Influence of tunnelling on the thermodynamics of an eight-site order-disorder ferroelectric model

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Abstract
A Hamiltonian with tunnelling and dipole-dipole interaction terms is written for an eight-site order-disorder model for ferroelectrics. Free energy, internal energy, spontaneous polarization, and transition entropies are calculated in the spirit of the mean-field approximation. It is shown that, even with tunnelling, lattice distortions are necessary to occur to allow the existence of more than one ordered phase. Transition entropies are found to decrease for increasing values of the tunnelling, becoming zero for the maximum value of the tunnelling compatible with the ordering of the system.