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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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Room-temperature crystal structures of triammonium hexafluoroaluminate, (NH(4))(3)AlF(6) (I), and triammonium oxopentafluorotitanate, (NH(4))(3)TiOF(5) (II), were refined, and the crystal structures of dirubidium potassium oxopentaflourotitanate, Rb(2)KTiOF(5), at 297 K (III) and 218 K (IV) were determined using single-crystal X-ray diffraction techniques. In ammonium fluoroelpasolites [(I) and (II)], the ligand (O, F) atoms are located in the mixed 24e + 96j position of the Fm3m unit cell. The 24e position is occupied by the ligand atoms predominantly in (III) and fully in (IV). 'Ordered' N1 and Rb atoms are tetrahedrally displaced from the 8c position into the 32f site, and the H atoms of the disordered ammonium group N2 are statistically distributed in the 96k and 32f positions. The Ti atoms in (II) and (IV) are shifted from the 4a position to 24e thus allowing identification of the O and F atoms in the octahedron on a local scale. The disorder in the crystals is of a dynamic nature. Unique Raman spectra of Rb(2)KTiOF(5) under the laser beam of 1064 nm indicate fast octahedral reorientations resulting in physical equalizations of the Ti-O and Ti-F distances as well as in the appearance of totally synchronous Ti-O and Ti-F stretching vibrations at 750 cm(-1). This phenomenon is assumed to also take place under X-rays.