Pseudosymmetry and ferroelectric phase transitions in the KTP structure type

The characteristic pseudosymmetry observed in the crystal structures of KTiOPO4-type ferroelectrics have been analyzed. The low-symmetry phases of all the crystals under consideration are characterized by a pronounced pseudosymmetry. The transition temperature monotonically decreases with an increase of the pseudosymmetry parameter of the crystal structure of the ferroelectric phase. The approach suggested to evaluate the transition temperature is more precise and universal than the Abrahams-Jamieson-Kurtz criterion.