Superprotonic conductivity in Cs , ( HS 04 ) 2 ( H , PO , )

Investigations of solid solutions of CsHSO, and CsH2P04 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 2.5X lo-’ a-’ cm-’ to 6.3XlO-3 f12-’ cm-‘. Powder diffraction data suggest that the hightemperature 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 CsHS04, it is likely that hightemperature proton conduction in Cs3( HS0,)2( H,PO,) is facilitated by sulfate and/or phosphate ion reorientations. Keywordr: Conductivity-superprotonic; CS~(HSO.,)~(H,PO,)