LATTICE-DYNAMICS AND STABILITY OF ANHARMONIC CRYSTALS

The self-consistent phonon theory (SCPT) is formulated using Green's function method to describe both the dynamical and thermodynamical properties of highly anharmonic crystals and to investigate the dynamical stability of anharmonic lattices. The results of calculations for the phonon spectrum, elastic constants, PV-diagrams, etc., are presented for a simple model of the fcc lattice. The phenomenon of lattice instability due to the volume fluctuations at sufficiently high temperatures or as a consequence of a high zero-point energy is investigated. An application of the SCPT to structural (or ferroelectric) phase transitions (PT) is also discussed. In that case taking into account the fluctuations of the order parameter by the SCPT results in the renormalization of the Landau expansion and in a first order PT. A unified model for ferroelectric PT, where both the statistical disorder and the instability of lattice in the PT are taken into account, is shortly reviewed. © 1978 with the authors.