A Method of Preparing Crystals that Undergo Changes under Electron Bombardment in an Electron Microscope

Abstract

OBSERVATIONS of pseudo-structures in the electron microscope have been reported by several authors. This phenomenon has been fully reported by Burton et al.¹ and by Watson and Preuss² for sodium chloride crystals. We have devised a reproducible method for the preparation of sodium chloride crystals which undergo such changes in an RCA electron microscope, model EMU, fitted with an unbiased gun. It was observed that crystals obtained by drying aqueous solutions of sodium chloride of analytical-reagent grade rarely showed the effect. Hence it was thought that the change was facilitated by a favourable crystalline habit of the sodium chloride. This favourable habit was obtained by adding formalin and calcium chloride to the aqueous solution of sodium chloride and drying this solution under given conditions. Formalin was added to the sodium chloride solution because it had been observed that it produces the liquid inclusions necessary for the observation of the changes in sodium chloride crystals. The idea of adding calcium chloride was suggested by the paper of Bunn and Emmett³, who dealt with the property exhibited by calcium chloride of making uniform and thickening the layers of sodium chloride crystals when they are forming, thus maintaining, as we believe, the liquid inclusions under the vacuum of the electron microscope.