Dielectric phase transition and symmetry change of constituent molecules in protonndash;deuteron mixed crystals of squaric acid

摘要

We have investigated effects of random substitution of proton sites with deuterons on the temperaturehyphen; and pressurehyphen;induced phase transitions in squaric acid crystal, which consists of ferroelectrically ordered molecular sheets of lsqb;C4O4rsqb;2minus;(SQ2minus;) skeletons and intervening protons or deuterons. In the isotopically mixed crystals (H1minus;xDx)2SQ, the critical temperatureTcfor the dielectric phase transition was found to increase linearly with the deuterium concentrationx, while the critical pressurePcincreases nonlinearly withxat 300 K. Around the pressure sim;20 kbar abovePc, the constituent SQ2minus;skeletons in the paraelectric phase show a symmetry change from the asymmetric (C1h) to centrosymmetric (C4h) form, which is likely caused by the deformation of the correlated proton (deuteron) potential.