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 Editorial Secretary: Miss J. E. Chapman Materials Chemistry Editorial Board Anthony R. West (Aberdeen) (Chairman) C. Richard A. Catlow (London) David A. Rice (Reading) David A. Dunmur (Sheffield) Rodney P. Townsend (Bebington) H. Monty Frey (Reading) Allan E. Underhill (Bangor) John W. Goodby (Hull) Graham Williams (Swansea) John D. Wright (Canterbury) International Advisory Editorial Board M. A. Alario-Franco (Madrid) D. Kohl (Aachen) K.Bechgaard (Copenhagen) M. Lahav (Rehovot) J. D. Birchall (Runcorn) A. J. Leadbetter (Daresbury) D. Bloor (Durham) P. M. Maitlis (Sheffield) A. K. Cheetham (Oxford) J. S. Miller (Wilmington) E. Chiellini (Pisa) P. S. Nicholson (Hamilton) M. G. Clark (Wembley) M. Nygren (Stockholm) P. Day (Grenoble) V. Percec (Cleveland) D. Demus (Halle) C. N. R. Rao (Bangalore) B. Dunn (Los Angeles) M. Ratner (Evanston) W. J. Feast (Durham) J. Rouxel (Nantes) A. Fukuda (Tokyo) R. Roy (University Park, PA) D. Gatteschi (Florence) J. L. Serrano (Zaragoza) A. M. Glass (Murray Hill) J. N. Sherwood (Glasgow) J. B. Goodenough (Austin) J. Simon (Paris) G. W. Gray (Poole) J. F. Stoddart (Sheffield) A. C. Griffin (Cambridge) S. Takahashi (Osaka) S-i.Hirano (Nagoya) G. J. T. Tiddy (Bebington and Salford) P. Hodge (Manchester) B. J. Tighe (Birmingham) H. lnokuchi (Okazaki) Yu. D. Tretyakov (Moscow) W. Jeitschko (Munster) R. J. P. Williams (Oxford) 0. Kahn (Orsay) R. Xu (Changchun) Journal of Materials Chemistry (ISSN 0959-9428) is published six times a year 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, The Distribution Centre, Blackhorse Road, Letchworth, Herts SG6 1 HN, UK. 1991 Annual subscription rate EC (inc. UK) f175.00, USA $395.00, Rest of World fl95.00. Air freight and mailing in the USA by Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11 003.USA Postmaster: send address changes to Journal of Materials Chemistry, Publications Expediting Inc., 200 Meacham Avenue, Elmont, NY 11003. Second Class postage paid at Jamaica, NY 11431. 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, 1991. 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. 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ISSN:0959-9428    

DOI:10.1039/JM99101FX001

出版商:RSC    

年代:1991

数据来源: RSC

ISSN:0959-9428    

DOI:10.1039/JM99101BX003

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 5-10 Mesogen exhibiting a Ch-A*-A Phase Sequence, a Liquid- crystalline Analogue of the Abrikosov Phase Andrew J. Slaney and John W. Goodby* School of Chemistry, The University, Hull HU6 7RX, UK ~ ~~~ A number of optically active liquid-crystalline compounds have been prepared that have a chlorine atom attached to their asymmetric centres. Many of these materials exhibit unusual properties at their transitions to the liquid state. In this article we report the properties of one of these compounds: S-2-chloro-4-methylpentyl 4’-(4”-n-nonyloxybenzoyloxy)biphenyl-4-carboxylate.From thermal and optical studies it appears that this material undergoes a transition from a cholesteric phase to a smectic A phase via an intermediary twisted phase.The intermediary A* phase is believed to have a structure where there is helical ordering of the molecules occurring in the planes of the molecular layers. Theoretical models suggest that this twist is effected by the inclusion of screw dislocations in the structure of the phase in the form of a lattice of defects. This lattice is thought to be similar in nature to the Abrikosov flux lattice found in superconductors. Keywords: Liquid crystaal; Optical activity; Screw dislocation; Phase transition 1. Introduction In certain circumstances the melting of a cholesteric meso- phase to the isotropic liquid may be mediated via the forma- tion of Blue Phases (BPs). Typically, Blue Phases are detected when the pitch of the cholesteric mesophase is relatively short (54000 A).Although Blue Phases are homogeneous on macroscopic scales larger than the pitch, on smaller, micro- scopic, scales they are thought to be inhomogeneous. The director configuration, that is locally an energy minimum called double twist,’ cannot fill space efficiently and the formation of a periodic array of defects is required in order to stabilize a large-scale homogeneous phase [see Fig. l(a)]. (i) Id (ii) Fig. 1 Schematic representation of some frustrated liquid crystal phases. In each case the twisted structure of the phase is stabilized by defects. (a)The structure of Blue Phase I (02symmetry); (b)the structure of the A* phase (spiral layer order); (c) the structure of a polymer analogue of the A* phase: (i) smectic A polymer, (ii) twisted A* polymer Consequently, Blue Phases are regarded as examples of frus- trated phases.Recently Renn and Lubensky2 showed theoretically that the cholesteric-smectic A phase transition can also be mediated by the formation of a frustrated, intermediary phase. They proposed that at the transition there is competition between the stabilization of the layered structure of the A phase and the desire for the molecules to form a helical macrostructure as in the cholesteric phase. They suggested that this competition can be resolved by the generation of a novel helical smectic A* phase. Subsequently, the phase was described as the liquid-crystalline analogue of the Abrikosov flux phase observed in some supercond~ctors.~~~ Independent of these theoretical studies we showed4 that certain members of the R-and S-l-methylheptyl 4’-(4”-n- alkyloxypropioloyloxy)biphenyl-4-carboxylates exhibit smec- tic A* phases on cooling from the isotropic liquid. X-Ray diffraction, differential scanning calorimetry, thermal optical microscopy, rotary dispersion, and electrical and alignment studies clearly show that this phase has a helical structure combined with a lamellar ordering of the molecule^.^^^ For example, the n-tetradecyloxy homologue4 (14P1 M7) was found to have the following transitions is0 -A*-C*-S3-S4 93.8 89.7 53.4 42.5 where A* is the twisted variant of the A phase, C* is a typical helical, ferroelectric smectic, S3 and S4 are two hitherto unidentified phases, and is0 is the isotropic liquid.Surpris- ingly, the axis of the helix in the A* phase was found to lie in the plane of the layers. The proposed structure for this phase is also shown in Fig. l(b). The twist in the layer structure was suggested to be effected by the formation of defects’ or fluid-like region^.^^^ Similar results to those found for low molar mass liquid crystals have been reported previously in polymer systemsa6 For example, in cholesteryl-substituted polymethacrylate side- chain polymers, helical smectic A* phases have been found. The structure of the proposed phase is shown in Fig. l(c) (ii). The twist is parallel to the layers and the polymer backbone is believed to coil between the layers.In this case, the stabilization of the helical phase is expected to be critically dependent on the degree of polymerization. Theoretical models2p7 show, for second-order phase tran- sitions of the type described above, that there is competition between the twist penetration distance (A,) and the SA corre-lation length (0.When the ratio AT/( (the Ginzburg parameter) is greater than (J2)- a frustrated phase is predicted. Stabiliz- ation of the phase is proposed to occur through the formation of a lattice of screw dislocations that are described as ‘twist grain boundaries’.2 In this present study we report the discovery of a material that exhibits cholesteric, smectic A*, and Blue phases. Thus, the transitions that bound the upper and lower temperature limits of the cholesteric phase are both mediated by the formation of frustrated phases. 2.Experimental The material synthesized for this study was S-2-chloro-4-methylpentyl 4-(4”-n-nonyloxybenzoyloxy)biphenyl-4-carb-oxylate (902C14M5) Cl which has a similar structure to the first compound reported to exhibit a smectic A* phase.4 The major structural differ- ences between compounds 1 and 2 are the larger dipole at the chiral centre for compound 2 and the greater freedom of rotation of this chiral centre over that in compound 1. The material was prepared by treating the diazonium salt of S-leucine with hydrogen chloride in order to form S-2- chloro-4-methylpentanoic acid. The resulting acid was reduced in the presence of lithium aluminium hydride, and the chiral alcohol formed was then esterified with 4-methoxycarbonoyl- oxy-4’-hydroxybiphenyl in the presence of diethyl azodicar- boxylate.The carbonate-ester produced was purified by column chromatography and then reacted with an ethanol- ammonia mixture in order to remove the protecting carbonate moiety. The resultiiig product, S-2-chloro-4-methylpentyl 4’-hydroxybiphenyl-4-carboxylate,was esterified with 4-n-nonyloxybenzoic acid to give the final liquid crystal 2. The final product was carefully purified by flash chromatog- raphy over silica gel (200-400 mesh) using dichloromethane as the eluant. The collected fractions showed a single compon- ent by thin-layer chromatography. After removal of the solvent the product was recrystallized successively from light petrol (40-60°C) and acetonitrile until constant transition tempera- tures were obtained.The purity of the product was investigated further by normal- and reverse-phase high-pressure liquid chromatography. Normal-phase chromatography was carried out over silica gel (5 pm pore size, 25 x 0.46 cm, Dynamax Scout column) using acetonitrile as the solvent for the mobile phase. Reverse-phase chromatography was performed over octadecylsiloxane (5 pm pore size, 25 x 0.46 cm, ODS Micro- sorb Dynamax 18 column) using both acetonitrile and methanol-water (9:l) as the mobile phase. Detection of the eluting products was made using a Spectroflow 757 UV-VIS detector (i= 254 nm).For both forms of chromatography the material was found to have a chemical purity greater than 99%. The chemical structures of the intermediates and final product were determined by a combination of IR, NMR, and mass spectral analysis; the results for the intermediates and final products were found to be consistent with predicted structures. The transition temperatures (fO.l°C) and initial phase assignments for S-2-chloro-4-methylpentyl 4’-(4”-n-nonyloxy-benzoyloxy)biphenyl-4-carboxylatewere determined by ther- mal optical microscopy using a Zeiss Universal Polarizing J. MATER. CHEM., 1991, VOL. 1 light microscope equipped with a Mettler FP52 microfurnace and FP5 control unit. Alignment, polarization, and tilt-angle studies were carried out using both thin (2 pm) and thick cells (10 pm) constructed from electrically conducting (ITO-coated) glass.The inner surfaces of the thin cells were coated with polyimide, whereas in the thick cells polybutyleneterephthalate was used as the aligning agent. The coated inner surfaces of the cells were unidirectionally buffed prior to construction in order to induce a homogeneous, planar alignment of the liquid crystal. Electrical contacts were made directly to the internal surfaces of the glass so that the polarization direction and tilt angle measurements could be studied in an applied electric field in a Linkam THM 600 hot-stage attached to a polarizing microscope. The values of the tilt angles are reported to within &lo. Temperatures and heats of transition were determined by differential scanning calorimetry using a Perkin-Elmer DSC- Lab I calorimeter in conjunction with a thermal analysis data station (TADS).As a check on instrumental accuracy an indium standard was run at 10.0”C min-’. The measured latent heat, 28.36 J g-’, compared well with that normally obtained for indium of 28.45 J g-’. The material was studied at various heating and cooling rates (1, 2, 5 and 10°C min-’), in both aluminium and graphite pans. Miscibility studies were performed by weighing out the individual mixture components onto a glass slide and heating them into their isotropic liquids. The materials were then mixed thoroughly and allowed to cool into their liquid- crystalline phases. Phase identification was then carried out in the usual manner.3. Results 3.1 Thermal Optical Microscopy S-2-chloro-4-methylpentyl 4’-(4”-n-nonyloxybenzoyloxy)bi-phenyl-4-carboxylate (902C14M5) was found to have the following phases and transition temperatures m.p. is0 -+ BPIII-+ BPI[ -+ BPI 74.9 150.2 149.9 148.6 -+ Ch -+ A* + A -+ C* -+ recryst. 145.8 145.4 144.6 121.0 41.8 where m.p. is the melting point (/“C), is0 is the isotropic liquid, and recryst. is the recrystallization temperature. On cooling the compound between untreated glass plates, the transition from the isotropic liquid to Blue Phase I11 was very difficult to detect optically. However, the transitions to Blue Phase 11, and subsequently to Blue Phase I were very clear and characterized by the formation of an iridescent platelet defect texture. Further cooling resulted in the forma- tion of an iridescent cholesteric phase (pitch xO.4-0.5 pm).This phase was identified from its focal-conic and Grandjean plane textures. Upon slow cooling (O.l°C min-’) the choles- teric phase transformed into a smectic A phase via the intermediary chiral smectic A* modification. The transition was characterized by the formation of filaments, thus produc- ing a vermis texture. The filaments grew rapidly from the Grandjean plane texture on cooling (Fig. 2). Rotation of the polarizer showed that the intermediary phase was helical.? This was confirmed by the observation of pitch bands perpen- dicular to the long axes of the filaments. As the temperature t Professor H.Stegemeyer recently communicated to us that he and W. U. Muller have also observed a similar phenomenon occurring at the cholesteric to smectic A phase transition in mixtures of cholesteryl oleyl carbonate and cholesteryl chloride. Their results are reported in W. U. Muller, Ph.D. Thesis, University of Berlin, 1974. J. MATER. CHEM., 1991, VOL. 1 Fig. 2 The formation of the A* phase of 902C14M5 on cooling from the cholesteric phase. The cholesteric phase appears in its Grandjean plane texture at the top (x 50) was slowly lowered the filaments formed a homeotropic texture. Only on certain occasions was a typical focal-conic texture of the A phase observed (Fig. 3). The filament texture of the A* phase4 obtained for S-1-methylheptyl 4'-(4-n- tetradecyloxypropioyloxy)biphenyl-4-carboxylate (Fig.4) can be compared with that for the compound under investigation as shown in Fig. 5. Further cooling produced a transition to a normal chiral smectic C* phase, which exhibited both broken focal-conic and plane textures. The plane texture of the phase also appeared somewhat iridescent. Furthermore, as the tempera- ture was lowered the phase showed some abnormal changes in its plane texture; these were assumed to be due to a large variation in tilt angle with respect to temperature (see later). Free-standing films of 902C14M5 prepared in the smectic A phase exhibited a normal homeotropic texture. Cooling produced a schlieren texture at the transition to the helical chiral C* phase.Upon further cooling this texture quickly Fig. 3 The filament texture of the A* phase and the focal-conic texture of the smectic A phase of 902C14M5. Filaments form at the transition as shown around an air bubble in the plate (x 50) Fig. 5 The filament texture of S-2-chloro-4-methylpentyl 4-(4"-n-nonyloxybenzoyloxy)biphenyl-4-carboxylate(902C14M5) (x 50) gave way to a plane texture. From this texture the twist rotation of plane-polarized light through the sample was found to be laevo(f). Hence the direction of the helix was catagorised as right-handed. As the chiral centre is removed from the rigid core by an even number of atoms (8,even parity), the material can be classified as See with a negative inductive effect at the chiral centre (-1).This result is in agreement with the previously reported hypothesis. Heating of the free-standing film to a temperature close to the transition to the cholesteric phase resulted in the formation of a droplet which was supported by a film of smectic A. The droplet was clearly in the cholesteric phase, but at the edges the filament texture of the chiral smectic A* phase was observed. The temperature differentials in the oven caused the fila- ments in the free-standing film to grow at the edge of the droplet at which point they transformed back into the A phase. At the centre of the film the A phase was thought to be converted into the cholesteric phase, thereby producing a convectional current in the film.Spiral growth of the filaments consequently resulted in the rapid rotation of the droplet on the surface of the film. At higher temperatures the droplet underwent a transition into Blue Phase I, at which point the rotation ceased. 3.2 Thermal Investigations The material (902C14M5) was studied by differential scanning calorimetry at various heating and cooling rates. Typical thermograms are shown in Fig. 6 and 7 for heating and 0.5 BPJJ -+ BPmor IS0 Fig. 6 Differential scanning calorimetry thermogram for the initial Fig. 4 The filament texture of S-l-methylheptyl4-(4-n-tetradecyloxy-heating cycle of 902€14M5. Heating rate= 2°C min-'; samplepropioloyloxy)biphenyl-4-carboxylate(14PlM7) (x 50) weight =6.24 mg 8 0.8 smectic A \7 0.6 2\ -g 0.4 c c Q).c 0.2 0 TI"C Fig.7 Differential scanning calorimetry thermogram for the first cooling cycle of 902C14MS. Cooling rate = 2°C min- '; sample weight = 6.24 mg cooling rates of 2°C min- In these figures only the transitions occurring near the clearing points are shown. The heating cycle (Fig. 6) shows five peaks over a 6°C temperature range. These phase changes were found to corre- spond to the A to A*, A* to Ch, Ch to BPI, BPI to BPII, and BPI1 to BPI11 or isotropic liquid transitions. The fact that the BPI1 to BPI11 transition could not be detected at all is in agreement with results obtained by more sensitive techniques.* The A* to cholesteric, cholesteric to BPI and BPll to BPlll or isotropic transitions appeared to be first order; the order of the A to A* and BPI to BPI1 transitions could not be gauged by this technique.The fourth differential suggests that both phase changes could be first order, but it remains a possibility that the A to A* transition is in fact second order. The cooling cycle showed a slight depression in transition temperature due to decomposition and supercooling. In this case the transition to the cholesteric mesophase from BPI supercools substantially (Fig. 7) with respect to the other phase changes. This is in agreement with thermal optical studies. Once again the isotropic to BPI,, BPll to BPI, and A* to A transitions can be seen clearly. The BPI to cholesteric phase change, however, is amalgamated with the cholesteric to A* transition.For heating cycles, the sample was either heated slowly (5°C min-') from room temperature, or preheated to 130°C and then slowly heated (2°C min-') through the phase transitions to a temperature of 165°C. The second method was generally found to produce better results because it reduced the possibility of thermal decomposition of the sam- ple. Cooling cycles were simply the reverse of heating runs. In order to investigate the extent of the decomposition of the specimen, thermal cycles were also made using graphite pans. The results obtained were found to be almost identical to those observed for aluminium pans. Thus, the results were found to be both reproducible, and consistent from one form of specimen holder to another.3.3 Electrical and Alignment Studies The material (902C14M5) was investigated using two types of cell: one with a glass-plate separation of 2 pm and the other with a separation of 10 pm. The material was introduced into the cells by capillary action while in its isotropic phase. In each case the material was allowed to cool slowly into its smectic C* phase before an electrical field was applied. Good alignment was obtained in the 2 pm cell by this method; J. MATER. CHEM., 1991, VOL. 1 however, for the thicker cell the alignment was of a poorer q~ality.~The inferior quality of the alignment in the thicker cell was primarily due to the pitch of the cholesteric mesophase being considerably shorter than the cell plate separation.In the thinner cell this was not the case as the pitch was comparable to the cell spacing gap. The tilt angle was determined as a function of temperature in both cells." The results obtained were found to be essen- tially consistent, with the tilt angle being slightly higher in the 2 pm cell (ca. 2", which is outside the experimental error). The tilt angle was found to saturate at a value close to 35", as shown in Fig. 8, and the polarization direction was deter- mined to be Ps(+) from the switching direction relative to the polarity of the applied electric field. The result for the polariz- ation direction obtained is in agreement with previous hypo theses. Switching in both cells was found to become complex as the voltage was increased.Hydrodynamic instabilities and electrical breakdown were found to occur in thick cells when the electric field intensity was increased above the relatively low level of ca. 2 V pm-l. Twist walls and splayed states were clearly visible in thick cells where switching occurred with poor contrast. Cumulative switching sometimes resulted in no contrast between the switched states at all; however, the switching process was detected from the transitory responses seen on polarity reversal. It appears that cumulative pulses result in the reorientation of the layers via hydrodyn-amic instabilities, hence only a transitory response is seen as the material is switched. This tends to suggest that the layers are relatively weak or poorly defined in this particular smectic C* phase. Observations of the alignment of the material on cooling from the isotropic liquid provided only a few examples where filaments were obtained at the transition to the A phase.This may have been because the surface forces were strong enough to prevent the formation of a helical structure. 3.4 Phase Miscibility Studies In order to identify conclusively the phase occurring between the A and cholesteric phases a miscibility study was attempted 40 60 120 140 TI% Fig. 8 Plot of the tilt angle as a function of temperature in the ferroelectric smectic C* phase of 902C14M5 [Set' Ps(+)]. The angle was measured optically in a cell with a 2 pm plate separation.A d.c. field of f10 V was applied to the specimen to induce polarization reversal J. MATER. CHEM., 1991, VOL. 1 CH, 160 1160 ts,* / smectic A / 1 601 4 60 I4O 0 20 40 60 80 100 100 ‘10 A 100OlO B composition (YO) Fig. 9 Miscibility phase diagram of binary mixtures (wto/o) between S-1-methylheptyl 4-(4-pentadecyloxypropioloyloxy)biphenyl-4-carb-oxylate (A) and S-2-chloro-4-methylpentyl 4(4”-n-nonyloxybenzoyl-oxybiphenyl-4-carboxylate(B) with S-1 -methyl heptyl 4’-(4”-n-pentadecyloxypropioloyloxy)-biphenyl-4-carboxylate (1 5PlM7), which was reported as exhi- biting a smectic A* phase.5 The phase diagram is shown in Fig. 9 for weighed mixtures of the two compounds. The figure shows that the C* phase of both materials is continuously miscible across the phase diagram, thus confirming the presence of a C* phase with a right-hand helix in the test material.The cholesteric phase was found to be present across half of the composition range. The temperature range of the cholesteric phase decreased as the amount of the standard material 15PlM7 was increased. At the lower temperature boundary of the cholesteric phase, filaments were formed at the transition to the smectic A phase. The temperature range for which the filaments were found to occur was determined to be independent of the concentration of the two components in the binary mixture. This suggests that the formation of filaments is not due to impurity effects. Once the concentration of the standard material had reached a level of more than 50% by weight in the binary mixtures, the cholesteric phase was no longer observed.Furthermore, the filaments also disappeared and the transition from the isotropic to the smectic A phase (5-50% by weight of 902C14M5 region of the phase diagram) took place in a normal fashion with the formation of typical focal-conic defects. In the region of 95-100% of the standard material, 15PlM7, the smectic A* phase was again found. On cooling of this phase a direct transition to the smectic C* phase occurred, without the formation of an intermediary A phase. The phase diagram exemplifies a number of points. First the A* phase is only seen accompanying a cholesteric-to-smectic A phase change or in close proximity to a transition to a C* phase. Secondly, the filaments observed are not due to phase separation caused by chemical impurities as some- times observed in other systems.’’ Thirdly the A*-C* tran-sition temperatures are depressed on adding the test material to the standard.Consequently, the A* phase abruptly disap- pears, suggesting that the A* phase is stabilized by the adjacent helical C* phase. Thus, continuous miscibility of the A* phase across the phase diagram for the two materials is probably not practical for these two types of material. This is because in the centre of the figure there is no cholesteric phase, and the C* phase is depressed too much to affect the formation of a smectic A* phase; hence miscibility of the A* phases fails.4. Discussion The optical and thermal results indicate that a novel phase mediates the cholesteric to smectic A phase transition in S-2-chloro-4-methylpentyl 4-(4-n-nonyloxybenzoyloxy)bi-phenyl-4-carboxylate. The defect texture of this phase is almost identical to that observed for the chiral smectic A* phase of S-1-methylheptyl 4’-(4”-n-tetradecyloxypropioloyloxy)bi-phenyl-4-carboxylate. Optical studies show that the intermedi- ary phase is indeed helical, and therefore fundamentally different in nature from the normal smectic A phase. The growth of filaments to give a uerrnis texture shows that the phase has long-range order and is not a cholesteric phase. Pitch bands normal to the long axes of the filaments, similar to those observed for 14PlM7, show that the helical axis of the phase possibly runs parallel to the long axes of the filaments, as shown in Fig.10. These observations are all consistent with the formation of an intermediary A* phase. Alternatively, the growth of the filaments can appear to be limited in the direction perpendicular to their long axes. Coalescence of the filaments (edge-to-edge) did not take place readily, thereby suggesting that a fluid-like defect region bounds the sides of the filament. Thermal studies clearly show enthalpies of transition for the various phases described. Transitions to, from, and between Blue Phases (I and 11)are all first order, a result that is consistent with heat-capacity studies.* The A*-A transition may be first or second order in nature; neither possibility is precluded by theory.” It is surprising that transitions such as these are clearly discernable by differential scanning calor- imetry.However, they are reproducible and consistent from one sample holder to another. Confirmation of the classification of the A* phase by miscibility was not found. This may be due to the selection of the standard material as the cholesteric phase disappears in the middle of the phase diagram and the fact that the A phase has an extended temperature range due to the depression of the A-C* transition temperatures. Therefore, neither the cholesteric nor the smectic C* phase is able to influence phase formation in the centre of the phase diagram.Unfortunately, as examples of the new A* phase are limited, a range of miscibility standards is not yet available in order to try to alter the ranges of the cholesteric and A* phases in the centre of the phase diagram. So, although miscibility studies do not show continuous miscibility of the two A* phases, neither do they preclude it. One interesting point that is brought out by miscibility studies is the need for pretran- sitional effects to be present before the A* can be stabilized. Purity studies show the material to be better than 99% helix axis -filament structure Fig. 10 The structure of filaments that grow in a typical defect texture at the cholesteric to A transition chemically pure. However, our initial studies show that the optical purity of the compound is ca.95% of the S isomer and 5% of the R isomer. Miscibility studies show that the formation of filaments is not dependent on chemical purity as in other systems.” However, there are some indications that the formation of filaments, and hence the temperature range of the A* phase, may be dependent on optical purity. Similarly, this is thought to be the case with Blue Phases where the temperature range of the phases are suspected to be dependent on optical purity.I3 Certainly, as the optical purity decreases towards formation of the racemate the helical A* phase diappears because the twist can no longer be stabilized. It is not clear, however, what happens with increas- ing optical purity. Preliminary studies suggest that increasing the optical purity also appears to reduce the temperature range of the phase.This hints at the possibility that the optical isomer in lower concentration stabilizes the formation of the phase. This could be achieved possibly by the migration of this isomer to the cores of the defects; thus the cores in this phase would become populated by the racemic species. This would have the effect of lowering the interfacial energy between the defect and smectic A-like region [see Fig. l(b)]. If the cores of the screw dislocations are populated by the racemic form, this means that the other regions in the phase will be enriched in the predominant optical isomer. Thus, the phase would be inhomogeneous with respect to the concen- tration of enantiomers.Such an outcome would be difficult to accept, but it should be remembered that enantiomers have been shown to be inhomogeneous in other systems.14 5. Conclusion We believe that S-2-chloro-4-methylpentyl 4’-(4”-n-nonyloxy-benzoyloxy)biphenyl-4-carboxylate ex hi bi ts a choles teric- smectic A*-smectic A phase sequence. This sequence has been theoretically predicted by Renn and Lubensky to occur at all cholesteric to smectic A phase transitions. Indeed, examination J. MATER. CHEM., 1991, VOL. 1 of other related materials shows the presence of the A* phase in all materials where the chirality of the material is strong and the layer ordering weak. The authors are extremely grateful to J. S. Patel (Bell Com- munications) and A.Samra (RSRE) for supplying them with aligned cells and to M. Turner for detailed HPLC studies. We would also like to thank Drs. J. S. Patel, R. Pindak and P. Styring for stimulating discussions and for their interest in this work. We are also indebted to the SERC for their financial support to A.J.S. References 1 S. Meiboom, J. P. Sethna, P. W. Anderson and W. F. Brinkman, Phys. Rev. Lett., 1981, 46, 1216. 2 S. R. Renn and T. C. Lubensky, Phys. Rev. A, 1988,38, 2132. 3 G. Srajer, R. Pindak, M. A. Waugh, J. W. Goodby and J. S. Patel, Phys. Rev. Lett., 1990, 64, 13. 4 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and J. S. Patel, Nature (London), 1989, 337, 449. 5 J. W. Goodby, M. A. Waugh, S. M. Stein, E. Chin, R. Pindak and J. S. Patel, J. Am. Chem. SOC., 1989, 111, 8199. 6 Ya. S. Freidzon, Ye. G. Tropsha, V. V. Tsukruk, V. V. Shilov, V. P. Shibaev and Yu. S. Lipatov, J. Polym. Chem. (USSR), 1987, 29, 1371. 7 P. G. de Gennes, Solid State Commun., 1972, 10, 753. 8 R. N. Kleinman, D. J. Bishop, R. Pindak and P. Taborek, Phys. Rev. Lett., 1984, 53, 2137. 9 J. S. Patel and J. W. Goodby, J. Appl. Phys., 1986, 59, 2355; J. S. Patel, T. M. Leslie and J. W. Goodby, Ferroelectrics, 1984, 59, 137. 10 J. W. Goodby, E. Chin, T. M. Leslie, J. M. Geary and J. S. Patel, J. Am. Chem. SOC.,1986, 108, 4729. 11 R. B. Meyer and P. Palffy-Muhoray, personal communication. 12 T. C. Lubensky, personal communication. 13 P. Taborek, J. W. Goodby and P. E. Cladis, Liq. Cryst., 1989, 4, 21. 14 J. F. Dreiding, personal communication. Paper 0/02342F; Received 25th May, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100005

