. Beveridge, L. McGhee, S. G. McMeekin, 1990,94,2954. M. I. Robertson, A. Ross and J. M. Winfield, J. Muter. Chem., 21 A. S. Kovalenko, B. L. Druz, E. I. Gusakova and L. P. Tikhonova, 1994,4, 119. J. Appl. Chem., USSR (Engl. Transl.), 1992,65.466. 16 L. McGhee, P. R. Stevenson and J. M. Winfield, unpublished 22 P. F. Vengel, V. N. Tomashik and A. V. Fornin, J. Appl. Chem., work, 1992. USSR (Engl. Transl.), 1992,65,751. 17 L. C. Adam, 1. Fabian, K. Suzuki and G. Gordon, Inorg. Chem., 1992,31,3534. Paper 3/04289H; Received 2 1st July, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400029

出版商:RSC    

年代:1994

数据来源: RSC

摘要:

J. MATER. CHEM., 1994, 4( l), 35-39 Dynamic Properties of the Urea Molecules in a,cu=Dibromoalkane/ Urea Inclusion Compounds Investigated by *H NMR Spectroscopy Abil E. AlievJ Sharon P. Smart and Kenneth D. M. Harris*t Department of Chemistry, University of St. Andrews, St. Andrews, Fife, UK KYI 6 9ST 2H NMR spectroscopy has been used to investigate the dynamic properties of the urea molecules in Br(CH,),Br/[2H,]- urea inclusion compounds (n=7-1 0) and in pure crystalline [*H,]urea. In the Br(CH,),Br/urea inclusion compounds, the urea molecules form an extensively hydrogen-bonded host structure, which contains linear, parallel tunnels within which the Br(CH,),Br guest molecules are located. The urea molecules undergo a two-site 180" jump motion about their C=O axes, on the 2H NMR timescale, at temperatures greater than ca.200 K for the Br(CH2),Br/[2H,]urea inclusion compounds and at temperatures greater than ca. 300 K for pure crystalline [2H,J~rea. Quantitative details relating to the dynamic properties of the urea molecules in these solids are presented. We are currently interested in the structural, dynamic and chemical properties of urea inclusion compounds containing a diverse range of organic 'guest' molecules. In these inclusion compounds,192 the urea molecules form an extensively hydro- gen-bonded 'host' structure containing parallel one-dimensional tunnels that are densely packed with guest mol- ecules. The internal diameter of the tunnels in the urea host structure is ca. 5.1-5.9 A, and 'guest' molecules based on a sufficiently long n-alkane chain can be accommodated within these tunnels, provided that the degree of substitution of this chain is small.Appropriate guest molecules include alkanes and derivatives such as a,o-dihalogenoalkanes, diacyl per- oxides, carboxylic acids and carboxylic acid anhydrides. We have shown by X-ray diffraction3y4 and other techniques that urea inclusion compounds containing functionalized alkane guest molecules exhibit interesting structural proper- ties, particularly concerning the three-dimensional packing arrangement of the guest molecules. In particular, it has been shown4 by single-crystal X-ray diffraction that, at room temperature, a,o-dibromoalkane guest molecules [Br( CH2),Br with n =7-10] exhibit a characteristic three- dimensionally ordered packing arrangement in which As = cg/3, where cg denotes the periodic repeat distance of the guest molecules along the tunnel and Ag denotes the offset, along the tunnel axis, between the positions of guest molecules in adjacent tunnels. This guest structure is rhombohedral, and a given single crystal of the inclusion compound usually contains two domains of this guest structure, differing in orientation with respect to the host structure.Furthermore, the Br (CH,),Br/urea inclusion compounds also contain regions in which the guest molecules are ordered only along the tunnel axis; the periodic repeat distance, cg, is the same (within experimental error) for the one-dimensionally ordered regions and the three-dimensionally ordered regions discussed above.In contrast to this situation for the Br(CH,),Br guests, the molecular packing arrangement in the three-dimensionally ordered regions of the guest structure in alkanelurea inclusion compounds has A, =0. It is interesting and important to speculate whether the presence of terminal bromine atoms as opposed to methyl groups on the guest molecule gives rise to a similarly marked difference in the dynamic properties of the guest molecules. Similarly, it is possible that the presence of different functional groups on the guest molecule could exert an important t Present address: Department of Chemistry, University College