K. Parlinski et Y. Kawazoe, FERROELASTIC PHASE-TRANSITION IN PB-3(PO4)(2) STUDIED BY COMPUTER-SIMULATION, Journal de physique. I, 7(1), 1997, pp. 153-175
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.