A modified molecular-field approximation is presented in which a dependence of the interaction parameters on the number of misoriented ion pairs is explicitly included. The usual second-order phase transition becomes first order for a sufficiently strong dependence of the interaction parameters on the number of misoriented pairs. Values of the parameters for a simple lattice necessary to account for the transition in (NH4)2SO4 are given. A qualitative explanation of the dependence of the crystal volume on temperature is proposed based on the hydrogen-bond structure of (NH4)2SO4.
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O’Reilly et al. (1967) studied this question.
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