London, 20 Gordon Street, London, UK WClH OAJ.influence on the dynamic properties of the urea molecules within the host structure. It is known5 that alkane/urea inclusion compounds undergo a phase transition from a low-temperature phase in which the host tunnel structure is orthorhombic to a high-temperature phase in which the host structure is hexagonal. This phase transition is also believed to be associated with an abrupt change in the dynamic properties of the alkane molecules, and it has been shown that these molecules undergo considerable motion in the high-temperature phase. Differential scanning calorimetry has shown that the Br(CH,),Br/urea inclusion compounds undergo a phase trans- ition similar to the well established phase transition for the alkane/urea inclusion compounds.Powder X-ray diffraction' has shown that this transition is associated with the same distortion of the host tunnel structure as that established previo~sly~,~for the alkane/urea inclusion compounds, Although many studies, by NMR and other techniques, have been carried out to probe the motion of the guest molecules in urea inclusion compounds, little attention has been devoted to studies of the dynamic properties of the urea molecules. Recently, Heaton et al. have studied the dynamic properties of the urea molecules in the nonadecane/C2H4]- urea inclusion compound by powder' and single-crystal" ,H NMR spectroscopy. This work led to the proposal that the urea molecules undergo 180"jumps about their C=O axes, with no evidence (on the 2H NMR timescale) for rotation of the NH, groups about the C-N bonds (although rapid librational motion about the C-N bonds cannot be ruled out).It is possible that the exact nature of the guest molecules (and particularly the presence of different types of functional group on the guest molecules) could have a significant bearing upon the urea jump motion. In pure crystalline urea, 180" jumps of the urea molecule about its C=O axis are also believed to above ambient temperature, and it has been proposed that simultaneous rotation about the C-N bond may also occur.11,12,15 In this paper we report ,H NMR investigations of the dynamic properties of the urea molecules in Br(CH,),Br/[2H4] urea inclusion compounds (n =7-10) and in pure crystalline C2H4]urea.Experimental Inclusion compounds containing Br (CH,),Br guest molecules (n= 7-10) in L2H4]urea were prepared by slowly cooling warm solutions of Br(CH,),Br and C2H4]urea in CH,OD. The degree of deuteriation of the urea molecules was shown by infrared spectroscopy to be greater than ca. 98%. Powder X-ray diffraction confirmed that all the crystals had the characteristic tunnel host structure of the conventional urea inclusion compounds. The phase-transition temperatures for these inclusion compounds, established by differential scan- ning calorimetry, are in the range ca. 145-170 K. 2H NMR spectra were recorded at 76.78 MHz on a Bruker MSLSOO spectrometer, using a standard Bruker 5 rnm high- power probe.The stability and accuracy of the temperature controller (Bruker B-VT1000) were ca. & 2 K. Temperature calibration of the probe was established via 2H NMR studies of the melting transitions in CD,OD (175 K) and D20 (277K). 2H NMR spectra were recorded using the conven- tional quadrupole echo [( 90"),-z-( 90"),-z-acquire-recycle] pulse sequence,16 with 2H 90" pulse duration=2.4 ys, echo delay z =13 ps, and recycle delay =60 s. Typically, 2048 data points were accumulated, with a dwell time of 0.5 ys. The 2H NMR spectra have not been artificially symmetrized. 2H NMR spectra were recorded for all Br(CH2),Br/[2H,]- urea inclusion compounds (n=7-10) at 293 K, and for Br(CH2),Br/C2H,] urea at several temperatures between 140 K and 293 K.2H NMR spectra were recorded for pure crystalline ['H,]urea at 293, 333 and 358 K. Simulations of 2H quadrupole echo NMR spectra were obtained using the program MXQET.17 The spectral simu- lations were obtained by Fourier transformation of calculated echo decays, with both Gaussian and Lorentzian apodization applied before Fourier transformation. The calculations using the MXQET program include effects arising from the virtual free induction decay and from the echo, and corrections for imperfect spectral coverage (due to finite pulse power) are also considered. The program also takes into