Effects of diagonal strains and H-bond geometry in antiferroelectric squaric acid crystals

The proton ordering model of the phase transition and physical properties of antiferroelectric crystals of squaric acid is modified by taking into account the influence of diagonal lattice strains and of the local geometry of hydrogen bonds, namely of the distance delta between the H-sites on a bond. Thermal expansion, the spontaneous strain epsilon(1) - epsilon(3), and specific heat of squaric acid are well described by the proposed model. However, a consistent description of hydrostatic pressure influence on the transition temperature is possible only with further modifications of the model.