Impurity sensitive etching in sodium fluoride crystals

Abstract

The recently introduced acetic acid etchant is shown to be a useful tool for investigating the impurity content of individual dislocations and grain walls in NaF crystals, since the degree of their contamination can be judged by visual inspection. In pure NaF crystals individual dislocations do not contain sufficient impurity to be sensed by the etchant, but grain walls can show large variations in their impurity content which may be regulated by the character of their dislocations. To a remarkable extent the contamination in each wall is constant, not changing sensibly with change of wall direction, but showing abrupt changes upon passing through intersections. Impure NaF crystals show impurity-saturated etching at all dislocations. This suggests that the core binding states of edge and screw dislocations are equivalent. The negative etching action of this etchant is shown to be due to the presence of polyvalent metallic impurities such as calcium.