Effects of pressure on the dielectric properties and phase transitions of the 2-D antiferroelectric squaric acid: (H2C4O4 and D2C4O4)

Abstract

The pressure dependences of the dielectric properties and phase transitions of squaric acid, H₂C₄O₄, and its deuterated analog, D₂C₄O₄, were investigated. The transition temperatures at 1 bar are 375.5±1.0 K and 527.5±2.0 K, respectively, and decrease with pressure at initial rates of 105.6±2.0 K/GPa and 102.5±2.0K/GPa. It is argued that the large hydrogen isotope effect on T_c can be accounted for on the basis of the elongation of the O–H⋅⋅⋅O bond resulting from deuteration. This elongation causes a modification of the double‐well potential along this bond. It is not necessary to invoke proton tunneling. The lack of an appreciable isotope effect on dT_c/dP supports the absence of tunneling. An interesting aspect of the results is the appearance of a dome‐shaped feature in the real part of the dielectric constant just above T_c in both crystals. This feature, which is reminiscent of the behavior of the magnetic susceptibility of antiferromagnets above the Néel temperature, is discussed. The results on D₂C₄O₄ suggest the possible existence of a pressure‐induced phase transition at ∼0.8 GPa. The dielectric loss of this crystal is large both below and above T_c. This is attributed to the high conductivity of D₂C₄O₄ on approaching the melting (or dissociation) temperature. The activation energies for conduction and their pressure dependences both above and below T_c are determined and discussed.