Pressure and Temperature Dependence of Dielectric Properties of Squaric Acid H₂C₄O₄

Abstract

The pressure (0–10 kbar) and temperature (10–130°C) dependence of the dielectric properties of squaric acid (H2C4O4) were investigated. From the large isotope effect on the transition temperature of Tnd/Tn=1.38 (the superscript d stands for deuteration) and the large decrease of Tn with pressure of dTn/dp=-13°C/kbar, the phase transition mechanism for H2C4O4 is discussed microscopically using the tunneling model of the proton on the O–H–O bond accepted for KH2PO4-type crystals. The temperature dependence of the permittivity under constant pressure is interpreted in terms of the antiferroelectric ordering of the proton using the pseudospin model incorporating proton tunneling.