J O U R N A L O F C H E M I S T R Y Materials Determination of the fluorite related structure of Mn3Ta2O8 using synchrotron X-ray powder and electron diVraction data Saeid Esmaeilzadeh,a Jekabs Grinsa and Andy Fitchb aDepartment of Inorganic Chemistry Arrhenius Laboratory Stockholm University SE-106 91 Stockholm Sweden bESRF BP 220 F-380 43 Grenoble Cedex France Received 29th June 1998 Accepted 4th August 1998 The Mn2+-containing oxide Mn3Ta2O8 has been synthesised at 1200 °C in Ar atmosphere and its structure has been solved from X-ray synchrotron powder data (l#0.65 A° ) by direct methods. The structure was refined by the Rietveld method to RF=6.3% with space group I41/a a=11.2728(2) c=9.8030(3) A° V=1243.47 A° 3 from 1190 reflections with d0.65 A° . It is related to the fluorite structure with a#Ó5af and c#2af.The Ta atoms are octahedrally coordinated by oxygen atoms and the three crystallographically diVerent Mn atoms by 7 4 +4 and 4 oxygen atoms. Electron diVraction patterns show the presence of weak superstructure reflections corresponding to a primitive unit cell with a¾=a and c¾=6c. The melting point of Mn3Ta2O8 is 1470 °C in Ar atmosphere. It is a semiconductor with an activation energy of 1.2 eV and a conductivity s=3.7×10-5 S cm-1 at 600 °C. The magnetic susceptibility shows a maximum at 23 K and a Curie–Weiss behaviour at higher temperatures with meff=5.7(1) mB per Mn atom.When Mn3Ta2O8 is oxidised at 1100 °C in air an Mn–Ta oxide forms which has a wolframite type structure with unit cell a=4.7574(5) b=5.7296(6) c=5.1133(4) A° and b=91.202(9)°. 900 °C for 12 h and then at 1200 °C for 12 h with intermediate Introduction grinding and re-pelleting.The obtained sintered pellets were The phase relations of Mn-Ta oxides have been investigated brown. Higher and lower Mn contents yielded materials with by Turnock,1 at 1200 °C and partial pressures of oxygen from Mn4Ta2O9 and MnTa2O6 respectively as secondary phases. 10-17 to 1 atm. The oxides observed were the orthorhombic Corresponding syntheses above 1350 °C yielded partially columbite-type MnTa2O6 Mn4Ta2O9 with a corundum-related melted materials that contained Mn4Ta2O9 as the major phase. structure and four compounds MnTaO4 Mn1.4TaO4.2 The Mn3Ta2O8 phase was described by Turnock1 as Mn1.4TaO3.9 and Mn6Ta2O11 with structures undetermined at Mn1.4TaO3.9 stable only above 1160 °C and oxygen partial the time.Two additional Mn–Ta oxides have been reported pressures below ca. 2.5×10-2 atm. at 1200 °C. by Scho�nberg,2 namely the metallic compound Mn3Ta3O with A Guinier–Ha�gg camera with Cu-Ka1 radiation was used the g-carbide type structure and Mn2TaO3 with an alleged for collection of powder diVraction patterns for phase identifi- CoSn (B35 type) structure. The latter of these phases appears cation. The films were measured with a computer-controlled to be quite unique and questionable considering the low microdensitometer. oxidation state of Ta. X-Ray powder diVraction data for structure determination Several of the above Mn2+-containing Mn–Ta oxides were and refinement were collected from a sample contained in a prepared by us for use as precursor materials in the synthesis 0.2 mm diameter spinning capillary on beam line BM16 at the of new Mn–Ta oxynitrides.3 Since the structures of most of ESRF Grenoble with l=0.652 782(3) A° in the 2h range them including ‘Mn6Ta2O11’ and ‘Mn1.4TaO3.9’ were not 1–57°.The detector arm with its nine detectors was scanned known we have investigated their structures and properties. at a continuous rate of 0.5 degrees min-1 and the electronic A study of ‘Mn6Ta2O11’ showed that the correct composition scalers and 2h encoder were read every 200 ms. The data were is Mn11Ta4O21.4 The crystal structure is trigonal (space group) subsequently normalised and rebinned and the counts from with a=5.3776(2) c=34.040(2) A° and can be described as the nine channels