• Prasenjit Saha

      Articles written in Bulletin of Materials Science

    • Alpha-quartz 1. Crystallography and crystal defects

      Prasenjit Saha N Annamalai Tarun Bandyopadhyay

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      Crystallography of alpha-quartz is discussed with special reference to the existing ambiguities regarding handedness of its enantiomorphic forms and a mnemonic has been suggested. Previous x-ray diffraction topographic studies of synthetic quartz are critically reviewed and analysed to understand the origin, nature and location of dislocations. It is suggested that dislocations associated with cell boundaries, characteristic of the Z-zone grown portions of synthetic quartz, are pure a-type edge dislocations but possibly with an alternating non-conservative climb component associated with the predominating glide component.

    • Alpha-Quartz 2. Crystal chemistry, and nature and distribution of impurities

      Prasenjit Saha Tarun Bandyopadhyay

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      Crystal chemistry of alpha-quartz is discussed in relation to the nature, concentration and distribution of the common impurities. Two schemes for incorporation of impurities in the host structure,viz., the charge compensation model for untwinned natural quartz, and the broken-bond model for synthetic quartz to accomodate excess impurity residue after charge compensation, are critically reviewed. Another model for the presence of Al-H defects independent of alkali association in natural quartz is suggested. It is pointed out that the growth pressure influences the nature and distribution of impurities as well as generation of planar and line defects in synthetic quartz. It is also established that mechanicalQ (which is a measure of the acoustic loss) of synthetic crystals grown at low pressure deteriorates with increase in impurity content in excess of what is required for satisfying the charge compensation rule.

    • Alpha-quartz 3. Origin of cellular structure of synthetic quartz

      Prasenjit Saha Tarun Bandyopadhyay

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      Surfaces of optically flawless and untwinned quartz single crystals, both natural and synthetic, were dissolved both at room temperature and at high temperatures in autoclaves using appropriate etchants. The features that develop have been optically studied, and x-ray transmission projection topographs of some of the specimens from the same natural quartz were also taken and studied. A model has been proposed to explain the anomalies detected earlier by other workers regarding the nature of line defects found in as-grown synthetic quartz, and the absence of lateral strain in the polygonised microcracked layer of the specimens of the present series of experiments.

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