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

Yu V Gerasimova; A S Oreshonkov; N M Laptash; A N Vtyurin; A S Krylov; N P Shestakov; A A Ershov; A G Kocharova

doi:10.1016/j.saa.2017.01.004

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

Spectrochim Acta A Mol Biomol Spectrosc. 2017 Apr 5:176:106-113. doi: 10.1016/j.saa.2017.01.004. Epub 2017 Jan 4.

Authors

Yu V Gerasimova¹, A S Oreshonkov¹, N M Laptash², A N Vtyurin¹, A S Krylov³, N P Shestakov³, A A Ershov⁴, A G Kocharova³

Affiliations

¹ 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.
² Laboratory of Optical Materials, Institute of Chemistry, FEB RAS, Vladivostok 690022, Russia.
³ Laboratory of Molecular Spectroscopy, Kirensky Institute of Physics, Federal Research Center KSC SB RAS, Krasnoyarsk 660036, Russia.
⁴ 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.

PMID: 28088082
DOI: 10.1016/j.saa.2017.01.004

Abstract

Two ammonium oxofluorovanadates, (NH₄)₃VO₂F₄ and (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 VO₂F₄^3- (C_2v) and the C_4v geometry of VOF₅^3-. 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 VOF₅^3- 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.

Keywords: Ammonium oxofluorovanadates; Dynamics; Infrared; Order-disorder; Phase transitions; Raman.

Publication types

Review