Phase transitions and dynamic effects in crystals characterized by single-cell potentials with a multi-well excited state

A study has been made of phase transitions and phase states in crystals whose unit cell potentials have a multi-well excited state. New phase states compared to the conventional order-disorder-type phase-transition models have been revealed. A phase diagram has been constructed. The applicability criteria of the mean field approximation employed are analyzed. A region of parameter variation where the system is close to the tricritical point has been found. It is shown that microdomains of the new phase can efficiently transfer to the original phase within this region, and vice versa, by resonant tunneling. This tunneling has a relaxational nature. Interaction of such a relaxor with an oscillator (the soft mode) creates in this system an efficient mechanism of formation of the central peak. Besides, this model includes a possibility of coexistence of the order-disorder-type and displacive behavior. This coexistence manifests itself, in particular, in an induced phase transition associated with interaction of the order-disorder-type soft mode with the displacive mode for oscillations in the same potential wells. This induced phase transition may serve as a microscopic model of the improper ferroelastic phase transition in the Hg2Cl2 model system. This transition may produce a long-range incommensurate phase involving formation of the corresponding domain system, which is likewise in agreement with the case of Hg2Cl2. The model developed here can be used also in describing phase transitions in oxygen-octahedron perovskites, where the relative low-symmetry minima of the single-cell potentials can be related to the charge-transfer vibronic excitons.