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

Two ammonium oxofluorovanadates, (NH4)3VO2F4 and (NH4)3VOF5, have been in-vestigated by temperature-dependent infrared and Raman spectroscopy methods to determine the nature of phase transitions (PT) in these compounds. Dynamics of quasioctahedral groups was simulated within the framework of semi-empirical approach, which justified the cis-conformation of VO2F43– (C2v) and the C4v geometry of VOF53–. The observed infrared and Raman spectra of both compounds at room temperature (RT) revealed the presence at least of two crystallographically independent octahedral groups. The first order PT at elevated temperatures is connected with a complete dynamic disordering of these groups with only single octahedral state. At lower temperatures, the octahedra are ordered and several octahedral states appear. This PT is the most pronounced in the case of (NH4)3VOF5, when at least seven independent VOF53– octahedra are present in the structure below 50 K, in accordance with the Raman spec-tra. Ammonium groups do not take part in PTs at higher and room temperatures but their reorientational motion freezes at lower temperatures