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 11-15 Synthesis, Structure and Ionic Conductivity in Nitrite Sodalites Mark T. Weller,* Simon M.Dodd and M. R. Myron Jiang Department of Chemistrx The Universitx Southampton SO9 5NH, UK Nitrite sodalite [Na8(AISi04)6(N02)2.XH20] has been synthesised by precipitation from an aluminosilicate solution, and a series of cation-exchange reactions with silver have been carried out on this parent compound so as to obtain materials of general stoichiometry [Ag,Na8- y(AISi04)6(N02)2.XH20] where 0 <y< 8 and xx 2. The effects of substituting silver for sodium have been studied using powder X-ray diffraction, IR spectroscopy, *'Si magic angle spinning NMR, and thermogravimetric analysis. Results indicate that substitution of sodium by silver leads to reorientation of the nitrite ion within the cage.Ionic conductivity has been characterised in two of these compounds using a.c. conductivity methods; at 500°C typical cation conductivities are in the range 10-5-10-4Qt-'cm-' with the fastest ion conduction observed with partial substitution of silver for sodium. Keywords: Sodalite; Silver ion exchange; Ionic conductivity The sodalite family of materials may be expressed by the general formula Mn8 [SiA1O4I6Xm2-where M represents + mono-or di-valent cations of overall charge 8+ and X represents anionic species of total charge 2 -.l Materials with differing silicon to aluminium ratios are also known, for example pure aluminate derivatives,2 where the levels of cations and anions in the cages change to maintain electroneu- trality.The compositional flexibility of the sodalites results from the ability of the structure to vary the dimensions of the sodalite cage by changes in the intertetrahedral bond angle along the Si-0-A1 direction in the range 120-150". This permits satisfactory co-ordination by oxygen to a range of different sized cations, e.g. Li' and Ag', which can in turn form a reasonable tetrahedral co-ordination geometry around the anion which is located at or near the cage centre. The relationship of the cage contents to framework geometry has been studied3- in a number of stoichiometric sodalites con- taining simple monatomic anions by diffraction and NMR methods. The sodalite structure has been shown to undergo cation exchange rapidly at high temperatures by direct reaction between solids;6 the rate of these reactions indicates that sodalites are reasonably good ion conductors at these tem- peratures.High-temperature ion-exchange reaction has been used to prepare lithium halide sodalites which have sub- sequently been studied by powder neutron diffraction. Ion exchange at relatively low temperatures in aqueous solution was initially reported by Jaeger.' More recently, Ozin et aL8 have described the properties of a number of silver sodalites, in particular their optical characteristics; these materials have been obtained by silver ion exchange from aqueous solution into sodalites and their thermal decomposition to sodalites containing silver micro- domains has been studied.In this paper we describe such facile ion exchange from aqueous solutions in nitrite-containing sodalites where sodium has successfully been replaced by silver. The products have been characterised in order to determine the effect of silver for sodium substitution on the structure. Changes in ion conductivity have been studied by a.c. methods. Experimental The parent sodalite was synthesised by precipitation from an aqueous aluminosilicate solution following a hydrothermal method detailed by Hund.'.'' A sodium aluminate solution was obtained by dissolution of aluminium powder in ca.8 mol dm-3 sodium hydroxide. The cavity salt, sodium nitrite, was then added followed by a solution of sodium silicate (12% Na20, 30% Si02) in sodium hydroxide, introduced through a dropping funnel.The solution was then refluxed for 24 h at 105-1 10°C to obtain a white crystalline product, which was filtered off, washed with distilled water and dried at 110°C overnight. The introduction of silver for sodium was achieved by an ion-exchange reaction carried out using silver nitrate (AnalaR) in a pressure bomb (Parr 4748); reaction under these con- ditions was found to proceed reasonably rapidly. Product stoichiometry was controlled by varying the AgNO, to parent sodalite ratio. The reactants were placed in the bomb along with sufficient water to cover them, the bomb was then assembled and placed in an oven at 130°C for 24 h. The ion- exchanged product was washed and dried as above.Silver Analysis Each of the compounds (including the parent sodalite as a 'blank') was analysed in order to determine the silver content. About 0.15 g of compound was added to a solution of conc. HN03 (9 cm3) and distilled water (30 cm3) in order to dissolve the aluminosilicate framework. After ca. 5 min boiling (to remove any nitrogen oxides still present) H202 (9 cm3 30%) is added to oxidize the nitrite ions (liberated from the cage) to nitrate and thus prevent precipitation of silver as the sparingly soluble nitrite. The resulting solution was acidified by addition of 5 cm3 5 mol dm-3 HN03 and then titrated against KSCN, previously standardized against AgN03 using NH4Fe(S0J2 as an indicator.The value of y in the general formula Ag,Na, -y(A1Si04)6(N02)2.2H20 was thus obtained and the results are summarized in Table 1. All products were characterized using a Siemens 8-28 D5000 diffractometer and software in order to determine sample purity; cell parameters were determined by a least- squares fit using a single lattice parameter in all cases. Solid- state NMR spectra were obtained using the SERC Varian VXR300 spectrometer with sample spin speeds of 4000 Hz in order to give high-resolution spectra with low-intensity spin- ning side bands. IR data for all compounds were collected over the range 4000-500 cm-' (Perkin Elmer PE1600 FTIR) using pressed CsI discs; in particular, changes in the major bands assignable to the cavity anion, the v3 and v1 stretches of the NOz- anion at 1100-1400 cm-', were monitored.Thermogravimetric analysis was carried out on a Stanton Redcroft TG 1000 system comprising an electrobalance, micro- J. MATER. CHEM., 1991, VOL. 1 Table 1 Ag content ( y) and refined cell parameter for Ag,Na, -y(A1Si04)6(N02)2 series ~~~~~ ~ molar ratios of AgNO, to sodalite in reaction mixture sample Y 0 0.050(1) 0.226(1) 0.539(1) 0.879( 1) 1.066( 1) 1.391(2) furnace and temperature controller connected to a chart recorder for graphical output; weight measurements accurate to 1Opg were achieved. In a typical analysis a platinum crucible was loaded with 20-50mg of sample and heated to 1000°C in air. A.c. impedance data were obtained using an HP 4284A Precision LCR meter in the frequency range 20 Hz-1 MHz.Pellets of 6 mm diameter and ca. 0.7 mm thickness were prepared from the compound using a Specac Hydraulic Press under 2 tonne of load. These pellets were then sintered at 500°C for 2-3 days in ambient atmosphere. The conductivity measurements were thus performed on anhydrous materials. After being cooled gradually to room temperature, the pellets were coated evenly with several layers of a silver-based conductive paint on both sides to form disc electrodes. Plati- num wires were attached to each electrode of a pellet as they were constructed to serve as contacts to the external measure- ment circuit. A conductivity jig similar to that described by Bruce and West" was used in this work, the pellet to be measured was attached to the two connectors in the jig by a combination of the weight of the pellet and adhesion to the Ag paint.Very good mechanical and electrical contacts between the pellet and the measurement system were generally obtained until the silver-based paint lost its mechanical strength at above 600°C. Results The parent sodalite Na8[SiA104]6.(N02)2 was single phase as shown by powder X-ray diffraction; all peaks could be indexed on a unit cell of dimensions 8.938 1$in good agreement with the value given by Hund. The IR pattern shows a strong peak at 1275 cm-' and a much weaker feature at 1410 cm-' which can be assigned to the v3 and v1 modes of the nitrite ion, respectively. In the ion-exchange reactions six different reactant compo- sitions of the molar ratios shown in Table 1 were investigated.The separated products were very pale grey but readily darkened on exposure to light; immediately after filtering, materials were sealed in the dark and handled thereafter, where possible, in the absence of light. A linear relationship between composition and cell parameter was found (Fig. 1) which could be used to determine approximate sample compo- sitions without recourse to lengthier volumetric analysis. IR Spectroscopy The vibrational characteristics of the nitrite ion in various inorganic salts have been the subject of a number of investigations.' 2-'4 There are three active vibrations for the nitrite anion of CZvsymmetry, the symmetric stretch, vl, the asymmetric stretch, v3, and the deformation mode, vt.There has been some debate as to the assignments of these modes in various nitrites but normal ranges are 1370f60cm-' for vl, 1250 & 40 cm-' for v3 and 810 cm-' for v2. In sodalites the deformation mode is obscured by the framework 0 0.39(3)1.48( 3) 3.28(4)5.13(5) 6.46(9) 7.64( 8) n aa ,"._"I 8.981 .d- ' 8.93 , 1 0 1 2 3 4 cell parameter/A 8.938( 1) 8.939( 1) 8.946( 1) 8.955( 1) 8.968(1) 8.975( 1) 8.986( 1) , 567y in Agy Na,-, [Si AIO,], (NO,), Fig. 1 Variation of cell parameter with Ag,Na, -y(A1Si04)6(N02)2.2H20 system composition in the vibrations. The positions of the bands have been found to be very dependent upon sample preparation.12 The IR spectra of NaN02 and AgN02 show few differences, though a shift to lower frequency of the bands in the silver salt has been interpreted in terms of the difference in structure of the two salts.In NaNO,, sodium is more closely co-ordinated to oxygen, whilst in AgN02 silver interacts more strongly with nitrogen. As the v1 and v3 symmetric and asymmetric stretches of NO2-are the major observable bands of the nitrite at ca. 1270cm-' the IR study centred on the region 1500-1000 cm-'. The spectrum of each of the samples is given in Fig. 2. For the samples with a high sodium content A, B, C and D (y=O, 0.39, 1.48 and 3.28), the v3 band was found at ca.1275 cm-' with a weaker shoulder, possibly the vl, at 1460 cm- '.However, for samples F and G (y =6.46 and 7.64) both bands shifted noticeably towards lower wavenumber (ca.1220 cm -' and 1360 cm -').Sample E (y =5.13) provides a link between the two sets of data: for this sample both v3 modes are present. This shift to lower wavenumbers parallels the behaviour seen in the simple nitrite salts. This shift could thus tentatively be assigned to a displacement of the nitrite ion position in the cavity at the highest silver contents possibly to allow silver to co-ordinate to nitrogen rather than oxygen. Thermogravimetric Analysis (TG) A typical TG trace from a nitrite sodalite is shown in Fig. 3; two weight losses occur, the first in the region 100-200°C and a second above 650°C.An onset temperature for the second process was determined from the differential of the trace. The first weight loss corresponds to water loss, probably from both the surface of the material and the intracage molecules; the second loss corresponds to a collapse of the aluminosilicate framework and decomposition of the silver nitrate to silver and nitrogen oxides. Water contents were estimated from the initial weight loss, though these may be J. MATER. CHEM., 1991, VOL. 1 I 1600 1400 1200 wavenurnberhn-Fig.2 IR spectra in the frequency range 1000-1500 cm-' for the materials Ag,Na, -y(A1Si04)6(N0,)2.2H,0, labels refer to com-pounds in Table 1. (a) A, (b) B, etc. O1-.-Ell .-, I I I I I 0 200 400 800 800 loo0 ternperature/"C Fig.3 Thermogravimetric data obtained from Ag,.,,Na,,,, (AISi04),(N0,),~2H20 heated in air to 1000°C slightly exaggerated by surface absorption which it was impossible to deconvolute from the former. These processes were verified by powder X-ray diffraction; little change was observed in the powder patterns recorded from samples heated to below 650°C, but above this temperature the typical 012345670 Ag content Fig. 4 Variation of decomposition temperature of the aluminosilicate framework with composition in the Ag,Na, -y(A1Si04)6(N02)2.2H20 system patterns of nepheline (NaA1Si04) and silver metal, the decomposition products of the sodalite, were observed in the powder profiles. A plot of the temperature of onset of decomposition against Ag content is shown in Fig.4. The materials become less thermally stable with increasing silver content; again the change is not linear but seems to occur in two stages: an initial rapid decrease as silver is first introduced, followed by a second more rapid change as the silver content approaches its limit. MAS Solid-state NMR Studies Spectra were obtained for each of the materials listed in Table 1 and in all cases a single resonance was observed. The frequency of this resonance as a function of composition is shown on Fig. 5. Previous studies by us3 have shown a linear relationship between resonance frequency and Si-0-A1 bond angle, and hence lattice parameter, in the sodalite system. This correlation has been interpreted in terms of changes in the s and p orbital contributions to the Si-0 bonds." The frequency range observed in these mixed silver sodium sodalites is rather limited as expected from the small variations observed in the lattice parameter on silver inclusion; however, these changes in resonance frequency are significant. The variation is not linear as expected but rather shows a more rapid change in frequency around the Ag3.2,Na4.72(A1Si04)6(N02)2composition which would indicate that some additional process above small changes in the silicon oxygen aluminium bond angle is occurring.% 84.9 0 1 2 34 5 6 7 a y in Agy Na8-y [Si AIO,], (NO,), Fig.5 "Si MAS NMR frequency as a function of cornposition for Ag,Na, -y(A1Si04)6(N02)2.2Hz0 14 Complex Impedance and Conductivity Study A typical impedance diagram for a sodalite pellet with silver paint electrodes is shown in Fig.6. It can be divided into two well defined parts; a high-frequency semicircle of depressed nature and a low-frequency straight line at slightly less than 45" to the real impedance axis. Such complex impedance diagrams have been discussed in considerable detail by various workers.'6.' The depressed semicircle has an extended high- frequency intercept at the origin and another intercept at low- frequency values on the real axis. This low-frequency intercept is regarded as the bulk impedance of the pellet arising from the ionic conduction in the electrolyte. The inclined straight line at low frequency is the electrolyte/electrode interface impedance arising from its geometric microstructure.' Owing to the limited frequency range available with the HP 4284A LCR meter, the measurable portion of the complex impedance diagram changes from the inclined straight-line portion at higher temperatures to the depressed semicircle at the lower temperatures investigated.This behaviour is shown in the set of complex plane impedance plots presented in Fig. 6. The appearance of only one depressed semicircle at high frequency, followed immedi- ately by an inclined straight line at lower frequencies in the temperature range studied indicates that the effect of inter- granular impedance (i.e. the grain boundary effect) on the total conductivity is negligible.Thus the d.c. conductivity at a particular temperature was derived from the resistance obtained by extrapolating the depressed semicircle to the real axis of the impedance diagram. 400 1 Ic 300 I t I 40 t 1 (3,g 20301 UI I 0 10 20 30 40 50 60 70 80 Z( real)/kR 2.5 1 0.5 -J. MATER. CHEM., 1991, VOL. 1 -37 1 1.2 1.4 1.6 1.8 2 2.2 103~1~ Fig. 7 Variation of conductivity with temperature for Na,(AISiO,), (NO,), :*, sample A; +, sample B Two nitrite sodalites samples have been studied, sample A (y =0), in order to obtain an idea of the cation conductivities in a typical sodalite, and sample B (y=O.39), in order to observe the effect of partially increasing the framework dimen- sion and replacing sodium by silver.log CT vs. 103/T plots were obtained for both samples (Fig. 7) yielding an Arrhenius activation energy of 81.6k0.8 kJ mol-' for sample A with CT =3.9 kO.1 x R-cm-' at 500°C and an activation energy of 77.3k0.5 kJ mol-' for sample B with c= 5.6 0.2 x 10-R-cm-at 500°C. The above results indicate that substitution of silver into nitrite sodalite reduces the thermal activation energy required for ion conduction and enhances bulk ionic conductivity. However, it is still too early at this stage to discuss the individual conduction contribution from either the silver or the sodium ion. The effects of water content on conductivity measurement could become quite significant at low temperatures, especially below 100°C.The existence of water molecules in sodalites, either on the surface or in the cages, has been shown using TGA. If desired, the effect of water absorption can be elimin- ated by flushing the cell system with an inert dry gas such as dry nitrogen. Further ionic conductivity studies on other samples and full studies regarding the effect of water content upon the cell conductivity are in progress and will be reported in due course. Discussion Silver may successfully replace sodium in the sodalite structure through a facile ion-exchange reaction in aqueous solution. The similar sizes of these species (1.16 A Agf and 1.13 A Naf) means there is only a minor change in the framework dimen- sions and geometry. This is readily seen from the powder X-ray data which show only a small linear expansion of the cell dimension, in line with the ionic sizes, as silver replaces sodium. The behaviours of the NMR, IR and thermogravi- metric data are more complex; in each case a change in characteristics is seen as silver becomes the major cation in the cavity.With respect to the NMR data, although there is a mono- tonic decrease in the resonance frequency as silver replaces sodium the variation is not linear. A contraction of the unit cell from 8.986 to 8.938 A would be expected to result in a decrease of resonance frequency of 1.0 ppm from previous measurements on the sodalite ~ystem.~ A change close to this value of 1.05 ppm is indeed observed; however, the data indicate a more sudden change between the two limits at the mid-point composition with slower variations when the com- J.MATER. CHEM., 1991, VOL. 1 position is dominated by either sodium or silver. Such a variation would indicate a sudden change in the silicon environment as one cation becomes the more dominant. Support for this behaviour comes from the IR data and to a more limited extent from the thermogravimetric results. A distinct shift in the nitrite stretching frequency as the silver content increases from Ag,,,,Na4,,2(A1Si04)6(N02)2to Ag,. 3Na2.87(A1Si04)6(N02)2 correlates with a rapid change in the 29Si NMR resonance frequency and a slower decrease in the sample decomposition temperature. It seems likely that, as no abnormal change is observed in the dimensions of the aluminosilicate framework, the changes in sample behaviour are associated with the cage species.The most likely source of this is the orientation of the nitrite ion. The accepted geometry of this ion, CZvwith N-0 of 1.24A and 0-N-0 angle of 115", is incompatible with the high symmetry of the sodalite cage centre. It is likely that the ion can adopt a variety of off-centre positions within the cage with respect to the tetrahedrally diposed cations and water molecule. Replac- ing sodium by silver probably results in a different preferred orientation when the silver level becomes greatest. Materials containing both silver and sodium are likely to have a statistical distribution of the two types of cation throughout the cages, so the contents of each cage may not reflect exactly the stoichiometry of the material.This will result in a less clear-cut change between the two nitrite ion orientations as different cages within the same compound may have a variety of nitrite ion positions. Powder neutron diffaction studies are planned in order to study the cage ion distributions in more detail but are only likely to provide information on the average structure. The cation conductivity of sodalites is reasonably high as expected from the facile exchange behaviour of materials from this family. The origin of the enhanced conductivity may be due to the slight expansion of the aluminosilicate framework that occurs as the larger silver ion replaces sodium.Any transport between the sodalite cages would involve migration of the cations through the bottleneck presented by the six- membered rings. Increasing the Si-0-A1 bond angle will increase the dimensions of this constriction allowing faster hopping of the cations between cages. Further conductivity studies of sodalites containing a wide variety of anions and cations sizes are in progress. We wish to thank SERC, The Ford Motor Company and A.B. Electronic Products for grants in associtation with this research. References 1 C. M. B. Henderson and D. Taylor, Spectrochimica Acta, Part A, 1977, 33, 283. 2 W. Depmeier, Acta Crystallogr., Sect. B, 1988, 44, 201. 3 M. T. Weller and G. Wong, J. Chem. Soc., Chem. Commun., 1988, 1103. M. T. Weller and G. Wong, Solid State Ionics, 1989, 32-33, 430. S. Ramdas and J. Klinowski, Nature (London), 1984, 308, 521. D. Taylor, Contrib. Mineral Petrol., 1975, 51, 39. F. M. Jaeger, Trans. Faraday SOC., 1929, 25, 320. G. A. Ozin, A. Kuperman and A. Stain, Angew. Chem. Int. Ed. Eng., 1989, 28(3), 359. 9 F. Hund, 2. Anorg. Allg. Chem., 1984, 509, 153. 10 F. Hund, Z. Anorg. Allg. Chem., 1984, 511, 225. 11 P. G. Bruce and A. R. West, J. Electrochem. SOC., 1983,130, 662. 12 R. E. Weston and T. F. Brodasky, J. Chem. Phys., 1957, 27, 683. 13 J. A. A. Ketelaar and C. J. H. Schutte, Recueil Trav. Chem. Pays Bas, 1961, 80, 721. 14 F. Vratny, M. Tsou and F. Gugliotta, Nature (London), 1960, 188, 484. 15 G. Engelhardt and R. Radeglia, Chem. Phys. Lett., 1984, 108, 271. 16 W. I. Archer and R. D. Armstrong, Specialist Periodical Reports, Electrochemistry, 1980, 7,157. 17 J. B. Bates, J. C. Wang and Y. T. Chu, Solid State lonics, 1986, 18/19, 1045. Paper Of0254363 Received 7th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100011

