Antiferroelectric phase transition in titanite: Excess entropy and short range order

The antiferroelectric A2/a <-> P2(1)/a phase transition in titanite may be described using a thermodynamic model where the principal contribution to t he excess entropy is assumed to be configurational rather than vibrational; G = A/2 Q(2) + B/4 Q(4) + lambda T [(1 + Q) ln (1 + Q) + (1 - Q) ln (1 - Q )]. Such a model is likely to be a valid description of a phase transition where strain effects are large enough to maintain mean field behavior, but not so large that vibrational effects dominate in the excess entropy. Best- fit parameters for A/lambda and B/lambda are determined from X-ray measurem ents of the order parameter. The magnitudes of the three parameters are the n determined using calorimetric data, with the results A = -337.6 J/mol, B = -112.5 J/mol, lambda = 0.34 J/mol-K. The model is compared with measureme nts of dielectric susceptibility, and is found to give good, but not perfec t, agreement with experiment. The excess entropy associated with the transi tion is far smaller than expected for a simple order-disorder model. This i s interpreted as evidence for significant short-range order above T-C.