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Orientational disorder and phase transitions in ammonium oxofluorovanadates, (NH4)3VOF5 and (NH4)3VO2F4

ABSTRACT: The high-temperature elpasolite-like crystal structure of (NH4)3VOF5 with six spatial orientations of the VOF53? octahedron undergoes an order–disorder phase transition with the existence of two octahedral dynamic states at room temperature. Single crystals of (NH4)3VOF5 and (NH4)3VO2F4 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 (NH4)3VOF5 and (NH4)3VO2F4, respectively] with similar unit-cell parameters and two independent vanadium atoms. Above RT [at 350 and 440?K for (NH4)3VOF5 and (NH4)3VO2F4, respectively], the compounds undergo reversible phase transitions into high-symmetry dynamically disordered elpasolite-like (

SUBMITTER: Udovenko A

PROVIDER: S-EPMC5744410 | biostudies-literature | 2017 Nov

REPOSITORIES: biostudies-literature

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