Abstract
The heat capacity, unit cell parameters, permittivity, optical properties, and thermal expansion of the (NH4)2TaF7 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 → (T 1 = 174 K) orthorhombic → (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.
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