Ferroelectric phase transition in DKDP: A macroscopic order-disorder model

The order parameter (twin angle) and excess entropy for the ferroelectric phase transition in an 80% deuterated DKDP have been measured. Comparison of these data indicates that the classical Landau relationship DeltaS proportional to Q(2) should be replaced by a spin mixing model when describing the thermodynamic behaviour of this transition. Fitting of the DeltaG(Q) curve calculated from the experimental measurements of the excess enthalpy and entropy leads to the free energy expression DeltaG = AT/2 (] +Q)ln(1 +Q) + (1-Q)ln(1-Q)) - AT(C)/2 Q(2) + B/4 Q(4)+ C/6 Q(6) + D/12 Q(12), with A = 5.77(4) J K-1 mol(-1), B = -399.7(1) J mol(-1), C = -336.1(5) J mol(-1), D = -465.7(4) J mol(-1), T-C = 206.45(1) K. This model reproduces the saturation behaviour seen at low T / high Q better than previous macroscopic models.