Superprotonic behavior of Cs2(HSO4)(H2PO4) – a new solid acid in the CsHSO4–CsH2PO4 system

Investigations into the CsHSO4–CsH2PO4 system have yielded a new solid acid, Cs2(HSO4)(H2PO4), with a superprotonic phase transition that occurs over the temperature range 61–105°C. In the room temperature structure, the SO4 and PO4 groups are randomly arranged on a single tetrahedral anion site. Hydrogen bonds are distributed through the structure so as to generate a two-dimensional network quite different from that of other cesium sulfate phosphate solid acids. The transition in Cs2(HSO4)(H2PO4) takes place by a unique two-step process, occurs at an unusually low temperature, is accompanied by a large heat of transformation, ΔH=44±2 J/g, and exhibits significant hysteresis. High temperature X-ray powder diffraction (XRD) and infrared (IR) spectroscopy revealed that the high temperature phase is cubic, with ao=4.926(5) Å, and likely takes on a CsCl structure, with Cs atoms at the corners of a simple cubic unit cell, and XO4 groups (X=P or S) at the center. The conductivity in the high temperature phase at 110°C is 3×10−3 Ω−1 cm−1, and the activation energy for proton transport is 0.37(1) eV. These values suggest that proton transport is facilitated by rapid XO4 group reorientations in the cubic phase of Cs2(HSO4)(H2PO4), as is known to occur in the high temperature, tetragonal phase of CsHSO4.