This new solid acid exhibited a superprotonic phase transition that occurred at 61 to 105C. In the room-temperature structure, SO4 and PO4 groups were randomly arranged on a single tetrahedral anion site. The H bonds were distributed throughout the structure so as to generate a 2-dimensional network that was quite different to that of other cesium sulfate phosphate solid acids. The transition took place here via a unique 2-step process, occurred at an unusually low temperature, was accompanied by a large heat of transformation (44J/g) and exhibited a significant hysteresis. High-temperature X-ray powder diffraction and infra-red spectroscopy revealed that the high-temperature phase was cubic (a = 0.4926nm) and probably had a CsCl structure with Cs atoms at the corners of a simple-cubic unit cell and XO4 groups (where X was P or S) at the center. The conductivity in the high-temperature phase at 110C was equal to 0.3S/cm, and the activation energy for proton transport was 0.37eV. These values suggested that proton transport was facilitated by rapid XO4-group reorientation in the cubic phase of Cs2(HSO4)(H2PO4); like that which was known to occur in the high-temperature tetragonal phase of CsHSO4.

Superprotonic Behavior of Cs2(HSO4)(H2PO4) - a New Solid Acid in the CsHSO4-CsH2PO4 System. C.R.I.Chisholm, S.M.Haile: Solid State Ionics, 2000, 136-137, 229-41