Optical evidences for an intermediate phase in relaxor ferroelectric Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-PbTiO₃ single crystals

The mechanism of low-temperature structural transformation and evolution of polar nano-structures in relaxor ferroelectric Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (x = 0.33, 0.35, and 0.42) single crystals have been investigated with the aid of temperature dependent low-wavenumber Raman scattering (LWRS) and photoluminescence (PL) spectra. The E(TO1) phonon mode reveals the characteristic relaxational polarization fluctuations associated with the reorientation of either polar nano-regions or polar nano-domains. It was found that these mechanisms are not independent and they can be ascribed to the phonon localization. In addition, a short-range monoclinic phase (M_c) can be found below 250 K in the tetragonal phase region by LWRS, which is always associated with the morphotropic phase boundary (MPB) and excellent electromechanical properties. It is interesting that PL spectra confirm these results. The present work indicates that external field modulation and change of composition can result in the monoclinic phase and co-existence of multi-phase.