Interrelationship between superprotonic conductivity and strain in CsHXO4

In order to clarify the mechanism of superprotonic conductivity (SPC) for the hydrogen-bonded compound CsHXO₄ (X=S, Se), we have investigated the elastic properties of CsHXO₄ with various transition temperatures (T_c), at which CsHXO₄ exhibits a structural phase transition from a low-temperature monoclinic phase with low protonic conductivity (phase II) to a high-temperature SPC phase (phase I). It was found that, in CsHS_xSe_1 − xO₄ (x=0–1), the transition temperature T_c decreases in proportion to the square of spontaneous strain b in the a_II–b_II plane of phase II. This result indicates that the spontaneous strain in the a_II–b_II plane plays an important role in the appearance of SPC in CsHXO₄. Moreover, thermodynamic analyses suggest that proton migration in phase I is induced by the release of the strain energy. From these results, it is deduced that the phase transition temperature in CsHXO₄ is determined by the competition between the kinetic energy of proton and the released-strain energy.