Optical Investigation of Structural Phase Transitions in the Oxide Perovskites {KT}a$_{(1-x)}${N}b$_x${O}$_3$ and ({N}a$_{0.5}${B}i$_{0.5}$){T}i{O}$_3$}

Abstract

This thesis studies static and dynamic optical effects through the phase transitions of two perovskite materials, potassium tantalate niobate (KTN) and sodium bismuth titanate (NBT). In the first part, the temperature dependence of the birefringence of three different potassium tantalate niobate, or KTN, crystals is examined upon approaching the phase transition from cubic to tetragonal. This birefringence, which is in violation of crystalline symmetry conditions, is caused by the strain originating from the difference in unit cell size of the end materials, KNbO$_3$ and KTaO$_3$. This strain is either a gradient or a modulation due to the growth striations in the material, and diffraction is observed from the volume phase gratings via the difference in refractive indices. Upon approaching the phase transition, the birefringence displays rapid temperature dependence due to the formation of polar nano-domains which are preferentially oriented perpendicular to the striations. The variation of the birefringence with temperature is reported and discussed in the context of the existing literature. A sample of K$_{0.977}$Li$_{0.003}$Ta$_{0.845}$Nb$_{0.155}$O$_3$ or KLTN15.5 with well defined striations, KTa$_{0.73}$Nb$_{0.27}$O$_3$or KTN27) with no well defined striations and a sample of KLTN36 doped with copper and vanadium and diffuse striations are examined with respect to their optical behaviors in the cubic paraelectric phase.In the second part, Raman spectroscopic measurements of sodium bismuth titanate (Na$_{0.5}$Bi$_{0.5}$TiO$_{3}$ or NBT) were carried out upon cooling from 1000 K to 80 K to study its two phase transitions. Full spectral deconvolution was performed, comparing the results from several fitting models, with a particular emphasis on the evolution of the central peak and low frequency phonon modes. The central intensity profile is found to be composed of two well-defined Lorentzian peaks, the temperature dependence of which suggests the presence of fluctuations in M-point and R-point rotations of the oxygen octahedra as well as fluctuations in cation displacements related to polar nano-domains. The temperature behavior of both central peaks is discussed in relation to the (tilting or shifting) transition behavior in similar perovskite systems. The four observed low frequency phonon modes indicate a different symmetry assignment than the commonly assigned R3c for the low temperature phase