Interrelationship between Number of Mobile Protons, Diffusion Coefficient, and AC Conductivity in Superprotonic Conductors, CsHSO₄ and Rb₃H(SeO₄)₂

Abstract

Using quasielastic neutron scattering (QENS), we investigated the proton dynamics for two superprotonic conductors, CsHSO 4 and Rb 3 H(SeO 4) 2. To evaluate the self-diffusion coefficients and the number of mobile protons on both superprotonic and normal phases, we focused on proton dynamics not only in the phase above T c , but also in the phase below T c. In Rb 3 H(SeO 4) 2 , the self-diffusion of protons was observed even below the T c phase. In contrast to popular belief, no large changes in the self-diffusion coefficients were observed across T c. Nevertheless, the increase in the number of mobile protons across T c was about 14.5 times, which was estimated from the integrated intensity of QENS spectra, and this change could not account for the increased magnitude of proton conductivity, which is about 500 times. As a large translational self-diffusion coefficient has not been reported in previous works by QENS experiments, there are still unknown factors that contribute to the Nernst-Einstein relation that need to be discovered.