Quantum paraelectricity and subsequent disappearance of bond alternation of molecule caused by proton dynamics in squaric acid crystal

The temperature-pressure phase diagram was determined for a crystal of squaric acid which consists of molecular sheets of [${\mathrm{O}}_4$ ${\mathrm{O}}_4$ ${]}^{2\mathrm{-}}$ skeletons and intervening protons. Under hydrostatic pressures, the paraelectric phase composed of the polar (${\mathit{C}}_{1\mathit{h}}$-distorted) molecules is realized even near 0 K due to a joint quantum motion of the protons and deforming molecular skeletons. Further application of pressure causes a disappearance of the π-bond alternation of the constituent molecule at all temperatures caused by enhanced motion of the protons.