Acentricity and phase transitions for some AM2X6 compounds

Mat. Res. Bull. Vol. 13, pp. 1247-1250, 1978. Pergamon Press, Inc. Printed in the United States.

ACENTRICITY AND PHASE TRANSITIONS FOR SOME AM2X 6 COMPOUNDS A. W. Sleight, F. C. Zumsteg, J. R. Barkley, and J. E. Gulley Central Research and Development Department E. I. du Pont de Nemours and Company Wilmington, Delaware 19898

(Received September 26, 1978; Refereed)

ABSTRACT Phases of the type AM2X 6 with a pyrochlore related structure were studied by x-ray diffraction, DSC, and second harmonic techniques and in one case by T1205 NMR. Although these phases are ideally centric and cubic, several were found to be acentric. Two, TINbWO 6 and RbNbWO 6 were found to be noncubic, but transitions to higher symmetry were observed for both phases above room temperature.

Introduction Phases of the type AM2X 6 with a pyrochlore related structure were first reported by Babel, et al. (i). In this structure A is a univalent cation such as Rb; X is oxygen and/or fluorine, and M is a cation octahedrally coordinated by X. The chemistry of these phases has been extensively studied by Raveau and coworkers (2). These ~M2X 6 phases are ideally cubic and centric with_the space group Fd3m. T h e structure may be viewed as a (M2X6) l~~yWi~et~+A~:t~°~eS~dl~s~e~tia~h~t~

e

readily ion exchanged (2), and we have reported on ionic conductivity for some of these phases (3). The purpose of this paper is to report on departures from ideal symmetry which we have observed for several of these AM2X 6 compounds. Experimental The compounds were prepared according to previously described procedures (2,3). X-ray diffraction patterns were

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obtained on all samples using a H~gg-Guinier camera using CuK~ radiation and an internal standard of high purity KCI (a = 6.2931~ at 250C). The acentric character was determined by measuring the second harmonic radiation generated in apparatus similar to that described by Perry and Kurtz (4) but using a variable temperature cell. Differential scanning calorimetry (DSC) measurements were made with a Du Pont 990 thermal analyzer. NMR measurements on TINbWO 6 were made with a broadline cw spectrometer. Results Second harmonic signals were observed for several of the AM206 phases. The values relative to quartz for these cases are: TINbW06, 217; TITaW06, 2; RbNbW06, 16; RbTaWO 6, 2; and CsNbWO 6, 4. Thus, these phases are clearly acentric at 25°C. The second harmonic signal was either undetected or too small to be definitive for CsTaW06, CsSbWO 6, CsNb2OsF, CsTa205F, RbSbWO 6, RbNb205 F, RbTa205F, RbNbTeO 6, KTaTe06, KNbTeO 6, TINb2OsF, and TITa205F. Thus, these phases are probably centric at 250C. No departure from cubic symmetry could be detected in the x-ray patterns for any of the above phases except TINbW06 and RbNbWO~. Tetragonal sxmmetry was indicated for RbNbWO 6 with a = I0.360A and c = I0.379A for the face-centered cell. The symmetry was even lower for TINbWO 6, but no definite conclusion concerning the actual symmetry was made. The temperature dependence of the second harmonic signal was studied for TINbWO 6 and RbNbWO 6. The results for TINbWO 6 are shown in Figure i. Similar results were found for RbNbWO 6 indicating that both phases are centric by 120°C. The loss of second harmonic signal is gradual; thus, the actual transition temperatures are difficult to determine by this method. A definite peak was found for TINbWO 6 by DSC at 85°C, and this would be our best definition of the transition temperature. A definite transition by DSC was not observed for RbNbWO 6.

20

FIG. i

T1205 NNR linewidth AH (GAUSS)

(solid line) and second harmonic signal (broken line with arbitrary units) vs. temperature.

