Cubic-tetragonal phase transition in SrTiO3 revisited:: Landau theory and transition mechanism

Existing experimental data for the antiferroelastic phase transition in strontium titanate are reviewed and analysed using a Landau free energy of the form Delta G = 1/2 A theta(S) (coth theta(S)/T-C - coth theta(S)/T)Q(2) + 1/4 BQ(4) + 1/6 CQ(6), with A = 0.6472 J K-1 mol(-1), B = 29.12 J mol(-1), C = 39.27 J mol(-1), T-C = 105.6 K, theta(S) = 60.8 K. The temperature dependence of the critical exponent is found to be due to the delicate balance between the Q(4) and Q(6) terms in the free energy expansion, and the saturation of the order parameter at low temperatures. The spontaneous strains observed in this phase transition are not consistent with simple rotation of the TiO6 octahedra around [001]. An alternative model is proposed, where these octahedra expand in order to preserve the Volume of the twelve-fold co-ordinated Sr site and the spacing between SrO3 pseudo-closepacked layers.