Ferroelastic phase transitions in (NH$_4$)$_2$TaF$_7$

The heat capacity, unit cell parameters, permittivity, optical properties, and thermal expansion of the (NH$_4$)$_2$TaF$_7$ compound with a seven-coordinated anion polyhedron have been measured. It has been found that the compound undergoes two successive phase transitions with the symmetry change: tetragonal $\to$ ($T_1$ = 174 K) orthorhombic $\to$ ($T_2$ = 156 K) tetragonal. The ferroelastic nature of structural transformations has been established, and their entropy and susceptibility to hydrostatic pressure have been determined.