Abstract
Single crystals of (NH(4))(3)VOF(5) and (NH(4))(3)VO(2)F(4) were obtained from aqueous fluoride solutions and phase transitions in these compounds were investigated using X-ray diffraction, differential scanning microcalorimetry (DSM) and vibrational spectroscopy. The room-temperature (RT) phases of these compounds belong to orthorhombic symmetry [Immm and I222, Z = 6, for (NH(4))(3)VOF(5) and (NH(4))(3)VO(2)F(4), respectively] with similar unit-cell parameters and two independent vanadium atoms. Above RT [at 350 and 440 K for (NH(4))(3)VOF(5) and (NH(4))(3)VO(2)F(4), respectively], the compounds undergo reversible phase transitions into high-symmetry dynamically disordered elpasolite-like ([Image: see text], Z = 4) structures with six and 12 spatial orientations of the vanadium octahedron for (NH(4))(3)VOF(5) and (NH(4))(3)VO(2)F(4), respectively. The ligand atoms are distributed in a mixed (split) position of 24e + 96j, one of the ammonium groups is disordered on the tetrahedron 32f site, but another one forms eight spatial orientations due to disorder of its hydrogen atoms in the 96j position. DSM and spectroscopic data enable the phase transition from high temperature to room temperature to be connected with the transition from isotropic orientations of the octahedron to its two different dynamic states.