Superprotonic conductivity of (NH4)3H(SO4)2 in the high-temperature phase

Impedance measurements were performed on single crystals of (NH4)3H(SO4)2 in the temperature range of 295–441 K. Anisotropy of protonic conductivity was found in the monoclinic room-temperature phase (space group C2/c) as well as in the rhombohedral high-temperature phase (space group R 3 ¯ m ). Above the high-temperature phase transition at 413 K, conductivity in the hexagonal (001) plane yields a value of ~ 10− 2 S cm− 1, which is typical for superionic conductors. A verification of a sample quality after the impedance measurements, using X-ray powder diffraction, showed minor additional peaks of (NH4)2(SO4). This suggests a beginning of thermal decomposition at high temperature. The possible pathways of proton conduction were discussed according to crystal structure analyses by single-crystal neutron diffraction method.