LANDAU THEORY AND MARTENSITIC PHASE TRANSITIONS

Landau theory proved itself appropriate for describing phase transitions in systems such as ferroelectrics and ferromagnets. Primarily Landau established the theory for second order phase transitions. Later on Devonshire generalized Landau's approach to first order transitions. The essential point of Landau theory is a power series expansion of the free energy, depending on temperature and on an order parameter describing the phase transition. In order to deal with phase boundaries the theory has been generalized to Ginzburg-Landau theory by adding a term depending on the gradient of the order parameter. Inspite of the success of Landau and Ginzburg-Landau theory in the systems mentioned above only little work has been done concerning martensitic phase transitions. Difficulties arise from the fact that the deformation of the unit cell does not coincide with the macroscopic strain. Considerations for overcoming this problem are discussed. It seems that even in the case of martensitic phase transitions Landau theory may be used as a starting point to obtain deeper insight into phenomena such as soft modes, nucleation, and the role of lattice defects.