title: Nature of phase transitions in ammonium oxofluorovanadates, a vibrational spectroscopy study of (NH4)3VO2F4and (NH4)3VOF5.
abstract: Two ammonium oxofluorovanadates, (NH4)3VO2F4and (NH4)3VOF5, have been investigated 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 C4vgeometry 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 50K, in accordance with the Raman spectra. Ammonium groups do not take part in PTs at higher and room temperatures but their reorientational motion freezes at lower temperatures.
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia; Institute of Engineering Physics and Radio Electronics, Siberian Federal University, Krasnoyarsk 660079, Russia.
authors.forename: Yu V
authors.initials: YV
authors.lastname: Gerasimova
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia; Institute of Engineering Physics and Radio Electronics, Siberian Federal University, Krasnoyarsk 660079, Russia.
authors.forename: A S
authors.initials: AS
authors.lastname: Oreshonkov
authors.affiliation: Laboratory of Optical Materials, Institute of Chemistry, FEB RAS, Vladivostok 690022, Russia.
authors.forename: N M
authors.initials: NM
authors.lastname: Laptash
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia; Institute of Engineering Physics and Radio Electronics, Siberian Federal University, Krasnoyarsk 660079, Russia.
authors.forename: A N
authors.initials: AN
authors.lastname: Vtyurin
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia.
authors.forename: A S
authors.initials: AS
authors.lastname: Krylov
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia.
authors.forename: N P
authors.initials: NP
authors.lastname: Shestakov
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia; Institute of Engineering Physics and Radio Electronics, Siberian Federal University, Krasnoyarsk 660079, Russia. Electronic address: jul@iph.krasn.ru.
authors.forename: A A
authors.initials: AA
authors.lastname: Ershov
authors.affiliation: Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia.
authors.forename: A G
authors.initials: AG
authors.lastname: Kocharova
keywords: Ammonium oxofluorovanadates; Dynamics; Infrared; Order-disorder; Phase transitions; Raman