New superprotonic crystals with dynamically disordered hydrogen bonds: cation replacements as the alternative to temperature increase

Investigations of new single crystals grown in the K 3 H(SO 4 ) 2 –(NH 4 ) 3 H(SO 4 ) 2 –H 2 O system from solutions with different K:NH 4 concentration ratios have been carried out. Based on the X‐ray diffraction data, the atomic structure of the crystals was determined at room temperature taking H atoms into account. It has been determined that [K 0.43 (NH 4 ) 0.57 ] 3 H(SO 4 ) 2 crystals are trigonal at ambient conditions such as the superprotonic phase of (NH 4 ) 3 H(SO 4 ) 2 at high temperature. A distribution of the K and N atoms in the crystal was modelled on the basis of the refined occupancies of K/N positions. Studies of dielectric properties over the temperature range 223–353 K revealed high values of conductivity of the crystals comparable with the conductivity of known superprotonic compounds at high temperatures, and an anomaly corresponding to a transition to the phase with low conductivity upon cooling.