Hardening of CaF2 single-crystal surfaces due to contamination by atmospheric oxygen

Abstract

Edge dislocation mobility and Vickers microhardness measurements have been made on pairs of matching, cleaved {111} surfaces of CaF₂ single crystals. While one part of each pair was preserved in the contamination‐free condition by immersion in a nonpolar liquid ambient, the matching counterpart surface was aged at room temperature in various ambients such as air, oxygen, nitrogen, or a moisture‐saturated atmosphere. Comparison of microhardness and edge dislocation mobility of the matching surfaces indicate that the atmospheric oxygen is mainly responsible for the observed surface hardening of the contaminated crystal. Oxygen contamination of ``air''‐aged surfaces is attributed to possible diffusion of O²⁻ ions into surface layers along dislocations, while the observed hardening is attributed to interactions between associated O²⁻ impurity‐F⁻ vacancy complexes in the surface layers.