Thermal cycle etching of willemite (0 0 0 1): effects of surface premelting, dislocation outcrops and polygonization

Thermal exposure of willemite (α-Zn2SiO4) (0 0 0 1) surface at 1250 °C caused a surface-premelt layer having silica-rich composition, yet with similar structural units as willemite according to energy-dispersive X-ray and reflection infrared spectroscopic analyses. Shallow hexagonal pits nucleated subsequently at dislocation outcrops beneath the surface-premelt layer, as a result of accumulation of inhibitor impurities to maintain undersaturation of etched components at dislocation outcrops, thus allowing etch pits to develop at these high energy sites. Thermal cycling between ambient and the selected temperature (1250 °C) caused slight deepening of hexagonal pits with progressively corrugated terrace steps, as well as the formation of hillocks at their centers, in accordance with the inhibition effect of impurities at dislocation outcrops. Polygonization, pore faceting and crack healing also occurred for thermally cycled crystals according to transmission electron microscopy observations.