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

The domain structures and phase transitions of relaxor-based piezo-/ferroelectric (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 [PMNT] single crystals, grown from high temperature solutions, have been analyzed by polarized light microscopy using an optical crystallographic approach. The substitution of Ti4+ ions for the B-site complex (Mg1/3Nb2/3)4+ ions in the relaxor PMN results in a long-range symmetry breaking, as revealed by the formation of birefringent macrodomains. The domain structures of the PMNT80/20 crystals indicate a primitive trigonal symmetry at room temperature with a weak birefringence. PMNT crystals with 50% Ti show tetragonal domain structures, reflecting the symmetry of PbTiO3. Single crystals of PMNT65/35 exhibit complex morphotropic domain structures, which are composed of both the trigonal and the tetragonal phase intimately mixed together. The domain structures, the sequence and temperature of phase transitions have been interpreted in relation to the morphotropic phase boundary behavior of the PMN-PT system. It is shown that in situ analysis of domain structures and phase transitions can be used as a nondestructive analytical tool for evaluating the local morphotropic composition and the properties, which sensitively depend upon the individual domains and the Ti4+ concentration. The analysis of morphotropic domain structures and phase transitions provides a better understanding of the microscopic mechanisms of the enhanced piezoelectric properties recently disclosed in PMNT and other relaxor-based piezocrystals.

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