X-ray topographic investigations of solid state reactions. I. Changes in surface and bulk substructure during incipient thermal decomposition in sodium chlorate monocrystals

Abstract

On heating, sodium chlorate monocrystals undergo a limited degree of thermal decomposition in the solid state (0.1-1 % mass loss) between 450 K and the melting point, ca . 535 K. The degree to which this occurs depends on the perfection of the sample, the more perfect crystals suffering lower mass loss. Examination of thermally treated pristine surfaces with a microscope reveals distributions of decomposition sites characteristic of the reaction initiating where emergent dislocations intersect the crystal surface. There is little correlation, however, between dislocation etch-pits and decomposition centres. X-ray topographic studies of changes in bulk substructure during thermal treatment confirm that ‘grown in’ dislocations play little or no part in the nucleation of the decomposition process in the bulk. There is some evidence, however, that they may contribute to the surface reaction. In the bulk, the potential nuclei are strained regions in the lattice. From these, and possibly as a consequence of decomposition, dislocation loops are punched out into the surrounding lattice along <100> and <110> directions. Similar defects are emitted from localized regions at the surface and it is possible that these give rise to the surface decomposition patterns. Impurity doped crystals show considerably enhanced decomposition.