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, l(1) 17-21 Synthesis and Structure of Ba,CaCu,.,,O,.,,(Co~)~.~, a Perovskite containing Carbonate Anions, and Related Phases Colin Greaves* and Peter R. Slater Superconductivity Research Group, School of Chemistrx University of Birmingham, Birmingham B15 2TT, UK Calcination of appropriate mixtures of BaCO,, CaCO, and CuO has been shown to result in a tetragonal [P4lmmm; a= 5.799(2)A, c= 7.992(3)A] perovskite-related phase containing C0:- anions. The unit-cell compo- sition is Ba,CaCu,,,O, 96(C0,), 5, and powder neutron diffraction has demonstrated that the octahedral sites are occupied by Ca, Cu and C in an ordered fashion. The structure is compared with the cubic phase Ba,CaCu,O,,,. Related phases in which the Ca ions are replaced by other cations are also reported.Keywords: Barium calcium copper oxide; Perovskite; Oxide carbonate; Neutron diffraction The superconducting properties of YBa,Cu307 have stimu- lated numerous studies of the Y,O,-BaO-CuO phase dia- gram. Of particular relevance to the present paper are the phases that have been observed with Ba:Y:Cu ratios of ca. 3:1:2. Preliminary studies'.2 of such materials suggested the existence of two closely related structures, one tetragonal with unit-cell dimensions a=5.81 A, c=8.02 A, and the other cubic. Dysprosium analogues of these phases were sub-sequently found to play a significant role in the formation of epitaxial thin films of DyBa2Cu307 using molecular-beam epitaxy,, and similar phases have been found recently in the Yb,O,-BaO-CuO ~ystem.~The cubic phase containing Y has been shown to have the composition Ba4YC~308,5+x, and a unit cell (a=8.12 A) comprising eight perovskite sub- cek5 Studies of barium calcium copper oxides have recently revealed an analogous phase, Ba4CaCu308.6 1, with a related structure.6 A tetragonal modification was also synthesised, and since no reliable structural information has been reported on such tetragonal phases, a full structural analysis has been performed using neutron diffraction, and the results are pre- sented here.The possibility of replacing Ca by other cations has also been examined. Experimental Samples were synthesised from intimate mixtures of high- purity BaCO,, CaCO, and CuO. The mixtures were heated in air or oxygen for 14 h at 950 "C, re-ground and heated again in air or oxygen at 1000 "C for 14 h.Powder X-ray diffraction (XRD) examination of the products revealed that a single-phase tetragonal material was obtained for Ba:Ca:Cu ratios of ca. 4:1:2. Similar synthetic conditions were employed to examine the possibility of substituting Mg, Y, Sr or Eu for Ca. Ambient temperature time-of-flight powder neutron diffrac- tion data were collected on the diffractometer POLARIS at ISIS, Rutherford Appleton Laboratory, using a vanadium container. Structure refinements used the least-squares program GDELSQ, which is based on the Rietveld method and uses the Cambridge Crystallography Subroutine Librar~.~.~ Scat-tering lengths (/lo-', cm) of 0.525, 0.490, 0.7718 and 0.5805 were assigned to Ba, Ca, Cu and 0,respectively. Results and Discussion The cation ratio Ba,CaCu, produced a black phase with a powder XRD pattern, indicative of a tetragonal unit cell [a =5.800(2)A, c =7.990(3)A] related to a perovskite subcell (size up) with a=J2ap, c=2uP.This contrasts to the body- centred cubic superstructure (a=2up) observed previously for Ba4CaCU308.61.6 For this cubic phase, the ordering of Ca and Cu on the octahedral sites was similar to that exhibited by Li/Na and Sb in Ba4LiSb3OI2 and Ba4NaSb3OI2,' and shown in Fig. l(a). A similar tetragonal supercell to that observed in the present study was previously described for Ba4LiBi301,-x, where a different cation order exists." This structure [space group P4/mmrn, Fig.l(b)] with Ca substituted for Li and Cu for Bi, was therefore adopted for the initial stages of structure refinement using powder neutron diffraction data. During the early stages of refinement it became clear that the central Cu site [Cu(3) at (9,4,+)I was incompletely occupied by Cu, with an apparent site occupancy of 0.65. Vacancies on this site were consistent with the Cu content of the initial reaction mixture being less than for the nominal Ba,CaCu3 ratio required for this sample model. The O(3) and O(4) positions [see Fig. l(b)] around this site were also only partially occupied (around 0.5 and 0.25, respectively), which was consistent with incomplete occupancy of Cu(3). However, they were too close to Cu(3) (1.67 and 1.32 A) for conventional Cu-0 bonds, and the temperature factor for O(3) was very high (7 A2).The introduction of anisotropic temperature factors for this atom suggested that the primary problem related to its position along the [llo] direction, and conse- quently this site was split to give O(3) [ca. (0.25, 0.25, OS)] and O(5) [ca. (0.33, 0.33, OS)] either side of the original position. After the refinement, which resulted in reduced 'R factors, the Cu(3)-0(3) distance (1.9 A) was consistent with a Cu-0 bond, but Cu(3)-0(5) was very short and similar to Cu(3)-0(4) (1.4 and 1.3 A, respectively). Since these distances are very similar to C-0 bonds in metal carbonates (ca. 1.32 A), it was assumed that residual C from the BaCO, and CaCO, starting materials was located on this site, and was covalently linked to three oxygens in adjacent O(4) and O(5) positions.Scattering length considerations (C, 0.6648 x lo-', cm) indicated that the central Cu(3) site was still only partially occupied: ca. 50% by C, 25% by Cu. Two constraints were applied to aid convergence during subsequent refinements, owing to the complex nature of the defect structure in the region of the Cu(3) position. First, it was assumed that Cu at the Cu(3) site was linked only to O(3) to give 4-co-ordinate planar stereochemistry (relaxation of this restriction in a later refinement confirmed its validity). The three 0 atoms of the COS- ions were also positioned statistically on the six octahedral bonds around the (4, +,+) site, i.e.one in O(4) and two in O(5)sites. Although this J. MATER. CHEM., 1991, VOL. 1 ?= I I I7: I I L: oSb eLi \L, 001and0200 Q 003 (a) 804 (b) Fig. 1 The cubic and tetragonal perovskite-related structures of (a) Ba,LiSb,O,,, and (b) Ba,LiBi,O,,-,. For clarity the Ba ions, which occupy central positions in the perovskite subcells. are not marked. The subject of the present study has a structure related to (b) with Ca replacing Li and Cu replacing Bi constraint cannot be in accordance with the real structure since it requires 0-C-0 angles of 90 and 180°, the implied random orientation of the carbonate groups was supported by the previous refinement which indicated an O(5):O(4) ratio of 2:l.The refinement converged to give a central site occu- pancy of 0.52 C and 0.25 Cu, and an overall composition of Ba4CaC~2,2507(C03)o,52.The model provided good agree- ment between observed and calculated neutron diffraction profiles (Fig.2) except for small peaks (marked by arrows), which were attributed to CaO impurity. The regions corre- sponding to the most intense of these peaks were excluded in the final refinement. A refinement in which the Cog- ions were constrained to the geometry of free ions was slightly inferior (higher R factors), and in view of the unusual environment provided by the perovskite structure for such ions, and the possibility of consequential distortions, the 0-C-0 angle constraints were not maintained.Owing to the complex stoichiometry of this phase, two additional samples were synthesised from slightly different mixtures of starting materials (Ba:Ca:Cu ratios 4:1.06:2 and 4:1:1.87), and both appeared single phase from XRD analysis. Since virtually identical results were obtained from all three neutron diffraction data sets, averaged values of the refined parameters are presented in Table 1. The profile R factors, R,, R,, and RE,are indicative of a reliable structural model; the slightly higher values of RI are thought to be due to the influence of the CaO impurity on the weak reflections at low d-spacings. Bond distances and bond strengths around the Ca and Cu ions were calculated using the parameters of Brown and Altermatt," and are given in Table 2.Owing to the resulting high bond-strength sum at Cu( l), alternative calculations using an appropriate Cu3 + parameter' were also performed for this site as indicated in the table. Thermogravimetric reduction (in a 10% H2 in N2 mixture at 930°C to give BaO, CaO and Cu) produced a mass loss of 7.6%, which is in reasonable agreement with that expected (7.0%) for the phase composition suggested by the structure refinements. However, the crystallographic structure implies a higher Cu content than expected from the cation ratios in the pre-fired mixtures. Although this is supported by the presence of small CaO peaks in the neutron diffraction profiles, no Ba-containing impurities were detectable.It there- fore seems likely that some volatilisation of Ba may occur during the synthesis, as was also observed during the prep- aration of Ba4CaCU308.6,.6 The most striking feature of the structure is the presence of C, Cu, and vacancies in a 2:l:l ratio on the central site at (4, 1, 5). Although the presence of car-bonate anions in perovskite structures is uncommon, the complete replacement of layers of octahedral cations by C03 groups has been reported in the perovskite-related phase Sr2C~02(C03).'3The presence of lattice carbonate anions in Ba,CaCu2.2406,96(C03)0.5 is supported by three additional observations: (1) dissolution in dilute mineral acids resulted in the evolution of larger amounts of C02 gas than expected for possible traces of BaC03 contamination.(2) Attempts to synthesise the material from BaO, CaO, and CuO in C02- free oxygen environments were unsuccessful. Subsequent heat- ing of the products in air, however, resulted in the formation of the required phase. (3) Treatment at 350 "C in 200 bar oxygen, which formed part of a wider programme to investi- gate the effects of such conditions on mixed copper oxides, resulted in phase decomposition, and XRD clearly indicated a significant quantity of BaCO, in the products; the carbonate could have originated only from the sample under investi- gat ion. Owing to the disordered nature of the C03 groups in the structure, the proposed model is necessarily a simplistic representation of the true structure since it imposes 0-C-0 bond angles of 90 and 180" instead of the expected 120". In reality, some of the 0 atoms linked to C [0(4) and 0(5)]are likely to be shifted from the positions indicated by the refinement (Table l), in order to achieve appropriate bond angles, and this is supported by the high temperature factors observed for the O(5)atoms.The lower value observed for O(4) suggests that the C03 groups may be oriented with one C-0 bond [to 0(4)] parallel to [OOl], and the two O(5) atoms being displaced from the ideal (x,x,3)sites, mainly along [OOl]. Given this problem of locating the carbonate oxygen atoms accurately, the observed C-0 distances, Table 2, are not dissimilar to those found in other inorganic carbonates (around 1.32 A).It is interesting to note that despite the substantial oxygen deficiency in this perovskite-related phase, the 0(1) and O(2) sites, which are co-ordinated to Ca, are fully occupied. Octahedral stereochemistry is therefore maintained around the Ca ions, which was also observed in the cubic phase Ba4CaCU308.61.6 The Cu(3) site is co-ordinated to four coplanar O(3) atoms, and bond-strength calculations, Table 2, support Cu2+ at this position. The Cu(1) and Cu(2) sites, however, are co-ordinated to some sites that are only partially J. MATER. CHEM., 1991, VOL. 1 -Ill I Ill Ii IIIIII Ill ll ll Ill Ill ll ll I ll I t i ll I i I I I I I1 II I 'I-1.4--I 1.2 --1.0--4 L 0.8-I h P dspacinglA I I 1 I 1 1 1I 1 0.0. 4 L Q,a g 0. 2 c 3 Q) 0. t r n -1 0.4 ' 015 ' 016 ' 017 018 0'.9 ' l'.O l'.l dspacing/A Fig. 2 Observed (dots), calculated (continuous line) and difference neutron diffraction profiles 7atom position Y Y BjA' site occupancy 4i 0 ~21 0.2391(4) 0.64(4) 1 la 0 0 0 0.87( 13) 1 lc 21 21 0 0.15(5) 1 lb 0 0 -1 2.06( 13) 1 1 ~Id -1 21 2 0.86(18) 0.24(3) 1Id 21 + 2 0.86(18) 0.50(4) 2g 0 0 0.2799(8) 2.12( 11) 1 4J 0.2745(4) 0.2745(4) 0 0.77(5) 1 4k 0.267(4) 0.267(4) + 2.8(6) 0.24(3) 2h -1 -1 0.333(3) 2.0(4) 0.25(2) 4k 0.329(4) 0.329(4) 1 3.2(4) 0.232) aTetragonal, P4/mmm, a =5.799(2) A, c =7.992(3) A. Sample I: R, =6.9%, R, =2.8%, R,, =3.2%, RE 2.0%. Sample 2: R, =8.6%, R, =3.0%, RWP=3.4%, RE=2.3%.Sample 3: R,=9.2%, R,= 3.2%, Rwp= 3.5'/0, R,=2.1%. 20 J. MATER. CHEM., 1991, VOL. 1 Table 2 Selected bond lengths and bond-strength summations for Ba,CUCU,,,406,96(C03)0,~ bond bond length/A bond bond length/A bond bond length/A Ca-O( 1)-W)Cu(2)-O( 1) -0(3) -0(5) 2.237(6) 2.251(3)1.759(6) 2.19( 3) 2.69(3) (x 2) (x 4) (x 2) (x 4) (x 4) C~(l)-0(2) 1.85q3) (X 4)-0(4) 2.67(2) (x 2)CU(3)-O( 3) 1.9 l(3) (x 4) bond strength summations" C-0(4) -0(5) 1.33(2) 1.41(3) (x 2) (x 4) Ca 2.56 [for ro(Cu2f)= 1.6793 2.94 [for ro(Cu3+)= 1.7312] occupied, and bond-strength summations were appropriately weighted to account for this feature. Cu(1) has four short coplanar bonds to 0(2),.and in the average unit cell a more distant interaction with a partly occupied O(4) of a carbonate group.Cu(2) has two short bonds at 180" to 0(1), an average of one long bond to O(3) and an even longer bond to an O(5) of a carbonate group. The Cu( 1) and Cu(2) sites appear to be Cu3+ and Cu2+, respectively, on the basis of bond-strength considerations (Table 2). This assignment gives an average Cu oxidation state of 2.44 +, which is in reasonable agreement with the value of 2.20(11) determined by the phase compo- sition. The co-ordination around Cu(2) appears unusual, and ordering of the Cu, C and vacancies in the (3, 3, f-)sites may occur to provide a more favourable co-ordination for Cu(2). Such an ordering process, which must occur over relatively short distances since superstructure reflections are not ob- served, is consistent with the relatively high thermal par- ameters observed for Cu(2), and 0(1) and O(3) to which it is bonded.It is interesting to note that the bond-strength sum at Ca is higher than expected, and this overcompensation of charge also occurs at the octahedral Ca/Y sites in Ba4CaCU308.61 (Ca bond strength sums 2.67) and Ba4YCu308., +x (average Y bond strength sums 3.41 for x =0.5, and 3.76 for x =0 using the data of de Leeuw et a/.'). This feature may, in fact, be fairly common for the occupancy of octahedral sites by Ca or Y in perovskite phases, since a high value for Ca (3.33) is also calculated for the hexagonal perovskite Ba3CaRu2O9.I4 The distribution of Ca ions on the octahedral sites is clearly different in Ba4CaCU2.2406.96(C03)0.~from Ba4CaCU308.61, and the two types of order are closely related to that ob- served in Ba4LiBi30 -and Ba,LiSb,O t,9 respectively.Madelung energy calculations" suggested that the two structures have similar electrostatic stabilities, and subtle effects therefore determine the actual order adopted. For the cubic and tetragonal forms of the copper-containing phases, it is thought that the presence of cation vacancies and Cog- anions within the structure is primarily responsible for the adoption of the tetragonal structure determined here. This possibility can be rationalised by recognising that a common structural feature of both cubic Ba4CaCu308.61 and tetra- gonal Ba4CaC~2.2406.96(C03)0.5is the complete occupancy of the 0 sites co-ordinating to Ca, such that 0 vacancies form links only between two Cu positions.If some of the Cu sites are vacant or occupied by C atoms of Cog -groups, the co-ordination sphere around these sites will be modified such that the co-ordination preference of Ca can be satisfied only with the tetragonal structure. Comparison of the two struc- tures (Fig. 1)shows that only the (+,& f-)site in the tetragonal unit cell can readily accommodate such defects, since it is connected via anions to six Cu sites and allows the Ca co- ordination to remain unchanged. In contrast, the presence of defects on other Cu sites would have a direct influence on 1.91 2.14 Table 3 Unit cell dimensions of related phases ~~ cation ratio in initial reagents alA c/A Ba4MgCu2.25 5.6542) 8.033(3) Ba4Mg1.0Scu2.16 5.661(2) 8.002(3) Ba4YCu, 5.779( 2) 8.O19( 3) Ba4EuCu2.2 5 5.820(2) 8.067( 3) Ba4SrCu,., 5.707(2) 7.903(3) the stereochemistry of adjacent Ca ions and result in incom- plete co-ordination or elongated bonds. Accordingly, it appears relevant that vacancies and Cog- ions are found only at the central site in the tetragonal material. Attempts to substitute Ca for other cations resulted in the formation of several phases with closely related tetragonal structures, and the materials listed in Table 3 were all obtained as single-phase or nearly single-phase products (estimated purity >95%) according to XRD analysis. All these materials are believed to contain lattice carbonate anions, since prepara- tive routes involving pure oxide starting materials and C02- free conditions were always unsuccessful.The Mg, Y and Eu containing samples are thought to be essentially isostructural with Ba4CaCU2.2406.96(C03)0.5, and this is supported for the Y analogue by neutron diffraction data which will be reported elsewhere. The unit cell dimensions (Table 3) are in accordance with ionic radii considerations, except for the sample containing Ba, Sr and Cu (4:1:2.4), which appears anomalous. In particular, the unit cell is smaller than that of Ba4CaCU2.2406.96(C03)0.~,and is consistent with the partial substitution of Ba by Sr on the large, nominally 12- co-ordinate perovskite sites.It is therefore thought to be more closely related to Sr2Cu02C03, in which a complete layer of Cu cations has been replaced by Cog- ions.I3 Our studies of Sr2Cu02C03 have indicated a unit cell with a=5.524(1)A and c =7.496 (2) A, which, as expected, is smaller than that observed for the mixed Ba/Sr phase. A more detailed structural examination of layered phases of this type is planned. We are grateful to SERC for providing financial support, neutron diffraction facilities and a studentship to P.R.S.; we also thank ICI for additional funding. We are grateful for the advice and assistance of S. Hull during the collection of neutron diffraction data. References 1 R. S. Roth, K. L. Davis and J. R. Dennis, Adv. Ceram. Muter., 1987, 2, 303.2 K. G. Frase and D. R. Clarke, Adv. Cerum. Muter., 1987, 2,295. 3 E. S. Hellman, D. G. Schlom, A. F. Marshall, S. K. Streiffer, J. S. Harris Jr., M. R. Beasley, J. C. Bravman, T. H. Geballe, J. N. Eckstein and C. Webb, J. Muter. Res., 1989, 4, 476. J. MATER. CHEM., 1991, VOL. 1 21 4 J. Liang, X. Chen, S. Wu, J. Zhao, Y.Zhang and S. Xie, Solid State Commun., 1990, 74, 509. 10 C. Greaves and S. M. A. Katib, J. Solid State Chem., 1990, 84, 82. 5 6 7 8 9 D. M. de Leeuw, C. A. H. A. Mutsaers, R. A. Steeman, E. Frikkee and J. W. Zandbergen, Physica C, 1989, 158, 391. C. Greaves and P. R. Slater, Solid State Commun., 1990, 73, 629. J. C. Matthewman, P. Thompson and P. J. Brown, J. Appl. Crystallogr., 1982, 15, 167. P. J. Brown and J. C. Matthewman, Rutherford Appleton Lab- oratory Report RAL-87-010, 1987. J. A. Alonso, E. Mzayek and I. Rasines, Muter. Res. Bull., 1987, 22, 69. 11 12 13 14 I. D. Brown and D. Altermatt, Acta Crystallogr., Sect. B, 1985, 41, 244. I. D. Brown, J. Solid State Chem., 1989, 82, 122. Hk. Miiller-Buschbaum, Angew. Chem. Znt. Ed. Engl., 1989, 28, 1472. J. Darriet, M. Drillon, G. Villeneuve and P. Hagenmuller, J. Solid State Chem., 1976, 19, 213. Paper 0102565H; Received 8th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100017

