High pressure Raman scattering measurements are performed on hydrogen-bonded molecular system, crystal of squaric acid H2C4O4. Specific intramolecular vibrational modes of squaric dianion C4O42- are found to disappear under pressures due to vanishment of the molecular distortion. This indicates that application of presure deformed the proton potential from double-minimum to single-minimum type. We fourther found a novel quantum paraelectric state, where deformed dianions can not show any ordering down to the lowest temperature, in the pressure range of 3.0-4.5 GPa.
