  We have applied a shell model potential developed by fitting first principle
results to describe the behavior of the relaxor-ferroelectric PMN-xPT as
function of concentration. The solid solution exhibits three regions with
different characteristics according to Ti content in agreement with the
experiments. At low Ti the polar and structural disorder dominates, and the
symmetry is rhombohedral. The intermediate region is ferroelectric with easy
rotation of the polarization direction, and the symmetry appears orthorhombic.
Finally, in the high Ti content region, the solid solution adopts a
ferroelectric behavior similar to PT, with tetragonal symmetry.
