I. Phys.: Condens. Mattcr zyxwvuts 5 (1993) A315-A316. zyxwvu Printed in zyxwv the zyxwv UK Order-disorder behaviour in a two-dimensional non-linearly polarizable model M Stachiotti, A Dobry and R Migoni Instituto de Fish Rosario, 27 de Febrero 210 Bis, 2WO Rosario. Argentina Abstract. A crossover from displacive to order-disorder behaviour is foundin atwo-dimensional non-linearly polarizable model with a ferroelectrictransition. The dynamicsof a shell model with double-well core-shell interaction is simulated by means of the molecular dynamics technique. Snapshots of the lattice displacement pattem reveal the presence of precwr order regions in the high-tempemure phase, while the dynamical sh’nclure factor exhibits a quasielastic component in addition to the ferroelectric son mode peak. The local panicle dynamics is characterired by WO time scales corresponding to the coexistence of inter- and intra-well motions. Several experiments on the paraelectric phase of ferroelectric zyx AB03 perovskites reveal that when zyxwvutsr Tc is approached a relaxational dynamics of the B ion between six equivalent off-centre positions appears zyxwvu [1,2], while the ferroelectric mode becomes overdamped [3]. A realistic description of the dynamical properties of these compounds is given by a shell model with non-hear 02- polarizability (NP) [4]. This, however, has been treated in the self-consistent phonon approximation (SPA), which cannot account for relaxational dynamics. We avoid this limitation by using the molecular dynamics (MD) technique. Figure 1. nvwiimensional diatomic NP model. The mows on shells and cores in the bottom left corner of the figure indicate their respective displacements in the ferrcelecnic phase. Figure 2. Dynamical structure factor in the paraelectric phase for T/T, = 11.4 (curve a), 2.9 (curve b) and 1.4 (curve e). The mow indicates the SPA valne of the ferroelectric mode in each case. The intensity maximum of each spectrum is set equal to unity. ~53-8984/93/SA0315c02$07.50 @ 1993 IOP Publishing Ltd A315