A previously unreported ferroelectric phase has been discovered in a highly homogeneous sample of PbZr0.52Ti0.48O3 by high-resolution synchrotron x-ray powder diffraction measurements. At ambient temperature the sample has tetragonal symmetry (at=4.037 Å, ct=4.138 Å), and transforms below ∼250 K into a phase which, unexpectedly, has monoclinic symmetry (am=5.717 Å, bm=5.703 Å, cm=4.143 Å, β=90.53°, at 20 K). The intensity data strongly indicate that the polar axis lies in the monoclinic ac plane close to the pseudocubic [111] direction, which would be an example of the species m3m(12)A2Fm predicted on symmetry grounds by Shuvalov.

Synchrotron x-ray powder diffraction measurements have been performed on unpoled ceramic samples of (1Ϫx)Pb(Mg 1/3 Nb 2/3)O 3 ϪxPbTiO 3 ͑PMN-xPT͒ with 30%рxр39% 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 ͑2000͔͒ and Singh and Pandey ͓J. Phys. Condens Matter 13, L931 ͑2001͔͒, 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 xϭ31% to xϭ37% 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 1Ϫx Ti x O 3 or (1Ϫx)Pb(Zn 1/3 Nb 1/3)O 3 ϪxPbTiO 3. These observations are in good accord with an optical study of xϭ33% by Xu et al. ͓Phys. Rev. B 64, 020102 ͑2001͔͒, who observed monoclinic domains with several different polar directions coexisting with rhombohedral domains, in the same single crystal.