consideration effects on the spectrum which can ariseI8 when exchange processes occur on a timescale comparable to z. Results and Discussion In ['H,] urea" and in r2H4] urea inclusion compounds'q2 there are two crystallographically inequivalent deuteron environments which are denoted here as 'axial' and 'equatorial' deuterons; the N-D bond for the axial deuterons forms an angle P~l80" with the axis of the C=O bond, whereas the N-D bond for the equatorial deuterons forms an angle P~z60' with the axis of the C=O bond (Fig.1). In principle, the axial and equatorial deuterons could have different values of the static quadrupole coupling constant and the static asymmetry parameter (and perhaps also different values of isotropic chemical shift). Before undertaking studies of the dynamic properties of the urea molecules in the B~(CH,),BI-/[~H,] urea inclusion com- pounds, 2H NMR studies of pure crystalline C2H,]urea were carried out in order to corroborate the known dynamic properties of this material.2H NMR spectra of pure crystalline C2H,]urea recorded at 293, 333 and 358 K are shown in Fig. 2(a), (b) and (c), respectively. The spectrum recorded at 293 K was fitted well by a spectrum [Fig. 2(d)] simulated assuming no motion of the deuterons, and with static quadru- 0 Fig. 1 Molecular structure of the ['HJ urea molecule, indicating the designation of the two different types of deuteron as 'axial' and 'equatorial' J. MATER. CHEM., 1994, VOL. 4 pole coupling constant x =212 kHz and static asymmetry parameter q=0.15. We conclude from this that the static quadrupole interaction parameters do not differ significantly for the two crystallographically distinguishable deuteron environments. Thus, in the spectral simulations carried out in this work, it was assumed that all deuterons in the [2H4]urea molecule have the same values of the static quadrupole interaction parameters.These values of x and q are in close agreement with those determined previously12 from single-crystal 2H NMR studies of C2H4]urea at room temperature (for the two crystallograph- ically distinguishable deuterons, the following values were obtained: x=210.8_+ 1.0 kHz and q=0.139_+0.010; x= 210.7_+ 1.0 kHz and q =0.146 +O.OlO). Similar values were also obtained previously' from 2H NMR lineshape analysis for a polycrystalline sample of C2H4]urea at 303 K (x= 212 f2 kHz and y =0.145 k0.005). 2H NMR spectra of C2H4]urea recorded at 333 and 358 K [Fig.2(b) and (c)] are clearly not static powder patterns, although the total widths of these spectra are only slightly less (ca. 2%) than that of the spectrum recorded at 293 K. The 'inner' powder pattern with intensity maxima at f. 14 kHz and the shoulders at k 99 kHz confirm the existence of mobile deuterons at 333 and 358 K. The existence of two crystallo- graphically distinguishable deuteron environments, and the spectral features observed at 333 and 358 K, suggest that the lineshapes measured at higher temperatures can be interpreted as a superposition of two powder patterns (one of which is similar in appearance to the powder pattern of a static system). It should be noted that at all temperatures, a single peak at zero frequency is also present, and its intensity increases gradually as temperature is increased.This is probably due to ['H,] urea molecules undergoing isotropic motion (e.g. in the gas phase). The changes in lineshape with temperature for C2H4] urea are characteristic of the changes in lineshape for a two-site 180" jump motion about an axis forming an angle of 60" with respect to the direction of the z axis (principal component) of the electric-field gradient tensor of the deuteron. Analogous lineshapes (with ca. 15-30 kHz separation between the 'inner' intensity maxima) have been reported for such motions in nonadecane/C2H,] urea,9 1,4-disubstituted benzenes'* and other systems.20 Based on this interpretation, the 2H NMR lineshapes at 333 and 358 K have been simulated successfully on the basis of a dynamic model consisting of a two-site 180' jump motion about the C=O axis of the urea molecule.Although both types of deuteron necessarily undergo jumps at the same rate, the N-D,, vector is almost parallel to the C=O vector (the jump axis), and the D,, deuterons do not significantly change their orientation with respect to the applied magnetic field during the jump