Vibrational and structural study in phase I of Rb3H(SO4)2

Raman and powder X-ray measurements were performed for high temperature and high pressure phase I of Rb3H(SO4)2 up to 5 GPa and 540 K. We determined phase I to be a trigonal-R3¯m structure. Proton conductivity in phase I increased with pressure as previously reported. We measured the change in the hydrogen bonding state and the lattice parameters of phase I against pressure and found that the proton conductivity increased despite the strengthening of hydrogen bonding. The reason was that the interlayer hydrogen bonds were formed dynamically as the pressure increased, and the hydrogen bonds became an additional proton conduction path along the interlayer direction.