The macroscopic symmetry of Pb(Mg_1/3Nb_2/3)_1−<i>x</i>Ti_<i>x</i>O₃in the morphotropic phase boundary region (<i>x</i>= 0.25–0.5)

Abstract

The macroscopic symmetry of Pb(Mg_1/3Nb_2/3)_1-xTi_xO₃ (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 T_c in crystals with x = 0.27-0.3. A refined phase diagram is presented.