Thermodynamic properties of vanadium oxypentafluoride (IV), (NH$_{4}$)$_{3}$VOF$_{5}$

The (NH$_{4}$)$_{3}$VOF$_{5}$ crystals have been synthesized and their homogeneity and single-phase structure has been established by the X-ray diffraction, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy studies. The investigations of the temperature dependences of specific heat, entropy, strain, and pressure susceptibility show the occurrence of three phase transitions caused by the structural transformations in the (NH$_{4}$)$_{3}$VOF$_{5}$ crystals. The $T$ – $p$ phase diagram shows the temperature limits of stability of the crystalline phases implemented in (NH$_{4}$)$_{3}$VOF$_{5}$. The optical and dielectric studies disclose the ferroelastic nature of the phase transitions. An analysis of the experimental data together with the data on the isostructural (NH$_{4}$)$_{3}$VOF$_{5}$ crystal makes it possible to distinguish the physical properties of oxyfluorides containing vanadium of different valences (IV and V).