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

Abstract

Two ammonium oxofluorovanadates, (NH)VOF and (NH)VOF, 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 VOF (C) and the C geometry of VOF. 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 (NH)VOF, when at least seven independent VOF 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.

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

“…The infrared spectrum of (NH 4 ) 3 VOF 5 ( Fig. 11a) clearly shows two states for the VOF 5 3À (C 4v ) octahedron at room temperature (two stretching vibrations of the V-O bond at 948 and 933 cm À1 ) and the infrared spectrum of (NH 4 ) 3 VO 2 F 4 demonstrates a cis configuration of the VO 2 F 4 3À anion (C 2v ), deconvolution of which in the range 850-1000 cm À1 shows four lines in accordance with two states of this anion as it was also considered in our recent publication (Gerasimova et al, 2017).…”

“…Calorimetric measurements performed in a wide temperature range confirmed the presence of four heat capacity anomalies at T 1 = 438, T 2 = 244, T 3 = 210 and T 4 = 205 K associated with the firstorder phase transitions (Fokina et al, 2009). Our recent vibrational spectroscopy data (Gerasimova et al, 2017) indicate a rather dynamic nature of the phase transitions, especially well pronounced in the case of (NH 4 ) 3 VOF 5 .…”

“…of dynamically disordered octahedra but not complete in accordance with the ÁS value (R ln 3:2 from R ln 6) at these phase transitions. Further cooling increases the number of lines to at least seven, which indicates about seven different local units of VOF 5 (at 50 K) meaning full ordering of the octahedral sublattice (Gerasimova et al, 2017). Thus, we can assume dynamic disorder in our complexes connected with isotropic orientations of octahedra in cubic phase and two different octahedral reorientations in orthorhombic phases.…”

Влияние Дейтерирования На Фазовые Переходы В Диоксотетрафториде Ванадия

“…Действительно, несмотря на триклинную симметрию фазы ниже T 4 , вполне можно было допустить существование изменения структуры с повышением симметрии при понижении температуры, как это, например, наблюдалось в двойных солях фторидов с аналогичной исходной фазой Fm3m [31]. Однако при исследованиях спектров КР и ИК вплоть до T ≈ 8 K не обнаружено каких-либо доказательств в пользу существования фазовых переходов помимо известных [32].…”

Термодинамические Свойства Оксипентафторида Ванадия (IV), (NH-=SUB=-4-=/SUB=-)-=SUB=-3-=/SUB=-VOF-=SUB=-5-=/SUB=-

“…В аммонийных оксифторидах (NH 4 ) 3 VOF 5 и (NH 4 ) 3 VO 2 F 4 лиганды распределены одинаковым образом по двум кристаллографическим позициям 24e + 96 j, однако анионные полиэдры [VO 2 F 4 ] 3− и [VOF 5 ] 3− характеризуются разным числом пространственных ориентаций: соответственно двенадцать и шесть [6]. Результаты исследований методом колебательной спектроскопии указывают на динамический характер ориентационного разупорядочения анионных групп, который более выражен в случае (NH 4 ) 3 VOF 5 [7]. Аммонийные группы также оказываются разупорядоченными.…”