Elastic interaction of tunneling impurities in solids

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Abstract
The tunneling model for dipolar impurities in solids is applied to a study the static collective behavior of systems with large impurity concentrations. A rigorous formal result for the elastic constants is derived predicting anomalous softening near the phase transition corresponding to orientational ferroelastic ordering of the impurities. The nature of the phase transition is studied within the mean field approximation and examples of second and first order transitions are given for typical cases of impurity equilibrium orientations. Application to mixed KCl1−xCNx crystals and to pure KCN is discussed in the light of recent experimental and theoretical work.