No abstract

The macroscopic symmetry of Pb(Mg1/3Nb2/3)1−xTixO3 (PMN–PT) crystals with x = 0.25–0.5 has been studied by optical microscopy. Precise data on the temperature dependence of the birefringence and optical extinction directions have been obtained. Two different low-symmetry phases separating rhombohedral and tetragonal phases have been observed in the compositional range x = 0.3–0.47. The optical extinction in the range x = 0.3–0.37 is consistent with Cm space group symmetry, while for x = 0.37–0.47 Pm symmetry is appropriate. Gradual rotation of the optical indicatrix has been found at temperatures just below Tc in crystals with x = 0.27–0.3. A refined phase diagram is presented.

The phase diagram of (PbMg 1/3 Nb 2/3 O 3 ) 1−x -(PbTiO 3 ) x (PMN-PT) has been determined to high accuracy over the whole compositional range using birefringence imaging. A steep change has been detected in the thermal hysteresis of phase transitions in the morphotropic phase boundary region at x = 0.295 at the boundary between relaxor and non-relaxor phases. At the same composition at a temperature about 30 • C below T c a monoclinic phase appears. The phase transition between non-relaxor and the monoclinic phase is characterized by a pronounced thermal hysteresis and a large jump in birefringence, showing that this is of first order. A significant change in slope for the compositional dependence of T c has been found for x 0.45, suggesting a boundary between the tetragonal phase and a phase of lower symmetry. The temperature dependence of the birefringence has been measured for a number of different compositions in the range 0.3 < x < 1, in which it is found that the birefringence of the monoclinic phase is much higher than that seen in the other phases for x < 0.5.

Since lead titanate was ®rst grown in single-crystal form in the 1930s, several independent reports of its linear birefringence have been published. Whilst there is overall qualitative agreement on its behaviour with temperature, the absolute values show some degree of variation. At 298 K they range from À0.009 to À0.013, a relative spread of 40%, whilst at 673 K the discrepancy is even greater. This paper reports unexpected results obtained from independent measurements of a selection of crystals that have been grown recently, in which the birefringence is some four times greater than hitherto believed. This has important consequences for the use of birefringence measurement as a means of studying phase transitions in crystals. Intrinsic values of birefringence for lead titanate are measured for the ®rst time.

A detailed phase diagram of Pb(Mg1/3Nb2/3)(1−x)TixO3, conventionally abbreviated to PMN‐PT, has been constructed around the morphotropic phase boundary from measurements taken from one single crystal possessing a composition gradient. Optical birefringence imaging was employed as a function of temperature to plot retardation and orientation information as a function of temperature and position within the crystal. Electron‐probe microanalysis measured the composition across the crystal and this enabled the phase boundaries to be plotted to high precision. Cubic (C), rhombohedral (R), tetragonal (T) and monoclinic (M) phases were observed, with the a C–R–T critical point at a composition x = 0.275 (4) at 394.7 (5) K on heating and at the same composition at 386.5 (5) K on cooling, in close agreement with phase diagrams suggested by published dielectric and diffraction experiments.