Reconstructive phase transition in (NH 4 ) 3 TiF 7 accompanied by the ordering of TiF 6 octahedra

An unusual phase transition P 4/ mnc → has been detected after cooling the (NH 4 ) 3 TiF 7 compound. Some TiF 6 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 TiF 6 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 P 4/ mbm and could be a high‐temperature phase and a parent phase, respectively, in (NH 4 ) 3 TiF 7 . Therefore, the sequence of phase transitions can be written as → P 4/ mbm → P 4/ mnc →. The interrelation between (NH 4 ) 3 TiF 7 , (NH 4 ) 3 GeF 7 and (NH 4 ) 3 PbF 7 is found, which allows us to suppose phase transitions in relative compounds.