MODELING OF THE PHASE-TRANSITIONS SEQUENCE IN KNBO3 AND BATIO3

Interatomic potentials are determined in the framework of the nonlinea r oxygen polarizability model to describe the structural behavior of K NbO3 and BaTiO3 as a function of temperature. To this purpose, the adi abatic potential is evaluated for different ferroelectric distortions and the model potential parameters are improved by comparing with tota l energies from full-potential LMTO calculations. Phonon dispersion cu rves are computed to test if the model reproduces the pronounced two-d imensional character instabilities found by first-principles lattice d ynamics using a linear response approach. Finally, the phase diagram f or KNbO3 is obtained through a constant-pressure molecular dynamics si mulation.