Conductivity, its anisotropy and changes as a manifestation of the features of the atomic and real structures of superprotonic [K_1–x (NH₄) _x ]₃H(SO₄)₂ crystals

Single crystals of [K1–x (NH4) x ]3H(SO4)2 (x ≥ 0.57) grown in the K3H(SO4)2–(NH4)3H(SO4)2–H2O water–salt system are studied. The atomic structure including H atoms was determined at room temperature using X-ray structural analysis. [K1–x (NH4) x ]3H(SO4)2 (x ≥ 0.57) crystals have trigonal symmetry and disordered hydrogen-bond networks at ambient conditions similar to the high-temperature phases of K3H(SO4)2, (NH4)3H(SO4)2 and other superprotonic compounds M 3H(AO4)2. Impedance measurements performed on single crystals show high values of conductivity characteristic for superprotonic phases. Using the methods of impedance spectroscopy and atomic force microscopy, a significant anisotropy of the conductivity of crystals has been detected. It was also shown that there is a qualitative correlation of bulk and local conductivity measured for samples of the same composition and orientation at room temperature, which is due to the peculiarities of their crystal structure.