combined to yield the equivalent scan with built up from corundum-type Mn4Ta2O9 blocks alternating a step of 0.002°.The direct methods program SIRPOW917 with single MnO layers of octahedra. was used for solving the structure and the GSAS program The compound Mn3Ta2O8 is the oxide described by Turnock package8 for structure refinements. as ‘Mn1.4TaO3.9’. The present study comprises a determination Metal elemental analyses were made in a JEOL 820 SEM of its structure and characterisation of its thermal magnetic (scanning electron microscope) with the EDX (energy and electrical properties. The structure is found to be related dispersive X-ray) analysis system LINK 10000. to the cubic MX2 fluorite type. A vast variety of structure A JEOL 2000 FX microscope operated at 200 kV and with types can be derived from fluorite by ordered removal of the a double tilting goniometer with limitations of ±45° was used X atoms and/or ordering of metal atoms among them the for collecting electron diVraction (ED) patterns.The specimens A2B2X7 (MX1.75) pyrochlore5 and the rare-earth RE2X3 were crushed dispersed in butanol and then transferred to (MX1.5) C-type.6 Mn3Ta2O8 (MX1.6) is a new type within the holey carbon-coated copper grids. class of MX2 fluorites and related structures. Thermal analysis was carried out with a SETARAM Labsys TG-DTA16 instrument. The recordings were made in both Ar atmosphere and air and with heating/cooling rates of Experimental 5–10 °Cmin-1. The magnetic susceptibility was measured with a weak-field Samples of Mn3Ta2O8 were prepared by solid state reaction ac-susceptometer (Lake Shore 7130) in the temperature range in Ar atmosphere using a mixture of appropriate amounts 10–300 K using a magnetic field of 500 A m-1 and a frequency (3:1) of Mn(C2O4) and fine-grained Ta2O5 heated in Ni crucibles in a graphite furnace.The samples were fired at of 500 Hz. J. Mater. Chem. 1998 8(11) 2493–2497 2493 Table 2 Atomic coordinates for Mn3Ta2O8; [tetragonal a= The electrical conductivity was determined with an auto- 11.2782(2) A° c=9.8030(3) A° I41/a Z=4]. mated impedance spectrometer.9 Data were collected at 400–600 °C in the frequency range 10 Hz–1 MHz. The Atom Site x y z U/A° 2 measurements were made in N2 atmosphere on sintered discs 0.5 cm2 in area and 0.1 cm thick with a porosity of ca. 20% Ta 16f 0.42257(7) 0.05661(9) 0.1300(1) 0.00641(2) Mn1 16f 0.3780(5) 0.0505(3) 0.5949(3) 0.0077(1) and furnished with gold electrodes.Mn2 4a 0 1/4 1/8 0.0073(2) Mn3 4b 0 1/4 5/8 0.0071(2) Results O1 16f 0.362(1) 0.198(1) 0.221(1) 0.0043(3) O2 16f 0.460(1) 0.893(1) 0.013(1) 0.0043(2) Compositional analysis O3 16f 0.196(1) 0.312(1) 0.997(1) 0.0056(3) O4 16f 0.310(1) 0.961(1) 0.243(1) 0.0048(2) SEM investigations of polished surfaces of sintered Mn3Ta2O8 compacts revealed an average crystallite size of 10 mm. Twenty EDX point analyses on individual grains yielded a metal composition of 61(1) % Mn and 39(1)% Ta in agreement atoms. The remaining fourth oxygen atom was later located with the starting composition. from diVerence Fourier maps. The structure was refined using 1190 reflections in the 2h X-Ray powder diVraction range 5–57° (d0.65 A° ). The background was fitted by 20 The Guinier–Ha�gg powder pattern of Mn3Ta2O8 was indexed Chebyshev polynomial coeYcients.The half-width of the with a body-centred unit cell using the TREOR90 version of Bragg peaks was 0.014° at 2h=21.4°.MnTa2O6 and Mn4Ta2O9 the indexing program TREOR.10 The cell dimensions a= were included as secondary phases with collective thermal 11.2728(2) c=9.8030(3) A° were obtained using Si as internal parameters lattice parameters and phase fractions refined. A standard and 80 reflections for 2h<88°. The indexed powder total of 74 parameters were refined including 18 positional pattern is given in Table 1 for the first 20 observed lines. It and 8 thermal parameters for Mn3Ta2O8. The obtained