I0

;

5'0

,;o

,;o I" (°C)

2~o

2~o

3ou

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AM2X6 COMPOUNDS

1249

The NMR linewidth for T1205 in TINbWO~ is also shown in Figure I. The observed line narrowing indlcates that the TI I+ cations are loosely held above about 80°C and appears to be correlated with the SHG and DSC data. Attempts were made to grow crystals of CsNbWO 6 and RbNbWO 6 from fluxes of cesium tungstate and rubidium tungstate, respectively. Crystals with the desired structure were indeed grown in this way. However, inspection of these crystals indicated that they did not possess the ideal stoichiometry. Nonstoichiometry of the type Al+xNbl+xWl-xO 6 is known (5,6) for these phases where x can take on positive or negative values. Apparently this type of nonstoichiometry occurred for our crystals. The crystals of Csl+~Nbl+xWl_x06 appeared cubic by optical examination as expected-]- However,-these crystals did not show a second harmonic signal. This suggests that x is nonzero since polycrystalline samples of stoichiometric CsNbWO 6 show a second harmonic signal. The crystals of Rbl_xNbl+xWl_xO 6 were not homogeneous. Some crystals were entirely birefringent, but others contained regions that were apparently cubic. Several crystals which appeared homogeneously birefringent were heated on a hot stage of a polarizing microscope. The birefringence began to decrease at about 85°C and had basically disappeared by about i15°C. This optical examination showed a definite hysteresis for the cubic-noncubic transition. Hysteresis was also observed in the SHG experiments on polycrystalline RbNbWO 6. Thus, this transition is first order. The refractive index of a Rbl+xNbl+xWl_xO 6 crystal was found to be 2.14. Attempts to generate a ferroelectric loop for RbNbWO 6 at room temperature were unsuccessful. Thus, ferroelectricity has not been demonstrated. The dielectric properties of all these AM206 phases are complicated by the significant ionic conductivity of A (3). Discussion The structure of the phases under consideration here is ideally centric and cubic with the point group m3m. However, we have discovered that some of these phases are not centric and that two are not cubic. The acentric cubic phases must have point groups of ~3m or 23 since these are the only two cubic point groups which allow a second harmonic signal. Phases in the Sn-Nb-O and Sn-Ta-O systems having pyrochlore related structures are also acentric and apparently cubic at room temperature (7). The broadness of the transitions for TIWNbO 6 and RbWNbO 6 near 90°C is probably due to the inherent disorder caused by the statistical occupancy of Nb and W on the octahedral sites. Similar broad transitions are observed for many perovskites where two or more different cations statistically occupy the octahedral sites (8).

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If the second harmonic signal of a niobate A-M2X6 phase is compared to its tantalate counterpart, the higher §i~nal is always observed for the niobate. This is consistent with the fact that Nb-O bonds have a greater nonlinear polarizibility than Ta-O bonds (9). The higher second harmonic signal when A is TI + would similarly be attributed to the higher polarizibility of TI +. References i. D. Babel, G. Pausewung, and W. Viebahn, 22, 1219 (1967).

Z. Naturforshung.

2. See for example, C. Michel and B. Raveau, Mat. Res. Bull. 8, 451 (1973). A complete listing is given in Reference 3. 3. A. W. Sleight, J. E. Gulley, and T. Berzins, Centennial ACS Meeting of the American Chemical Society, New York, NY, April 1976, Abstract INOR 67 and "Solid State Chemistry of Energy Conversion and Storage" Advances in Chemistry Series, No. 163, American Chemical Society, Washington, DC, 1977, pp. 195-204. 4. S. K. Perry and T. T. Kurtz, J. Appl. Phys. 39, 3798 (1968). 5. G. Allais, C. Michel, 1625 (1972).

and B. Raveau,

Comptes rendus 274C,

6. R. Sabatier and G. Baud, J. Inorg. Nucl. Chem. 34, 873 (1972). 7. T. Birchall and A. W. Sleight, (1975). 8. G. A. Smolensky,

J. Solid State Chem. 13, 118

J. Phys. Soc. Jap. 28, 26 (1970).

9. J. G. Bergman and G. R. Crane, J. Solid State Chem. 12, 172 (1975).