Bragg diffraction at structural phase transitions: a molecular dynamics study

Correlations in Space and Time for a Soft-mode Phase-transforming System

“…This is at variance with experimental MSD data for the soft-mode systems K 2 SnCl 6 (Mair 1984) and CsPbCl 3 and CsPbBr 3 (Sakata et al 1980), which show that the MSD of the halide ion, which is the ordering species, is linear with temperature at temperatures well above Tc' A multiple-well potential with quadratic walls gives the experimentally observed linear behaviour for the MSD at high temperatures. This has been shown for a range of harmonically coupled quadruple-quadratic potentials by Mair (1986) using MD for a two-dimensional lattice of particles (and for the one-dimensional case by Mair 1983Mair a, 1983.…”

“…An anti-ferrodistortive phase transition occurs at temperature Tc = 0·029 (scaled by the inverse Boltzmann constant), the soft-mode driving the phase transition having wave vector qs = (1T/ao)(e l +e 2 ), where a o is the lattice constant and e l and e2 are unit vectors directed along the x and Y axes defining the square grid [ Fig. 1 of Mair (1986) shows the displacements of the atoms in going from the high to the low temperature phases].…”

“…The details of the model and numerical methods are treated in Mair (1986) (model 1B of that paper is the one used here), so only an outline of these will be given here.…”

“…1 a of Mair (1986) for a diagram of the ordered structure]. For two-particle properties, the spatial average is over all pairs of particles.…”

“…In the present paper, one of the quadruple-quadratic models of Mair (1986), with an anti-ferrodistortive structural phase transition, is adopted for calculation of the space and time dependent correlation properties. As the quadruple-quadratic potential has a cusp-shaped barrier, a modification with a smoothed barrier is also considered.…”