Crystal perfection and silver electromigration in α-quartz

The diffusion of silver through plates of natural quartz cut parallel to (0001) has been studied at temperatures between 270 and 450°C when direct electric fields ranging from 5.5 to 11 kV. cm⁻¹ were applied between silver chloride electrodes pressed on to the plate surfaces. Optical and X-ray topographic examinations of specimens were performed before and at various stages during electrolysis experiments. Both highly perfect and relatively imperfect specimens, as assessed by X-ray topography, were used. In all electrolysis experiments silver was deposited internally to form pillars and threads parallel to [0001] and from these there often grew out thin plates parallel to (0001). Optical microscopic and X-ray topographic studies showed no correlation of the silver deposition patterns with the distribution of lattice defects such as dislocations, precipitates, and impurity layers producing moderately strong X-ray diffraction contrast. Twenty specimens were electrolysed, some repeatedly. Only once was evidence obtained for preferential growth of silver deposits in imperfect crystal regions and this involved growth parallel to the boundary of a phantom which was detectable by light-scattering as well by intense X-ray diffraction contrast. X-ray topographs showed an abundance of fine lines parallel to [0001] in regions too weakly electrolysed to produce silver deposits detectable by microscopy or light-scattering.