Phonon modes and local polar distortions in nanoclusters of Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb₂/3)O₃-PbTiO₃ crystals probed by terahertz reflectance spectra and Raman scattering

Optic phonons and lattice vibrations of Pb(In_1/2Nb_1/2)O₃-Pb(Mg_1/3Nb₂/3)O₃-PbTiO₃ single crystals near the morphotropic phase boundary (MPB) are determined by terahertz (THz) reflectance spectra in a temperature range of 5.5-300 K. Raman scattering is measured with a He-Ne laser with a wavelength of 632.8 nm as the exciting source. On cooling from 300 K, the A1 component of the splitting TO1 mode hardens, which follows the Cochran law with a critical softening temperature of 761-861 K. Moreover, the E component of the soft mode is heavily damped around 0.81 THz (27 cm^{- 1}). Additional polar phonon modes, which are forbidden in the cubic structure, are activated due to the broken cubic symmetry in polar clusters.