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Abstract
This dissertation contains two sections. The first deals with uniaxial stress effects on K$\sb2$SeO$\sb4$. This material is known to undergo a structural phase change at 129K to an incommensurately modulated structure. We have studied this transition with the techniques of optical birefringence and Brillouin scattering under uniaxial stress. In the birefringence experiment we have measured the dependence of the transition temperature on stress for all three unique axes. In the Brillouin scattering experiment, we have demonstrated that at the phase transition, certain of the third-order elastic constants show anomalous behavior which is possibly related to the anharmonic properties of the lattice.
In the second part is a theoretical simulation of the A$\sb2$BX$\sb4$ layered perovskite-like compounds. In particular Rb$\sb2$CaCl$\sb4$ was examined. This family of compounds may be thought of as being constructed from normal perovskite unit cells only with a different stacking arrangement. In recent months, this structural family has become extremely important since it is the parent structure from which the new class of high temperature superconductors comes from. The simulation was done using ab initio potentials with no adjustable parameters. Our calculations show a specific heat anomaly at 400K which corresponds to an observed pseudo-rotation of the CaCl$\sb6$ octahedral group. This rotation is similar to the structural phase transitions observed in the perovskite structure family. We also postulate a dynamically disordered high temperature structure for this compound. Finally, we compare our results for Rb$\sb2$CaCl$\sb4$ to similar structural studies on La$\sb2$CuO$\sb4$
