Superprotonic behavior of Cs_2(HSO_4)(H_2PO_4) – a new solid acid in the CsHSO_4–CsH_2PO_4 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 a₀=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