Thermally-induced changes in the defect substructure of pure and doped magnesium oxide

Three independent nondestructive elastic light scattering and imaging techniques; ultramicroscopy, bulk scattering angular distributions, and large angle bulk scattering as a function of crystal orientation were used to self consistently examine the precipitated defect substructure of MgO. Computational routines were developed to fit the bulk scattering angular distribution measurements and determine the particle size distributions, as well as the total particle population, using Mie theory or other approximation schemes. The aggregation, precipitation, growth, and development of structures beyond the point defect stages up to macroscopic dimensions were followed through a number of controlled heat treatments. The progressive changes seen in the size of particulates within the bulk of the samples showed effects similar to coarsening of Oswald ripening. The theoretical size distributions for each ripening case were used to predict the time evolution of the angular dependence of scattered light from precipitates in an ionic crystal at a particular annealing temperature. Magnesium oxide crystals doped with nickel and cobalt were used as samples to test the theoretical ripening model.