First-principles based atomistic modeling of phase stability in PMN-xPT

We have performed molecular dynamics simulations using a shell model potential developed by fitting first-principles results to describe the behavior of the relaxor-ferroelectric (1 - x)PbMg(1/3)Nb(2/3)O(3)-xPbTiO(3) (PMN-xPT) as a function of concentration and temperature, using site occupancies within the random site model. In our simulations, PMN is cubic at all temperatures and behaves as a polar glass. As a small amount of Ti is added, a weak polar state develops, but structural disorder dominates, and the symmetry is rhombohedral. As more Ti is added the ground state is clearly polar and the system is ferroelectric, but with easy rotation of the polarization direction. In the high Ti content region, the solid solution adopts ferroelectric behavior similar to PT, with tetragonal symmetry. The ground state sequence with increasing Ti content is R-M(B)-O-M(C)-T. The high-temperature phase is cubic at all compositions. Our simulations give the slopes of the morphotropic phase boundaries, crucial for high-temperature applications. We find that the phase diagram of PMN-xPT can be understood within the random site model.