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

Abstract

Ongoing studies of the CsHSO4–CsH2PO4 system, aimed at developing novel proton conducting solids, resulted in the new compound Cs2(HSO4)(H2PO4) (dicesium 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) Å3 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 F 2 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 (1¯01). 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.