Abstract

A mean-field model is proposed for the calculation of thermodynamic properties and order parameters of multicomponent solid solutions of molecular crystals with orientational and substitutional disorder. Intermolecular interaction in a static lattice is calculated using the atom–atom approximation. Lattice state probabilities are supposed to be those of an ensemble of noninteracting particles. Absence of correlation between the order parameters is assumed. The expression for the lattice entropy at partial order is chosen to be similar to the one at complete disorder. The number of order parameters is reduced with the help of symmetry relations between them, which can be chosen in accordance with computational capacity. Order parameters and lattice (free) energy are found by minimization of the expectation value of the lattice free energy. High temperature and lower temperature expressions are derived. The model is illustrated by four simple examples, in which reasonable values are predicted for the order parameters.

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