Very High Resolution 17O NMR Evidence for Displacive Behavior in Hydrogen-Bonded Solids: Squaric Acid

Abstract

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 . The peak assignment was supported by quantum theoretical calculations of the 17O isotropic chemical shifts using a pentamer model of the crystal structure. There was a clear break in the isotropic part of the chemical shift on lowering the temperature through TC , implying that the phase transition involves a distortion of the whole H2C4O4 framework, and not just the order-disorder rearrangement of the H's i.e., future models of the transition should include a displacive component, in addition to an order-disorder part. The observation of the double at T > TC implies that the two O─H⋅⋅⋅O chains retain their difference in the paraelectric phase as well. This is consistent with the one-dimensional Ising chain model, in contrast to the more prevalent, two-dimensional, C4O4 square-lattice model.