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 23-28 Investigation of Defect Ordering in the Perovskite System SrCro,, Feo,903 -,,by Mossbauer Spectroscopy Terence C. Gibb School of Chemistry, The University, Leeds LS2 9JT, UK The oxygen-deficient perovskite system SrCro.lFeo,903- has been studied by Mossbauer spectroscopy and X-ray powder diffraction techniques. The material obtained is strongly dependent upon the conditions of preparation. Partially oxidized materials annealed in air and quenched are simple cubic perovskites with considerable cation and oxygen vacancy disorder. Annealing at 1200°C in vacuo and then cooling step-wise produces an ordered brownmillerite superlattice with the chromium ordered onto octahedral sites. However, rapid cooling gives a microdomain-textured material containing a random distribution of cations; the introduction of chromium stabilizes the high-temperature state of Sr2Fe205 which has never been quenched to room temperature.Annealing in an argon stream at 1200°C gives a partially oxidized crystalline brownmillerite in which the chromium (and extra oxygen) are incorporated in the tetrahedral layers. The magnetic ordering temperatures are correlated with the observed site distribution of the chromium. Keywords: Mossbauer spectroscopy; Defect ordering; Brownmillerite; Microdomain; Perovskite The perovskite SrFe03 is of particular interest because it is one of the few oxide phases to contain iron in the +4 oxidation state.' It readily loses oxygen, ultimately producing the Fe3 + oxide SrFe02,5 (usually referred to as Sr,Fe205), which has been shown to have a grossly oxygen-deficient perovskite- related structure of the brownmillerite with alternate layers of iron in six and four co-ordination to oxygen. Work in this laboratory4 and subsequently has revealed the existence of the mixed-valence phases SrFe02.875 and SrFeO,,,,, both having an ordered arrangement of oxygen vacancies, although there remains considerable confusion over the precise atomic arrangement.Material quenched from high temperatures to room temperature seems to contain an intimate mixture of at least two of these four phases, probably with considerable intergrowth. There is considerable e~idence~-~ to support the existence of a cubic phase at high temperatures over the whole composition range.This phase, which may well have a microdomain texture, is accompanied by the formation of an averaged valence state for the iron at a temperature which rises with increasing Fe3+ content. The cubic form of Sr2Fe205 is believed to be a microdomain-textured brownmill- erite,6-8 although it has proved impossible to preserve this phase by quenching. Work in this laboratory on the brownmillerite Sr,CoFe05 has shown that the cobalt can be incorported either at random or preferentially on the tetrahedral sites, depending on the conditions of preparation.'.' Partially oxidized material, pre- pared by quenching, contained microdomains of the brownmil- lerite with excess oxygen accommodated in the domain walls. SrFe03-, and SrCro~,Fe0.,O3-, were ground together in a ball mill, pressed into a pellet, and initially fired in a platinum crucible at 1200°C for 11 days with two intermediate grindings.Aliquots of this material were then annealed for several days under a variety of conditions, as detailed in the text. Initial characterization in each case was by X-ray powder diffraction recorded with a Philips diffractometer using nickel- filtered Cu-Kcr radiation. Chemical analyses for nominal Cr4+/Fe4+ content were carried out as described previously.' ' Mossbauer data were collected in the temperature range 78 < VK <600 using a 57Co/Rh source matrix held at room temperature; isomer shifts were determined relative to the spectrum of metallic iron. Results and Discussion The initial choice of a 10% substitution of iron by chromium was prompted by the failure of the earlier investigation to observe a substituted brownmillerite, analogous to those observed in the equivalent calcium system at up to 28% substitution.' Furthermore, ordered compounds such as Ca2CrFe05 and Sr,CrFe05 do not appear to exist.Samples were prepared from the initial material by annealing for several days in air (or argon or in uacuo) at a controlled temperature before quenching into liquid nitrogen (from air) or cooling quickly. The chemical analyses and phase analysis by X-ray powder diffraction and Mossbauer spectroscopy are summarized in Table 1. More recently, a detailed study of the system SrCr,Fe, -x02.5+yPreparations in Air (x=O.25, 0.33, 0.50) has been carried out" which revealed a number of perovskite-related phases including a novel 15R- rhombohedra1 phase, the nature of which remains elusive.However, there was no evidence at that time to support the existence of a Cr-substituted brownmillerite phase. This paper reports a study of the composition SrCro.lFeo.903-, which has revealed the existence of a num- ber of unusual brownmillerite-related phases, and the effects of Cr substitution on the SrFeO3_,, system are discussed. Experimental Accurately weighed amounts of spectroscopic grade Fe203, Cr203, and SrC03, with stoichiometric ratios appropriate for The Mossbauer spectra at 290 K of equivalent pairs of materials for SrFe03-y and SrCro.,Fe0~,O3 -y quenched from 1200, 1O00, 800, 600"C, and cooled very slowly in air are illustrated in Fig.1-3. All the chromium-substituted materials gave a very sharp X-ray pattern that could be easily indexed as a simple cubic perovskite pattern, the cell parameter being given in Table 1. The oxygen content increases (y decreases) at lower temperatures, and the cell parameter decreases. The Mossbauer spectra (Fig. 1) for the 1200, lo00 and 800°C quenches comprise very broad magnetic hyperfine patterns that are completely different from those for the parent system shown in Fig. 2. The latter show a superposition of a typical magnetic brownmillerite pattern (from Sr2Fe205) and a cen- 24 J. MATER.CHEM., 1991, VOL. 1 Table 1 Chemical analyses and X-ray phase analysis for samples prepared under a range of experimental conditions SrFeO, -SrCrO. 1Fe0.~03-y conditions y phases" y phases" 1200°C air/quench 0.452 BM-(P) 0.353 P (a =3.932 A)1ooo"C air/quench 0.412 BM-(P) 0.314 P (a=3.934 A)800°C air/quench 0.337 BM-(P) 0.273 P (a=3.923 A)600°C air/quench 0.230 split-P 0.209 P (a=3.913 A)slow-cool in air 0.120 split-P 0.126 P (a= 3.905 A) C1200°C argon --0.422 BM .-1200°C in uucuo --0.500 BM u) (step-cooled) .-1200°C in uacuo --0.497 microdomains (fas t-cooled) E CI "Phases identified as: BM, brownmillerite; P, perovskite; split-P or microdomains (see text). . lllllllllll -12 -0 -4 0 4 8 12 velocity/mm s-' Fig. 2 The Mossbauer spectra at 290 K of samples of SrFeO,-, annealed in air at (a) 1200, (b) 1OOO and (c) 800°C and quenched into liquid nitrogen W Fig.1 The Mossbauer spectra at 290K of samples of SrCro,lFeo,903-yannealed in air at (a) 1200, (b) 1OOO and (c) 800°C \jand quenched into liquid nitrogen tral paramagnetic component from SrFe02., 5. The computed area ratios were used to estimate an oxygen content which agrees within experimental error with the chemical analyses, .-and these materials can best be described as multiphase with -: no signs to suggest the presence of microdomain intergrowths. 3 The chromium-substituted samples appear to be single- -----7, !\i?L'cphase, the broad magnetic patterns being indicative of con-siderable disorder in the oxygen vacancies.There are some indications of fine structure, which is consistent with a vari- ation in the iron co-ordination. There is no conclusive evidence to suggest the presence of any iron in oxidation states higher I I I I I I I than +3, which are required by the stoichiometry. The -4 -2 0 2 4 spectrum at 800°C is partially collapsed because of a lowering of the magnetic ordering temperature, and the 600°C quenches velocity/mm s-' and slow-cooled samples shown in Fig. 3 are paramagnetic Fig.3 The Mossbauer spectra at 290K of samples ofat 290 K. The two iron materials have chemical compositions SrCro,lFeo,903-y and SrFeO,-, annealed in air at 600°C andand STF~O~,~,~, very close to the nominal ones for STF~O~.,~ quenched into liquid nitrogen [(a)10% Cr, and (c)], or slowly cooled respectively. The X-ray patterns (which show superlattice in air [(b) 10% Cr, and (d)] J.MATER. CHEM., 1991, VOL. 1 lines) and the Mossbauer spectra are similar to those described confirming the identification. However, the equi- valent Cr-substituted samples are very different. The Mossbauer spectra show no evidence for the ordered-vacancy phases. There is still some doubt as to the correct analysis of the spectra for STF~O~,,~ and SrFe02.875 because of the difficulty in obtaining high-purity samples; the spectrum of the latter (at the bottom of Fig. 3) shows clear evidence for contamination by the former. However, it is widely accepted that the ideal phases contain 1:l mixtures of the nominal cation states Fe3 +/Fe4+ and Fe3.' +/Fe4', respectively.An analysis of the nominal iron states in the Cr-substituted material is not possible, but it is evident (and confirmed by numerical evaluation) that despite a similar level of oxidation, the mean isomer shift of the iron has shifted significantly to a higher value. This implies a significant reduction of the iron at the expense of chromium which may be presumed to be oxidized to at least the 4 + state. This observation is consistent with the results at higher Cr content.'' It will be clear from the above that the substitution of chromium has suppressed the tendency to form ordered defect phases, and that the products are highly disordered and therefore difficult to study in detail.However, the materials annealed under argon or in U~CUOproved to be much more amenable, and produced some surprising results which are now described. Preparations in vacuo A sample of the Cr-substituted material was annealed in U~CUO at 1200°C for 6 days before annealing at 100°C intervals for 24 h down to 600°C and then cooling to room temperature. The product was a brownmillerite without additional oxygen, and the good quality X-ray pattern was indexed with the orthorhombic cell a =5.665(2)A, b =15.509(5)A, c = 5.523(2)A and a cell volume, r! of 485.2(5) A3, compared with a= 5.672(2) A, b =15.561(5) A, C= 5.525(2)A, V=487.6(5) A3 for the parent Sr2Fe20S. The parameters for Sr2Fe2O5 are in good agreement with published data.2y3 The substituted material shows a reduction in all parameters consistent with the introduction of the smaller Cr3+ ion.The Mossbauer spectrum as a function of temperature is shown in Fig. 4.The pattern at 78 K is clearly that of a brownmillerite, but with a substantial reduction in the inten- sity of the lines from the octahedral sites. The area ratio of the octahedral/tetrahedral sites was computed to be 0.84(9), compared with an ideal figure of 0.80 if all the Cr is on octahedral sites and 1.25 if on tetrahedral sites. Therefore, within experimental error the chromium is effectively ordered onto the octahedral sites. The spectrum at 290 K is particularly interesting because it shows considerable fine structure, which can be interpreted directly in terms of the individual nearest-neighbour environ- ments. Such effects have not been reported previously in a brownmillerite system, although they have been observed in the ~rthoferrites.'~,'~ A magnified portion of a typical data analysis is shown in Fig.5, and shows clear evidence for three octahedral and two tetrahedral sites with different hyperfine parameters. To simplify the analysis it was assumed that the linewidths, r,isomer shifts, 6, and quadrupole perturbations, E, are the same for a given co-ordination to oxygen. The computed parameters and area ratios are shown in table 2, together with the ideal probabilities, calculated assuming that the Cr is exclusively on octahedral sites. It can be seen that there are in fact only five significant combinations for this model, and that the observed areas are in reasonable agree- ment with prediction.Most of the discrepancy is almost certainly due to a difference in the recoilless fractions. Our I l l l l l l l l l l -12 -8 -4 0 4 8 12 velocity/mm s-' Fig,4 The Mossbauer spectra at various temperatures of SrCro,lFeo,902,5annealed in V~CUOat 1200°C and then step-cooled to 600°C. T/K: (a) 600, (b)570, (c) 550, (d)500, (e)400,(f)290, I I I I I I I -10 -9 -a -7 -6 -5 -4 -3 -2 velocity/mm s-' Fig. 5 Part of the Mossbauer spectrum at 290 K of SrCr,,lFeo,902~5 as shown in Fig. 4. The curve-fit shows the existence of three distinct octahedral and two tetrahedral iron sites produced by the ordering of the chromium onto the octahedral sites J.MATER. CHEM., 1991, VOL. 1 Table 2 The Mossbauer data analysis at 290 K for SrCr,., Fe,,, 02,5prepared by step-wise cooling in uucuo octahedral sites' tetrahedral sitesb no. Cr neighbours ideal probability observed area BIT no. Cr neighbours ideal probability observed area BIT 0 0.182 0.175 48.8 0 0.356 0.394 40.8 1 0.182 0.162 46.5 1 0.178 0.199 37.9 2 0.068 0.070 43.9 2 0.022 --3 0.01 1 4 0.001 "6=0.38, E= -0.35 mm s-l, r=0.43 mm s-'; bd=0.16, E= +0.30mm s-', r=0.43 mm s-'. own data on various brownmillerites suggest that the recoilless fraction is usually marginally greater at the tetrahedral site, and it is reasonable to assume that introducing a smaller atom in the octahedral layers may accentuate this difference.The temperature dependence of the spectrum is otherwise as expected for a solid solution system. The collapse seen immediately below the Neel temperature of 575 K is typical, and there is no suggestion of a second phase present. Above 575 K, the spectrum comprises a quadrupole doublet; the superimposed doublets from the two site symmetries are not resolved because of nearly identical quadrupole splittings. In conclusion, this sample represents a single-phase brown- millerite with ordering of the Cr onto the octahedral sites and a Neel temperature of 575 _+ 5 K. A second aliquot of material was annealed in uucuo at 1200°C in the same way, but was cooled quickly in the anticipation that this might preserve a cation-disordered form.The X-ray pattern was completely different, with intense broad lines in the correct place for a simple cubic perovskite. However, there were also many weaker and broader features. Close comparison with the pattern for the ordered brownmill- erite suggested that these features were indeed derived from the superlattice lines of the brownmillerite cell. A possible explanation is that the material contains domains of the brownmillerite lattice, which are small enough to cause aver- aging of the X-ray scattering, i.e. with a mean size of the order of 200A. This conclusion was reinforced by the Mossbauer characterization. The Mossbauer spectrum as a function of temperature is shown in Fig.6. The patterns strongly resemble the expected brownmillerite type, but with additional broadening and unsymmetrical lineshapes. It is difficult to determine the octahedral/tetrahedral area ratio accurately, but various com- putations suggest that they are approximately equal as would be expected for a cation-disordered structure. The temperature dependence is similar to that in Fig. 4, except that the Neel temperature has increased to 600+5 K. Once again, the paramagnetic phase shows only a single doublet without resolution of the two sites. There are no signs of extensive relaxation in the magnetic hyperfine spectra, as seen pre- viously9,' for the microdomains found in Sr2CoFe05 +y and Ca,LaFe,O, +y.One can deduce that the magnetic cation lattice is coherent over much greater distances than the X-ray lattice, which shows an averaging of the brownmillerite struc- ture over distances of the order of 200A. The possibility of magnetic coherence in microdomain materials containing oxygen vacancies and microdomains has been discussed pre- viously.' It would appear that the introduction of Cr into Sr2Fe205 has at last enabled the successful preservation of the high- temperature 'cubic' form at room temperature. The brownmill- erite cell is determined by the order of the oxygen vacancies along a [1lo] axis of the perovskite cell to give an orthorhom- bic supercell in which the cation layers normal to the b axis have their spins aligned along the c axis,2 although the Mossbauer spectrum is insensitive to any rotation of the spins I l l l l l l l l l l -12 -8 -L 0 4 8 12 velocity/mm s-' Fig.6 The Mossbauer spectra at various temperatures of SrCro,lFeo,902,5annealed in uucuo at 1200°C and then rapidly cooled. T/K: (a) 610, (b)600, (c) 580, (d) 500, (e)400,(f) 290, (g) 78 in the a-c plane. Microdomains can be produced by a periodic change in the choice of the [llO] axis for vacancy ordering, This will imply that there must also be a rotation of the spin axis across at least some of the domain boundaries associated with the effective rotation of the crystal axes, and will contrib-ute to the broadening seen in the spectrum. A study by electron diffraction should throw more light on the nature of the microdomain behaviour, and further work in this direction will be pursued.Preparations in Argon It is possible to prepare Sr2Fe205 under an argon atmosphere, and an attempt was made to obtain the equivalent Cr- substituted material in the same way by heating for 5 days at 1200°C under an argon stream before cooling by switching off the furnace. The product was, however, partially oxidized in accord with earlier experience at higher Cr concentrations. Nevertheless, the X-ray data showed a crystalline brownmiller- ite with the lattice parameters a =5.635(2)A, b =15.652(5)A, c =5.532(2)A, V=487.9(5) 81,. The b parameter is significantly larger than that for the in uucuo preparation, or indeed for the parent Sr2Fe20S, and this is convincingly shown by the substantial movement of the (080) reflection to lower angle.J. MATER. CHEM., 1991, VOL. 1 1 1 I 1 - 1 I I 1 I I I I -12 -8 -4 0 4 8 vetocity/rnrn s-' Fig. 7 The Mossbauer spectra at various temperatures of SrCro,lFeo,902,5,8annealed in flowing argon at 1200°C and then rapidly cooled. Note the reversal in the relative intensities of the outer lines compared to Fig.4 caused by the transfer of chromium from the octahedral to the tetrahedral sites layers. T/K: (a) 650, (h)630 (c) 62.5, (d)620, (e) 600, (-0500, (8)400, (h) 290, (i) 78 The Mossbauer data in Fig. 7 are again consistent with a brownmillerite structure, but with a further increase in ordering temperature to 630+ 5 K, and an apparent decrease in the fraction of tetrahedrally co-ordinated iron.There is no evidence to suggest that the extra oxygen is accommodated by producing a second phase as has been found in the parent system. Ignoring for the moment the question of the additional oxygen, various computed fits to the data at 78 and 290K give the fraction of tetrahedral sites to be only 0.43 compared to an ideal probability for complete order of Cr onto the tetrahedral sites of 0.444. Furthermore, there is evidence to suggest that there are at least two distinct types of octahedral site. At 78 K a fraction of ca. 0.38 of the iron occupies sites with a field of 52.4 T and parameters generally consistent with octahedral co-ordination in a brownmillerite, compared to a prediction of 0.356 of the iron with six Fe nearest neighbours.A further 0.20 of the iron has an average field of 53.6T but with a much smaller quadrupole perturbation, and presum- ably represents 0.178 of sites with one Cr neighbour on tetrahedral sites (plus 0.022 with two Cr neighbours). There appears to be less distinction between the 'tetrahedral' sites. There is no evidence for higher oxidation states of iron, and the excess oxygen corresponds approximately to that required for oxidation of chromium to the +4 state (a repeated preparation from new starting material gave y =0.446 and very similar spectra). The excess oxygen can be presumed to occupy the tetra- hedral layers, and be responsible for the increase in the length of the b axis.This is believed to be the first time that evidence has been found for the introduction of a significant excess of oxygen into the brownmillerite structure without forming a textured intergrowth or microdomain phase. The octahedral sites adjoining the Cr4+ sites presumably experience a signifi- cant change in electric field gradient, which is seen in the Mossbauer spectra. It is possible that an added oxygen ion tends to be situated between a pair of the smaller Cr4+ ions in 5-co-ordination, and this would leave the iron co-ordination in these tetrahedral layers unchanged. A similar preparation at 1000°C under argon showed a higher oxygen content, but was clearly a mixed phase with the introduction of a cubic perovskite phase.Effects of Order-Disorder on the Nkel Temperature Further support for these explanations can be gained by a closer examination of the critical temperatures for magnetic ordering. The Neel temperature of Sr,Fe,O, is 700K. The partial replacement of some of the iron by another cation will affect the exchange interactions and modify this temperature, usually depressing it significantly. The exchange interactions are essentially short range between nearest cation neighbours via the oxygen anions, and an increasing dilution of the iron with a diamagnetic cation will effectively begin to isolate some iron atoms completely from the long-range exchange. These effects have been expressed quantitatively by Gilleo,' and demonstrated in the related perovskite system EuFe03, for Co and Cr sub~titution.'~~'~ The Fe-0-Cr interaction is apparently rather weak compared with the Fe-0-Fe inter-action, and to a first approximation can be ignored as if it were a simple diamagnetic replacement.This may reflect the competition between ferromagnetic and antiferromagnetic coupling which is strongly dependent on the bond angle.18 The Sr2Fe205 lattice is more complex because of the two co- ordinations, but a similar treatment can be given to investigate the effects of ordering of the cations on these sites. For an arbitrary site occupation by Cr, the formula can be written as Sr,(Cr,Fe, -.Jo(Cr,Fel with an average of five inter- actions per magnetic cation.Following Gilleo, the probability that an ion of co-ordi- nation n is linked with rn magnetic ions when the occupation probability is x is given by n! -x)" (1)PAM)=rn! (n-rn)!x-(l The probability of linkage to only 0 or 1 Fe ions (which precludes magnetic coupling to the bulk material, assuming that the couplings to Cr are very weak) is E =nxn-l( 1-x) +xn and the proportion of Fe ions contributing to magnetism is only (1 -x) (1-E). The proportion of octahedral sites mag- netically active is thus No=(1-~)(1-6x5 +5x6) and the proportion of tetrahedral sites magnetically active is NT =(1-y)(1-4y3+3Y4) The total of 10 exchange pathways comprise four oct-O-oct, four oct-O--tet, and two tet-O-tet interactions.Thus, the number of active interactions is 4N02 +4NoNT+2NT2. The Neel temperature as a function of composition is thus given by 4N; +4N0NT+2N-f700_-TN = NO +NT 5 (2) If all the Cr cations are on octahedral sites (x=O.2, y=O) then TN=603 K, if they are completely random (x =0.1, y = 0.1) then TN=629 K, and if they are all on tetrahedral sites then TN=655 K. Thus simple theory predicts a variation of Neel temperature of some 50°C with change in site occupation. It is significant to note that the predicted magnitude and variation in the Nee1 temperature is in good agreement with the observations made on the three phases. This is not unreasonable since the magnetic exchange interactions are short range and dominated by the Fe-0-Fe linkages, making other factors such as coherent domain boundaries and Cr4+ defect clusters comparatively ineffective. Conclusions The primary result of this work is the observation that chromium can be incorporated into the Sr,Fe,O, lattice in three very different ways according to the experimental con- ditions.Annealing in mcuo at ca. 600°C to prevent oxidation and facilitate cation ordering produces a brownmillerite with chromium concentrated onto the octahedral sites. A similar preparation with rapid cooling from 1200°C results in a random site occupation and the stabilization of a high-temperature 'cubic' phase containing microdomains of brown-millerite with a coherent cation interface. Annealing in argon at 1200°C to allow a limited degree of oxidation introduces chromium and presumably excess oxygen into the tetrahedral layers and causes elongation of the b axis.Preparations allowing a higher degree of oxidation produce a cubic perovsk- ite with considerable vacancy and cation disorder, and the introduction of chromium seems to suppress the formation of the ordered vacancy phases which are known in the parent SrFeO, -y system. I am grateful to Mr. A. Hedley for chemical analyses, and to SERC for financial support. J. MATER. CHEM., 1991, VOL. 1 References 1 P. K. Gallagher, J. B. MacChesney and D. N. E. Buchanan, J. Chem. Phys., 1964, 41, 2429. 2 C. Greaves, A. J. Jacobson, B. C. Tofield and B. E. F. Fender, Acta Crystallogr., Sect. B, 1975, 31, 641.3 M. Harder and Hk. Muller-Buschbaum, Z. Anorg. Allg. Chem., 1980,464, 169. 4 T. C. Gibb, J. Chem. SOC., Dalton Trans., 1985, 1455. 5 Y. Takeda, K. Kanno, T. Takada, 0. Yamamoto, M. Takano, N. Nakayama and Y. Bando, J. Solid State Chem., 1986,63,237. 6 M. Takano, T. Okita, N. Nakayama, Y. Bando, Y. Takeda, 0. Yamamoto and J. B. Goodenough, J. Solid State Chem., 1988, 73, 140. 7 L. Fournes, Y. Potin, J. C. Grenier, G. Demazeau and M. Pouchard, Solid State Commun., 1987, 62, 239. 8 J. C. Grenier, N. Ea, M. Pouchard and P. Hagenmuller, J. Solid State Chem., 1985, 58, 243. 9 P. D. Battle, T. C. Gibb and S. Nixon, J. Solid State Chem., 1988, 73, 330. 10 P. D. Battle, T. C. Gibb and P. Lightfoot, J. Solid State Chem., 1988, 76, 334. 11 T. C. Gibb and M. Matsuo, J. Solid State Chem., 1990, 86, 164. 12 T. C. Gibb and M. Matsuo, J. Solid State Chem., 1990, in the press. 13 T. C. Gibb, J. Chem. SOC., Dalton Trans., 1983, 873. 14 T. C. Gibb, J. Chem. SOC.,Dalton Trans., 1983, 2031. 15 T. C. Gibb, J. Solid State Chem., 1988, 74, 176. 16 P. D. Battle, T. C. Gibb and S. Nixon, J. Solid State Chem., 1989, 86, 86. 17 M. A. Gilleo, J. Phys. Chem. Solids, 1960, 13, 33. 18 T. C. Gibb, J. Chem. SOC., Dalton Trans., 1984, 667. Paper 0/02644A; Received 13th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100023

