FERROELASTIC PHASE-TRANSITION IN PB-3(PO4)(2) STUDIED BY COMPUTER-SIMULATION

A model of lead phosphate which describes its rhombohedral-monoclinic improper ferroelastic phase transition is proposed. It contains a redu ced number of degrees of freedom but it is constructed consistently wi th symmetry changes at the phase transition. Potential parameters of t he model are derived from available experimental data. The crystallite s of 25 x 25 x 25 and 121 x 121 x 25 unit cells have been simulated by the molecular-dynamics technique. The results determine the phase tra nsition at the L point of reciprocal space, the order parameter, and t he temperature behavior of monoclinic lattice parameters. In the rhomb ohedral phase the calculated dynamical structure factor shows inelasti c peaks from which a soft branch of underdamped phonons has been estab lished. The model has been used to calculate a diffuse scattering func tion which shows above T-c a maximum at an incommensurate wave vector located along the L - F line of the Brillouin zone. The mentioned line is parallel to the ternary symmetry axis. On the basis of the above r esults we were able to vizualize the nature of the dynamical monoclini c microdomains persisting in the high-temperature rhombohedral phase. It has been shown that above T-c the fluctuations can be treated as te mporary orientational monoclinic microdomains. Each type of microdomai ns always contains an irregular sequence of antiphase domains.