Synthesis and crystal structure of α-Ba2ZrF8 and Pb2ZrF8 determined ab initio from synchrotron and neutron powder diffraction data

α-Ba₂ZrF₈ 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 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 also examined. An unexpected relationship with NaBaZrF₇ is discussed.