Temperature dependent spectroscopic ellipsometry and Raman scattering of PbTiO₃-based relaxor ferroelectric single crystals around MPB region

To discover intrinsic relationship between optical bandgap and structural transformations in relaxor ferroelectric single crystals, electronic band structures and dielectric functions of xPb(In_1/2Nb_1/2)O_3- (1 - x - y)Pb(Mg_1/3Nb_2/3)O_3-yPbTiO₃ single crystals (x~0.27-0.28, y~0.29-0.35) around morphotropic phase boundary have been investigated by variable-temperature (200-750 K) spectroscopic ellipsometry. It was found that the discontinuous evolution from the second derivative of dielectric functions corresponds to structural transformation patterns. Using the SCP (standard critical point) model, four typical interband transitions (E_a~2.8 eV, E_b~3.6 eV, E_c~4.6 eV, and E_d~5.4 eV) can be uniquely assigned. These interband transitions are mainly attributed to the contributions from B-O bonds and multiphase coexistence. Furthermore, a modified phase diagram based on interband transition variations with the temperature and PT composition for PIMN-PT crystals was provided. In order to verify the accuracy of phase transition temperature, temperature-dependent low-wavenumber Raman scattering was used as a support. The present results provide important supports for the theoretical model, which establish a quantitative relationship between the electronic transition and structural transformation for ferroelectric oxides.