atomic agrees with that given by Turnock1 but contains additionrdinates are given in Table 2.Selected interatomic weak reflections and the recorded d values are more accurately distances and bond valence sums11 are given in Table 3 determined. Systematic absences in the powder pattern (hk0 together with expected bond distances calculated from h,k<2n;00l l<4n) indicated the space group I41/a to be Shannon–Prewitt ionic radii:12 O2-(IV)=1.38 A° Ta5+(VI )= possible. The pattern showed furthermore that the sample 0.64 A° Mn2 +(HS,IV )=0.66 A° Mn2 +(HS,VII )=0.90 A° contained small amounts of Mn4Ta2O9 and MnTa2O6 as Mn2+(HS,VIII)=0.96 A° . The fit between observed and calcuimpurities. lated patterns is illustrated in Fig. 1. The corresponding refinement indices are Rwp=0.100 Rp=0.071 DwD=0.90 Structure determination and refinement RF=0.063 and x2=4.8. The refined phase fractions of MnTa2O6 and Mn4Ta2O9 were 8.7(1) and 1.5(1) wt.% The synchrotron powder pattern did not exhibit any more respectively corresponding to a total Mn content of 58.3%.Bragg reflections than those observed in Guinier–Ha�gg films and could thus not validate the lower symmetry and larger unit cell implied by the ED study (see below). It was therefore decided to solve the structure using the unit cell obtained from Table 3 Bond distances (A° ) and bond valence sums for Mn3Ta2O8. the X-ray data with a#11.3 and c#9.8 A° . Ta–O3 1.94(2) Mn1–O3 1.98(2) Mn2–O4 4×2.24(1) Observed integrated intensities were extracted from a part O1 1.95(1) O3 2.06(1) O4 4×2.63(1) of the synchrotron data and converted to |F|2 values. A partial O4 2.00(1) O1 2.18(1) mean 2.44 expected 2.34 structure was then derived in space group I41/a by direct O2 2.01(1) O1 2.34(1) methods.The SIRPOW91 program successfully located the O4 2.02(1) O2 2.44(1) Mn3–O2 4×2.00(1) positions of the metal atoms and three out of the four oxygen O2 2.21(1) O4 2.47(1) mean 2.00 expected 2.04 O4 2.57(1) mean 2.02 expected 2.02 mean 2.29 expected 2.28 Table 1 Observed and calculated 2h values for the Guinier–Ha�gg bond valence sums Ta +4.74 O1 -1.86 diVraction pattern of Mn3Ta2O8 up to the twentieth observed line. Mn1 +2.07 O2 -1.93 D2h=2hobs-2hcalc. [l=1.5406 A° cell figure-of-merit M20=98 F20= Mn2 +1.51 O3 -2.12 133 (0.0060 26)]. Mn2 +2.29 O4 -1.86 hkl 2hobs/degrees D2h dobs/A° I/I0 1 0 1 11.955 0.001 7.40 7 2 0 0 15.704 -0.006 5.64 1 1 2 1 19.785 -0.002 4.484 43 1 1 2 21.256 -0.007 4.177 8 2 2 0 22.292 0.005 3.985 7 3 0 1 25.363 -0.001 3.509 14 1 0 3 28.424 0.009 3.137 6 2 3 1 29.964 -0.011 2.980 2 3 1 2 30.977 -0.017 2.885 100 2 1 3 32.629 -0.002 2.742 11 4 1 1 34.017 0.003 2.633 3 4 2 0 35.576 -0.011 2.521 24 3 0 3 36.418 0.010 2.4651 1 0 0 4 36.636 -0.002 2.4509 9 3 3 2 38.501 -0.009 2.3364 1 3 2 3 39.881 0.007 2.2587 3 4 2 2 40.203 0.006 2.2413 2 5 0 1/4 3 1 41.044 -0.002 2.1973 11 4 1 3 43.112 0.007 2.0966 3 Fig.1 Observed (crosses) calculated (solid line) and diVerence 2 2 4 43.308 0.002 2.0875 1 (bottom) X-ray diVraction patterns of Mn3Ta2O8 for 2h=17–29°. 2494 J. Mater. Chem. 1998 8(11) 2493–2497 Electron diVraction group symmetries. The radius of the first order Laue zone indicated furthermore a c axis of ca. 30 A° . The crystal was ED patterns of Mn3Ta2O8 showed reflections that matched then tilted around the b axis.In the [1039] zone [Fig. 1(b)] the I-centred unit cell with a#11.3 and c#9.8 A° that was there are relatively strong superstructure reflections with indiderived from the X-ray data but also sets of weaker reflections ces such as 1 0 1/3 and 2 0 2/3 which can be accounted for that implied both a lower symmetry and a larger unit cell. These by a primitive cell with a tripled c axis. In the [1902] zone superstructure reflections were yielded by all crystallites [Fig. 1(c)] there are weak reflections present with indices such studied. as 1 0 1/2 implying a doubled c axis. The smallest unit cell DiVraction