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 29-35 Synthesis, Structure, and Spectroscopic and Electrochromic Properties of Bis(phthalocyaninato)zirconium(w)t Jack Silver,*" Peter Lukes," Stuart D. Howea and Brendan Howlinb a Department of Chemistry and Biological Chemistry, University of Essex, Wivenhoe Park, Colchester C04 3SQ, UK Department of Chemistry, University of Surrey, Guildford GU2 5XH, UK The crystal structure of the title Fompound [Zr(pc),] has been solved by X-ray diffraction techniques. The cell is triclinic, Pi; a= 13.410(2) A, b= 13.400(7) A, c= 16.340(11) A, a=68.68(1)", fi=65.92(1)", y=74.74(1)", R=0.039 and R'=0.058 for 6576 reflections. The infrared spectrum and details of the solvent dependence of the electronic absorption spectrum are also given. [Zr(pc),] has the most distorted rings of all the metal bisphthalocyanines so far reported.The distortion is primarily caused by the Zr-N distances (average 2.30 A), which though long for such Zr bonding, are very small when compared to other M-N bonds in similar structures. Cyclic voltammograms of [Zr(pc)J are presented and discussed. The distorted structure has a detrimental effect on the electrochromic properties of the molecule, aiding decomposition during oxidation. Keywords: Bisphthalocyanine; Zirconium; Crystal structure; Cyclic voltammetry Since their discovery by Kirin, Moskalev and co-workers,' the bis(phthalocyaninato)lanthanide(rn)complexes have gener- ated a great deal of interest. Their electrochromic properties are the main driving force for the sustained interest, and several different groups of workers have systematically explored these compounds.' -6 Electrochromism is the term describing the phenomenon of an electrochemically induced colour change in a material.We and others have achieved better than lo7 colour cycles in bis(phthalocyaninato)lanthanide(IrI) device^.'^.^ From the many papers on these materials that have appeared,'-' the present consensus of opinion is that the neutral green parent material can be formally written as [M(pc)(p~*)]~ (where pc= the phthalocyaninato 2- anion, and pc. is the phthalocyanin- ato 1-mono radical anion); the red oxidised form contains two radicals [M(pc.),]+ and the blue mono reduced species is [M(pc),]-. There is also a further reduced purple [M(pc),12- species though the exact nature of this is not yet clear.Several structures of metallo-bisphthalocyanine complexes have been characterised by X-ray diffraction studies. Main block,8 lanthanide'-' ' and actinide' 2,1303b metals have been found in such structures. There have been suggestions that the heavier transition metals may also form such complexes, but to date no such compounds have been fully character- ised.14 The known structures of all the metal bisphthalocyan- ines show the same type of non-perfect square antiprism geometry around the metal centre. However, there are some subtle differences both in the planarity of the phthalocyanine rings [they can both be dianion (pc), or one can be a dianion and the other a monoanion radical (pc.)] and in the fact that the metals may be either tri- or tetra-valent.In our e~perience'~ the Sn" material did not display all the colours derived in the lanthanide(r1r) materials,' though others have suggested that it does.5 Because of this, we investigated the electrochromic properties of bis(phtha1ocyani-nato)zirconium(rv) and, as for the Sn" compound, found t Supplementary data available from the Cambridge Crystallo- graphic Data Centre: see Information for Authors, J. Muter. Chem., 1991, Issue 1. only limited colour changes available; on oxidation the materials lost its colour.' We therefore undertook a structural investigation of bis(phthalocyaninato)zirconium(Iv) which is reported here along with spectroscopic studies and cyclic voltammetry on this material.We discuss the structure in relation to other similar structures and also in relation to its electrochromic properties. Experimenta1 We have reported the synthesis of bis(phtha1ocyaninato) zirconium(rv), [Zr(pc),], e1se~here.I~Crystals suitable for X-ray diffraction were grown in a tube furnace from crude material by passing the material several times through the furnace with a slow flow of nitrogen. We wish to add a cautionary note here, if one attempts to prepare [Zr(pc),] from zirconium acetate rather than ZrC1, as in the reaction of [LU(~C)~CH&~~],'~ then a Zr material which has an electronic spectrum similar to [Lu(pc)(OAc)( H ,O),] H,0.2CH ,C1, is always present as a major contaminant.This is difficult to remove. We believe this material probably contains [(pc)Zr(OAc),] but did not pursue it further. Its solution spectrum in CH2Cl, contains two equally intense peaks at 627 and 658 nm. X-Ray Structural Investigation Crystal Data: C64N16H32Zr,M = 1124.36, triclinic, a = 13.410(2)A, b= 13.400(7)A, c= 16.340(11)A, a=68.68(1)",fi= 65.92( ly, y =74.74( l)', V= 2474.4 A3 (by least-squares refine- ment of 25 automatically centred reflexions, ;L =0.71069 &), space group Pi,Z =2, D, =2.509 g cm -'. Approximate crystal dimensions 0.2 mm x 0.2 mm x 0.1 mm, F(000)= 1152, p(Mo-Ka)=2.78 1 cm -'. Data Collection and Processing:' Enraf-Nonius CAD4 diffractometer, omega/2@ mode with omega scan width = 0.80+0.35 tan@, omega scan speed 3.33 min-', graphite-monchromated Mo-Ka radiation; 8 124 unique reflexions mea- sured (1 <@/" <24) yielding 6576 with I >2.58a(I). Structure Analysis and Rejnement: Direct methods and Pat- terson (Zr atom) followed by normal heavy-atom methods.Full-matrix least-squares refinement with all non-hydrogen atoms anisotropic and hydrogens in calculated positions. The weighting scheme w =Lp/[a2(I) +(0.6 Z)2]* where I =raw intensity, gave satisfactory agreement analyses. No reflexions were omitted because of extinction. Final R and R’ values were 0.039 and 0.058, respectively. Programs and computers used and sources of scattering factor data are given in ref. (16). Atomic co-ordinates are given in Table 1.Cyclic Voltammetry An EG & G applied research model potentiostat, connected to a Philips PM8271 XYT recorder, was used to record cyclic voltammograms of thin films of [Zr(pc),]. The working electrode was a gold film deposited onto a glass slide which had previously been coated with a thin layer of chromium metal. All [Zr(pc),] films were deposited under vacuum (10- Torrt). Other working electrodes were indium- doped tin oxide (ITO) covered glass on which the [Zr(pc),] was sublimed. The counter electrode was a 1 cm2 Pt gauze, and a silver wire constituted the pseudo-reference electrode. The voltammetry was recorded in deionised water using 5% KC1 as the supporting electrolyte. Prior to recording the voltammetric data the solution was degassed, and an N2 atmosphere was maintained throughout the experiment.Infrared Spectroscopy The infrared spectrum of [Zr(pc)J was collected on material sublimed from single crystals directly onto KBr plates. It was also collected on a Perkin-Elmer 17 10 Fourier Transform infrared spectrometer from single crystals ground with KBr and pressed as a disc. Both spectra agreed with each other in band positions and intensities (see Table 2). Table 2 also contains the data for the [Sn(pc),] p-phase” which is very similar to the data of [Zr(pc),]. UV-VIS Spectroscopy We have previously shown the electrochromic absorption spectra of a thin film of [Zr(pc),] sublimed on indium-doped tin oxide covered glass (ITO) recorded by a Perkin-Elmer Lambda 5 spectrophotometer.15 Two Q bands are apparent at 702 and 637nm in the spectrum of the neutral complex, which may indicate the presence of two phases; however, no evidence for two phases was apparent in the infrared data discussed above.Also, on oxidation of the film both Q bands disappear together, suggesting that if more than one sublimed phase is present their electrochromic behaviour is similar. A better explanation of the appearance of the two bands in the Q region is that they arise from different vibrational com-ponents of the same transition.I8 This implies that the [Zr(p~)~]molecule is asymmetric about the xy plane. This finding is borne out by the crystallographic data. The electronic absorption spectrum of [Zr(pc),] shows solvent dependence; in chloronaphthalene bands are observed at 687.0 nm and a shoulder at 635 nm (relative intensities 4:l); in dimethylformamide bands shift to 678.0 and 613 nm (rel. int.4:1), in CH2Cl, the bands are at 684 and 618 nm (rel. int. 5:1). t 1 Torr ~133.322Pa. J. MATER. CHEM., 1991, VOL. 1 Table 1 Table of positional parameters and their estimated standard deviations“ atom X Y Z Bj A’ Zr 0.52206(2) 0.2045 3( 2) 0.74244( 2) 2.83 1 (6) N1A 0.6955( 2) 0.2400(2) 0.7 109( 2) 3.24(5) CIA 0.72 13(2) 0.3329(2) 0.7098( 2) 3.62(7) C7A 0.881 6( 2) 0.2518(3) 0.6261(2) 3.87(7) C8A 0.7931(2) 0.1873(2) 0.6632(2) 3.47(7) N2A 0.812I(2) 0.0889(2) 0.6559(2) 3.64(6) C9A 0.7390(2) 0.0214(2) 0.7077( 2) 3.32(7) ClOA 0.7679( 2) -0.0926(2) 0.7 145(2) 3.62(7) C11A 0.8610(3) -0.1509( 3) 0.665 l(2) 4.73(9) C12A 0.8620(3) -0.2608(3) 0.69 15(3) 5.7(I) C13A 0.7769( 3) -0.3114(3) 0.7645( 3) 5.6( 1) C14A 0.6858(3) -0.2533(2) 0.8136(2) 4.51(8) C15A 0.681 l(2) -0.1425(2) 0.7861(2) 3.5q7) C16A 0.5985(2) -0.0574(2) 0.8194(2) 3.19( 6) N3A 0.63 3 3( 2) 0.041 l(2) 0.7682(1) 3.15(5) N4A 0.5047( 2) -0.0783(2) 0.8902( 2) 3.32(5) C17A 0.4337(2) -0.0019( 2) 0.9250(2) 3.19(6) C18A 0.3343(2) -0.0260(2) 1.0053(2) 3.42( 6) C19A 0.2872(3) -0.121 8(3) 1.0547( 2) 4.26(8) C20A 0.1852(3) -0.1 145(3) I.1242(2) 4.8 2(9) C21A 0.1334(3) -0.0175(3) 1.145 l(2) 5.1(1) C22A 0.18 1 l(3) 0.0757( 3) 1.0980(2) 4.32(8) C23A 0.2830(3) 0.0700(2) 1.0263(2) 3.44( 7) C24A 0.3525(2) 0.1507(2) 0.9602(2) 3.24( 6) N5A 0.4399( 2) 0.1069( 2) 0.8956( 1) 3.08(5) N6A 0.3 343( 2) 0.2499(2) 0.9660(2) 3.41(5) C25A 0.4067( 2) 0.3191(2) 0.91 16(2) 3.3 3( 6) C26A 0.3960(2) 0.4201(2) 0.9288(2) 3.73(7) C27A 0.3159(3) 0.47 15(3) 0.993 1 (2) 4.41(8) C28A 0.3 350(3) 0.5688(3) 0.99 14(2) 5.14(9) C29A 0.4279(3) 0.61 37(3) 0.9292( 2) 5.30(9) C31A 0.4906( 2) 0.4659(2) 0.866 1(2) 3.70( 7) N7A 0.5007(2) 0.3056(2) 0.8376( 1) 3.16(5) N8A 0.6563( 2) 0.4062( 2) 0.7525(2) 3.73( 6) C32A 0.5560( 2) 0.39 15(2) 0.8 1 18(2) 3.42(6) C30A 0.5072(3) 0.564 1 (3) 0.8652(2) 4.65(8) C6A 0.9933(3) 0.234 l(3) 0.5728(3) 5.09(9) C5A 1.0564( 3) 0.3113(4) 0.5543(3) 6.3( 1) C4A 1.0125( 3) 0.4004(3) 0.5877( 3) 64 1) C3A 0.9025(3) 0.4 185(3) 0.6396(3) 5.7( 1) C2A 0.8366( 2) 0.34 1 7( 3) 0.6567(2) 4.19(8) N7B 0.421 l(2) 0.0995( 2) 0.7299( 1) 3.3 l(5) C1 B 0.2483(2) 0.2482( 3) 0.81 39( 2) 4.1 l(7) NIB 0.3440(2) 0.29 36( 2) 0.76 56( 2) 3.39(5) N5B 0.608 l(2) 0.1776(2) 0.5965(1) 3.28(5) C24B 0.6200(2) 0.0823(2) 0.5772(2) 3.57(7) C16B 0.6226(2) 0.3993(2) 0.5556( 2) 3.44(6) C8B 0.3 122(2) 0.4026(3) 0.7595(2) 3.86(7) N2B 0.3715(2) 0.48 19(2) 0.71 12(2) 3.90(6) C9B 0.4716(2) 0.4673(2) 0.6505(2) 3.46(6) N3B 0.5321(2) 0.37 17(2) 0.6339(2) 3.26(5) C15B 0.6245(2) 0.5154(2) 0.5250(2) 3.89( 7) C14B 0.6992( 3) 0.58 12( 3) 0.45 12( 2) 4.68(8) C13B 0.6743(3) 0.6906( 3) 0.4426( 2) 5.42(9) C12B 0.5 804( 3) 0.7331(3) 0.5025(2) 5.43( 9) CllB 0.5043( 3) 0.6686(3) 0.5750(2) 4.8 5(8) ClOB 0.5296( 2) 0.557 1( 2) 0.5857( 2) 3.85(7) N4B 0.6965( 2) 0.3 372( 2) 0.5056(2) 3.6 3( 6) C17B 0.6852(2) 0.2364(2) 0.5229( 2) 3.62( 7) C18B 0.753 3( 2) 0.1746(3) 0.4580(2) 3.99(7) C19B 0.8438(3) 0.197 l(3) 0.3752( 2) 5.1( 1) C20B 0.8890(3) 0.1193( 3) 0.3298(3) 5.9( 1) C21B 0.8469( 3) 0.0234( 3) 0.3618(3) 5.8( 1) C22B 0.7577(3) O.O006( 3) 0.4436(2) 5.07(9) C23B 0.7117(2) 0.0783(3) 0.49 13(2) 4.01(8) N6B 0.5581(2) 0.0040(2) 0.623 3( 2) 3.79(6) C25B 0.4651(2) 0.01 39( 2) 0.692 7( 2) 3.56( 6) C32B 0.3 145(2) 0.0826(2) 0.78 8 1 (2) 3.60(7) N8B 0.2348(2) 0.1486(2) 0.8294(2) 4.34(7) C31B 0.2944(2) -0.02 15( 3) 0.7699( 2) 4.23(7) C26B 0.3889(2) -0.0650(2) 0.7366(2) 4.05(7) C27B 0.3973(3) -0.1660(3) 0.7266(2) 5.41(9) C28B 0.3095(3) -0.2223(3) 0.7801(3) 641)C29B 0.2153(3) -0.1807(3) 0.84 13(2) 6.3( 1) J.MATER. CHEM., 1991, VOL. 1 Table 1 (continued) atom Y Y z B/A' C30B 0.2044(3) -0.0791(3) 0.8514(2) 5.40(9) C2B 0.1574(3) 0.3270(3) 0.8455(3) 5.16(9) C7B 0.1963( 3) 0.4231(3) 0.81 19(2) 4.97(9) C6B 0.1275(3) 0.5172(4) 0.8291(2) 7.q1)C5B 0.0192(4) 0.5093(4) 0.8829(5) 9.6(2)C4B -0.0209(4) 0.41 12(4) 0.9170(5) 10.4(2) C3B 0.0444(4) 0.3176(4) 0.9007(4) 7.7(2) a Anisotropically refined atoms are given in the form of the isotropic equivalent displacement parameter defined as 3[2aB( 1,l) + 2b B(2,2)+ 2cB(3,3)+ abB(1,2)cosy+ acB(1,3)cosP+ bcB(2,3)cosa] Table 2 Absorption frequencies mp4, P-Sn(Pch" relative relative frequency/cm-intensity frequency/cm-intensity 485 483 506 504 567 565 629 625 -640' 731 725 746 742 778 772 -796 815 811 8 70 866 894 890 950 946 1003 1002 1038 1041 -1059 1074 1075 1118 1119 1162 1166 -1209 1287 1287 -1321 1333 1335 1384 1383 1421 1425 -1450 1467 1477 1505 1508 1595 1550 1609 1613 Data from ref.(17). Relative intensities were not given but are estimated from Fig. 1 of that text. ' Not seen in Fig. 1 of ref. (17), but it is recorded in the text table. Results The structure of [Zr(pc),] is shown in Fig. 1, the bond lengths are given in Table 3 and the angles around the Zr atom in Table 4.To appreciate the structure it is best to first consider the deviations of the atoms from the mean pyrrole N, plane in the pc rings.It is apparent that these rings are very distorted and the overall structure resembles two back-to-back exagger- ated saucers. Table 5 gives a comparison of the distortions of the rings in this structure and those in some of the other known structures. Table 6 presents other major comparisons of the structures. As the Zr structure is the most distorted to date, we will refer to it henceforth as the 'wok-wok' structure. The Zr-N distances vary from 2.293(2) to 2.315(2) 8, with a mean distance of 2.30 A. This is smaller than those of the other known structures and presumably reflects the fact that Zr" is the smallest cation in eight co-ordination in the bis(phtha1ocyaninato) metal structures so far elucidated. This Fig.1 Structure of [Zr(pc),] with the numbering scheme used for the atoms in the molecule. The rings are indicated by the letters immedi- ately to their side is surprising in view of the conventionally quoted radii for Sn" and Zr"" and suggests that these do not hold for N-ligand co-ordination. The effect of the small Zr-N distances is that the pc rings are drawn in towards the Zr" cations, and the mean pyrrole N4 to pyrrole N4 plane distances (Table 7) are smallest in this structure.? This is the driving force behind the main buckling of the ring. Even though the two rings are staggered at an angle of 42", this close N4-N4 plane proximity means that the n clouds are interdigitating and add to the distortion of the rest of the pc rings from the N4 planes.In fact, it is only the outer six-membered carbon rings that are as far apart as would be expected for non-interpenetrating n clouds. In contrast, the rings of the other known structures, all of which have larger metals at their centre, are well separated at all atoms other than the inner N4 plane on the macrocycles. The staggering angle of 42" for the Zr" structure is the same as that found in the Sn" structure. For the other structures, the angle depends on the six-membered rings chosen between t We have calculated the mean distance between the N, planes of the two pc rings in [Zr( c),] on a graphics package using COSMIC. It was found to be 2.00 1.If we calculate the centroid of four nitrogen to centroid of four nitrogen distance using a simple geometric model we get a distance of 2.53 A.The centroid distance is larger owing to the uneven tilting of the nitrogens. Both these values compare favourably with the centroid-to-centroid distance of 2.20 A calculated using GEOSTAT. Hence, for the worst estimate the distance between planes is less than that found in the structure, at best it is significantly less than the tin. J. MATER. CHEM., 1991, VOL. 1 Table 3 Bond distanceslk atom 1 atom 2 distance atom 1 atom 2 distance atom 1 atom 2 distance Zr Zr Zr Zr Zr Zr Zr Zr N1A N1A C1A C1A C7A C7A C7A C8A N2A C9A C9A ClOA ClOA C11A C12A C13A C14A C15A C16A C16A N4A C17A C17A C18A C18A C19A C20A N1A N3A N5A N7A N7B N1B N5B N3B C1A C8A N8A C2A C8A C6A C2A N2A C9A ClOA N3A C11A C15A C12A C13A C14A C15A C16A N3A N4A C17A C18A N5A C19A C23A C20A C21A 2.308(3) 2.307(2) 2.305(2) 2.302(3) 2.301(3) 2.315(2) 2.304(2) 2.293(2) 1.369(5) 1.3 79( 3) 1.327(4) 1.444(4) 1.446(5) 1.399(4) 1.369(5) 1.3 15(4) 1.3 1 5( 4) 1.448(4) 1.38 1( 3) 1.393(4) 1.3 79(4) 1.374(5) 1.376(5) 1.3 77( 5) 1.377(4) 1.455(4) 1.365( 3) 1.328(3) 1.32 l(3) 1.447( 3) 1.373(4) 1.398(4) 1.38 l(4) 1.384(4) 1.393(5) C21A C22A C23A C24A C24A N6A C25A C25A C26A C26A C27A C28A C29A C31A C31A N7A N8A C6A C5A C4A C3A N7B N7B C1B C1B C1B N1B N5B N5B C24B C24B C16B C16B C16B C8B C22A C23A C24A N5A N6A C25A C26A N7A C27A C31A C28A C29A C30A C32A C30A C32A C32A C5A C4A C3A C2A C25B C32B N1B N8B C2B C8B C24B C17B C23B N6B N3B C15B N4B N2B 1.37 1( 5) 1.397(4) 1.436(4) 1.383(3) 1.3 19(4) 1.327(4) 1.440(5) 1.37q3) 1.39 5( 4) 1.3 89( 4) 1.38 3( 6) 1.365(5) 1.376(5) 1.449(4) 1.386(5) 1.376(4) 1.3 13( 3) 1.380(7) 1.383( 7) 1.3 76( 5) 1.409(6) 1.375(4) 1.380(3) 1.369(4) 1.3 13(5) 1.440(4) 1.388(4) 1.379(4) 1.365(3) 1.448( 3) 1.3 15(4) 1.368(3) 1.454(4) 1.32q4) 1.306(4) C8B N2B C9B C9B C15B C15B C14B C13B C12B C11B N4B C17B C18B C18B C19B C20B C21B C22B N6B C25B C32B C32B C31B C31B C26B C27B C28B C29B C2B C2B C7B C6B C5B C4B C7B C9B N3B ClOB C14B ClOB C13B C12B CllB C 10B C17B C18B C19B C23B C20B C21B C22B C23B C25B C26B N8B C31B C26B C30B C27B C28B C29B C30B C7B C3B C6B C5B C4B C3B 1.444(4)1.3 19( 3) 1.377( 4) 1.438(4) 1.394(4) 1.383(4) 1.382( 5) 1.375( 5) 1.3 87( 4) 1.404(5)1.3 1 2( 4) 1.448(4) 1.397(4) 1.376(5) 1.367(6) 1.377(6) 1.379(4) 1.389(5) 1.3 13(3) 1.448(4) 1.315(4) 1.437(5) 1.385(4) 1.400(5) 1.392( 5) 1.367(5) 1.382(5) 1.392(6) 1.356(6) 1.423(5) 1.397( 6) 1.367( 6) 1.386(8) 1.3 72( 7) " Numbers in parentheses are estimated standard deviations in the least significant digits.Table 4 Table of bond angles around Zr atom in degrees" Table 5 Comparison of displacements of the outermost carbon atoms ~~ ~~ ~~~~~ of each phenyl ring for some [M(pc),] and [M(pc)(pc.)] structures atom 1 atom 2 atom 3 angle least displaced most displaced refs. N1A Zr N3A 72.19(8) compound carbon atom/A carbon atom/A NlA Zr N5A 113.57(9) N1A Zr N7A 73.37(8) Ca-Sn(pc),I 0.18 1.01 8 N1A Zr N7B 146.42(8) [U(PC)21 0.49" 1.10" 12,13( b) NlA Zr N1B 139.21(9) CTh(PC),l 0.27 1.30 13(b)N1A Zr N5B 81.2q9) CLU(PCMPC.)I 0.25 1.10 10 N1A Zr N3B 76.42(8) CZr(pc),I 0.49 1.47 this work N3A Zr N5A 73.18( 7) N3A Zr N7A 114.16(9) " Calculated from data from the Cambridge database using COSMIC.N3A Zr N7B 81.42(9) N3A Zr N1B 146.85(8) N3A Zr N5B 75.96(8) which to measure it, though most of the angles lie between N3A Zr N3B 138.34( 7) N5A Zr N7A 71.98(8) 37 and 45". N5A Zr N7B 76.7q9) The distortions in the [Zr(pc)J structure are even noticeable N5A Zr N1B 81.49( 7) in the pc rings themselves (Table4). The atoms given in N5A Zr N5B 138.88(9) Table6 are defined as in the legend. The Niso-C, bond N5A Zr N3B 146.27(8) distances are similar to the other structures though theN7A Zr N7B 138.1 5( 7) 76.42(9) C,-C,, C,-C,, Ca-NiSO and Cphe-Cphe bond distancesN7A Zr N1B N7A Zr N5B 147.18(8) are all shorter.There are also some significant differences in N7A Zr N3B 81.23(8) the bond angles especially the C,-NiSo-C, and Nis0-C,-C, N7B Zr N1B 72.06( 9) angles. These findings are in keeping with the severe wok-like N7B Zr N5B 72.38(8) distortions found in the rings. N7B Zr N3B 1 13.60(9) It is worth noting at this point that, though this is the most NIB Zr N5B 112.99(9) distorted [M(pc),] structure to date, the pc rings are neutral, N1B Zr N3B 72.42(7) N5B Zr N3B 72.76(8) and not one electron oxidised, as found and discussed for [Lu(pc)(pc.)]lo. Thus ring distortion does not infer the pres- Numbers in parantheses are estimated standard deviations. ence of a pc radical; clearly large distortion can be a factor J.MATER. CHEM., 1991, VOL. 1 Table 6 Lengths and angles found in isoindole moieties of metallo-bisphthalocyanines a-Sn(pc)," U(PC)Zb U(PC)ZC Th(PC)Z' P-Nd(pcM~c*)~ LU(PCHPC*Y P-Zr(~c)z 1.375(6) 1.467(9) 1.387(8) 1.32 1 (7) 1.409(9) 108.1(7) 109.2( 4) 106.6(5) 128.7(5) 12 1.8( 6) 122.7(4) 115.3(6) 1 2 1.8( 7) 1.38(1) 1.46( 1) 1.4q 1) 1.32(1) 107.9(7) 109.4(7) 106.6(7) 127.6(7) 1 23.9( 7) 12 1.6(7) 116.8(7) 12 1 S(7) 1.40(1) 1.37 1.48 1.40 1.33 1.40 107.9 110.2 105.8 127.1 124.5 122.0 117.5 121.2 1.38 1.49 1.39 1.32 1.41 108.2 110.8 104.8 128.0 123.6 120.9 118.7 121.4 1.377(5) 1.472( 6) 1.398(8) 1.344(5) 1.40 1( 6) 107.5(4) 109.8( 3) 106.3( 3) 127.4(3) 123.2(9) 122.1(4) 116.2(4) 12 1.7(4) 1.376(3) 1.456(2) 1.390(4) 1.327(2) 1.389(3) 107.6(2) 109.6(2) 1 06.5( 2) 127.5( 3) 123.0(4) 12 1.2( 2) 117.3(3)12 1.2( 5) 1.3 75( 6) 1.445(5) 1.377( 7) 1.3 18( 5) 1.386(6) 106.1( 3) 11042) 106.5(4) 128.q5) 121.5( 3) 1 2 1.3( 5) 1 18.0(8) 12 1.8(4) " Ref.(8); ref. (12); 'ref. 13 (b); ref. 9(b); ref. (10). Numbers in parentheses are estimated standard derivatives; when not present, these are not given in the original papers. Niso=nitrogen atom of the isoindole groups; C, =a carbon atom of the isoindole group with respect to Niso; C, =P carbon atom of the isoindole group with respect to Niso;Cphe=phenyl carbon atom; N,= methine nitrogen atom. Table 7 Comparison of [M(pc),] and [M(pc)(pc)] structures mean N, plane to staggering average mean N4 plane angle of M-N bond Electrochromism and Cyclic Voltammetry We have previously described the colours and spectra ob- served on reduction of thin films of [Zr(pc),] on IT0 glass." The reaction that takes place is CZr(pc),l+ e --+ CZr(pc),l-cyan purple-red The spectral changes between neutral and reduced forms are reversible and more than lo3 colour cycles can be achieved.We have also reported that on oxidation an irreversible loss of colour of the film takes place, and we presented the spectra that show the colour loss." We now report some cyclic voltammograms that add to these data. The cyclic voltammograms for [Zr(p~)~] are presented in Fig. 2. The first cycle has two current peaks on the forward scan at -1.09 and -1.17 V, and one return wave at -0.75 is observed.On the second cycle a single current maximum is apparent at -1.05 V and it has a return wave at -0.69 V. A small maximum at -0.58 V can also be distinguished on the second forward scan, but this has no apparent return wave. The voltammograms show that the reduction of [Zr(pc),] occurs near the limit of the negative potential range of IT0 coated glass in aqueous solution. The very sharp peak on the first scan is typical of ion penetration of the film,23 though this is normally simultaneous with the reduction of the film material. An alternative expla- nation for the very sharp nature of this wave is that it represents a polarographic stripping wave which suggests the reduction of metal ions in the film. This type of wave, however, neither appears in the bare Cr/Au electrode nor is observed 001 mAI500 mV s-' / P +05 0 potential/\/ Fig.2 Cyclic voltammograms of a [Z~(pc)~] film on an Au electrode. (-) Initial scan showing ion penetration; (---) second cycle distance/A CTh(PC)Z1 2.96 CNd(pcHpc*)l 2.94 CU(PC),l 2.81 CLU(PC)(PC)l 2.69 Ca-Sn(Pc)zl 2.70 CZr(PC)zl 2.20" rings/" distance/A ref. 37 2.48 13" 38 2.47 9b 37 2.43 12 45 2.38 10 42 2.35 8 42 2.30 this work a Calculated from GEOSTAT. of structure alone. Here it is forced by the binding of the Zr atom. It is therefore obvious that in [M(pc),] and [M(pc)(pc.)] structures the bonding of the Nisoatoms to the metal must significantly distort the rings. There are not many Zr" structures known in which the Zr" cation is bonded to eight nitrogen atoms.In fact, in the Cambridge Crystallographic Database there are only two. One of these, tetraisothiocyanatobis(2,2'-bipyridine)zirconium(1~) [Zr(NCS)4(CloHsN2)2],20 has an 'average' Zr-N bond length of 2.297(4) 8, and is close to a D2d dodecahedron in structure. The other structure bis(2,2'-bi-2-2-thiazoline)tetrabis-(isothiocyanato) zirconium(1v) [Zr(NCS)4(C6H8N2S2)]21 has an average Zr-N bond length of 2.284(5) 8, and is intermedi- ate between an ideal dodecahedron and an ideal square antiprism. In both these structures each Zr" cation is sur- rounded by four monodentate and two bidentate ligands. The Zr" cation can thus dictate its geometry much more freely in these complexes than it can in [Zr(pc),].So it would appear that it is the presence of the two pc ligands that forces a square antiprismatic structure on the Zr" cation, and usually this early heavy transition metal much prefers dodecahedra1 co-ordination. The latter is particularly true for Zr" in non- nitrogen atom or partial nitrogen eight co-ordination. From Table 2 it is clear that our [Zr(pc),] is equivalent to the /3 phase of [Sn(pc),].17 It is clear that the [Zr(pc),] is not isostructural with c~-[Sn(pc)].~ These findings are in agreement with the fact that our crystals were grown by sublimation techniques. We have not compared this structure to those of the reduced lutetium phthalocyanines reported by Moussavi et a[.," as the latter are said to be influenced by the presence of the cation.We note that the protonated structure is very similar to that of the [Lu(pc)(pc.)] compound discussed here. in other phthalocyanine films deposited by vacuum subli-mati~n.~~ The voltammogram obtained by continuously cycling a [Zr(pc),]-coated Au electrode shows the same general features as seen in the second scan in Fig. 2. It was found that the features of the second scan remain, and that the current passed by the electrode falls continuously as the number of cycles increases. This is typical of observations made on similar materials23 and is due to the swelling of the films during the electrochromic process once ion penetration has been initiated. Visually, the electrochromic process continues throughout the cycling.On films on IT0 glass the cyclic voltammograms show similar behaviour in the same voltage range. On the first cycle ion penetration is again observed as a peak at -1.40 V and there is a broad return wave centred at -0.65 V. On the second scan reduction is observed at -1.2 V and the return wave is again broad. The greatest current loss is between the first and second cycle which is further evidence for the assignment of the first peak to ion penetrati~n.~~ If the voltage range is increased to +1.2 to -1.5 V (Fig. 3), then several new peaks are seen on the voltammogram. The peaks indicating reduction and reoxidation of the film to neutral are in keeping with those described previously. One new current peak is observed on the forward scan at f1.00 V and two peaks are seen on the reverse scan at -0.68 and +0.12 V.The peak at +1.O V is indicative of oxidation of the [Zr(p~)~]film and the two return peaks are reduction to the neutral state. Upon repeated cycling there is a rapid decrease in the current of both oxidation and reduction. On oxidation, the film goes first colourless, then a faint red is observed, which in turn returns colourless and then cyan- blue as the cycle continues towards neutral voltage. We have shown that if the films are cycled between +1,2 and -1.2 V more than about 30 times, they fade completely. We believe that the fading is due to the pc rings being chemically degraded (possibly by ring opening) on oxidation.We have suggested15 that the presence of MIv ions destabilises the pc rings owing to their polarising power (in the case of lanthanide ions many cycles can be achieved"). This explanation would be ia keeping with the highly distorted ring structure found for CZr(pc)*l.The exact nature of the destruction has not been elucidated but it is probably due to attack on the nitrogen bridges of the pc rings by OH-ions as has been previously suggested for [Lu(pc)(pc.)] films.' The very distorted [Zr(pc),?] molecules are more open to OH-ion attack at these positions than the less distorted [Lu(pc)(pc.)]. Fig. 3 Cyclic voltammogram of [Zr(pc),] film on ITO-coated glass. Reduction and oxidation peaks can be seen; there is a rapid decrease in current upon repeated cycling J.MATER. CHEM., 1991, VOL. 1 All the values quoted on IT0 were for a scan rate of 200 mV s-';peak positions varied at other scan rates. Collins and Schiffrin' have described peak-current dependence on sweep-rate for [Lu(pc)(pc.)] for both oxidation and reduction sweeps. They suggest that the shift in the voltammograms with change of scan rate is due to uncompensated cell resistance. We agree with their interpretation that the films behave as potential-dependent capacitors. In addition, such effects will be dependent on film thickness. The thickness of the films used were measured using a Tencor a-step 100 step height measuring device. All our films were between 1000 and 3000 8, thick.Conclusions The [Zr(pc),] structure contains the most distorted pc rings yet reported. The extreme wok-like ring distortions found in this compound are caused by their binding to the small Zr" cation. This cation has a large polarising power which causes the complex to be destroyed when it is oxidised. The mechan- ism of ring opening on oxidation probably involves OH-ions reacting with the exposed nitrogen bridges at the periph- ery of the pc rings.' We have found no evidence for the involvement of the metal in the electrochemical processes. J. S. thanks the British Technology Group for their continued support in this area and for studentships to P.L. and S.D.H. References (a) 1. S. Kirin, P. N. Moskalev and Yu A. Makaskov, Russ. J Inorg.Chem. (Engl. Transl.), 1965, 10, 1065; (b) Comprehensive list of relevant references by the above authors included in ref. l(c); (c) C. S. Frampton, J. M. O'Connor, J. Peterson and J. Silver, Displays, Techn. Appl., 1988, 9, 174. (a) M. M. Nicholson and F. A. Pizzarello, J. Electrochem. SOC., 1979, 126, 1490; (b) 1980, 127, 821; (c) 1980, 127, 2617; (d) 1981, 128, 1288; (e) M. M. Nicholson and T. P. Weismuller, J. Electrochem. Soc., 1984, 131, 231 1. A. G. Macklay, J. F. Boas and H. J. Troup, Aust. J. Chem., 1974, 27, 955. S. C. Dahlberg, C. S. Reinganum, C. Lundgren and C. E. Ric, J. Electrochem. SOC., 1981, 128, 2150. G. C. S. Collins and D. J. Schiffrin, J. Electroanal. Chem., 1982, 139, 335. A. T. Chang and J. C. Marchon, Inorg.Chim. Actu, 1983, 53, L241. M. T. Rion and C. Clarisse, J. Electroanal. Chem. Interface. Electrochem., 1988, 249, 18 1. W. E. Bennett, D. E. Broberg and N. C. Baenziger, Inorg. Chem., 1973, 12, 4. (a)K. Kasuga, M. Tsutsui, R. C. Petterson, K. Tatsumi, N. Van Opdenbosch, H. Pepe and E. F. Meyer Jr., J. Am. Chem. SOC., 1980, 102, 4836; (b) A. N. Darovskikh, A. K. Tsytsenko, 0. V. Frank-Kamenskaya, V. S. Fundamenskii and P. N. Moskalev, Sol;. Phys. Crystallogr., 1984, 29, 273. A. de Cian, M. Moussavi, J. Fischer and R. Weiss, Inorg. Chem., 1985, 24, 3162. A. N. Darovskikh, 0.V. Frank-Kamentskaya, V. S. Fundamen-ski and A. M. Golubev, Kristallografiya, 1988, 31, 279. A. Gieren and W. Hoppe, J. Chem. Soc., Chem. Commun., 1971, 413. (a) A. N. Darovskikh, 0. V. Frank-Kamentskaya, V. S. Funda-menskii and 0. A. Golynskaya, Kristallograjya, 1985, 30, 1085; (b) I. S. Kirin, A. B. Kolyadin and A. A. Lychev, Zh. Struk. Khim., 1974, 15, 486. L. G. Tomilow, N. A. Ovchinnikova and E. A. Luk'yanets, Zh. Obschch. Khim., 1987, 57, 2100. J. Silver, P. J. Lukes, P. K. Hey and J. M. O'Connor, Polyhedron, 1989, 8, 1631. S. D. P. Plus program suite, version 1.1, B. A. Frenz and Associates Inc., College Station, Texas, USA. W. J. Kroenke and M. E. Kenny, Inorg. Chem., 1964, 3, 696. J. MATER. CHEM., 1991, VOL. 1 35 18 19 20 M. Gouterman, in The Porphyrins, ed. D. Dolphin, Academic Press, New York, 1978, vol. 3, pp. 1-165. R. D. Shannon, Acta Crystallogr., Sect. A, 1976, 32, 751. E. J. Peterson, R. B. Van Dreel and T. M. Brown, Znorg. Chem., 1976, 15, 309. 22 23 24 M. Moussavi, A. de Cian, J. Fischer and R. Weiss, Znorg. Chem., 1988, 27, 1287. H. Abruna, Coord. Chem. Rev., 1988,86, 135. J. Silver, P. Lukes and S. D. Howe, unpublished results. 21 R. A. Johnson, R. B. Van Dreel and T. M. Brown, Inorg. Chem., 1984, 23, 4302. Paper 0/02753G; Received 20th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100029

