The cubic-tetragonal phase transition in strontium titanate: excess specific heat measurements and evidence for a near-tricritical, mean field type transition mechanism

The transition between the cubic and tetragonal phase in SrTiO3 shows an excess specific heat of 0.0035 J g(-1) K-1. Comparison between the temperature evolution of the excess entropy Delta S = f(C/T) dT and the structural order parameter Q shows Delta S proportional to Q(2) within experimental errors (gamma = 1.004+/-0.006). The apparent order parameter exponent <(beta)over bar> = 0.35+/-0.02 was confirmed and analysed using a Landau-type expression for the excess Gibbs free energy Delta G = A theta(s)(coth(theta(s)/T) - coth(theta(s)/T-c))Q(2) + 1/4 BQ(4) + 1/6 CQ(6) with A = 0.70 J K-1 mol(-1) B = 31.22 J mol(-1), C = 42.17 J mol(-1), T-c = 105.65 K and theta(s) = 60.75 K. The closeness to the tricritical point is seen by B < C; all thermodynamic data between 85 K and T-c could be described selfconsistently using this approach although small deviations cannot be excluded in a temperature interval of less than 1 K around T-c and a small tail of excess entropy at T > T-c.