An unusual phase transition P4/mnc → Pa\bar 3 has been detected after cooling the (NH4)3TiF7 compound. Some TiF6 octahedra, which are disordered in the room-temperature tetragonal structure, become ordered in the low-temperature cubic phase due to the disappearance of the fourfold axis. Other TiF6 octahedra undergo large rotations resulting in huge displacements of the F atoms by 1.5-1.8 Å that implies a reconstructive phase transition. It was supposed that phases P4/mbm and Pm\bar 3m could be a high-temperature phase and a parent phase, respectively, in (NH4)3TiF7. Therefore, the sequence of phase transitions can be written as Pm\bar 3m → P4/mbm → P4/mnc → Pa\bar 3. The interrelation between (NH4)3TiF7, (NH4)3GeF7 and (NH4)3PbF7 is found, which allows us to suppose phase transitions in relative compounds.

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Ferroelastic phase transitions (PT's) have been found, studied and confirmed in chlorides of Cs2NaB3+Cl6 and fluorides of A2BDyF6 types with the elpasolite structures. Symmetry of distorted phases has been determined. Behaviour of thermodynamic properties was analyzed in frames of the phenomenological theory. Coefficients of thermodynamical potential were obtained.