The structure of the VO2F3−4 ion: Crystal structure of (NH4)3VO2F4

Nature of phase transitions in ammonium oxofluorovanadates, a vibrational spectroscopy study of (NH4)3VO2F4 and (NH4)3VOF5

“…Ferroelectric and proton conducting behavior of a new vanadium oxide fluoride (NH4,K)3VO2F4 was recently reported [17]. The compound belongs to a large structural family, namely, elpasolite and crystallizes in an orthorhombic lattice (Immm) at ambient temperature, similarly to (NH4)3VO2F4 [18]. The existence of the VO2F4 3ion in solution as well as in solid state was established by nuclear magnetic resonance (NMR) [19] and electron paramagnetic resonance (EPR) [20,21] as well as by x-ray diffraction (XRD) studies [18].…”

“…The compound belongs to a large structural family, namely, elpasolite and crystallizes in an orthorhombic lattice (Immm) at ambient temperature, similarly to (NH4)3VO2F4 [18]. The existence of the VO2F4 3ion in solution as well as in solid state was established by nuclear magnetic resonance (NMR) [19] and electron paramagnetic resonance (EPR) [20,21] as well as by x-ray diffraction (XRD) studies [18]. Structural studies from single crystal of (NH4)3VO2F4 has been reported with four kinds of disordered NH4 groups and two types of VO2F4 3ions (one comparatively regular than the other) [18].…”

“…The existence of the VO2F4 3ion in solution as well as in solid state was established by nuclear magnetic resonance (NMR) [19] and electron paramagnetic resonance (EPR) [20,21] as well as by x-ray diffraction (XRD) studies [18]. Structural studies from single crystal of (NH4)3VO2F4 has been reported with four kinds of disordered NH4 groups and two types of VO2F4 3ions (one comparatively regular than the other) [18]. In accordance with the EPR data [21], one third of octahedra are dynamically disordered while the other two third represent a rigid ordered form of VO2F4 3-.…”

“…The longer distances 1.97(1) x 2 from V2-(O,F) bond lengths were assigned to F atoms, and the octahedra were assumed to have an asymmetric cis-arrangement, as observed in the case of (NH4)3VO2F4. Trans-octahedral structure of VO2F4 in the latter case was excluded according also to experimental IR and Raman spectra of (NH4)3VO2F4 [18]. Crystal structure of Ag3VO2F4 [6] supports only cis-configuration of the anion.…”

“…The crystal structure of the former was not determined, while the detailed single crystal structure determination of the latter performed at room temperature has revealed the orthorhombic symmetry (space group Immm, Z = 6) [18]. Clear evidences for the occurrence of the phase transitions at 418 and 215 K were obtained by variable temperature X-ray powder diffraction [18] as well as by EPR study on γ-irradiated (NH4)3VO2F4 [21]. The phase at T > 418 K was determined to have the cubic cryolite type structure [18].…”