A Thermodynamic Theory of the Improper Antiferroelectric Phase Transition in ADP

The thermodynamic theory of the elastic and dielectric properties of ammonium dihydrogen phosphate is developed on the basis of the improper phase transition. By the representation analysis the transformation properties of the two‐component order parameter are investigated and the thermodynamic potential of the Landau type describing both elastic and dielectric properties (triggered by the order parameter) is derived, which correctly describes the space group symmetry change at the transition point. Its minima and their stability conditions are also studied. The temperature dependence of the spontaneous deformations and material constants is analysed in detail and consistency with the available experimental data is briefly discussed.