Crystallochemical Aspects of the Theory of Ferroelectric Phenomena

In Chaps. 8 and 9, while considering the microscopic theory of phase transitions, we used simple model Hamiltonians: the Hamiltonian (7.1) for displacive-type phase transitions and the Hamiltonian (8.8) for order-disorder phase transitions. The form of these Hamiltonians was specified so as to render the theory as simple as possible. The only requirement was the occurrence of a phase transition in a model system. Naturally, the coefficients in the Hamiltonians remained indeterminate; only certain plausible assumptions have been made as to the order of magnitude of their values. Of course, such an approach is insufficient for explaining the predictions of the properties of particular crystals. The problem therefore is to find Hamiltonians, though not as simple as (7.1) and (8.8), such that they would reflect more accurately the properties of concrete crystals.