Chapter 1The Thermodynamic Approach 1
Despite several decades of studying microscopic modeling, no theory is yet reliable enough to explain and fully predict the behavior of perovskite in thin films although the ab initio or “from first principle” approach is beginning to give very good results [JUN 03].
However, the phenomenological approach based on thermodynamics, which is a statistical view of the problem, helps to explain a very large proportion of the numerous behaviors of perovskites. Although the case of thin films is more delicate due to extrinsic effects such as residual stresses, interfaces, composition inhomogeneities, lattice, or domain wall motion effects, it is often possible to understand, at least qualitatively, how a device that integrates a perovskite material will behave by using the thermodynamic formalism.
Therefore, in this chapter, we develop the equations that enable us to use this formalism. This energetic method is very useful to understand the several possible couplings that arise in perovskites. The general idea is to have the most general description of the system energy and, therefore, to quantify the conversions of the thermal, mechanical, electrostatic, or magnetic energies.
This theory is often referred to as Landau Ginzburg Devonshire (LGD), all three having sequentially contributed to developing it in the first half of the 20th Century. Landau’s work focused on second-order phase transitions, which are those that do not ...