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

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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 Tc 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 dTc/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 Tc 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 Tc. 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 Tc are determined and discussed.