X-ray and neutron diffraction studies of single-crystal cubic Cs2(HSO4)(H2PO4)

Abstract

Superprotonic CsH₂PO₄ is of interest as an electrolyte for a range of electrochemical devices. To date, the proton position has not been experimentally revealed in the phase of interest. Here, structurally analogous crystals of Cs₂(HSO₄)(H₂PO₄) were grown at high temperature, quenched, and then studied by X-ray and neutron diffraction. The compound crystallizes in a CsCl-related structure-type with Cs atoms at the unit cell corners and rotationally disordered (S/P)O₄ tetrahedra located at the unit cell centers. At ambient temperature, the cubic phase is metastable for ∼3 h, whereas at 100 K, the cubic phase shows no signs of transformation after 65 h. The neutron diffraction analysis, carried out at 100 K, indicates that the hydrogen resides at the Wycoff 3c position (at the unit cell face center), directly midway between tetrahedral groups. In addition to resolving the hydrogen position, the single crystal studies revealed a significant variation in the cell parameters between individual crystals. Chemical analysis of the crystal used for neutron diffraction studies indicates that cell expansion reflects the presence of Cs vacancies, as has been recently reported for CsH₂PO₄.