motion. Thus, their contribution to the spectrum is almost indistinguishable from that of static deuterons. In contrast, the N-D,, bond forms an angle of ca. 60" with the C=O vector, and the orientation of the D,, deuterons relative to the applied magnetic field changes appreciably during the motion.Thus, in the rapid motion regime, the spectrum due to the D,, deuterons is very similar to a static 2H NMR powder pattern, whereas the spectrum due to the D,, deuterons is substantially motionally averaged. The best-fit spectral simulation at each temperature is shown in Fig. 2(e) and (f)together with the value of the jump frequency (K). In order to achieve good agreement between simulated spectra and the experimental spectra recorded at 333 and 358 K, it was necessary to use slightly smaller values of the static quadrupole coupling constant x and the static asymmetry parameter q than those determined from the 'static' spectrum recorded at 293 K. The best-fit values of x and q J.MATER. CHEM., 1994, VOL. 4 I"""""""""'I''""'"'"""'''~ 200 0 -200 kHz Fig. 2 2H NMR spectra recorded for pure crystalline C2H4]urea at: (a) 293; (b) 333; (c) 358 K. The best-fit simulated 2H NMR spectra corresponding to the experimental spectra (a)-(c) are shown in (d)-(f)respectively. The simulated spectrum (d)was calculated assuming no motion of the deuterons, whereas (e)and (f)were calculated assuming the two-site 180" jump motion discussed in the text. The following parameters were used in these spectral simulations: (d) static quadrupole coupling constant x =212 kHz, static asymmetry parameter rj =0.15; (e)x=209 kHz, q=O.13, jump frequency IC=1.5 x lo5s-'; (f)x=208 kHz, q=O.13, IC=~x lo6 s-'. The agreement between the simulated and experimental spectra is noticeably degraded if x is changed by more than k1 kHz or if q is changed by more than kO.01 were found to be x=209 kHz, q=O.13 at 333 K and x= 208 kHz, q=O.13 at 358 K.An analogous decrease in the value of 2was found in the case of spectral simulations of a two-site 180" jump of the ND2 deuterons of p-nitroaniline,20 and it was proposed that this behaviour arises from an increase in the amplitude of restricted, rapid libration of the N-D bonds about their equilibrium orientations. It is also possible that the exact nature of the hydrogen bonding in these systems may have a critical influence on the temperature- dependence of x and q, particularly when the motion is a large-angle jump process involving breakage and formation of hydrogen bonds.The frequency separation between the 'inner' intensity maxima in the spectral simulations depends critically upon the angle p for the equatorial deuterons, and best-fit simu- lations for the experimental spectra recorded at 333 and 358 K were obtained with /3 =60.5f0.3". The corresponding angle determined from neutron diffraction studies" of crystalline C2H4]urea is 61.2'. In our spectral simulations, the angle /3 for the axial deuterons was fixed at 177', based on the value ( 176.6') obtained from these neutron diffraction studies. Additional spectral simulations showed that variation of /3 for the axial deuterons by as much as f5' gives no perceptible I change in the simulated 2H NMR lineshape. 2H NMR spectra recorded at 293K for the Br(CH,),Br/ C2H4] urea inclusion compounds with n=7-10 are shown in Fig.3. The temperature-dependence of the 2H NMR spectrum for Br(CH2)7Br/[2H4]urea is shown in Fig. 4. As for pure crystalline C2H4]urea, the 2H NMR spectra of the Br(CH,),Br/C2H4] urea inclusion compounds are consistent with the existence of two crystallographically distinguishable deuterons D,, and D,, in the host structure in the inclusion compound. In contrast to pure crystalline C2H4] urea (Fig. 2), however, the 'inner' powder patterns in the spectra of the Br(CH2),Br/C2H4] urea inclusion compounds recorded at 293 K (Fig. 3) have intensity maxima at f8 kHz and should- ers at +95 kHz. Fig. 3 'H NMR spectra recorded for different Br(CH,),Br/[2H4]urea To prove the existence of the two crystallographically inclusion compounds at 293 K: (a)n=7; (b)n=8; (c)n=9; (d)]'I=10 A I""""'""~""'~"""'""'"""'~ 200 0 -200 kHz Fig.4 'H NMR spectra recorded for the Br(CH,),Br/['H,]urea inclusion compound at: (a) 140; (b) 160; (c) 200; (d)240; (e)293 K distinguishable deuteron environments, 2H NMR spectra were recorded for the