Phase transitions in crystals with competitive Jahn-Teller subsystems

A statistical thermodynamic model of phase transformations caused by the cooperative Jahn-Teller effect in spinels containing two types of Jahn-Teller cations has been proposed. It has been shown that the specific features of the phase diagrams of compounds similar to Cu_{1 ? x} Ni _x Cr₂O₄, such as the presence of one orthorhombic and two anti-isostructural tetragonal phases, are associated with the competition between orderings of the subsystems of tetrahedra distorted due to the Jahn-Teller effect. The degree and character of the ordering of each subsystem of the solid solution have been determined. The main factors responsible for the thermodynamic properties and phase diagrams, in particular, the conditions for splitting of an isolated critical point into two triple points, have been investigated.