Investigations of solid solutions of these materials revealed a new compound with a sulfate/phosphate ratio of 2:1. Monocrystal X-ray data showed that the as-synthesized phase was monoclinic and was not isostructural with any known compounds. Alternating-current impedance measurements revealed that, during heating, the compound underwent transformation into a high-conductivity phase at about 119C. The activation energy fell from 0.90 to 0.45eV, while the conductivity rose from 0.000025 to 0.0063S/cm. Powder diffraction data suggested that the high-temperature phase was body-centered cubic. It was suggested that a Cs+ contribution to the conductivity at high temperatures was possible. Upon cooling, the compound remained in the high conductivity phase down to temperatures of between 60 and 80C. It then transformed into a third new phase, and the conductivity fell by almost 3 orders of magnitude but remained higher than that of the as-synthesized compound. It was proposed that high-temperature proton conduction in the Cs3(HSO4)2(H2PO4) was facilitated by sulfate and/or phosphate ion reorientation.

S.M.Haile, G.Lentz, K.D.Kreuer, J.Maier: Solid State Ionics, 1995, 77, 128-34