Dielectric phase transition and symmetry change of constituent molecules in proton-deuteron mixed crystals of squaric acid

We have investigated effects of random substitution of proton sites with deuterons on the temperature- and pressure-induced phase transitions in squaric acid crystal, which consists of ferroelectrically ordered molecular sheets of [C₄O₄]^2- (SQ^2-) skeletons and intervening protons or deuterons. In the isotopically mixed crystals (H _1-xD_x)₂SQ, the critical temperature T _c for the dielectric phase transition was found to increase linearly with the deuterium concentration x, while the critical pressure P_c increases nonlinearly with x at 300 K. Around the pressure ∼20 kbar above P_c, the constituent SQ^2- skeletons in the paraelectric phase show a symmetry change from the asymmetric (C_1h) to centrosymmetric (C_4h) form, which is likely caused by the deformation of the correlated proton (deuteron) potential.