Application of atomic force microscopy to study the structure and electrical properties of cesium hydrosulfate phosphate crystals

Cs₃(HSO₄)₂(H₂PO₄) and Cs₄(HSO₄)₃(H₂PO₄) crystals are being studied in connection with the prospects of their use as materials for electrochemical devices, including fuel cells, providing direct conversion of chemical energy into electrical energy in the temperature range of 300–500 K. The paper demonstrates the capabilities of atomic force microscopy (AFM) for the diagnosis of two similar in composition isostructural compounds with superprotonic conductivity. Measurements of local current–voltage characteristic (CVC) and surface potential of crystals under atmospheric conditions with controlled parameters were performed using conductive AFM (C-AFM) and Kelvin scanning microscopy (KPFM). With an increase in temperature, an increase in conductivity was detected and the presence of a transition to the superprotonic phase was confirmed. The current value increases by 1.5–2 orders of magnitude at 413 K relative to the low-temperature state for both samples. As the phase transition temperature approaches, the continuity of the surface layers of crystals is disrupted and a defective block structure is formed. The surface is characterized by a uniform distribution of positive electrostatic potential at the micro and nanoscales and is sufficiently resistant to the effects of the surrounding air atmosphere. In interpreting the topographic and electrical features of the monoclinic phases before and after exposure to temperature, neutronography data on the atomic structure and nature of hydrogen bonds are used.

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