Phase transition in ammonium hexafluorovanadate (III)

• Published in: Solid state communications 1985-01, Vol.53 (8), p.719-722
• Main Authors: Kobayashi, Kiyoaki, Matsuo, Takasuke, Suga, Hiroshi, Khairoun, Slimane, Tressaud, Alain
• Format: Article
• Language: English
• Subjects: Condensed matter: structure, mechanical and thermal properties; Exact sciences and technology; Physics; Thermal properties of condensed matter; Thermal properties of crystalline solids
• ISSN: 0038-1098; 1879-2766
• DOI: 10.1016/0038-1098(85)90386-2

Heat capacity of ammonium hexafluorovanadate (NH 4) 3 [VF 6] has been measured with a miniaturized adiabatic calorimeter from 20 to 300 K. A phase transition was found at 280.44 ± 0.05 K with the associated entropy change Δ trs S = 24.9 ± 0.5 JK −1 mol −1. The entropy transition is accounted for by the orientational order-disorder changes of hexafluorovanadate ion and ammonium ion occupying respective octahedral sites, as in the cases of (NH 4) 3AlF 6 and (NH 4) 3FeF 6 crystals. Changes in infrared spectra relative to v 3 vibrational mode of [VF 6] 3− ion can be explained by an orientational disorder of the anions in the high-temperature phase (HTP). The dependence of cubic root of the unit-cell volume of a family of ammonium cryolites on their transition temperatures is discussed in relation to the nature of interactions which induce the phase transition.