Why does Devonshire's theory work so well?

Abstract

The great success of Devonshire's phenomenological theory in explaining a wide variety of data connected with ferroelectric phase transitions in BaTiO₃, has led to a belief that in this system, fluctuation effects are unimportant and can be neglected. However, as has been shown by Vaks, there are a number of anomalous features associated with such a mean field description of the phase transition. Recently the authors have presented a calculation for the free energy of a compressible ising system taking fluctuation effects into account. In this paper we show that the success of the Devonshire theory can be attributed to the fact, that the form of the free energy functional that is postulated closely mimics the true free energy taking fluctuation effects into account. This also provides a very satisfactory explanation for all the anomalous features encountered.