Temperature-dependent Raman scattering and multiple phase coexistence in relaxor ferroelectric Pb(In_1/2Nb_1/2)O₃- Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ single crystals

We report direct observation for the structural transformations of relaxor ferroelectric Pb(In_1/2Nb_1/2)O₃-Pb(Mg _1/3Nb_2/3)O₃-PbTiO₃ single crystals with the aid of temperature-dependent polarized Raman scattering and X-ray diffraction. The cubic to tetragonal phase transition is observed at 460 K and tetragonal to rhombohedral structural transformation takes place at 390 K, which are consistent with the results determined by dielectric spectroscopy. In addition to two well-known phase transitions above room temperature, anomalous structure deformations involving octahedral rotations and tilting angle of polarization can be found around 200 K. A comparison of experimental results with the Devonshire expansion of the free energy by Vanderbilt [Phys. Rev. B 63, 094108 (2001)] allows us to elucidate the peculiar characteristic as the variation of volume fractions among coexistence of three phases, including a first-order phase transition between the orthorhombic and rhombohedral phases and a second-order phase transition between the monoclinic and orthorhombic ones at low temperature.