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Superprotonic conductivity in Cs_3(HSO_4)_2(H_2PO_4)

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Investigations of solid solutions of CsHSO4 and CsH2PO4 have led to the discovery of a new compound with a sulfate to phosphate ratio of 2:1. Single-crystal X-ray data showed the as-synthesized phase to be monoclinic and not isostructural with any known compounds. Ac impedance measurements revealed that, upon heating, the compound undergoes a transformation into a phase of high conductivity at ~119 °C. The activation energy dropped from 0.90(2) eV to 0.45(5) eV, while the conductivity jumped from 2.5 X 10^(-5) Ω^(-1) cm^(-1) to 6.3 X 10^(-3) Ω^(-1) cm^(-1). Powder diffraction data suggest that the high-temperature phase is body-centered cubic. Although unlikely, the possibility of a Cs+ contribution to the conductivity at elevated temperatures cannot be precluded. Upon cooling, the compound remained in the high conductivity phase until 60-80 °C. At this point it transformed into a third, new phase and the conductivity dropped almost three orders of magnitude, but remained higher than in the as-synthesized compound. Given the similarities between this new compound and CsHSO_4, it is likely that high-temperature proton conduction in Cs_3(HSO_4)_2(H_2PO_4) is facilitated by sulfate and/or phosphate ion reorientations.

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