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 isot ropic C-13 chemical shifts of crystals of squaric acid (H(2)SQ) and its deu terated compound (D(2)SQ) yields convincing evidence that order-disorder an d displacive features coexist at the antiferroelectric phase transition tem perature. The experimental data are analyzed within a coupled pseudospin-la ttice model which combines order-disorder and displacive dynamics and consi stently describes the dynamics and the huge isotope effect observed. The re sults are considered to be of general importance for systems with collectiv e proton ordering which is common to many organic compounds.