Superprotonic conductivity in Cs₃(HSO₄)₂(H₂PO₄)

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 × 10^-5 Ω^-1 cm^-1 to 6.3 × 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 CsHSO₄, it is likely that high-temperature proton conduction in Cs₃(HSO₄)₂(H₂PO₄) is facilitated by sulfate and/or phosphate ion reorientations.