Lattice dynamics and phase transitions in a crystal Ba$_2$ZnTeO$_6$

The lattice dynamics of Ba$_2$ZnTeO$_6$ double perovskite in three phases has been studied. Vibrational spectra and dynamic charges are calculated. In the rhomobohedral phase $R\bar{3}m$ , soft modes were found; after their condensation, the $C2/m$ phase was obtained, which agrees with the experiment. An assessment was made of the dependence of the enthalpy of different phases on pressure; it showed that the monoclinic phase is more favorable at 0 pressure, but as the pressure increases to 2.9 GPa, the cubic phase $Fm\bar{3}m$ becomes advantageous.