Ferroelectric Phase Transition of first Order in BaTiO3

A microscopic model of BaTiO₃ is treated to describe static phenomena near the ferroelectric phase transition. In addition to the polarization, changes of the lattice constant at fixed pressure are also considered. That leads to a new explanation of the first order character of the transition, which is carried out not only qualitatively but also quantitatively. We investigate the dependence on pressure and external field. Agreement with the experiment through the adjustment of the parameters of the theory is obtained.