High temperature series expansions in cooperative Jahn-Teller T-systems

We are concerned with the phase transition behaviour of the cooperative Jahn-Teller T-e system and the T-t system in a simple cubic lattice. High temperature series expansions are derived for the specific heat and the elastic constants. In the latter case the symmetry has to be broken by applying an external stress field. The critical point Tc of the phase transition and the respective critical exponent are obtained by means of the Padé approximants method. A comparison with the results of other approaches is also given.