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 37-41 Phase Transition of Tetrakis(octy1thio)tetrathiafulvalene (TTC,-TTF) and Crystal Structure Analysist Chikako Nakano," Kenichi Imaeda," Takehiko Mori," Yusei Maruyama," Hiroo Inokuchi," Naoko lwasawab and Gunji Saito" a Institute for Molecular Science, Myodaiji, Okazaki 444, Japan Institute for Solid State Physics, University of Tokyo, Roppongi, Tokyo 106, Japan " Department of Chemistry, Faculty of Science, Kyoto University, Sakyo-Ku, Kyoto 606, Japan Tetrakis(octy1thio)tetrathiafulvalene (TTC,-TTF) crystallizes into two different phases (Forms I and 11). Form I undergoes a slightly endothermic, phase transition at ca. 33°C with a steep increase in the resistivity and a decrease in the drift mobility. Powder X-ray diffraction confirms that this transition is the result of a structural transformation from Form I to Form 11.The microscopic origin of the phase transition is ascribed to a subtle repulsive motion in the nearest-neighbour octyl chains which results in the decrease in the transfer integrals between the neighbouring n-electron moieties of TTC,-TTF molecules within a segregated column. Keywords: Phase transition; Tetrathiafulvalene; Resistivity jump; Organic semiconductor; Crystal structure The long-chain compounds of tetrakis(alky1thio)tetrathiaful-valenes (TTC,-TTF) are unique organic semiconductors with noticeably low resistivities (for n>8 ca. lo5 S2 cm).'Y2 In these compounds, an electron-rich n moiety, tetrathio-TTF (C&) skeleton, is linked to four polyethene-like chains and the specific packing of the alkyl chains in the crystal is supposed to play an important role in the novel electrical properties.It is well known that long-chain paraffins exhibit rotational phase transitions near their melting p~int.~,~ These transitions originate from a change from the monoclinic phase (even carbons) or the orthorhombic phase (odd carbons) to the hexagonal phase. In TTC,-TTF (n>8), the presence of solid- solid phase transitions just below the melting point was confirmed by DSC experiment^.^ Since one end of each alkyl chain of TTC,-TTF is bound to the tetrathio-TTF (C,S,) skeleton, the character of the transition is probably somewhat different from that of n-paraffins. In this paper, we report the preparation of two kinds of single crystals of TTC8-TTF (molecular structures shown in Fig.1) and the crystal structure analysis of these two forms. The phase transition due to the subtle movement of the long alkyl chains was found in Form I to accompany substantial changes in the solid-state electronic properties. Experimental The synthesis of TTC8-TTF was reported in the preceding paper.6 Two kinds of orange crystals were crystallized from a mixed solvent of hexane and ethanol (the composition is about 1:l) by slow cooling and evaporation. Plate crystals (Form I) were grown when the temperature of the solution was maintained below -5"C, while the needle crystals (Form 11) were obtained from solution above 0°C.Crystal data and the conditions of data collection for both phases are given in Table 1. The reflection data were collected on an Enraf-Nonius CAD4 diffractometer with graphite- monochromatized Cu-Kx radiation (2 <20/"<120) by the w-20 scan technique. The measurement for the needle phase was performed at 24"C, and that for the plate phase was at OT, since in this case, at room temperature, all the Bragg t Supplementary data available from the Cambridge Crystallo- graphic Data Centre: see Information for Authors, J. Mater. Chem., 1991, Issue 1. Fig. 1 Molecular structure of TTC,-TTF. (a) Form I; (b)Form I1 peaks were considerably broadened to make the structure refinement difficult. The crystal structures were solved by the direct method using MULTAN 82* and, after absorption correction, refined by a block-diagonal least-squares pro- cedure {w = [o:+(O.0l5FJ2]-'>.All non-hydrogen atoms were refined anisotropically. Since the intensity of reflections was decreased by X-ray damage during the measurement, the R factor of the needle crystal was 0.131 in spite of the decay correction. (This R factor for Form I1 is quite large, and could be due to the disorder of the octyl chains.) The electrical conductivity was measured by a two-probe Table 1 Crystal data and conditions of data collection Form I Form I1 chemical formula C38H68S8 molecular weight 781.47 shape plate needle colour orange orange space roup PI Pi a1x 7.8443 (6) 8.794(1) blA 28.095(3) 25.990 (7) CIA 5.115(1) 5.141 (2) 4" 94.08(1) 92.37(3) Pl" 101.72(1) 103.39 (2) Yl" 89.523 (7) 97.03(2)uiA3 1101 (3) 1131 (8) z 1 1 DJg cm-3 1.179 1.147 p (Cu-Ka)cm-' 38.74 37.72 range of h, k, 1 -8<h<8 -9<h<9 -31<k<31 -29<k<29 0<1<5 OGlG5 reflections measured 3638 3789 reflections used 3069 2020 ( IF,I > 34~~1)R 0.035 0.131 Rw 0.059 0.137 crystal size/mm 0.3x 0.05 x 0.4 0.05 x 0.03x 0.4 temperature/"C 0 24 method along the stacking axis under a vacuum of ca.Pa to avoid oxidation. The powder X-ray diffraction patterns were measured by a Rigaku RAD-IIC diffractometer at room temperature. Thermal properties were investigated with a Du Pont 9900 differential scanning calorimeter (DSC) between room temperature and T, +15°C (T, =melting point) on heating at 2°C min-'.Results and Discussion Final atomic coordinates of the TTC,-TTF plate crystal (Form I) and the needle crystal (Form 11) are shown in Tables 2 and 3; the bond distances and angles for both crystals are given in Tables 4 and 5. Fig. 1 shows the molecular structures and the numbering scheme of the atoms. The shape of the Table 2 Final fractional atomic coordinates (x lo4) of the TTC8-TTF crystal (Form I) ~~ atom X Y Z s1 832 (1) 489 (1) 3564 (1)s2 -2648 (1) 197(1) 450 (1) s3 -571 (1) 1178 (1) 7303 (1) s4 -4549 (1) 809 (1) 3964 (1) c1 -373 (2) 145 (1) 815 (4) c2 -987 (2) 763 (1) 4533 (4)c3 -2566 (2) 625 (1) 3141 (4) c4 442 (3) 1657 (1) 5914 (5) c5 569 (3) 2092 (1) 7867 (5)C6 1392 (3) 2523 (1) 6936 (5)c7 1430(3) 2962 (1) 8862 (5)C8 2216 (4) 3403 (I) 8001 (6) c9 2179 (4) 3839 (1) 9905 (6)c10 2917 (5) 4286 (1) 9040 (7) c11 2846 (6) 4719 (1) 10963 (9) c12 -5572(3) 1097 (1) 944 (4) C13 -4632 (3) 1546 (1) 474 (4) C14 -4582 (3) 1954(1) 2613 (5) C15 -3758 (3) 2404 (1) 1899 (5)C16 -3730 (3) 2828 (1) 3908 (5)C17 -2932 (4) 3275 (1) 31 16 (6) C18 -2934 (4) 3709 (1) 5045 (7)C19 -2154 (5) 4147 (1) 4177 (9) J.MATER. CHEM., 1991,VOL 1 Table 3 Final fractional atomic coordinates (x lo4) of the TTC8-TTF crystal (Form 11) s1 5751 (4) 814 (1) 1474 (6) s2 3259 (3) 40 (1) 2414 (5)s3 4911 (5) 1676 (1) 4617 (6) s4 2099 (4) 776 (1) 5912 (5)c1 4773 (12) 180 (4) 770(1 8)c2 4511 (13) 1026 (5) 3358 (21) c3 3358 (13) 670 (5) 3795 (19) c4 4333 (25) 1999 (6) 1533 (32)c5 4297 (25) 2563 (7) 2103 (29) C6 3741 (26) 2839 (7) -441 (33)c7 3712 (29) 3397 (8) 118 (38) C8 3133 (31) 3674 (8) -2445 (42) c9 3160 (41) 4248 (10) -1875 (52) c10 2576 (45) 4524(11) -4299 (70) c11 2520 (58) 5090 (1 2) -3888 (93) c12 195 (14) 650 (5) 3415 (28) C13 -31 (14) 1066 (5) 1394 (24) C14 -189 (17) 1595 (5) 2674 (27) C15 -580 (20) 1981 (6) 418 (29) C16 -767 (23) 2512 (6) 1630 (33)C17 -1201 (26) 2878 (7) -702 (37) C18 -1362 (33) 3414 (8) 496 (49) C19 -1841 (37) 3780 (10) -1756 (60) molecules in both forms is chair-like and centrosymmetric. The four octyl chains in Form I elongate almost parallel to each other, while one chain is bent and a little apart from the neighbouring chain in Form 11.Crystal structures of the two forms are shown in Fig. 2. The stacking of the TTC8- TTF molecules along the c axis is uniform and similar in both forms. The differences are in the direction of the four alkyl chains and in the angle between the c6sg plane and the stacking axis. Furthermore, there exist significantly short intercolumnar S---S contacts [the shortest distance is 3.7008 (8) 8,] along the a axis in Form I, but there is no such short intercolumnar S---S contact in Form 11. These differences lead to characteristic differences in the structural and physical aspects of the forms, and in their transport properties. Even though the central c6s8 groups in both forms are completely planar, the distances between the least-squares planes of the c6sg parts along the stacking axis are substantially different, i.e. 3.43 8, (Form I) and 3.50 8, (Form 11).This difference is significant even if we allow for a thermal shrinkage of 0.02 8, in this temperature range., Moreoever, the molecular packing should be looser in Form I1 because of its larger unit cell volume [the volume at room temperature of each crystal is 1108 A3 (Form I) and 1131 A3 (Form II)].The intermolecular overlap in Form I is larger than in Form I1 (Fig. 3). Accordingly, the calculated overlap integrals based on the extended Hiickel molecular orbitalsg between the HOMOS of TTC,-TTF in the plate crystal are ca. 1.3 times larger than those in the needle crystal (-4.89 x 10-and -3.66 x 10-', respectively).This fact may be due to the smaller interplanar distance in Form I compared with Form 11. The existence of the phase transition is clearly shown in the DSC thermogram (Fig. 4). A small endothermic signal was observed at 33.7"Cas an extrapolated onset. The heat of transition is evaluated to be 6.7 kJ mol-', which is much smaller than the heat of fusion of 96.9 kJ mol-' at 48.1"C (m.p.), and it is rather small compared with the heat of transition of n-paraffins, 29.3 kJ mol- (n-C33H6,).'0Although the external shape of the heated crystals of Form I did not change, the colour became slightly darker. In order to make clear the nature of the phase transition, we tried to apply X-ray diffraction analysis for the crystal after the J.MATER. CHEM., 1991, VOL 1 39 Table4 Bond lengths/h and angles/" for the TTC,-TTF crystal (Form I) s1-Cl 1.757 (2) C6-C7 1.520 (3) Sl-C2 1.757 (2) C7-C8 1.521 (4) s2-c1 1.762 (2) C8-C9 1.512 (4) S2-C3 1.753 (2) C9-C10 1.518 (4) S3-C2 1.747 (2) CIO-c11 1.518 (5) s3-c4 1.826 (2) C12-C13 1.526 (3) s4-c3 1.756 (2) C13-C 14 1.521 (3) S4-C 12 1.829 (2) C14-Cl5 1.526 (3) Cl-Cl* 1.341 (3) C 15-C 16 1.514 (3) C2-C3 1.343 (2) C16-C17 1.523 (4) c4-c5 1.513 (3) C 17-C18 1.513 (4) C5-C6 1.525 (3) C18-Cl9 1.512 (5) symmetry operation: *(-x, -y, -z) c1-s1 -c2 95.33 (8) C5-C6-C7 112.3 (2) Cl-S2-C3 95.34 (8) C6-C7-C8 114.4 (2) Fig. 2 Crystal structures of TTC8-TTF. (a)Form I; (b)Form I1 c2-s3-c4 100.4 (1) C7-C8-C9 113.1 (3) c3-s4-c 12 101.0 (1) C8-C9-C10 113.9 (3) s1-c1 -s2 114.4 (1) c9-c1o-c11 112.9 (3) Sl-C2-S3 116.7 (1) s4-c 12-c 13 114.2 (1) SI-C2-C3 117.4 (1) c12-c13-c14 114.9 (2) s3-c2-c3 125.9 (2) c13-c14-c15 112.0 (2) s2-c3-s4 117.63 (9) C14-Cl5-Cl6 114.5 (2) s2-c3-c2 117.4 (2) C15-C 16-C17 113.4 (2) s4-c3-c2 124.8 (2) C 16-Cl7-CI 8 114.6 (3) s3-c4-c5 107.8 (2) C17-C18-C 19 113.1 (3) C4-C5-C6 113.2 (2) @:S Table 5 Bond lengths/A and angles/" for the TTC8-TTF crystal (Form 11) 0:c s1-Cl 1.75 (1) C6-C7 1.47 (3) Sl-C2 1.74 (1) C7-C8 1.54 (3) s2-c1 1.75 (1) C8-C9 1.51 (3) S2-C3 1.74 (1) C9-C10 1.48 (4) S3-C2 1.75 (1) c10-c11 1.49 (4) s3---c4 1.82 (2) C12-Cl3 1.53 (2) s4-c3 1.76 (1) C13-C 14 1.54 (2) S4-C 12 1.84 (1) C14-Cl5 1.57 (2) c1--c1* 1.36 (2) C15-Cl6 1.53 (2) C2-C3 1.35 (2) C16-C17 1.57 (3) c4-c5 1.49 (2) C17-C18 1.54 (3) C5-C6 1.53 (2) C18-Cl9 1.55 (4) symmetry operation: *( 1-x, -y, -z) CI-Sl-C2 95.8 (6) C5-C6-C7 112.5 (14) Fig.3 Intermolecular overlaps along the stacking axis. (a) Form I; c1-s2-c3 95.9 (5) C6-C7-C8 112.5 (16) (b)Form I1 c2-s3-c4 100.4 (6) C7-C8-C9 112.3 (18) 1c3-s4-c 12 98.9 (6) C8-C9-C 10 113.7 (21) s1-Cl -s2 114.0 (6) c9-c 10-c 11 116.9 (29) T, = 33.7 "Cs1-c2-s3 116.9 (7) s4-c 12-c 13 113.1 (8) Sl-C2-C3 117.2 (9) C12-Cl3-Cl4 112.8 (1 1) s3-c2-c3 125.8 (10) C13-Cl4-Cl5 109.2 (1 1) s2-c3-s4 117.7 (7) C14-Cl5-Cl6 110.5 (12) s2-c3-c2 116.7 (10) C15-Cl6-Cl7 108.6 (14) s4-c3--c2 125.2 (10) C16-Cl7-Cl8 109.0 (16) s3-c4-c5 111.3 (11) C17-Cl8-Cl9 1 10.4 (20) C4-C5-C6 112.5 (13) transition.However, it was almost impossible to gather the 30 35 40 45 50,single-crystal reflections because of the poor single-crystal 5 quality after heating. Therefore, we carried out powder X-ray TI"C diffraction measurements, the results for which are shown in Fig. 5 together with those of each pristine crystal. The Fig. 4 DSC thermogram of the TTC8-TTF (Form I) diffraction patterns of the pristine crystals of Form I and I1 are drawn in Fig. 5 (a)and 5 (b),respectively. All the reflections transition. The octyl chains in Form I should be more closely can be indexed by using the cell parameters of the single- packed than in Form 11, because the thermal vibrations are crystal data.Fig. 5 (c)shows the pattern of Form I at room somewhat suppressed in the low-temperature crystallization. temperature after heating at 40°C for 30 min. This pattern From this point of view, Fig. 6 (projection along the octyl exactly corresponds to that of the pristine needle crystal of chain direction) is helpful to provide insight into the phase Form 11. This means that the transition is caused by a transition mechanism. The four octyl chains in the TTC8-structure transformation from Form I to Form 11. TTF molecule [Fig. 6 (a)] have a regular orientation. Each Now let us turn to the structural origin of this phase octyl chain interacts with the intercolumnar one in the a+c 10.0 20.0 30.0 40.0 50.0 2810 Fig. 5 Powder X-ray diffraction patterns of TTC,-TTF. (a) Form I; (b) Form I1 (x 10).(c) the crystals (Form I) heated at 40°C for 30 min direction (marked by an arrow), where there exist many short C---C separations of ca. 4 8, [shortest distance=4.019(3) A]. The shortest H- --H contact in this direction is 2.52 8, which is about twice the hydrogen van der Waals radius (1.25 8,) in n-alkanes (C33H68).11*12 When the crystal is heated, the thermal vibrations become large and the repulsive force between the hydrogen atoms occurs in the nearest-neighbour octyl chains. Consequently, these chains in Form I1 tend to separate from one another to avoid collision [Fig. 6 (b)].The intercolumnar C---C distances are larger than those in Form I [the shortest distance is 4.25(4) A], since one octyl chain is a little bent.Simultaneously the c6s8 skeletons may follow the motion of the octyl chains. In this way, Form I transforms to Form I1 triggered by the octyl chain motion to looser packing. The microscopic structural analysis mentioned above is compatible with the fact that the electrical conductivity of Form I1 is lower than that of Form I (Table 6). In fact, the room-temperature resistivity and the activation energy of Form I are smaller than those of Form 11: 1.2 x lo5 R cm and 0.18 eVt uersus 7.0 x lo7 R cm and 0.21 eV, respectively. Although Form I1 did not exhibit any phase transition up to 48.1"C(melting point), Form I underwent a phase transition at ca. 33"C, as shown in Fig. 7. The resistivity increased by ca.10' to become nearly the same as the resistivity of Form 11. This phase transition was irreversible; after the transition, repeated cooling and heating runs of the resistivity measure- ment followed the line of the high resistivity phase. The results -f 1 eV~1.602~10-'~J J. MATER. CHEM., 1991, VOL 1 Fig. 6 Crystal structures of TTC,-TTF projected along the octyl chain direction. The shaded chains elongate downwards, and the non- shaded chains upwards. The arrows indicate the direction of the shortest intercolumnar C- --C contacts. (a) Form I; (b)Form I1 Table 6 Differences between Forms I and I1 Form I Form I1 shortest intercolumnar shortest intercolumnar C---C contact/A 4.019 (3) 3.7008 (8) 4.25 (4) - S---S contact/A interplanar distance/A electrical resistivity/Q cm activation energy/eV hole mobility/cm2 V-' s-l 3.4344(3) 1.2 x 105 0.18 6.4 3.495(3) 7.0 j, 107 0.21 0.77 of the DSC measurements show that Form I is the stable phase at room temperature, so we should consider that the reverse transformation could be kinetically hindered.The resistivity change accompanying this transition could be elucidated as follows. The significant change in the inter- molecular overlap integrals at the transition may give rise to a change in the electronic bandwidth. This effect may result in changes in the bandgap energy and the charge-carrier mobility. Actually, the activation energy for the electrical conduction in the high-temperature phase (deduced from Fig.7) is 0.28 eV, while that in the low-temperature phase is J. MATER. CHEM., 1991, VOL 1 T/"C 50 40 308 -7 E G P v P, 0-6 0 0 00 OO-ooaooooooooo 5 3.1 3.2 3.3 3.4 103 KIT Fig. 7 Temperature dependence of the electrical resistivity along the c axis for TTC,-TTF (Form I). The temperature of 48°C at which the resistivity suddenly increases, corresponds to the melting point (Tm) 0.16 eV. Also, the drift mobility measurements, indicate that the mobilities change abruptly around the phase-transition temperature; the hole mobilities are 6.4 cm2V-'s-' at 22°C and 0.77cm2V-'s-' at 45"C.I3Taking the changes in acti- vation energy and mobility into account, we can quantitatively elucidate the resistivity jump at the phase transition.For TTC,-TTF and TTC1,-TTF, which are isostructural with TTC,-TTF (Form I), there occur phase transitions at 53 and 56"C, respectively, accompanied by changes in the electrical resistivity.* The transition temperature shifts to the high-temperature side with increasing length of alkyl chain. This fact supports the above-mentioned conclusion that the phase transition is due to the motion of the alkyl chains. In summary, the long alkyl chain compounds of TTC,- TTFs exhibited phase transitions just below their melting points. A similar phase transition is known in the long-chain n-paraffins, in which the crystal structure changes to a different crystal system since the whole chain can rotate freely. In the TTC,-TTFs, these free rotations are prohibited because one end of the alkyl chain is fixed to the tetrathio-TTF skeleton.Thus, the phase transition in this case takes place via a relatively small movement of the alkyl chains and is slightly endothermic; the effect on the physical properties, however, is substantial. We thank Dr. M. Onoda of the Institute for Molecular Science (IMS) for his help in the powder X-ray measurement. We also thank Dr. T. Inabe of the IMS for helpful discussions. References 1 H. Inokuchi, G. Saito, P. Wu, K. Seki, T. B. Tang, T. Mori, K. Imaeda, T. Enoki, T. Higuchi, K. Inaka and N. Yasuoka, Chem. Lett., 1986, 1263. 2 K. Imaeda, T. Enoki, Z. Shi, P. Wu, N. Okada, Y. Yamochi, G. Saito and H. Inokuchi, Bull. Chem. Soc. Jpn., 1987,60, 3136. 3 S. Hosoya, J. Phys. Soc. Jpn., 1954, 9, 524. 4 D. W. McClure, J. Phys. Chem., 1968, 49, 1830. 5 Z, Shi, T. Enoki, K. Imaeda, K. Seki, P. Wu, H. Inokuchi and G. Saito, J. Phys. Chem., 1988, 92, 5044. 6 P. Wu, G. Saito, K. Imaeda, Z. Shi, T. Mori, T. Enoki and H. Inokuchi, Chem. Lett., 1986, 441. 7 P. Main, S. J. Fiske, S. E. Hull, L. Lessinger, G. Germain, J. P. Declercq and M. M. Woolfson, MULTAN 82, 1982, Univ. York and Louvain. 8 C. Nakano, in preparation. 9 T. Mori, A. Kobayashi, Y. Sasaki, H. Kobayashi, G. Saito and H. Inokuchi, Bull. Chem. Soc. Jpn., 1984, 57, 627. 10 W. Piesczek, G. R. Strobl and K. Malzahn, Acta. Crystallogr., Sect. B, 1974, 30, 1278. 11 H. M. M. Shearer and V. Vand, Acta Crystallogr., 1956, 9, 379. 12 P. W. Teare, Acta Crystallogr., 1959, 12, 294. 13 Y. Li, C. Nakano, K. Imaeda, Y. Maruyama, H. Inokuchi and G. Saito, Bull. Chem. Soc. Jpn., 1990, 63, 1857. Paper 0/02861D; Received 26th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100037

