Phase diagram of the ferroelectric relaxor (1-x) PbMg_1/3Nb_2/3O₃-xPbTiO₃

Synchrotron x-ray powder diffraction measurements have been performed on unpoled ceramic samples of (1-x)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PMN-xPT) with 30%≤x≤39% as a function of temperature around the morphotropic phase boundary, which is the line separating the rhombohedral and tetragonal phases in the phase diagram. The experiments have revealed very interesting features previously unknown in this or related systems. The sharp and well-defined diffraction profiles observed at high and intermediate temperatures in the cubic and tetragonal phases, respectively, are in contrast to the broad features encountered at low temperatures. These peculiar characteristics, which are associated with the monoclinic phase of M_C-type previously reported by Kiat et al. [Phys. Rev. B 65, 064106 (2000)] and Singh and Pandey [J. Phys. Condens Matter 13, L931 (2001)], can only be interpreted as multiple coexisting structures with M_C as the major component. An analysis of the diffraction profiles has allowed us to properly characterize the PMN-xPT phase diagram and to determine the stability region of the monoclinic phase, which extends from x=31% to x=37% at 20 K. The complex lansdcape of observed phases points to an energy balance between the different PMN-xPT phases which is intrinsically much more delicate than that of related systems such as PbZr_1-xTi_xO₃ or (1-x)Pb(Zn_1/3Nb_1/3)O₃-xPbTiO₃. These observations are in good accord with an optical study of x=33% by Xu et al. [Phys. Rev. B 64, 020102 (2001)], who observed monoclinic domains with several different polar directions coexisting with rhombohedral domains, in the same single crystal.