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The Slater KDP model is solved for all temperatures and with an electric field. Above T₂ the specific heat behaves like (T-{T₂) }^-1{2} and the polarizability like (T-{T₂) }^-1. There is a first-order phase transition at T₂ (latent heat). Below T₂ the free energy is simply -|E|d (E=electricfield, d=dipolemoment).
Élliott H. Lieb (Mon,) studied this question.