Br(CH,),Br/C2H4] urea inclusion compound with very short recycle delay (10ms) at 240 and 293 K (Fig. 5). With this very short recycle delay, there is a substantial decrease in the intensity of the powder pattern assigned to the axial deuterons (as a consequence of their longer spin- lattice relaxation times) relative to the intensity of the powder pattern assigned to the equatorial deuterons.Qualitative features of the 2H NMR spectra recorded at 293 K are identical for all of the Br(CH,),Br/[2H,]urea inclusion compounds studied, and we concentrate our dis- cussion on the spectra for Br(CH2),Br/['H4] urea. The spectra recorded (Fig. 4) between 140 and 200 K are characteristic of 'static' ,H NMR powder patterns; there is no discernible change in lineshape upon increasing the temperature in this range, even though the phase transition occurs within this range. The spectra recorded at 240 and 293 K can be con- sidered in terms of a superposition of two powder patterns: an 'inner' powder pattern (assigned to the D,, deuterons) and an 'outer' powder pattern (assigned to the D,, deuterons). A single peak at zero frequency is also present, and is probably due to [2H4] urea molecules undergoing isotropic motion (e.g.in the gas phase). The spectra recorded at 140 and 160 K are fitted well by a spectrum [Fig. 6(a)] simulated assuming no motion of the deuterons, and with static quadrupole coupling constant x =212 kHz and static asymmetry parameter q = J. MATER. CHEM., 1994, VOL. 4 ,I,,, I",'/ " I 1 200 100 0 -100 -200 kHz Fig. 5 'H NMR spectra recorded for the Br(CH,),Br/['H,]urea inclusion compound with a short recycle delay (10 ms) at: (a) 240; (b)293 K Fig. 6 Simulated 'H NMR spectra calculated (a) assuming no motion of the deuterons, and (b)-(d)assuming the two-site 180" jump motion discussed in the text. The following parameters were used in these spectral simulations: (a) static quadrupole coupling constant x = 212 kHz, static asymmetry parameter q =0.18 [compare with Fig.4(a)]; (b) x=207 kHz, q=0.17; jump frequency ~=2 xlo5s-' [compare with Fig. 4(d)]; (c) x=207 kHz, q=0.17; K= 1.5 xlo6s-' [compare with Fig. 3(a)]; (d) x=207 kHz; q=O.17; ~=4xlo6s-l [compare with Figs. 3(b), (c) and (d)].The agreement between the simulated and experimental spectra is noticeably degraded if x is changed by more than k1 kHz or if q is changed by more than +O.Ol J. MATER. CHEM., 1994, VOL. 4 0.18. These values of x and y are in close agreement with those (3: =208 k3 kHz and rj =0.155k0.005) determined pre- viouslyg from lineshape analysis of motionally averaged 2H NMR powder patterns recorded for the nonadecane/C2H4]- urea inclusion compound at 258-338 K. As in the case of pure crystalline C2H4]urea, the 2H NMR spectra recorded at 293 K (Fig.3) for the Br(CH,),Br/ C2H4] urea inclusion compounds have been simulated success- fully (Fig. 6) on the basis of a dynamic model consisting of a two-site 180" jump motion about the C=O axis of the urea molecule. The best-fit values of x and rj at 293 K were found to be x=207 kHz and y=O.l7. The best-fit of the frequency separation for the 'inner' powder pattern in the spectral simulations was obtained with the angle p for the equatorial deuterons in the range 59.5f0.3". As in our studies of pure crystalline ['H,]urea, the angle fl for the axial deuterons was fixed at 177".The spectra recorded at 293 K for the C2H4]urea inclusion compounds with Br(CH,),Br, Br(CH2)9Br and Br(CH,),,Br guest molecules were fitted well by a spectrum [Fig. 6(d)] simulated using jump frequency ic =4 x lo6s-'. For the Br( CH,),BI-/[~H~] urea inclusion compound, the best-fit value of K is 1.5x lo6 s-' at 293 K [Fig. 6(c)] and 2 x lo5 s-' at 240 K [Fig. 6(b)]. Concluding Remarks If the comparatively bulky bromine atoms on the Br(CH,),Br guest molecules hinder the jump motion of the urea molecules, the dynamics of the urea molecules should be expected to depend on the length (n) of the guest molecule. For shorter Br(CH,),Br guests, there is a higher 'density' of bromine atoms per unit length of tunnel, and the motion of a higher proportion of the urea molecules (i.e.those in the vicinity of bromine atoms) might be expected to be hindered. It is interesting that the jump frequency (K) is lower (at the same temperature) for Br(CH,),Br/['H,] urea than for the Br(CH2),Br/['H4] urea inclusion compounds