Studies of the growth and structure of alkali halides

Abstract

Existing techniques have been reviewed for the production of thin electron-transparent films of alkali halides for study by electron microscopy and a new technique has been developed. A novel apparatus has been designed and built by means of which samples prepared from the vapour phase in high vacuum may be transported to, and examined in an electron microscope without exposure to the atmosphere. Special specimen holders and a couple of heaters have been designed so that the effects of varying temperatures on the growth and subsequent behaviour of alkali halide crystals may be studied. Other factors examined are the pressure during growth, the effects of atmospheric moisture after growth and the electron irradiation in the electron microscope. The surface of the alkali halide crystals have been examined by decoration with evaporated gold and subsequent examination in the electron microscope and their crystallographic structures investigated by electron diffraction. Thicknesses of the crystals have been measured by extinction contour technique. The principal materials studied are CsBr, CsCl, CsF and Csl, as over growth on electron-transparent crystals of Csl and on two different types of mica: muscovite and biotite. These are the first studies of alkali halides on Csl and on biotite. The main results of investigations are: 1. CsI may be prepared as a flat, electron-transparent single crystal suitable for electron microscopic studies in situ. 2. Small crystallites are produced by vapour deposition of alkali halides in the early stages and after exposure to the atmosphere large single crystals are formed. 3. CsBr, CsCl, CsF and CsI grow epitaxially on Csl at room temperature. 4. CsF may grow in bcc form in thin layers in certain

circumstances, instead of the usual feeform in bulk. 5. No significant difference was observed in the growth of alkali halides on muscovite and biotite.