Coexistence of Order-Disorder and Displacive Features at the Phase Transitions in Hydrogen-Bonded Solids: Squaric Acid and its Analogs

The systematic analysis of high-resolution NMR data, bond lengths, and isotropic ${}^{13}\mathrm{C}$ chemical shifts of crystals of squaric acid ( ${\mathrm{H}}_2\mathrm{SQ}$) and its deuterated compound ( ${\mathrm{D}}_2\mathrm{SQ}$) 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.