with longer guest molecules. In the knowledge that the host structure does not differ significantly4 (within experimental error) between the different Br(CH,),Br/['H,] urea inclusion com- pounds investigated here, the fact that there is a measurable difference in the jump frequency of the urea molecules in Br(CH2),Br/[2H4]urea suggests that the nature of the guest molecules (e.g. the 'density' of bromine atoms along the tunnel) does indeed exert some influence upon the dynamic properties of the host.However, in view of the fact that the motions of the Br(CH,),Br guest molecules [for r1=8-10]~'*~~occur on a timescale that is several orders of magnitude shorter than the jump motion of the urea molecules established here, it is clear that there is no direct correlation between the dynamic properties of the host and guest molecules. Note that the dynamic properties of the guest molecules in Br(CH2),Br/[2H,] urea have not yet been investigated. Finally, it is interesting to recall that there is an incommen- surate structural relation~hip'?~ between the host and guest substructures in the Br(CH,),Br/urea inclusion compounds with n=7-10, and this may be expected to give rise to a distribution of K values for the jump motion of the urea molecules (a lower value of K may be expected for urea molecules in the vicinity of a bromine atom of the guest molecule).However, interpretation of the 2H NMR spectra reported in this paper did not require a distribution of ic values to be invoked. It is known21*22 that translational motions of the guest molecules along the tunnel occur on a timescale that is several orders of magnitude shorter than the timescale for the motion of the urea molecules, and it is possible that this translational motion of the guest molecules leads to each urea molecule experiencing the same average jump frequency over the timescale of the 2H NMR measure- ment. This provides a possible explanation for our ability to interpret the 2H NMR spectra of the Br(CH2),Br/C2H4] urea inclusion compounds on the basis of a single value of ic, even although each urea molecule has a different instantaneous environment as a consequence of the incommensurate relationship between host and guest substructures.We are grateful to the SERC (studentship to S.P.S.and general support to K.D.M.H.) and the Royal Society (postdoctoral fellowship to A.E.A.) for financial support. References 1 A. E. Smith, Acta Crystallogr., 1952,5,224. 2 K. D. M. Harris and J. M. Thomas, J. Chem. SOC.,Faradaj Trans., 1990,86,2985. 3 K. D. M. Harris and M. D. Hollingsworth, Proc. Roy. SOC. London, Ser. A, 1990,431,245. 4 K. D. M. Harris, S. P. Smart and M. D. Hollingsworth, J. Chem.SOC.,Faraday Trans., 1991,87,3423. 5 N. G. Parsonage and R. C. Pemberton, Trans. Faraday So,.., 1967, 63,311, and earlier references cited therein. 6 Y. Chatani, H. Anraku, and Y. Taki, Mol. Cryst. Liq. Cryst., 1978, 48,219. 7 K. D. M. Harris, I. Gameson and J. M. Thomas, J. Chern. SOC., Furaduy Trans., 1990,86,3135. 8 I. J. Shannon and K. D. M. Harris, in preparation. 9 N. J. Heaton, R. L. Vold and R. R. Vold, J. Am. Chern. Sol ., 1989, 111,3211. 10 N. J. Heaton, R. L. Vold and R. R. Vold, J. Mugn. Reson., 1989, 84, 333. 11 J. W. Emsley and J. A. S. Smith, Trans. Faraday SOC.,1961, 57, 1233. 12 T. Chiba, Bull. Chem. SOC.Jpn., 1965,38,259. 13 A. Zussman, J. Chem. Phys., 1973,58,1514. 14 H. H. Mantsch, H. Saito and I. C. P. Smith, Prog. NMR Spectrosc., 1977, 11,211. 15 T. P. Das, J. Chem. Phys., 1957, 27, 763; see also J. Chem. Phys., 1961,35,1897. 16 J. H. Davis, K. R. Jeffrey, M. Bloom, M. I. Valic and T. P. Higgs, Chem. Phys. Lett., 1976,42, 390. 17 M. S. Greenfield, A. D. Ronemus, R. L. Vold, R. R Vold, P. D. Ellis and T. R. Raidy, J. Magn. Reson., 1987,72, 89. 18 T. M. Barbara, M. S. Greenfield, R. L. Vold and R. R. Vold, J.Mugn. Reson., 1986,69, 311. 19 J. E. Worsham, H. A. Levy and S. W. Peterson, Acta Crysfullogr., 1957, 10, 319. 20 M. A. Kennedy, R. R. Vold and R. L. Vold, J. Magn. Reson, 1991, 91, 301. 21 S. P. Smart, F. Guillaume, K. D. M. Harris, C. Sourisseau and A. J. Dianoux, Physica B, 1992,180& 181,687. 22 F. Guillaume, S. P. Smart, K. D. M. Harris and A. J. Dianoux, J. Phys., Condens. Matter, in the press. Paper 3/04273A; Received 20th Julj, 1993

ISSN:0959-9428    

DOI:10.1039/JM9940400035

出版商:RSC    

年代:1994

数据来源: RSC