RESUMO
Compositionally disordered perovskite compounds have been one of the exotic topics of research during the past several years. Colossal piezoelectric and electrostrictive effects have been observed in disordered perovskite ferroelectric materials. The key ingredient in the physical behavior of disordered perovskites is the nucleation and growth of the local dipolar regions called polar nanoregions (PNRs). PNRs begin to nucleate far above the temperature of the dielectric maximum Tm and exhibit varied relaxation behavior with temperature. The evidence for the existence of various stages in the relaxation dynamics of PNRs was revealed through the study of the temperature evolution of optical phonons by Raman scattering. The quasi-static regime of PNRs is characterized by the strong coupling between the local polarization and strain with the local structural phase transition and the critical slowing of the relaxation time. Strong anomalies in the frequency and the width of the acoustic phonons, and emergence of the central peak in the quasi-static region of the relaxation dynamics of PNRs have been observed through Brillouin scattering studies. In this review, we discuss the anomalies observed in Brillouin scattering in selected disordered perovskite ferroelectrics crystals such as Pb(Mg1/3Ta2/3)O3, Pb(Sc1/2Ta1/2)O3, 0.65PIN-0.35PT and Sr0.97Ca0.03TiO3 to understand dynamical behavior of PNRs.
RESUMO
Congruent lithium niobate single crystals with a Ru:Mg co-dopant have been successfully grown using the Czochralski technique from the melt containing 0.02 mol % Ru with Mg of two varied concentrations (4.0 and 6.0 mol %). The effects of Ru and Mg co-doping on the crystalline quality were determined by high-resolution X-ray diffractometry, which confirmed that the crystalline quality is good and that the dopants are statistically distributed in the crystal. The Raman scattering analysis shows no change in the lattice vibration except a slight change in the peak width and intensity due to more asymmetry in the molecular charge, which leads to enhancement of the polarizability. The optical transmission spectra indicate that both the crystals have high optical transparency in the visible region, with a shift of the absorption edge toward shorter wavelengths, as compared to un-doped LN. The weak absorption band observed below 400 nm is attributed to Ru ions. The influence of co-doping in the electronic band gap energies is calculated by the Tauc relation. The refractive index is measured by using a prism coupler at two wavelengths (532 and 1064 nm). The calculated absorption coefficients and direct and indirect band gap energies for both the samples are found to be nearly the same within experimental error. A decrease in the birefringence is observed for the Ru:Mg(6 mol %) doped sample. The observed slight reduction in refractive indices with Ru:Mg co-doping is consistent with a rise in band gap energy, which is related to the change in absorption edge to the lower wavelength. The second harmonic generation (SHG) efficiency is measured by the Kurtz and Perry powder method, and a decrease in SHG efficiency for Ru:Mg(6 mol %) is observed.