Effect of hydrostatic pressure on phase transitions in perovskite-like ferroelastics

L.V.Kirensky Institute of Physics, Siberian Branch of the Russian Academy of Sciences, 660036, Krasnoyarsk, Russia

The haloid compounds with perovskite-like structure undergo as a rule ferroelastic phase transitions (PT's) related to the soft modes of lattice vibrations. It therefore follows that an external pressure is probably the only parameter that can change significantly the crystal potentials.

The analysis of the relationship between the susceptibility to external pressure at PT's from initial phase and the structural peculiarities has been carried out for some families of perovskite-like ferroelastics, namely: perovskites, elpasolites, antifluorites, compounds with the ordered ReO₃ structure and layered perovskites. The following main points have been found.

1. There is absent the simple correlation between the type of soft mode and the sign of the value (dT₀/dp)_p=0.
2. The nonlinearity of the function T₀(p) is due to the change of the coefficient of thermodynamic potential invariant describing the interaction of order parameter with the strain.
3. There are three variants of possible phase diagrams T-p for PT's close to the tricritical point.
4. In the case of layered structure with at least two coupled layers the value of T₀^-1*dT₀/dp is the same as for cubic structure.
5. The sign of the PT temperature shift under hydrostatic pressure is due to the definite changes of bond stresses between critical and noncritical ions.