The transition between the cubic and tetragonal phase in shows an excess specific heat of . Comparison between the temperature evolution of the excess entropy and the structural order parameter Q shows within experimental errors . The apparent order parameter exponent was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy with A = 0.70 J , B = 31.22 J , C = 42.17 J , K and K. The closeness to the tricritical point is seen by B < C; all thermodynamic data between 85 K and could be described selfconsistently using this approach although small deviations cannot be excluded in a temperature interval of less than 1 K around and a small tail of excess entropy at .