patterns along [001] [1039] and [1902] are shown able to index all reflections is thus found to be a primitive in Fig. 2. The strongest reflections in the <001> zone tetragonal with a¾=a and c¾= 6c#58.8 A° .The sets of super- [Fig. 1(a)] correspond to a fluorite-type subcell. The a axis of structure reflections have diVerent relative intensities. Those the I-centred unit cell is defined by the reciprocal lattice vector that indicate a tripled c axis are the strongest the ones that a*=1/5(2a*f+b*f) as illustrated. The presence of weaker indicate a primitive cell are moderately strong and the ones reflections of the type 100 shows however that the cell is indicating a doubled c axis are the weakest. primitive and thus also that the glide plane perpendicular to Attempts were made to refine the structure both with a the a axis is absent. This suggests P41 or P49 as possible space lower symmetry and/or larger unit cells as implied by the electron diVraction data.Space group symmetries such as I49 P49 and P41 and larger unit cells with c¾=3c and c¾=6c were tried. None of these refinements yielded any significant improvements. This was partly expected since the powder pattern manifests no reflections corresponding to a primitive or larger cell and any information about a superstructure is thus provided only by the main reflections. Refinements with anisotropic thermal parameters were also carried out in order to see if any atoms would show anomalous thermal displacements but none were revealed and these refinements yielded only insignificantly lower R values. Structure description The structure derived for Mn3Ta2O8 is illustrated in Fig. 3. There are four diVerent metal positions one occupied by Ta and three by Mn atoms.The metal atoms are nearly in cubic close packing and the positional shifts from the metal array found in a cubic fluorite structure are small. The metal cubooctahedra around the Mn and Ta atoms are only slightly distorted and the metal–metal distances have a mean value of 3.54 A° and range between 3.23 and 3.81 A° . The structure can be envisaged as built up from four layers of metal–oxygen atom polyhedra that are related to each other by the 41 axis in the c direction. One such layer is shown in Fig. 3(a). The Ta atoms are coordinated by six O atoms forming a distorted octahedron a common coordination polyhedron for Ta5+. The Ta–O mean distance is 2.023 A° which agrees well with an expected value of 2.02 A° . The structure contains pairs of Ta–O octahedra that share an edge and these pairs are in turn connected to each other by cornersharing.The framework formed by the Ta–O octahedra is illustrated in Fig. 3(b). The Mn1 atoms are coordinated by seven O atoms at distances of 1.98(2)–2.57(1) A° . The coordination polyhedra can be idealised as a cube with one corner missing. The mean distance 2.29 A° agrees well with an ionic radius sum of 2.28 A° for Mn2+(HS,VII ) and O2-(IV). The Mn1 atoms are located at positions that are displaced from the Ta atom positions by z#1/2. The structure thus contains strings of Mn1–empty–Ta–empty polyhedra along the c axis related to each other by a 49 symmetry axis. The Mn2 and Mn3 atoms are found on the special positions 4a and 4b respectively both with site symmetries 49 in the channels between the Mn1–Ta strings of polyhedra as seen in Fig.3(b). The Mn2 atoms are 4+4 coordinated by O4 atoms at distances of 2.24(1) and 2.63(1) A° . Each set of O4 atoms forms a tetrahedron and the resulting 8-coordination polyhedron may be described as a distorted cube. The mean Mn2–O distance is 2.44 A° which is longer than the expected value of 2.34 A° . Mn3 is tetrahedrally coordinated by four O2 atoms. The observed Mn3–O2 distance 2.00(1) A° is somewhat shorter than the expected value of 2.04 A° . Bond valence Fig. 2 Electron diVraction patterns for Mn3Ta2O8 along (a) [001] (b) [1039] and (c) [1902]. sums accord