Phase transformation and electrical behaviour of lead iodide polytypes

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Abstract
The electrical resistivity behaviour of lead iodide polytypic crystals has been investigated. The polytypes were obtained by inducing phase transformation through vacuum annealing (10-4 Torr, temperature 200-290 degrees C) in the as-grown crystals corresponding to 2H structures. The resistivities of the lead iodide polytypes exhibited two types of variations. The resistivities of the 4H crystals arising from the interpolytypic transformation of the 2H to 4H type were found to be higher than the initial 2H structures, a representative resistivity variation of this type being a change from 3.29*1011 Omega cm(2H) to 2.56*1012 Omega cm (4H). On the other hand, the resistivities of higher polytypes originating from the occurrence of the 2H to nH (n>4) transformation were always lower than the initial 2H structure. As for example, the resistivity of a 12H polytype was found to be 4.10*108 Omega cm whereas the initial 2H crystal had a resistivity of 2.83*1012 Omega cm. Arguments and evidence have been given to show that the foregoing resistivity variations become intelligible in terms of the changes in the interactions between layers and also the changes in the band structures and Brillouin zones, which result due to the formation of the polytypes consequent to interpolytypic conversion.