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The systematic analysis of high-resolution NMR data, bond lengths, and isotropic ^13C chemical shifts of crystals of squaric acid (H₂SQ) and its deuterated compound (D₂SQ) yields convincing evidence that order-disorder and displacive features coexist at the antiferroelectric phase transition temperature. The experimental data are analyzed within a coupled pseudospin-lattice model which combines order-disorder and displacive dynamics and consistently describes the dynamics and the huge isotope effect observed. The results are considered to be of general importance for systems with collective proton ordering which is common to many organic compounds.
Dalal et al. (Mon,) studied this question.