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 43-49 43 Mechanism of n-Alkylammonium Ion Intercalation into the Layered Host a-VOP04*2H20 Michael Morris,aJohn M. Adamsband Alan Dyer*" a Department of Chemistry and Applied Chemistry, The University of Salford, Salford M5 4WT, UK English China Clays International Ltd., John Keay House, St. Austell, Cornwall PL25 4DJ, UK The mechanism of redox intercalation reactions between alkylammonium iodides and the layered host a-VOP02-2H20has been investigated by synthesis of mixed- and single-ion intercalates, by ion-exchange experiments, and by EPR study of in situ reactions. Intercalation was found to occur first at the edge of crystallites. With alkyl chains of butyl and longer, intercalation of ions was seen to proceed throughout the interlayer galleries, resulting in a bilayer arrangement, with the chains making an average angle of 39"to the host layers.Smaller ions were not intercalated to any great degree, with reaction occurring only on crystal edges and faces. Intercalation of these smaller ions at crystal edges in mixed intercalation reactions prevented larger, co-present octylammonium ions from intercalating, indicating that they represented a barrier to free interlayer diffusion of further reactant. This barrier is suggested as being the result of smaller ions intercalating parallel to the host layers at crystallite edges. Keywords: Intercalation; Alkylammonium iodide; a-Vanadyl phosphate dihydrate The intercalation of guest molecules in layered hosts has received considerable attention in recent years'-4 owing to the potential that such materials show for catalysis, molecular sieving, ion exchange, and so on.Perhaps the most studied of guest molecules are n-alkylamines (whether protonated or electrically neutral), which have a remarkable ability' to penetrate the interlamellar regions of layered substances, an activity thought to arise from the combination of the polarity of the primary amino group and the ability of these molecules to alter their molecular shape by conformational changes of the alkyl chain^.^,^ Alkylamine intercalates of layered hosts display interesting structural properties, with multilayers of intercalated guest molecules being Such multi- layers are thought to be favoured by numerous van der Waals interactions between adjacent alkyl chains bonded to opposite layers,' the energy of interactions balancing, to some extent, that required for increasing the interlayer distance.The layered phase alpha-vanadyl phosphate dihydrate, a-VOP04*2H20, has a tetragonal unit cell (Z=2) with par- ameters a=b= 6.21 A." The c parameter corresponds to the basal spacing (BS) of the phase, i.e. the repeat distance perpendicular to the layers. The structure consists of distorted vanadium(v)-oxygen octahedra which are condensed with four phosphate tetrahedra in their equatorial planes. One of the axial groups in each octahedron is a short V=O bond, while the other is a replaceable water molecule.'2 The electri- cally neutral layers are held together by hydrogen bonds between the co-ordinated water molecules and other interlayer water molecules (Fig.1). The weak interlayer bonding means that a-VOP04 2H20 readily undergoes intercalation reac-tions with the uptake of small polar molecules such as ammonia, n-alkanols,' and n-alkylamines.' 3714 These species are intercalated as neutral molecules with no change in the uncharged nature of the layers being implied. A second type of intercalation reaction involves the reduction of a fraction of the vanadium(v) to vanadium(1v) with concomitant intercalation of cations to counterbalance the induced nega- tive layer charge. This 'redox intercalation' process may conveniently be carried out using iodides. These allow both control and determination of vanadium(v) reduction.Martinez-Lara et al.' ' intercalated n-alkylammonium ions Fig. 1 Schematic represenration of the structure of a-VOP0,.2H20: . small circle, V; large circle, interlayer H,O into a-VOP0,*2H20 by this method according to RNH31+ a-VOP04*2H20-+ X(RNH3)x(V02+),(V03+)1-xP04*nH20+(2-n)H20+-122 (1) The basal spacings of the products were found'' to increase linearly with the number of carbon atoms in the alkyl- ammonium ion. Martinez-Lara et al. suggested that the alkylammonium ions are orientated perpendicular to the vanadyl phosphate layers, but commented no further on the arrangement in the interlayer region. In addition, no mention was made as to the possibility of intercalating methyl- and ethyl-ammonium ions into the host by this method.The purposes of the present study are to investigate the mechanism of these redox intercalation reactions, and to determine the arrangement of alkylammonium ions in the products. To these ends, the original intercalation reactions, carried out by Martinez-Lara et al. were repeated, and the data supplemented by an electron paramagnetic resonance (EPR) study, mixed alkylammonium ion intercalation reac- tions, and ion-exchange experiments. Experimental Alkylammonium Iodide and Host Production Solid alkylammonium iodides and a-VOPO4 2H20 were prepared as described by Martinez-Lara et al.15 The identity and composition of the host were confirmed by powder X-ray diffraction (XRD) and by wet chemical and thermogravimetric (TG) analysis.Intercalation Reactions Batch intercalation reactions with individual alkylammonium iodides were carried out by dissolving the particular iodide in acetone (0.02 mol in 30-100 cm3 depending upon solubility), and stirring a-VOP04*2H20 (0.01 mol) with this solution for 30min at room temperature. The intercalate was then obtained by centrifugation and washed iodine-free with ace- tone. The liberated iodine was titrated with standard thios- ulphate to obtain a value for the degree of vanadium(v) reduction, x. This value was found to be consistent with that obtained by wet chemical analysis. For example, an x value of 0.70, obtained by thiosulphate titration, for a butylam-monium ion intercalate, was found to compare well with 0.69 determined by redox titration of vanadium(1v) and total vanadium present in the sample.Similar study of an octylam- monium intercalate gave x values of 0.77 and 0.76 for thio- sulphate titration and chemical analysis, respectively. Chemical analysis showed also that no molecular iodine had been included by the solid product. The above method was adapted also to produce butylam- monium (C4) and octylammonium (C8)intercalates of various x values, by reacting a-VOPO4 *2H20 with 0.2-2.0 equiva-lents of the appropriate iodide. Mixed alkylammonium ion intercalation reactions were carried out by treating 5 x mol samples of the host with an alkylammonium iodide mixture (containing a total of 0.01 mol) by the same method.The iodide mixtures contained C8 iodide and one other alkylammonium iodide, with the mole fraction of each iodide in the mixture being varied from zero to one. EPR Spectroscopic Study of Intercalation Reactions EPR study of single alkylammonium iodide reactions was carried out by placing 0.12 g of lightly ground a-VOP04*2H20 in an EPR tube and wetting this with 1cm3 of acetone. The tube was then shaken to obtain a suspension, and 1 cm3 of an acetone solution of the alkylammonium iodide (containing 2 equivalents) added. Shaking was resumed for 30 s, and the tube contents were allowed to settle for 30 s (note that this settling period was considered to be part of J. MATER. CHEM.,1991, VOL. 1 ation, to be followed qualitatively.The host compound yields an EPR spectrum (due to the presence of V" impurities) consisting of numerous hyperfine linesI6 arising from interac- tions of the isolated d' electrons with "V, I=$, nuclei. Redox intercalates of a-VOPO, *2H20,such as the proton interca- lates prepared by Zazhigalov et a1.,17 give spectra that show rapid collapse of hyperfine lines into singlets with increasing x, owing to exchange interactions between d' electrons. In the present case, butylammonium ion intercalates with x values as little as 0.16, gave spectra consisting essentially of singlets with some hyperfine structure superimposed. At x = 0.34, only a narrow singlet was observed. The intensity of these signals is directly related to the V" content of the intercalate.The spectrometer used in the present study, how- ever, yielded first-derivative spectra of the signals, making estimation of signal intensity difficult. Nevertheless, the dis- tance between the minima and maxima (or 'peak-to-peak height') of the first-derivative spectra was found to be directly related to the V" content of an intercalate, as is shown for a series of butylammonium ion intercalates in Fig. 2. A qualitat-ive method of following alkylammonium ion intercalation was therefore provided by EPR. It should be noted, however, that no calculations upon reaction kinetics were attempted owing to the relative crudity of the method. Instrumental XRD data were obtained using a Philips PW1050 diffractometer with Co-Ka radiation, and a Philips PW1730 instrument with monochromated Cu-Ka radiation.TG of samples was carried out from 298 to 1223 K using a Mettler TG 50 TG/DTG under still air. (n.b. Vanadium(1v) reoxidation was not found to be a problem under these conditions.) EPR 100 . " " " ' ' ' /€ i, the total reaction time for the purposes of this study). The 0 EPR spectrum was then recorded, and the above shaking- 0.15 0.25 0.35 0.45 0.55 0.65 settling cycle repeated until no further change was observed X in the EPR spectrum. This method allowed the reduction of Fig. 2 Relationship between EPR intensity ('peak-to-peak height')the EPR-invisible Vv to VIV (d' configuration) upon intercal- and vanadium(1v) content (x) of C4 intercalates J.MATER. CHEM., 1991, VOL. 1 spectra were recorded on a JEOL FE-FE3X spectrometer at room temperature. Results The redox intercalation reactions with single alkylammonium iodides of butylammonium (C4) and larger ions were found to proceed in a similar fashion to that described by Martinez- Lara et al.” The basal spacings (BS) of these products (Table 1) were found to be at least double that of the host itself, suggesting that the larger ions were propping the inorganic layers of the host apart. The x values of the materials confirmed also that a substantial degree of reduction had occurred, being in the range 0.7-0.8. On the other hand, the three smaller (IC3) alkylammonium iodides were found to give a low degree of reduction (x =0.1-0.2), and products with BS similar to that of the host itself.This indicates that the amount of alkylammonium ion intercalation was rather low in these cases, and contradicts the observation of Martinez- Lara et al. who found that propylammonium (C3) ions were intercalated to yield a product with a BS of 14.6 A and an x value of 0.5. Alkylammonium Intercalates We concentrate initially on the larger alkylammonium ion intercalates. A plot of the BS against the number of carbon atoms (24) in the alkyl chain of the ion yields a straight line of gradient (ABSlAn) 1.59A per carbon (Fig. 3). Since the maximum possible increment in an alkyl chain is 1.27 A per carbon atom, it is clear that a multilayer arrangement of alkylammonium ions must exist between the layers of the host.The simplest arrangement consistent with an increment of 1.59 8, would be a bilayer of straight, all-trans, alkylam- monium ions with their alkyl chains orientated at an angle, (6, of 39” to the inorganic layers. This value of (6 is extremely low. In comparable intercalates, (6 ranges from ca. 55” in a-zirconium phosphate (ZrP)”.” and KNiAs04,” to 90” (i.e. perpendicular to the layers) in silicic acidz1 and high layer- charge density clay minerals.” One of the few examples of all-trans, alkyl chains having (6 values of less than 40” is in alkylpyridinium ion-exchanged bent~nite.’~ In these materials, however, the low value of (6 seems to be determined by the interaction of the aromatic ring of the intercalated ion with the aluminosilicate layers.The angle which alkylammonium ions make with a layer depends on the ratio of the available interlayer surface area per unit charge to the cross-sectional area of the ions. In the present case, the surface area is 77.1 8,’ per unit cell and 38.6 per vanadium site (2=2). As the intercalates possess an x value of ca. 0.7 (Table l), the surface area available to each intercalated ion is 55A2,which can be compared with the cross-sectional area for an all-trans alkyl chain of only 22-24 It seems likely that the alkylammonium ions would be Table 1 The x [fraction of vanadium(v) reduced] values and major basal spacings of intercalates of different alkylammonium ions R in RNH,’ X basal spacing/A 0.18 7.19 0.09 7.40 0.18 7.19 0.70 15.55 0.72 17.40 0.7 1 18.66 0.74 20.74 0.77 2 1.83 22.51 4 5 6 7 8 number of carbon atoms in ion Fig.3 Plot of basal spacing of intercalates against number of carbon atoms in intercalated ion present as gauche-block structures, i.e. the alkyl chains contain isolated kinks.7 The loose packing of such conformations is known to allow the formation of the maximum possible number of van der Waals interactions and therefore be energetically favourable. Such structures have generally been found where alkyl chains have packing densities lower than 33 8,’ per chain.’ gauche blocks also enable the amine groups to improve their interaction with ‘ro~gh”~ or puckered layers such as are found in the present case.Alkylammonium Ion Mixtures As expected from the results obtained with single alkyl- ammonium iodides, the degree of reduction was high (ca. 0.7) when C4-C7 ions were used in mixtures with octylammonium iodide. Of special interest is the fact that XRD traces showed single basal reflections of comparable width to that of a pure, single-ion intercalate (e.g. Fig. 4). This suggested that the two types of ion were randomly distributed in each interlayer gallery. If interstratification, i.e. segregation of different ions into different interlayer galleries, had occurred the intercalate would possess more than one repeat distance parallel to its 001 plane, and a broad XRD peak would be expected.That random intercalation of ions occurred was supported by the observations that both the BS of the products and their total weight losses from TG increased linearly with the fraction of octylammonium iodide in the mixture (Fig. 5 and 6). The mixed-ion intercalation reactions with the smaller C1-C3 and C8 iodides were found to result in a considerably different type of material. The obtained products displayed x values of 0.15-0.30, suggesting low degrees of intercalation. It is apparent (Table 2) from the XRD results that the materials produced fell into two classes. The first was that produced by reactions with mixtures of mole fraction C8 50.6 (Cl) and J. MATER. CHEM., 1991, VOL. 1 10 5 28J" Fig.4 XRD traces (Cu-Ka) of (a) C8 and (b) mixed C6/C8 (mole fraction C8 iodide 0.8) intercalates 22 --21 -20 5 9) .-04 19--3 f2 18-17-0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 mole fraction of octylammonium iodide Fig.5 Plot of basal spacings of mixed C4/C8 intercalates against mole fractions of C8 iodide in the initial reaction mixture 39 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 mole fraction of octylammonium iodide Fig. 6 Plot of total TG weight losses of CS/C8 mixed intercalates against mole fraction of C8 iodide in the initial reaction mixture C8 I0.4 (C2,C3). These materials gave XRD traces with reflections of BS equal to, or slightly less than, that of the host. This low degree of intercalation was similar to that observed in the single, smaller ion intercalation reactions.The second type of material produced in Cl-C3/C8 mixed intercal- ation reactions was found when mixtures containing higher mole fractions of C8 were used (Table 2). As in the case of the first class of materials, their XRD traces were dominated (Fig. 7) by peaks with a BS of ca. 7 A from the unreacted host. For both classes of mixed intercalate this suggests that intercalation of ions is limited to crystallite faces and edges, with the bulk vanadium sites remaining unreacted. In the second class, however, low-intensity XRD peaks at ca. 17 8, (together with their second-order reflections at 8.5 A) were also found. Although it is not possible to assign these reflec- tions to a particular arrangement of alkylammonium ions with any certainty, the fact that the magnitude of the BS does not change with the size of the other ion in the mixture suggests that these regions may contain monolayers of C8 ions. The low degree of interaction observed with both single- and mixed-ion intercalation reactions of smaller ions should be contrasted with the results obtained (Table 3) from reacting the host with various amounts of C8 iodide (similar results were also obtained for the C4 salt).Here a correlation was found between the amounts of intercalate and unreacted host present in a material, and the number of equivalents of C8 iodide used in the reaction. The XRD trace (Fig. 7) of the product obtained by reaction of the host with 0.4equivalents of C8 iodide illustrated the point well.The material had an x value of 0.34, similar to that of the above-mentioned mixed intercalates, yet a peak with a BS of 21.76A can be seen in addition to that due to the unreacted host at 7.08 A. The BS of this material is therefore almost identical to that of the J. MATER. CHEM., 1991, VOL. 1 47 Table 2 Major basal spacings of mixed alkylammonium ion intercalates other alkylammonium iodide in mixture mole fraction of C8 iodide= 0.0 0.2 basal spacings/A 0.4 0.6 0.8 1.o methyl ethyl ProPYl 7.06 7.22 7.23 7.12 7.21 7.27 7.14 7.22 7.22 7.08 7.13" 7.13" 7.40" 7.18" 7.18" 21.83 21.83 21.83 butyl pentyl hexyl heptyl 15.16 17.26 18.70 20.7 1 16.80 18.11 19.21 20.94 18.80 18.96 19.27 21.34 20.74 20.36 20.74 21.55 21.14 2 1.04 20.27 21.87 21.83 21.83 21.83 21.83 a Small peaks at 17 and 8.5 8, also observed in XRD traces of these materials ;!.L I I 1 10 5 2eio Fig. 7 XRD traces (Cu-Ka) of (a)C8 intercalate produced with 0.4 equivalents of C8 iodide and (b) mixed C3/C8 intercalate, mole fraction C8 iodide in initial reaction mixture = 0.8 Table 3 x [fraction of vanadium(v) reduced] values and basal spacings of C8 intercalates produced by reaction of host with varying amounts of C8 iodide equivalents of C8 iodide x XRD peaks (relative intensity) 0.2 0.16 21.98(7) 7.38(17) 7.14(76) 0.4 0.34 21.76( 19) 7.08(81) 0.6 0.44 21.66(71) 7.08(3) 6.71(26) 0.8 0.53 21.87(81) 7.03(7) 6.69(12) 1.o 0.56 21.55(92) 6.99(2) 6.68(6) 2.0 0.76 21.55(97) 6.61(3) fully reduced x = 0.76 intercalate, suggesting that the arrange- ment of alkyl chains is the same in both compounds.The most likely explanation for this is that intercalation of C8 ions occurred initially at the edge of the crystallite and spread towards the centre of the interlayer gallery as reaction pro- ceeded. The alternative explanation is that individual crystal- lites consisted either of unreacted host or of fully reduced intercalate. This seems unlikely given the relative weakness of interlayer bonding in the host. Ion-exchange Experiments The ion-exchange experiments further demonstrated the difference in the properties of large and small alkylammonium ions with respect to intercalation in cc-VOPO4.2H20.Treat-ment of an octylammonium ion intercalate with butylam- monium ions (Fig.8) caused the BS of the intercalate to fall, suggesting exchange of the larger ions (this was confirmed by the total TG weight losses of intercalates decreasing as exchange occurred). Conversely, treatment of a butyl-ammonium intercalate with ethylammonium (C2)ions caused no change in BS. The C4/C8 exchange indicated that diffusion of the smaller ions was facile, suggesting that this should not be a problem for C2/C4, and other explanations are, therefore, required to explain why ion exchange did not occur. 519.0-.-18.5--(d 3 18.0-D -17.5 17.0--16.5 15 n .v.-I....,....0 1 2 3 4 5 6 7 8 9 10 equivalents of exchanging ion Fig.8'Plot of basal spacings of (a) C8 intercalate treated with C4 iodide and (b)a C4 intercalate treated with C2 iodide 48 EPR Measurements EPR study of intercalation reactions was found to be imposs- ible for Cl-C3 iodides as the presence of acetone solvent prevented the recording of the rather weak spectra of the resulting materials. The reactions of the larger ions could, however, be followed and these were found to have fast rates, being complete in minutes. C5 and larger ions gave almost immediate reaction, with maximum EPR intensity being found in the first recorded spectrum after 1 min (the reaction of C8 together with that for C4 is shown in Fig.9). The reaction with C4 iodide was found, however, to have a lag of 2min before a rapid increase to maximum vanadium(~v ) content after 3 min.C4 iodide, in addition to being the smallest alkylammonium iodide to yield a stable intercalate, is therefore unique in that its intercalation reaction with a-VOPO, 2H20 involves a short period when the degree of vanadium(v) reduction is low before a rapid reaction to yield the final intercalate. Discussion All the aata presented show that a-VOP04*2H20 discrimi- nates as an intercalation host between large (2C,) and small (5C,) n-alkylammonium ions. The larger ions are readily intercalated and form a stable bilayer between the inorganic layers of the host. Alternatively, the three smallest ions are not intercalated to any great degree and indeed inhibit the intercalation of C8 ions when mixtures of iodides are used for intercalation reactions.An explanation as to why the smaller ions have this effect is suggested by the XRD traces of the resulting materials (Table 1, 2 etc.), which indicate that intercalation has occurred at crystallite edges and faces only. 110 V 100 90 Q,-5 80-(d .-.c 5 70 23 Ln 60 I 0,.-2 50 Y Q,Q 4-6 40 2i Q,n c 30 n LLI 20 10 0 I 50 100 150 200 250 300 350 400 450 500 time/s Fig.9 Reaction of (a) C8 iodide and (b) C4 iodide with cr-VOP0,.2H20 (as followed by EPR) J. MATER. CHEM., 1991, VOL. 1 Results for C8 intercalates (Fig. 7) suggest that intercalation occurs initially at the edge of interlammellar galleries and spreads inwards towards crystallite centres. Intercalation of smaller ions at crystallite edges, however, prevents reaction with the bulk of the present vanadium(v) sites.It is interesting to speculate how these smaller ions could have this ‘poisoning’ effect on further intercalation. One possibility is that the small ions could take up a parallel orientation to the host layers when intercalated. Strong layer-ion-layer electrostatic inter- actions would then cause the layers to be held tightly together, and to effectively block the diffusion of additional reactant to the centre of interlammellar galleries. This ‘poisoning’ effect could account for the failure of both single- and mixed-ion intercalation reactions involving the smaller alkylammonium ions.The observed peaks at ca. 17A for certain mixed intercalates suggest, however, that the effect of the smaller ions may be modified somewhat if sufficient of a larger ion is used in the reaction mixture. EPR study of intercalation reactions demonstrated well how C4 ions represented the borderline between ions that successfully intercalated into the host, and those that ‘poi- soned’ subsequent intercalation reaction. The short time lag observed in the reaction of this iodide may be explainable by C4 ions being initially intercalated parallel to the host layers at the crystallite edges, thereby blocking the diffusion of further reactant. This situation would, however, be energeti- cally unstable with respect to a gauche-block arrangement of C4 ions as is present in the final intercalate, and a transform- ation (perhaps aided by unfavourable steric interactions between neighbouring parallel C4 ions) to gauche-block would occur.Diffusion of further reactant between the loose-packed C4 ions would then be facile, accounting for the fast reaction observed after the initial time lag. The failure of C2 ions to be introduced into a C4 intercalate by ion exchange may also be accountable for by a similar ‘poisoning’ effect of C2 ions at crystallite edges. XRD revealed, however, that the products of this ion exchange were still monophasic with no evidence of the regions of low BS that would be anticipated if edge-blocking occurred.Another possibility is that exchange did not occur because the product would be less thermodynamically stable than the original intercalate. C2 ions have been previously to form an all-trans bilayer between the layers of ZrP; however the layer charge density (i.e. the area per unit charge) of ZrP is greater than the diameter of the C2 ion. An all-trans bilayer of C2 ions is therefore dictated on stoichiometric grounds in ZrP.” The layer charge density of the present ions was considerably lower than that of ZrP, far exceeding the diam- eter of the C2 ion. To enjoy significant stabilisation uia van der Waals interactions therefore, an alkylammonium ion would need to contain kinks and form a loose-packed, gauche- block structure.C2 ions are not large enough to contain kinks and to form gauche-blocks. The mechanism of alkylammonium ion intercalation into a-VOPO, 2H20 was a considerably more complicated pro- cess than originally envisaged. The edge-first intercalation reaction observed here has been reported previously for ZrP by Alberti,25 and seems to be a general mechanism for most layered hosts (although not for the important clay mineral kaolin’). Indeed, Clearfield and TindwaIg observed that propylamine and butylamine were intercalated parallel to the layers of ZrP at low amine loadings. It was found also that the intercalated organic ions were forced into a more upright arrangement as further intercalation proceeded. The present study is believed, however, to be the first to detail an instance where an initial intercalation of alkylammonium ions at the edge of an interlayer gallery blocks further intercalation reactions.J. MATER. CHEM., 1991, VOL. 1 One of the authors (M.M.) would like to thank Laporte Inorganics, Widnes, and the Science and Engineering Research Council for their financial support of this work via a CASE grant. References 1 Intercalation Chemistry, ed. M. S. Whittingham and A. J. Jacob- son, Academic Press, New York, 1982. 2 G. Lagaly, Philos. Trans. R. SOC. London, Ser. A, 1984, 311, 315. 3 T. J. Pinnavaia, Science, 1983, 220, 365. 4 G. Alberti, in Recent Developments in Ion Exchange, ed. P. A. Williams and M. J. Hudson, Elsevier Applied Science, London, 1987, pp.233-248. 5 G. Lagaly, Solid State lonics, 1986, 22, 43. 6 G. Lagaly and A. Weiss, Angew. Chem., Int. Ed. Engl., 1971, 10, 558. 7 G. Lagaly, Angew. Chem., Int. Ed. Engl., 1976, 15, 575. 8 F. R. Gamble, J. H. Osiecki, M. Cais, R. Pisharody, F. J. Disalvo and T. H. Geballe, Science, 1971, 174, 493. 9 C. Rosner and G. Lagaly, J. Solid State Chem., 1984, 53, 92. 10 G. Alberti and U. Costantino, in Intercalation Chemistry, ed. M. S. Whittingham and A. J. Jacobson, Academic Press, New York, 1982, pp. 147- 1 80. 11 A. W. Hewat, M. Tachez, F. Theobold and J. Bernard, Rev. Chim. Miner., 1982, 19, 291. 12 J. W. Johnson, A. J. Jacobson, J. F. Brody and S. M. Rich, Inorg. Chem., 1982, 21, 3820. 13 G. Ladwig, 2.Anorg. Allg. Chem., 1965, 338, 266. 14 K. Beneke and G. Lagaly, Inorg. Chem., 1983, 22, 1503. 15 M. Martinez-Lara, A. Jimenez-Lopez, L. Moreno-Real, S. Brusque and E. Ruiz-Hitzky, Muter. Res. Bull., 1985, 20, 549. 16 D. Ballutaud, E. Bordes and P. Courtine, Muter. Res. Bull,, 1982, 17, 519. 17 V. A. Zazhigalov, A. I. Pyanitskaya, 1. V. Bachemkova, G. A. Komashko, G. Ladwig and V. M. Belousov, React. Kinet. Catal. Lett., 1983, 23, 119. 18 E. Michel and A. Weiss, Z. Nuturforsch, Teil B, 1965, 20, 1307. 19 A. Clearfield and R. M. Tindwa, J. Inorg. Nucl. Chern., 1979, 41, 871. 20 K. Beneke and G. Lagaly, Clay Miner., 1982, 17, 175. 21 G. Lagaly, Colloid Interface Sci., 1979, 11, 105. 22 G. Lagaly, Clay Miner., 1981, 16, 1. 23 G. Lagaly, H. Stange and A. Weiss, Proc. Int. Clay Conf. 1972, 1973, 693. 24 A. Grandin, M. M. Bore1 and B. J. Raveau, J. Solid State Chem., 1985, 60, 366. 25 G. Alberti, Acc. Chem. Rex, 1978, 11, 163. Paper 0/02933E; Received 29th June, 1990

ISSN:0959-9428    

DOI:10.1039/JM9910100043

出版商:RSC    

年代:1991

数据来源: RSC

摘要:

