PHASE-TRANSFORMATIONS IN LOCALLY ANHARMONIC SYSTEMS - SUSCEPTIBILITY APPROACH TO ORIENTATIONAL INSTABILITIES IN MOLECULAR-SOLIDS

A model of the structural transformation has been discussed, where the local potential is anharmonic but single-welled. Such potential repre sents a Hamiltonian to model the structural phase transition in charge -transfer molecular crystals A-TCNB (anthracene-tetracyanobenzene). Th e conditions for the instability and transformation have been found, u sing the static susceptibility approach, and compared with the variati onal approximation. The latter method gives a six times lower transiti on temperature. The fluctuations and entropy are pointed out as the ca use of this difference. The exact transition temperature has been calc ulated by simple Monte Carlo simulations. A comparison with a double-w elled case, which models the structural transition in N-TCNB molecular crystal (naphthalene-tetracyanobenzene), has been presented.