Synthesis and crystal structure of α-Ba 2ZrF 8 and Pb 2ZrF 8 determined ab initio from synchrotron and neutron powder diffraction data

Abstract

α-Ba 2ZrF 8 is prepared as systematically twinned crystals by hydrothermal synthesis (200°C) or as fine powders either on a sand bath (60°C) (aqueous HF medium) or by solid state reaction at 450°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. Neutron data were used for improving accuracy because some degree of non-stoichiometry was suspected. The cell is orthorhombic, space group Prima, Z = 4, with a = 9.7401(1) Å, b = 5.6147(1) Å and c = 11.8871(1) Å (synchrotron data, 25°C). The final neutron Rietveld refinement led to R P = 8.4 % and R B = 3.5 % for the stoichiometric Ba 2ZrF 8 formulation (sand bath sample). The structure is built up from [ZrF 8] bicapped trigonal prisms isolated in a kinked fluorite matrix. The isostructural Pb 2ZrF 8 compound prepared in solution is also examined. An unexpected relationship with NaBaZrF 7 is discussed.