Phase diagram of the relaxor ferroelectric (1 - x)Pb(Mg1/3Nb2/3)O3+xPbTiO3 revisited: a neutron powder diffraction study of the relaxor skin effect

We revisit the phase diagram of the relaxor ferroelectric PMN- [GRAPHICS] 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- [GRAPHICS] PT were prepared with nominal compositions of [GRAPHICS] , 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 [GRAPHICS] , 0.20, and 0.30 are best refined using the monoclinic [GRAPHICS] 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- [GRAPHICS] PT vanishes on the Ti-rich side of the morphotropic phase boundary.