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 th e structural order parameter Q shows Delta S proportional to Q(2) with in experimental errors (gamma = 1.004+/-0.006). The apparent order par ameter exponent <(beta)over bar> = 0.35+/-0.02 was confirmed and analy sed using a Landau-type expression for the excess Gibbs free energy De lta 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 = 4 2.17 J mol(-1), T-c = 105.65 K and theta(s) = 60.75 K. The closeness t o the tricritical point is seen by B < C; all thermodynamic data betwe en 85 K and T-c could be described selfconsistently using this approac h although small deviations cannot be excluded in a temperature interv al of less than 1 K around T-c and a small tail of excess entropy at T > T-c.