Diffuse scattering in orientationally disordered crystals: hard-core correlations treated by the weak-graph method

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Abstract
The authors propose a new method for the evaluation of elastic diffuse scattering of some orientationally disordered crystals. It is particularly appropriate for the study of the influence of the correlations induced by hard-core repulsions. For each Q vector the scattering is expressed directly, as a series expansion of connected graphs. The explicit expression is given for a simple model of orientational disorder in a plane. To show the usefulness and reliability of the method, applications to plastic succinonitrile and sodium cyanide are performed and compared with known scattering experiments. In the case of NaCN, this method makes the interpretation of the experimental scattering pattern by the correlation between the reorientation of CN- ions and the static displacement of Na+ ions possible. The significance of the graph calculation rules is discussed in the Ising adamantane case.