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

• Impedance measurements on TAHS show a pronounced anisotropy of proton conduction.
• Superprotonic conductivity was observed in the high-temperature phase of TAHS.
• Proton diffusion can take place along the energetically equivalent proton sites.

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 R3¯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.