MOLECULAR-DYNAMICS STUDY OF PBTIO3 USING NONEMPIRICAL POTENTIALS

Molecular dynamics simulations for finite clusters of PbTiO3 using non-empirical, many-body potentials were performed in order to better understand the origin of size effects in ferroelectrics' The interatomic potentials used are based on the ionic Potential Induced Breathing (PIB) model with correction terms fit to energy differences between PIB and linearized augmented plane wave (LAPW) total energies. Uncorrected PIB calculations give incorrectly a paraelectric cubic perovskite ground phase due to the neglect of covalency and polarizability. The new potentials, however, do give a correct ferroelectric tetragonal perovskite ground phase for a perfect lattice. Consistent with experiment, we find that a finite cluster of 1295 atoms is non-ferroelectric due to the presence of a large depolarization field. A metallic coating on the cluster was simulated in order to attempt to cancel out the depolarization field but no model was found that gave a realistic ferroelectric ground state. This may be due to a zone boundary instability for the fitted potential.