J. MATER. CHEM., 1991, 1(1), 51-57 Single-crystal Conductivity Study of the Tin Dichalcogenides SnS2- $ex Intercalated with Cobaltocene Carl A. Formstone, Mohamedally Kurmoo, Emma T. FitzGerald, P. Anthony Cox* and Dermot O’Hare* Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX 1 3QR, UK Single crystals of the n-type semiconducting tin dichalcogenides SnS2-,Se, (x=O, 0.3,0.5, 1.3, 1.85 and 2), which have a two-dimensional layered structure, have previously been intercalated with cobaltocene (CoCp2, Cp =q5-C5H5)to give the series of compounds SnS2 -xSex(CoCp2)o.33. Four-contact resistivity measurements have been carried out on these host and intercalate samples. The sulphur-rich intercalates (x=O.O, 0.3,0.7 and 1.3) were found to be semiconducting, whereas the selenium-rich intercalates (x= 1.85 and 2.0)were found to be metallic.These findings confirm earlier observations made in a photoelectron spectroscopy study. The resistivity of the semiconducting intercalates closely follows the functional form exp[( TolT)”4],characteristic of variable- range hopping. The values of To were in the range 3.4-3.7x lo8 K. The metallic intercalates show a transition to a superconducting state at 6.1 K for x=2.0 and 5.7 K for x= 1.8.In the diselenide case this has been confirmed by observation of a magnetic response appropriate to a type II superconductor below 6 K. This is the highest reported T, for a two-dimensional layered structure intercalated by an organometallic guest molecule. Keywords: Intercalation; Superconductivity; Cobaltocene; Variable-range hopping; Conductivity 1.Introduction Two-dimensional layered materials, such as the tin dichalcog- enides SnS,-,Se, (O<x<2), have been much studied with regard to their pronounced electronic and structural aniso- tropy.’?’ The SnS, -,Sex layered compounds crystallise in the Cd(OH),-type structure (space group Phl) to form an isostructural series of solid solution^.^ There are several other series of this kind, such as Tal-,W,S,(0~xIl), TaS, -,Se,(O I.xI2) and Zr, -,Sn,Se,(O Ix Il).4 The three- dimensional structure is built from repeatedly stacked XMX lamellae bound together by van der Waals interactions between adjacent planes of hexagonally close-packed chalcog- enide atoms (X).5 In SnX2 (X =S, Se) the metal atoms are co- ordinated in nearly octahedral sites.The layered MX, structure permits a variety of guest molecules to be inserted into the interlamellar gaps of the host materials.6 For example, the intercalation of metallic tantalum and niobium dichalcogenides by organic amines has led to the discovery of a new class of two-dimensional supercond~ctors.~This has raised questions as to the dimen- sionality of superconductivity in layered structures and the relationship between charge density waves (CDWs) and the superconducting BCS mechanisms.8 There are also examples of molecular crystals with a layered structure that exhibit superconductivity at low temperature, such as salts of bis(ethylenedithi0)-tetrathiafulvalene (BEDT-TTF).9 The importance of the process of charge transfer from the guest to the host in intercalation reactions has been recognised for some time.” In view of this, the possibility of ‘fine-tuning’ the electronic structure of the host material by intercalation has been utilised in several past studies.’ In fact, there are many examples of semiconducting layered structures being induced into a metallic state by intercalation of electron donor guest species, for example, LiTiS,,” Ko.5WS212 and Ko.5MoS2.13 Although a vast amount of effort has been expended on the characterisation of layered TX, materials (T =Ti, Mo, W, Ta, Nb; X=S, Se, Te) intercalated by hydrazine, organic amines and metallocenes,’ little attention has been paid to the intercalation of non-transition-metal dichalcogenides such as SnS, and SnSe,, especially in single-crystalline form.14y1 The synthesis of large single crystals (ca.2mmx 4 mm x 0.5 mm) of intercalated materials has often proved difficult. Given the poor kinetics of the general intercalation reaction,6 the majority of such reactions are only possible with microcrystalline host compounds. Consequently, single- crystal studies have been carried out mainly on the host16 rather than on the intercalate materials. Single crystals are particularly attractive for conductivity studies, which can give a great deal of information, especially when combined with other techniques such as solid-state photoelectron spectroscopy (PES).A prime example might be the extensive physical characterisation of single-crystalline phosphorus-doped SnS, and its cobaltocene (CoCp,) interca- late, S~S,(COC~~)~.,,, where Cp =q5-C5H5.17 Previous studies on the tin dichalcogenide hosts SnS, -,Sex have involved electrical measurements, such as the tem-perature dependence of the Seebeck coefficient or resistivity, on single-crystalline SnS2,18*19 SnSSe,” SnSo.7Sel,3,21 SnSo.lSel.921 and SnSe,.” The successful intercalation of cobaltocene (CoCp,) into the series of single crystals SnS2-,Se, (05x52) has been achieved in this laboratory.22 The widely differing properties of the host compounds coupled with their structural uniform- ity make them very interesting from an electronic viewpoint, especially in the intercalated form in which extensive electron transfer between the guest and host entities would be expected.Photoelectron spectroscopy has demonstrated a semicon-ductor-to-metal transition on moving from the sulphur-rich members to the selenium-rich members of the SnS,-,Se, (CoCp,),,,, intercalate series.” The aim of the present study was to confirm the differing electronic properties of these intercalate materials by using single-crystal four-contact a.c./ d.c. measurements in the temperature range 2-300 K. 2. Experimental 2.1 Preparation of SnS, -xSex Single Crystals The host single crystals SnSz-,Se, described in this work were all grown with the use of the iodine vapour transport method.’, The high-purity elements (>99.99%) with a 1YO molar quantity of phosphorus dopant and the transport agent Table 1 Growth conditions, appearance and c-spacing for SnS, -$ex temperature, composition T,,T,/"C growth time/h colour c-spacing/A SnS, SnS1,7Seo.3 685,645 670,630 12 48 orange red 5.928 5.953 SnS,,,,Se,,,, SnS,,,,Se,.,, SnSo.lsSe,,8, SnSe, 650,610 620,580 570,530550,510 48 100 50 72 dark red black black black 6.008 6.103 6.136 6.141 I, (5 mgcm-3) were sealed in evacuated quartz ampoules (10 cm x 1 cm).A three-zone furnace provided a stable tem- perature gradient between the reaction zone (7'') and the growth zone (T,) of the ampoule. Table 1 gives the growth conditions for some members of the series as well as the appearance24 and c-spacing of the crystals. The crystal structure was analysed using a Phillips PW 1710 powder diffractometer.The stoichiometry of the host crystals was determined using a JEOL FX 2000 analytical electron micro~cope.~~ The X-ray emission from microcrystal- line samples excited by a 200 keVt electron beam was detected and the end members of the series (SnS, and SnSe,) used as standards for the stoichiometry determination. 2.2 Synthesis and Characterisation of Intercalated Samples The synthesis and manipulation of the air-sensitive intercalate SnSz -xSex(CoCp2)o.33 single crystals were carried out under an atmosphere of dinitrogen. The acetonitrile solvent used was pre-dried over molecular sieves and then distilled over CaH,, followed by thorough degassing.The crystalline host (ca. 100 mg) was added to a solution of freshly sublimed CoCp, (ca. 150 mg) in acetonitrile (ca. 5cm3). In order to avoid damage to the brittle crystals, the reaction was carried out at ca. 65°C without stirring. After the reaction was complete, typically 5-21 days, the CoCp, solution was removed and the intercalate washed with aceto- nitrile (4 x 20 cm3) until the filtrate was colourless. The single crystals were then dried in vacua for several hours and characterised by X-ray diffraction under an N2 atmosphere within a sealed cell. The product crystals were deemed to be fully intercalated when all the host reflections had disappeared from the spectrum. Table 2 shows the lattice expansion, stoi- chiometries, appearance and reaction conditions for the intercalates produced in this study.The stoichiometry in each case was determined by elemental microanalysis. 2.3 Conductivity Experiment The air-sensitive intercalate materials were handled in a glove bag under an inert atmosphere. Under such conditions four contacts were attached to these samples, which had previously t 1 eV x1.602x J J. MATER. CHEM., 1991,VOL. 1 been cut into a parallelepiped geometry, typically 2 mm x 4mm xO.2 mm. A sample would then be sealed inside the specimen holder ensuring complete isolation from atmospheric oxygen and water. Conductivity data were, in fact, reproduc- ible over several days for a given sample sealed within the cryostat.The host crystals presented no special problems other than the necessity for gentle handling, in order that structural damage should be avoided. Colloidal Ag paint was used to attach the metallic contacts to the samples, the experimental results being identical when colloidal Au paint was used instead. This observation was taken as evidence that any reaction between the Ag paint and the chalcogen-rich crystal surfaces was unimportant. For all the samples measured, ohmic contacts were established within the chosen current range. Samples were occasionally rejected if cracks were noticed in them during routine investigation under a microscope. Resistance measurements were taken with a Hewlett-Pack- ard HP 3478A multimeter interfaced to a RML 3802 micro- computer, which managed the experiment over a wide range of temperature (2-300 K) in conjunction with an Oxford Instruments 3 120 temperature controller.The temperature changes within an Oxford Instruments cryostat were brought about as slowly as possible to allow equilibration of the temperature within the sample holder unit. The measurements were all four-contact in type, but only the SnSo.15Se,.85 and SnSe, host and intercalate crystals were studied using an a.c. method (15 Hz), which required the use of a Brookdeal 9503 'lock-in' amplifier. 3. Results and Discussion 3.1 Experimental Data for the Host Materials Fig.1 presents a plot of log,,(resistivity/n cm) versus tempera-ture/K for the entire series of host single crystals (phosphorus doped) SnS,-,Sex, where x=O.O, 0.3, 0.7, 1.3, 1.85 and 2.0.This plot is intended to give an impression of the wide variation in conductivity behaviour in the host structures as sulphur is replaced by selenium. A closer examination of the data presented in Fig. 1 suggests that the host materials can be divided into two distinct classes on the basis of their resistivity variation with temperature. For class I (x=O.O, 0.3, 0.7 and 1.3) the resistivity spans several orders of magnitude in the temperature range 100-300 K. For class I1 (x = 1.85 and 2.0) the resistivity decreases in the range 298 to ca. 150 K then increases down to 2 K. The lowest resistivity corresponds to 140 and 160 K for x= 1.85 and x=2.0, respectively. Materials in the first class (I) seem to be well described by an Arrhenius-type model for conductivity.Fig. 2 gives a logarithmic plot of resistivity us. inverse temperature for SnS,-,Se,, where x=O.O, 0.3, 0.7 and 1.3. The activation energies (E,) decrease steadily from 0.45 eV (x=O.O) to 0.09 eV (x= 1.3) across this series. The resistivity versus temperature plots for SnSz -,Sex, Table 2 Stoichiometries, reaction conditions, lattice expansion and appearance for the intercalated layered materials [host(CoCp,),] reaction conditions host temp./"C time/da ys stoichiometry (y) Ac/A colour SnS, 65 5 0.31 5.46 dark blue SnS1.,Se0.3 65 7 0.31 5.24 light blue SnS1sSe0.s 65 9 0.31 5.20 light blue SnS0.7Se1.3 65 14 0.33 5.26 black SnSo.15Se1.85 65 17 0.33 5.50 black SnSe, 65 21 0.33 5.56 black J.MATER. CHEM., 1991, VOL. 1 6*o 1 5.0 4.0 E 3.0 x c.'.-.-2> 2.0.-fn 1.0 0,-53 0 -12 1 o.loo/\ (b)/ 5 0.osol -2.0 1,0.0401-77 155 233 3 10 temperature/K Fig. 1 Plot of log (resistivity/R cm) us. temperature/K for the host single crystals, SnS, -,Sex, where 01x 2. (a) SnSe,; (b) SnSe1.85S0.15; (c) SnSe1.3S0.7 (d)SnSe0.7S1.3; (e)SnSe0.3S1.7;(f)SnS2 6-o 3 0.002 0.005 0.008 0.010 0.013 temperature- '/K -Fig. 2 Plot of log (resistivity/R cm) us. K/temperature for the host single crystals, SnS2-,Se,, where x=O.O, 0.3, 0.7 and 1.3. (a) SnSe1.3S0.7;(b)snse0.7s1.3; (c) SnSe0.3S1.7;(d)SnS2 where x= 1.85 and x=2.0, are given in Fig.3. The materials in the second class (11) can be modelled by considering them be very low activation energy semiconductors (E, z lo-, eV), so that the carrier mobility term (p) becomes important com- pared to the carrier concentration (n) at elevated tempera- tures (T>150 K). This would account for their metallic-like behaviour in the range 150-300 K. An attempt has been made elsewhere to model the resistivity us. temperature behaviour of an organic conductor, (NMP),TCNQ, according to:26 p(T)= l/(nep)=AT"exp(E,/kBT) (34 where E, is the activation energy and a is the mobility factor, relating to the type of scattering mechanism in operation. Fig. 3 presents an attempt at the best fit for the resistivity uersus temperature (100-300 K) experimental data with theor- etical data derived from eqn.(3.1) (solid line). Consequently, an estimate can be made for the activation energies (E,) and mobility factors (a) of these class I1 materials. Table 3(a) summarises the resistivity (p) at 298 K, the mobility parameter (a) and the activation energy (E,) for the host crystals SnS, -,Sex. 1 0.020 1,90 147 20 5 262 320 temperature/K Fig. 3 Plot of resistivity/R cm us. temperature/K for the host crystals SnS,-,Se,, where x= 1.85 and 2.0. (a)SnSe,; (b) SnSel,,,So,ls 3.2 Nature of the Host Material Conductivity Phosphorus doping was initially adopted as a means to produce SnS, samples of sufficient conductivity for X-ray and ultraviolet photoelectron spectroscopy (XPES and UVPES) studies.,, The phosphorus-doped SnX, samples are all n-type semiconducting on the evidence of this PES study.Electro- chemical measurements have shown P-doped SnS, electrodes to be n-type semiconducting on the basis of electrochemical impedance studies in aqueous sol~tion.~~*~* The undoped SnSo.7Sel.3, SnSo.l Se,., and SnSe, single crystals are n-type semiconducting according to previous Hall and Seebeck coefficient measurements.21 It is interesting to speculate as to the nature of the phos- phorus doping in these materials. One might imagine that P atoms are able to substitute for Sn atoms in the individual XSnX sandwich structure. This would be analogous to the situation found in the layered metal thiohypophosphate com- pounds, M2P2S629 (M=Fe, Ni, Mn, etc.), in which co-ordination sites between the sulphide layers are occupied by metal atoms (M) or P2 units.On the other hand, one might consider the possibility of P atoms occupying interlamellar positions, a situation that arises in the layered compound Po.2VS2.30 In both these cases P atoms would be acting as electron donors in accordance with the n-type semiconductiv- ity of the SnX, host materials. In support of the latter possibility, it was noticed that undoped SnS, was intercalated by cobaltocene in 1,2-dimethoxyethane (DME) solution at room temperature much less rapidly than P-doped SnS, under the same conditions. This could be explained by the presence of intersandwich P atoms disrupting the interlayer van der Waals bonding, thereby allowing cobaltocene molecules easier access to the interlamellar spaces.The experimental conductivity data presented above show a gradually decreasing activation energy in these host materials (Table 3) from SnS, (0.45 eV) to SnSe, (0.04 eV) via SnS0.,Se,., (0.09 eV). This is to be expected on the basis of Mott's impurity model for doped semic~nductors,~' which predicts that as the medium becomes more polarisable the energy required to ionise impurity electrons from the donor levels into the host conduction band will decrease. No work has been done to estimate the level of P doping throughout J. MATER. CHEM., 1991, VOL. 1 Table 3 Summary of important resistivity data on hosts and intercalates (4hosts (x) p/Qcm at 298 K E,/eV 0.0 387.6 0.45 0.3 7.67 0.37 0.7 2.87 0.28 1.3 1.21 0.09 1.85 0.114 0.05 2.0 0.054 0.04 0.0 3.90 1.54 0.3 38.6 7.48 0.7 69.2 9.77 1.3 61.7 12.5 1.85 1.1 x 2.0 1.1 x the series, but an estimate of 1015 cm-3 for the carrier density in P-doped SnS2 has been made.28 The conductivity behaviour of the so-called class I1 host materials (x= 1.85 and 2.0,Fig.1) can be interpreted in terms of the carrier mobility factors (p)in semiconductors with small activation energies (E, =lo-, eV). The model presented on the basis of eqn. (3.1) seems to work well above 100 K, but below this temperature there is some deviation possibly arising from additional conduction via impurity sites.It has previously been demonstrated that in two-dimen- sional layered systems the carrier mobility (p)is highly tem- perature dependent:, ’ P( Tl )/AT2)=(TIIT2)-“; a>1.5 (3.2) This strong temperature dependence above ca. 100 K has been related to an optical phonon scattering mechanism. This is unique to two-dimensional layered materials, since the carriers are confined to individual XMX layers with mainly short-range interactions coupling the carriers to the optical modes of the lattice. These vibrational modes involve modu- lation of the XMX sandwich thickness in layered materials. In this study the exponent (a)in the mobility temperature dependence expression in SnS2 -,Sex crystals was found to be a= 1.70 and 1.72 for x= 1.85 and 2.0, respectively.This correlates well with theoretical predications and other exper- imental data on layered systems,, suggesting that scattering of conduction electrons at T>75 K may well be related to this mechanism in these particular SnS, -xSex hosts. 3.3 Experimental Data for the Intercalate Materials As with the host materials it is convenient to divide the intercalate compounds into two distinct groups. Consider first the mainly sulphur-rich intercalate single crystals SnS2 -,Sex (CoCp,),.,,, where x=O.O, 0.3, 0.7 and 1.3. For these samples log resistivity is plotted against Tp1I4in Fig. 4 in order to demonstrate the excellent agreement between the experimental data and the Mott variable-range hopping (VRH) law.Least- U ----1.70 1.72 3.67 4.0 3.42 3.9 3.60 3.9 3.64 3.9 --5.00-4.00-9> c..-2 3.00-.-UJ2 Y 0 2 2.00-1.00-0.001 I I 1 , I I I , I I I 1 I 1 1 1 1 I I , 0.22 0.24 0.26 0.28 0.30 0.32 temperature-1’4/K”’4 Fig. 4 Plot of log (resistivity/Q cm) us. temperature-’’4/K-1/4 for the intercalates SnS, -xSex(CoCp2)o,31, where x =0.0, 0.3, 0.7 and 1.3. (a) SnS2{Co(Cp)2}; (b) SnSe0.3S1.7{Co(Cp)2}0.31;(c) SnSe0.7S1.3 {cO(cp2}0.32;(d) SnSel .~s0.~{c0(cP)2}0.31 intercalate materials a much reduced anisotropy is observed. (P 11 /PI =10).The resistivity versus temperature variations for the SnS2 -,S~,(COC~~)~~,~ compounds, where x = 1.85 and 2.0, are given in Fig.5 and 6, respectively. On cobaltocene inter- cala tion the room- temperature single-cr ystal conductivity increases significantly in both cases (see Table 3). On cooling these samples the resistivity decreases as expected for metallic samples. At 5.7 K (x = 1.85) the resistivity drops sharply (width 1.5 K)as in Fig. 5. At 6.1 K (x=2.0) a similar transition (width 0.7 K) is observed with the resistivity falling to zero as in Fig. 6. Measurements on several different samples indicate that these transitions generally take place in the range 4.8-5.7 K in the SnSo,15Sel.85 intercalates and 6.1-6.5 K in the SnSe, intercalates. Within this variety of samples were several crys- tals of undoped SnSe2 intercalated with cobaltocene.The absence of phosphorus seemed to make no difference. squares fitting of the resistivity data to the expre~sion~,-~~ P =PornTo)”2exPC(To/T)”1 (3.3) gives o=0.25f0.02. The values of p at 298 K,o,po and Tofor each composition are given in Table 3. Notice that the room-temperature resis- tivity increases for x=0.3, 0.7 and 1.3 upon intercalation, whereas for x=O.O the opposite is true. Experiments have demonstrated that there is considerable anisotropy in the host single-crystal conductivity (pll/pl =loo), whereas in the J. MATER. CHEM., 1991, VOL. 1 1.2x10-3 1 7 18, 141 9.0 x 0 77 155 233 310 temperature/K Fig. 5 Plot of resistivity/R cm us. temperature/K for the intercalate SnS0.15Se1,85(CoC~2)0.33 0 77 155 233 310 temperature/K Fig.6 Plot of resistivity/R cm us. temperature/K for the intercalate SnSe2(CoCPz)o33 The superconductivity has been confirmed in the diselenide case by magnetic susceptibility measurements, which have demonstrated the Meissner effect below 6 K appropriate to a type I1 supercond~ctor.~~After cooling to 4.2 K in zero applied magnetic field, an initial diamagnetic response was observed as in Fig. 7. At 4.2 K the magnetisation falls to a low value above 300 G; the presence of flux trapped within the sample is shown on returning to zero applied field. Below 3 K extensive hysteresis of the magnetic moment is observed. Table 3(b) summarises the data for the metallic intercalates (x= 1.85, 2) giving the resistivity at 298 K and the supercon- ducting transition temperature (T,).Finally, in order to illustrate clearly the difference in electrical properties of the intercalates at either end of the series, Fig. 8 gives the log,, resistivity uersus temperature variation for the entire intercalate single crystal series. The clear semiconductor-to-metal transition appears to occur in the stoichiometry range 1.3<x<1.85. 3.4 Intercalate Conductivity Data Previous UVPES measurements indicate that a transition from semiconducting to metallic character occurs on passing -750 -500 -250 0 250 500 7 field/G 2.00 4.00 6.00 8.00 10.00 Fig. 7 Plot of molar magnetisation us. applied magnetic field for the1.o x10-5 1,diselenide (x 2.0) intercalate at 2.5 K35= 0 77 155 233 310 ternperature/K Fig.8 Plot of loglo(resistivity/R cm) us. temperature/K for the intercalate single crystals, SnS, -,S~,(COC~~)~.~~, where 0 5x I 2. (') SnSel .8SS0.1dc0(cP)2)0.3 1; (b) SnSe,(Co(Cp)Z}0.3 1; (c) SnS2{c0(cP)2 )O.31; (d) SnSe0.3S1 .7{c0(cP)2}0,31; SnSe0.7Sl .3{c0(cP)2}0,31; (f) Snsel ,3s0.7{c0(cp)2~0.31 through the series of organometallic intercalates SnX, (COC~,),,,,.~~The present conductivity study has confirmed this observation, but at the same time several other points of interest have emerged. First, a change in conduction mechan- ism has been observed on going from the host to intercalate semiconductors (x=O.O, 0.3, 0.7 and 1.3), and, secondly, there has been an observation of superconductivity in the metallic members (x = 1.85 and 2.0) of the intercalate series.For the semiconducting SnS2 -xSe,(CoCp2),, intercalates (x=O.O, 0.3, 0.7 and 1.3) it has been established that a Mott VRH law can be successfully applied to the experimental resistivity data as in Fig. 4. Previous XPS experiments2, indicate that these intercalate materials are mixed valent in both tin and cobalt. Upon intercalation roughly 10% of the Sn" sites in the host materials are reduced to Sn" and roughly 66% of the cobaltocene molecules are oxidised (Co" to Co"'). This can be interpreted by considering each cobaltocene molecule to be an electron donor impurity sitting adjacent to a tin acceptor atom with chalcogen atoms separating them.A rigid band model, which would predict a partially filled Sn(5s, 5p) acceptor band,12 does not seem to apply in these semiconducting sulphur-rich cases. There must be strong localising influences affecting the ionised impurity electron, such as the distortion of the Sn-X bond lengths upon Sn reduction and the presence of an attractive [CoCp,] poten-+ tial. In fact, the UVPES data provided evidence for a localised band (Sn 5s2) situated just below the empty conduction band in the intercalates (x I1.3). An Anderson model,36 similar to that used to interpret the electronic structure of disordered solids, might be applicable to these intercalate materials. Evidence from crystallographic studies suggests that the cobaltocene molecules are close- packed between the SnX, layers.37 However, disorder may arise in several different ways; for example, there may be an uneven distribution of chalcogen atoms in the SnX, layers.However, recent STM investigations have revealed that the chalcogen distribution may be random in WSSe single-crystal solid solutions.38 Nevertheless, the energy of Sn 5s2 states may well be scattered over a small energy range to give an Anderson band as in amorphous semiconductors. The Fermi level (EF) may be situated above the mobility edge in these intercalate (x=O.O, 0.3, 0.7 and 1.3) systems with the Sn 5s2 states localised in the band-gap region as envisaged in the Anderson localisation model. The mixed valency observed in the intercalate XPS data2, was interpreted as involving a hopping of impurity electrons between the cobaltocene and tin sites, the XPS timescale being very fast relative to the timescale of the hopping process.A mechanism of thermally activated hopping conduction between the cobalt and tin sites seems entirely reasonable on the basis of this discussion. The experimental data give an extremely good fit to the exp[( T0/T)'/4] dependence. The hopping mechanism in 6 dimensions yields a T''('+@ expre~sion,~~so the intercalate experimental data strongly suggest an isotropic three-dimensional (T-'I4) hopping pro- cess rather than a two-dimensional (T-1/3) process. The experiments carried out to investigate the anisotropy of con- duction in these intercalate systems suggest that the current carriers are not confined to a single layer to such an extent as in the host systems.The higher resistivity of some of these intercalates relative to their host compounds (see Table 3), despite substantial electron transfer to the Sn atoms, may depend on the limiting nature of the thermally activated hopping process rather than the presence of a large energy gap. For the SnSo.15Sel.85 and SnSe, hosts there is a dramatic increase in conductivity upon intercalation; a change from a semiconducting to a metallic conduction mechanism is also observed (see Table 3). The impurity band in these intercalates may now overlap the conduction band to some extent to give metallic character. Moving through the series towards the selenium-rich materials, the increasing conduction band width, the increasing screening effect of the chalcogenide layers and the increasing degree of band filling are all factors that may contribute to this changeover. An Anderson (insulator-to- metal) transition provides a good model for the electronic changes in these intercalate systems.Within this scheme the mobility edge is assumed to move past the Fermi level, so that states at the bottom of the impurity band are still localised, whereas states at the top of the band are considered to be delocalised. A rigid band model still seems inadequate even for the metallic intercalates, as this would not explain the observation of mixed tin valency in SnSe2(CoCp,),,,, by xPs.22 The anomaly in the resistivity versus temperature behaviour of the diselenide intercalate (Fig.6) may be related to some sort of structural phase transition taking place at ca. 100 K. The periodic lattice distortions (PLDs) that occur in some low-dimensional metals are driven by strong electron-phonon coupling interactions. In metallic 2H-TaS, and 2H-TaSe2. anomalies are found to arise in the resistivity data39 consistent J. MATER. CHEM., 1991, VOL. 1 with the formation of superlattices in these low-dimensional systems. Similar anomalies have been observed in systems such as the charge-transfer salt K-(BEDT-TTF),CU(NCS),.~ In the tin dichalcogenide intercalates the superconducting transition temperature (T,)increases as sulphur is replaced by selenium. This change can be understood in terms of the enhanced degree of electron charge transfer to the empty Sn(5s, 5p) conduction band as suggested by XPS data.7 This can be related to the increasing polarisability of the medium as selenium is added, since screening of the ionised electron from the [CoCp,] attractive potential becomes more effect- + ive.This would probably lead to a greater value in N(EF)for the pure diselenide relative to the other case, which would be expected to lead to a decrease in the T, value as predicted by BCS theory. Previous XPS data2, have indicated that unoxidised cobaltocene is in electronic equilibrium with the oxidised form in the SnX, intercalates. The superconductivity does not seem to be destroyed by the presence of the paramagnetic cobaltocene molecules between the SnX, layers. This can be compared to the TaS2(CoCp2)o.25 and TaS2(CrCp2)o.25 intercalates in which T, is roughly the same (ca.4 K) despite the presence of diamagnetic [CoCp,] and paramagnetic + [CrCp,] cations between the TaS, layer^.^ Local magnetic + moments are generally thought to be responsible for the breaking of Cooper pairs in a spin-disorder scattering mechanism. 4. Conclusion For this work it was essential to prepare high-quality single crystals of the layered tin dichalcogenides SnS, -xSex interca- lated with cobaltocene. The variation of the sulphur and selenium content has produced an interesting change in electronic structure (semiconducting to metallic) through this intercalate series.The main result of this work has been the successful confirmation of this transition, which was originally observed using photoelectron spectroscopy.22 The resistivity measurements for the semiconducting intercalates obey an exp[( T,/T)]1/4 temperature dependence. This suggests that these intercalated materials may conduct via a variable-range hopping mechanism, in contrast to their respective host compounds that obey the Arrhenius law. The metallic intercalates have also proved to be extremely interest- ing, not least in the sense that the SnS,,,,Se,~,, and SnSe, host materials are themselves semiconducting. Such semicon- ductor-to-metal transitions induced by intercalation are uncommon,12 but it is very rare for the resulting metal to be superconducting (T,z6 K).Another example is the metal KxMoS, (T,z7 K), one of a family of intercalates produced by alkali-metal insertion into the semiconducting MoS2 host.13 However, the present case is important as it involves the intercalation of an organometallic species into a layered structure with paramagnetic CoCp, molecules sitting in the van der Waals spaces. Further work will concentrate on the superconducting materials. It may be possible to monitor the changes in the superconducting transition temperature (T,)by isotopic substi- tution. Magnetoresistance measurements are also envisaged 'in order to determine the upper critical field (Bc2)value in these type I1 superconductors.The intercalation of other guest molecules into the SnX, host structure will also be a primary concern, despite the failed attempts at chromocene (CrCp,) and ferrocene (FeCp,) intercalation into the pure disulphide material.' We acknowledge Dr. J. W. Hodby of the Clarendon Labora- tory, Oxford for performing the magnetic susceptibility measurements and SERC for providing financial assistance. J. MATER. CHEM., 1991, VOL. 1 57 References 21 Y. Frongillo, M. Aubin and S. Jandl, Can. J. Phys., 1985, 63, 1405. 1 2 3 4 R. H. Friend and A. D. Yoffe, Adv. Phys., 1987, 36, 1. Physics and Chemistry of Materials with Layered Structure, ed. P. A. Lee, Reidel, Dordrecht, 1976, vol. 5. H. P. B. Rimmington and A. A. Balchin, Phys.Stat. 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ISSN:0959-9428    

DOI:10.1039/JM9910100051

出版商:RSC    

年代:1991

数据来源: RSC