Phase diagram of the relaxor ferroelectric (1-x)Pb(Mg_1/3Nb_2/3)O₃+ x PbTiO₃ revisited: A neutron powder diffraction study of the relaxor skin effect

We revisit the phase diagram of the relaxor ferroelectric PMN-PT using neutron powder diffraction to test suggestions that residual oxygen vacancies and/or strain affect the ground state crystal structure. Powdered samples of PMN-PT were prepared with nominal compositions of, 0.20, 0.30, and 0.40 and divided into two identical sets, one of which was annealed in air to relieve grinding-induced strain and to promote an ideal oxygen stoichiometry. For a given composition and temperature the same structural phase is observed for each specimen. However, the distortions in all of the annealed samples are smaller than those in the as-prepared samples. Further, the diffraction patterns for, 0.20, and 0.30 are best refined using the monoclinic space group. By comparing our neutron diffraction results to those obtained on single crystals having similar compositions, we conclude that the relaxor skin effect in PMN-PT vanishes on the Ti-rich side of the morphotropic phase boundary.