Hydrostatic pressure effect on the phase transition of squaric acid H2C4O4

Abstract
Both the dielectric and the dilatometric measurements were made under various pressures up to 2 kbar. With increasing pressure, the transition temperature (Tn) decreases rapidly with the pressure coefficient dTn/dp=13 degrees C kbar-1, and this decrease in Tn supports the tunnelling model of the proton as a microscopic model for the phase transition. The large anomaly of the strain along the a(c) and the b axes was found close to Tn.