Very High Resolution17O NMR Evidence for Displacive Behavior in Hydrogen-Bonded Solids: Squaric Acid

Magic angle sample spinning (MAS)7O NMR studies of the paraelectric-antiferroelectric phase transition (at TC = 373 K) of the model hydrogen-bonded compound squaric acid (H2C4O4 ) reveal a significant displacive component in the microscopic mechanism of the transition. The high resolution (400% enhancement over conventional MAS) was obtained by utilizing single crystals. All four oxygens were clearly shown to be chemically different at T TC impli...