High resolution x-ray scattering studies of structural phase transitions

Abstract

This thesis is concerned with the application_ of very high-Q-resolution x-ray scattering techniques to the study of structural phase transitions in crystalline solids. By the use of perfect crystals to define the collimation of the incident and diffracted beam in a scattering experiment, a reciprocal space resolution of the order of 10⁻⁴,Å⁻¹ can be achieved. This is some two orders of magnitude higher resolution than is available through conventional x-ray, or neutron, diffraction techniques. A high-resolution x-ray spectrometer, based on this principle, has been constructed and the x-ray optical factors determining the resolution of the instrument are discussed.

Experimental studies of structural phase transitions in three materials, Ag₃AsS₃,BaMnF₄ and RbCaF₃, are presented. In each case, the uniquely high resolving power of the triple-crystal spectrometer has revealed new, and in some cases surprising, information on the nature of the phase transition in these materials. In particular, it is evident that defects, even in very low concentrations, play a significant role in determining the behaviour of crystals close to the phase transition temperature.