SYNTHESIS AND CRYSTAL-STRUCTURE OF ALPHA-BA2ZRF8 AND PB2ZRF8 DETERMINED AB-INITIO FROM SYNCHROTRON AND NEUTRON POWDER DIFFRACTION DATA

alpha-Ba2ZrF8 is prepared as systematically twinned crystals by hydrot hermal synthesis (200 degrees C) or as fine powders either on a sand b ath (60 degrees C) (aqueous HF medium) or by solid state reaction at 4 50 degrees C. Synchrotron radiation was used because of ambiguities in indexations from conventional X-ray (pseudo-hexagonal symmetry). The structure was determined ab initio from synchrotron powder data. Neutr on data were used for improving accuracy because some degree of non-st oichiometry was suspected. The cell is orthorhombic, space group Pnma, Z = 4, with a = 9.7401(1) Angstrom, b = 5.6147(1) Angstrom and c = 11 .8871(1) Angstrom (synchrotron data, 25 degrees C). The final neutron Rietveld refinement led to R-P = 8.4 % and R-B = 3.5 % for the stoichi ometric Ba2ZrF8 formulation (sand bath sample). The structure is built up from [ZrF8] bicapped trigonal prisms isolated in a kinked fluorite matrix.The isostructural Pb2ZrF8 compound prepared in solution is als o examined. An unexpected relationship with NaBaZrF7 is discussed.