with the above bond-length considerations. While J. Mater. Chem. 1998 8(11) 2493–2497 2495 Fig. 4 Molar magnetic susceptity per Mn atom (a) and its inverse (b) versus temperature for Mn3Ta2O8.conductivity determined at 600 °C was s=3.7×10-5 S cm-1 and the relative dielectric constant was calculated from the capacitance to be 30. The temperature dependence of the conductivity was found to be characteristic of a semiconductor with an approximate activation energy value of 1.2 eV. Fig. 3 An illustration of the structure of Mn3Ta2O8 (a) a polyhedral Thermal analysis representation of a (001) section of the structure; (b) a polyhedral A DTA recording for Mn3Ta2O8 heated and then cooled in representation of the Ta atom arrangement. The positions of the Mn1 inert Ar atmosphere at a rate of 10 °Cmin-1 is shown in and Mn2/Mn3 atoms are illustrated by filled and open circles respectively. Fig. 5. The weight change of the sample after the heating– cooling run amounted to less than 0.5%.Small endothermal peaks are observed upon heating at ca. 1340 and 1380 °C they are reasonably satisfactory for the Ta Mn1 and O atoms before Mn3Ta2O8 melts at ca. 1470 °C. These may either too high and too low values are obtained for Mn3 and Mn2 respectively. Magnetic susceptibility The magnetic susceptibility per Mn atom xM of Mn3 Ta2O8 and its inverse xM-1 are shown in Fig. 4 as functions of the temperature T. The susceptibility shows a well-defined maximum at 23 K and a Curie–Weiss law behaviour xM=C/(T-h) above ca. 100 K. The eVective number of Bohr magnetons per Mn atom (meff) was determined from the Curie constant C to be 5.7(1) mB which is close to the expected value of 5.9 mB for Mn2+ in a high-spin state. Electrical conductivity Impedance spectra for Mn3Ta2O8 were measured in a heating –cooling cycle between 400 and 600 °C.The data showed no polarisation at the electrodes and the conductivities were Fig. 5 DTA recordings for Mn3Ta2O8 heated and then cooled in Ar atmosphere. calculated from the low-frequency parts of the spectra. The 2496 J. Mater. Chem. 1998 8(11) 2493–2497 suggest that the phase is of the wolframite (FeWO4) type.13 Its structure composition and formation will be further investigated. Concluding remarks There are a large number of structures related to the fluorite structure which can be derived from it by ordered removal of oxygen atoms or ordering of metal atoms. Mn3Ta2O8 is a new member of this class of compounds. Among the cations of the first-row transition metals the largest ionic radius is found for Mn2+ and the crystal chemistry of oxides containing Mn2+ and Ta5+ ions is expected to be influenced by this fact and by the comparatively large diVerence in size of these cations.The stability of fluorite-related structures increases with the size of the cation and such structures may therefore be expected for higher Mn2+ contents. The metal-to-oxygen ratio decreases Fig. 6 TG curve for the oxidation of Mn3Ta2O8 in air. with increasing Mn2+ content however and the Mn-rich compounds Mn4Ta2O9 (MO1.5) and Mn11Ta4O21 (MO1.4)4 have structures that are related to corundum. The Mn3Ta2O8 originate from phase transitions of Mn3Ta2O8 or be associated compound is accordingly found at an Mn–Ta–O composition with the impurity phases MnTa2O6 and Mn4Ta2O9 present. that satisfies both a relatively high metal-to-oxygen ratio Upon cooling an exothermal reaction starts at approximately MO1.6 and a high Mn content.1440 °C characteristic of a passage through a liquidus curve We have not been able to determine the superstructure and subsequent eutectic or peritectic solidification. In addition implied by the ED studies of Mn3Ta2O8 from the present X- a smaller exothermal transition occurs at ca. 1320 °C indicatray synchrotron data. It is very likely however that the ing a solid-state phase transition. superstructure with c¾=6c is associated with either modu- A