Tavassoli, Mohammad Taghi (1978)
Studies of some alkali halide crystals by transmission electron microscopy.
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A new technique has been developed for producing mono-crystalline foils of some alkali halides for transmission electron microscopy. The formation mechanism of the crystals has been investigated. The foils, which have two free faces have been used as electron transparent substrates to grow epitaxial films of gold, on one or both surfaces of the substrates. It has been shown that by covering the above foils with a thin layer of another alkali halide, bending contours suitable for measuring the thickness of the foils, are produced. A chamber has been designed and constructed for transporting the specimen from the preparation vacuum system into the electron microscope without exposing it to the atmosphere. The variation of pressure inside the chamber has been measured and some possible modifications have been suggested. The device has been used to study the nucleation and growth from the vapour phase of LiF, NaF, NaCl, KOI, KBj, KI and Rbl, on electron transparent substrates of mica and potassium chloride. The crystallographic and atomic orientations of the above materials with respect to atomic arrangement of the substrates have been studied. It has been shown that the alkali halides form continuous films at average thicknesses below 2.5 nm on substrates of mica and KCl. The effect of the heat treatment during and after growth of the films has been studied. The effects of atmospheric moisture on the overgrowths have been studied in detail, and it has been shown that there are some similarities between the effects of atmospheric moisture and the heat treatment on the hygroscopic alkali halides. It has been shown that when thin deposits of NaCl on the substrate of KCl are heated, diffusion or re-evaporation of the deposit does not occur randomly. The consequent possibility of the existence of periodic defect sites has been discussed. From a general consideration of the results it is concluded that (a) Chemical interactions are a dominant factor in the epitaxial growth of alkali halides on mica and KCl. (b) The alkali halides deposited from vapour consist of flat crystallites at the earliest stages of growth at room temperature.
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in Royal Holloway Research Online.Last modified on 01-Feb-2017
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Institution: University of London, Royal Holloway College (United Kingdom).