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.

Recently, a new orthorhombic phase has been discovered in the ferroelectric system (1 Ϫx)Pb(Zn 1/3 Nb 2/3)O 3-xPbTiO 3 (PZN-xPT) for xϭ9%, and for xϭ8% after the application of an electric field. In the present work, synchrotron x-ray measurements have been extended to higher concentrations 10%рxр15%. The orthorhombic phase was observed for xϭ10%, but, surprisingly, for xу11% only a tetragonal phase was found down to 20 K. The orthorhombic phase thus exists only in a narrow concentration range with near-vertical phase boundaries on both sides. This orthorhombic symmetry (M C type͒ is in contrast to the monoclinic M A-type symmetry recently identified at low temperatures in the Pb(Zr 1Ϫx Ti x)O 3 ͑PZT͒ system over a triangle-shaped region of the phase diagram in the range xϭ0.46-0.52. To further characterize this relaxor-type system, neutron inelastic scattering measurements have also been performed on a crystal of PZN-xPT with xϭ15%. The anomalous soft-phonon behavior ͑''waterfall'' effect͒ previously observed for x ϭ0% and 8% is clearly observed for the 15% crystal, which indicates that the presence of polar nanoregions extends to large values of x.

Relaxor ferroelectrics, with their strong dependence of polarization on the applied electric field, are of considerable technological importance. On a microscopic scale, however, there exists competition as well as coexistence between short-range and long-range polar order. The conventional picture is that the polar nano-regions (PNRs) that appear at high temperatures beyond the Curie transition, form nuclei for the field-induced long-range order at low temperatures. Here, we report high-energy X-ray diffuse-scattering measurements on the relaxor Pb(Zn(1/3)Nb(2/3))O(3) (PZN) to study the short-range polar order under an electric field applied along the [111] direction. In contrast to conventional expectations, the overall diffuse-scattering intensity is not suppressed. On the other hand, the field induces a marked change on the shape of the three-dimensional diffuse-scattering intensity pattern, corresponding to a redistribution of PNRs in real space. We show that these surprising results are consistent with a model in which the PNRs with [110]-type polarizations, orthogonal to that of the surrounding environment, are embedded and persist in the [111]-polarized ferroelectric order of the bulk.

A ferroelectric monoclinic phase of space group Cm (M A type) has been discovered in 0.65Pb(Mg 1/3 Nb 2/3 )O 3 -0.35PbTiO 3 by means of high resolution synchrotron X-ray diffraction. It appears at room temperature in a single crystal previously poled under an electric field of 43 kV/cm applied along the pseudocubic

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