• P Krishna

      Articles written in Bulletin of Materials Science

    • Anomalous photovoltaic effect and disorder in ZnS crystals

      M T Sebastian P Krishna

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      Some crystals of ZnS are known to produce an anomalously high photovoltage, up to several hundred volts per cm, when illuminated by uv light in the absorption edge region. This has been attributed to the presence of alternate regions of hexagonal and cubic packing with charged dislocations at the interfaces producing built-in electric fields. Differential absorption of the incident light in the hexagonal and cubic regions is believed to create the necessary asymmetry in the built-in fields, causing an addition of tiny photovoltages at a series of interfaces which finally results in the abnormally high photovoltages observed.

      This paper investigates the possible mechanism by which disordered ZnS crystals containing alternating regions of cubic and hexagonal packing can result. X-ray diffraction studies show that such a disordered configuration results during the 2H to 3C phase transformation in ZnS. It is suggested that the transformation occurs by the non-random nucleation of deformation faults wherein the probability (α) of random nucleation of the faults is much less than the probability (β) for the faults to occur at two-layer separations.

    • Structural disorder and solid state transformations in single crystals of ZnxCd1−xS and ZnxMn1−xS

      M T Sebastian P Krishna

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      Single crystals of ZnxCd1−xS and ZnxMn1−xS were grown from the vapour phase at 1100°C in the rangex=0·9 to 1. X-ray characterization shows that polytypes and disordered structures occur in ZnxCd1−xS forx ≥ 0·94, whereas ZnxMn1−xS displays disordered and polytype structures in the entire rangex=0·9 to 1. It is observed that ZnxCd1−xS and ZnxMn1−xS undergo a 2H-6H solid state transformation on annealing in vacuum around 600°C. Experimental analysis of the intensity distribution along the 10·L reciprocal lattice row as recorded on a single crystal diffractometer from partially transformed crystals shows that the mechanism of the transformation cannot be explained in terms of the one-parameter models of non-random faulting reported earlier. A two-parameter theoretical model with α representing the probability of random insertion of a fault in the 2H structure and β representing the probability of the growth of the 6H nucleus, is developed both for a deformation mechanism and a layer displacement mechanism. It is found that the theoretical model of non-random deformation faulting with β ≫ α approximates the actual mechanism of transformation in these crystals.

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