X-RAY AND NEUTRON-DIFFRACTION STUDY OF NH4TAF6

NH4TaF6 was obtained by the reaction of Ta3N5 with dry HF gas at 553 K . It is an air-sensitive crystalline powder that hydrolyses readily vi a (NH4)2Ta2O3F6 to TaO2F. Dependent on temperature three crystalline m odifications of NH4TaF6 are observed. Above room temperature NH4TaF6 c rystallizes in the cubic space group Pm3BARm with a = 5.2718(1)angstro m. Between room temperature and 150 K a rhombohedral phase with the sp ace group R3BAR and a = 5.12(1)angstrom, alpha = 96.23(4)-degrees exis ts. The modification below 150 K, is presumably triclinic or monoclini c. The crystal structures of the cubic and the rhombohedral phases wer e determined by Rietveld refinements of X-ray and neutron powder diffr action data. In the cubic high temperature modification both the NH4and the TaF6- ions exhibit a rotational disorder, while in the interme diate rhombohedral phase only the NH4+ cation is disordered. For the l ow temperature phase a complete ordering is expected.