Structure and thermal behavior of the new superprotonic conductor Cs₂(HSO₄)(H₂PO₄)

Ongoing studies of the CsHSO₄-CsH₂PO₄ system, aimed at developing novel proton conducting solids, resulted in the new compound Cs₂(HSO₄)(H₂PO₄) (decesium hydrogensulfate dihydrogenphosphate). Single-crystal X-ray diffraction (performed at room temperature) revealed Cs₂(HSO₄)(H₂PO₄) to crystallize in space group P2₁/n with lattice parameters a = 7.856 (8), b = 7.732 (7), c = 7.827 (7) Å, and β= 99.92 (4)°. The compound has a unit-cell volume of 468.3 (8) ų and two formula units per cell, giving a calculated density of 3.261 Mg m^-3 . Six non-H atoms and two H atoms were located in the asymmetric unit, with SO₄ and PO₄ groups randomly arranged on the single tetrahedral anion site. Refinement using all observed reflections yielded weighted residuals of 0.0890 and 0.0399 based on F₂ and F values, respectively. Anisotropic temperature factors were employed for all six non-H atoms and fixed isotropic temperature factors for the two H atoms. The structure contains zigzag chains of hydrogen-bonded anion tetrahedra that extend in the [010] direction. Each tetrahedron is additionally linked to a tetrahedron in a neighboring chain to give a planar structure with hydrogen-bonded sheets lying parallel to (101). Thermal analysis of the superprotonic transition in Cs₂(HSO₄)(H₂PO₄) showed that the transformation to the high-temperature phase occurs by a two-step process. The first is a sharp transition at 334 K and the second a gradual transition from 342 to 378 K. The heat of transformation for the entire process (∼330-382 K) is 44 ± 2 J g^-1. Thermal decomposition of Cs₂(HSO₄)(H₂PO₄) takes place at much higher temperatures, with an onset of approximately 460 K.