18-03E

Some aspects of thermodynamics of the quantum lattice model with the local anharmonic potentials are considered for the case of deformed lattice. The effects, taking place under external pressure, are investigated in the framework of the Blume–Emery–Griffiths model corresponding to the local potential with three minima. The deformable BEG model is proposed for this purpose. Based on this model, the pressure dependences of the u = ΔV/V volume deformation are calculated on an example of the Sn₂P₂S₆ crystal. The presence of anomalies of u(p) function in the regions of ferroelectric phase transitions as well as the tricritical point is established; the behaviour of the volume compressibility in these cases is investigated.