Temperature-driven polarization rotation and triclinic phase at the morphotropic phase boundary of Pb(Mg_1/3Nb_2/3)O₃–PbTiO₃ crystals

Information about the crystal structures in the range of morphotropic phase boundary of ferroelectric perovskite solid solutions is important for understanding their intricate properties, which result in wide opportunities for practical applications. However, for the (1 − x)Pb(Mg₁/₃Nb₂/₃)O₃–xPbTiO₃ solid solution system, this information is contradictory. Different composition-temperature phase diagrams have been reported for this system in literature based on the investigations of single crystals and ceramics using various experimental techniques. In this work, we apply polarized light microscopy (PLM), x-ray diffraction (XRD) and dielectric spectroscopy to study the crystal structure and phase transitions in the 0.68Pb(Mg₁/₃Nb₂/₃)O₃–0.32PbTiO₃ single crystal. We confirm the monoclinic M_B symmetry (space group Cm) of the room-temperature phase. According to PLM, it transforms with increasing temperature into a triclinic (Tr) phase rather than the previously reported monoclinic M_C or tetragonal phase. XRD data are consistent with the presence of Tr phase. The Tr phase transforms to the monoclinic M_C (Pm) phase and then to the cubic phase. Ergodic relaxor behavior is observed above the Curie temperature. The unit cell in the M_C phase is pseudotetragonal with the lattice parameters a = b < c and a small monoclinic angle β. In the M_B phase, the direction of spontaneous polarization is temperature independent and close to the <111> pseudocubic direction. In the Tr and M_C phases, it changes with temperature so that near the Curie point it is close to the [001] axis. No significant anomalies in the dielectric properties or changes in the domain structure are observed at the M_B → Tr and Tr → M_C phase transitions. The domain structure changes dramatically when the temperature varies within the Tr phase, causing a sharp change in birefringence.