Rule of superprotonic phase transition in CsxRb1 − xH2PO4

We have investigated the relation between the superprotonic phase transition and elastic property in CsH2PO4 (CDP) type compounds. It was found that, in CsxRb1 − xH2PO4 (x = 0–1), the transition temperature Tc decreases in proportion to the square of spontaneous strain component d. From this result, it is indicated that the spontaneous strain plays an important role in the appearance of superprotonic conductivity in CDP type compounds. In addition, by thermodynamic analyses, proton transfer in superprotonic phase is induced by the release of the strain energy. These results indicate that the superprotonic transition temperature in CDP type compounds is determined by the competition between the kinetic energy of proton, the increase of entropy and the released-strain energy.