Pressure and Temperature Dependence of Dielectric Properties of Squaric Acid H2C4O4

The pressure (0–10 kbar) and temperature (10–130°C) dependence of the dielectric properties of squaric acid (H 2 C 4 O 4 ) were investigated. From the large isotope effect on the transition temperature of T n d / T n =1.38 (the superscript d stands for deuteration) and the large decrease of T n with pressure of d T n /d p =-13°C/kbar, the phase transition mechanism for H 2 C 4 O 4 is discussed microscopically using the tunneling model of the proton on the O–H–O bond accepted for KH 2 PO 4 -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.