Asymmetric-to-centrosymmetric structure change of molecules in squaric acid crystal: Evidence for pressure-induced change of correlated proton potentials

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High-pressure effects on vibrational modes have been investigated in crystals of squaric acid H2C4O4 (H2SQ) and its deuterated analog (D2SQ) which show two-dimensional networks of hydrogen bonds. Specific vibrational modes in Raman and infrared spectra were observed to anomalously disappear with the application of pressures above 30 kbar in H2SQ, but not in D2SQ, indicating that the H2SQ molecule in crystal shows the asymmetric (C1h)-to-centrosymmetric (C4h) structure change. This is possibly caused by pressure-induced deformation of the correlated proton potential essentially from the double-well to single-well type.