Effect of deuteration on phase transitions in vanadium dioxotetrafluoride

The crystals of (ND$_{4}$)$_{3}$VO$_{2}$F$_{4}$ with a high degree of deuteration ($\sim$87%) have been grown and a significant ($\sim$1.5%) increase in the unit cell volume has been detected. The decrease in the chemical pressure leads to a change in the sequence of phase transitions due to wedging-out of one of the rhombic phases observed in (ND$_{4}$)$_{3}$VO$_{2}$F$_{4}$. The thermal physical studies have been carried out and the entropies, deformations, and pressure coefficients related to structural transformations have been determined. The $T$–$p$ phase diagram has been built and the pressure and temperature boundaries of crystal phase stability have been determined. The dielectric studies demonstrate a nonferroelectric nature of the phase transitions in (ND$_{4}$)$_{3}$VO$_{2}$F$_{4}$. The experimental and model entropies are compared. Based on the decrease in the entropy as a result of deuteration, a hypothesis on significant but not limited anharmonism of vibrations of ammonia tetrahedral corresponding to the disordering is proposed.