Theory of the orientational glass state in mixed crystals M(CN)_{x}X _{1-x}. I. Statics

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Abstract
A microscopic model of mixed crystals with orientational disorder and translation-rotation coupling is proposed. The substitutional atoms act as static impurity centers which generate random strain fields. The static strains play the role of external random fields which couple to the orientational order parameter. In addition, the orientational modes couple to thermal lattice vibrations. The thermodynamics is studied within mean-field theory. Above a certain concentration of substitutional atoms, the system does not undergo anymore a transition to a phase with long-range orientational order. The theory accounts for experimental properties which are characteristic of the quadrupolar orientational glass state.