TG recording of the oxidation of Mn3Ta2O8 in air upon lations of oxygen atom positions and/or an extended ordering heating at a rate of 5 °Cmin-1 is shown in Fig. 6. The largest of oxygen atoms around Mn2 and Mn3.part of the oxidation occurs in the temperature range Finally it can be remarked that presently available data 300–600 °C and is exothermal. A following smaller increase in indicate a series of uncharacterised phases in the system weight is observed up to ca. 850 °C and two broad endother- Mn2+–Mn3+–Ta5+–O among them the wolframite type phase mal reactions take place above 600 °C. The calculated metal– mentioned above and the phases denoted by Turnock1 as oxygen compositions at the observed plateaus at 600 and MnTaO4 and Mn1.4TaO4.2. Studies of phase formation and 850 °C are MO1.72 and MO1.75 respectively. crystal structures in these systems are presently in progress. A Guinier–Ha�gg film of the sample heated to 1100 °C in We have recently characterised a new cubic fluorite phase the TG run showed that a new phase had formed.Its powder Mn0.6Ta0.4O1.65 by techniques which include XRPD selected- pattern could be indexed by a monoclinic unit cell with a= area ED and high-resolution electron microscopy.14 The ED 4.7574(5) b=5.7296(6) c=5.1133(3) A° b=91.202(9)° and patterns of the phase exhibit prominent diVuse scattering V=139.35 A° 3. The pattern also contained reflections from similar to that reported for other cubic oxygen-deficient fluorite tetragonal Mn3O4 (JCPDS No. 24-734) and three additional compounds. reflections with relative intensities below 6% which could not be attributed to any reported oxide containing Mn and/or Ta. The authors thank Dr. T. Ho� rlin for help with the conductivity The indexed pattern for the new phase is given in Table 4 for measurements.Prof. M. Nygren is thanked for help with the the first 20 observed lines. The unit cell systematic reflection thermal analysis and for support and valuable discussions. absences (h0l l<2n) and the reflection intensities strongly This work has been financially supported by the Swedish Natural Science Foundation. Table 4 Observed and calculated 2h values for the Guinier–Ha�gg diVraction pattern of the Mn–Ta wolframite type phase up to the twentieth observed line. D2h=2hobs-2hcalc [l=1.5406 A° cell figure- References of-merit M20=85 F20=92 (0.0063 35)]. 1 A. C. Turnock J. Am. Ceram. Soc. 1966 49 382. hkl 2hobs/degrees D2h/degrees dobs/A° I/I0 2 N. Scho�nberg Acta Metall. 1955 3 14. 3 J. Grins P.-O. Ka� ll and G. Svensson J. Solid State Chem. 1995 0 1 0 15.464 0.011 5.73 1 117 48.1 0 0 18.640 0.000 4.756 9 4 J. Grins and A. Tyutyunnik J. Solid State Chem. 1998 137 276. 0 1 1 23.305 0.004 3.814 8 5 M. A. Subramanian G. Aravamudan and G. V. Subba Rao Prog. 1 1 0 24.295 -0.006 3.661 50 Solid State Chem. 1983 1 55. -1 1 1 29.770 0.001 2.999 99 6 R. Norrestam Ark. Kemi 1968 29 343. 1 1 1 30.230 -0.008 2.954 100 7 G. Cascarano L. Favia and C. Giacovazzo J. Appl. Crystallogr. 0 2 0 31.183 -0.013 2.866 19 1992 25 310. 0 0 2 35.082 0.003 2.556 30 8 A. C. Larson and R. B. Von Dreele Los Alamos National 0 2 1 35.904 -0.001 2.4992 55 Laboratory Report No. LA-UR-86-748 1987. 2 0 0 37.796 -0.002 2.3783 17 9 T.Ho� rlin Chem. Scr. 1985 25 270. -1 0 2 39.656 0.010 2.2709 4 10 P.-E. Werner L. Eriksson and M. Westdahl J. Appl. Crystallogr. 1 2 1 40.934 0.002 2.2029 8 1985 18 367. -1 1 2 42.797 0.007 2.1113 9 11 I. D. Brown and D. Altermatt Acta Crystallogr. Sect. B 1985 1 1 2 43.494 0.018 2.0790 10 41 244. 2 1 1 45.196 -0.013 2.0046 3 12 R. D. Shannon Acta Crystallogr. Sect A 1965 32 258. 0 2 2 47.638 -0.004 1.9074 10 13 H. Weitzel Z. Kristallogr. 1976 144 238. 2 2 0 49.792 0.001 1.8298 11 14 S. Esmaeilzadeh J. Grins and A.-K. Larsson in preparation. 1 3 0 51.533 0.010 1.7720 35 -2 0 2 51.928 0.006 1.7595 18 Paper 8/04938K -2 2 1 52.828 0.004 1.7316 23 J. Mater. Chem. 1998 8(11) 2493&nda