X-ray single crystal, vibrational and phase transitions in the mixed Rb2(HSeO4)(H2PO4)

Abstract

Ongoing studies of the RbHSeO4–RbH2PO4 system, aimed at developing novel proton conducting solids, resulted in the new compound Rb2(HSeO4)(H2PO4) (dirubidium hydrogenoselenate dihydrogenophosphate). Single-crystal X-ray diffraction (performed at room temperature) revealed Rb2(HSeO4)(H2PO4) to crystallize in space group P21/n with lattice parameters a = 7.514 (3) Å, b = 7.658 (2) Å, c = 7.696 (3) Å and β = 101.20 (3)°. The compound has a unit-cell volume of 434.4 (2) Å3 and two formula units per cell, giving a calculated density of 3.149. Six non-H atoms and three H atoms were located in the asymmetric unit, with SeO4 and PO4 groups randomly arranged on the single tetrahedral anions site. Refinement using all observed reflections yielded residuals of 0.0971 and 0.0438 based on F 2 and F values, respectively. Anisotropic temperature factors were employed for all six non-H atoms and isotropic temperature factors for the three 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 neighbouring chain to give a planar structure with hydrogen-bonded sheets lying parallel to The rubidium atom is coordinated by nine oxygen atoms with Rb–O range from 2.868 (4) to 3.364 (5) Å. The infrared spectra of the new compound Rb2(HSeO4)(H2PO4) recorded at room temperature in the frequency range 4000–400 cm−1, confirm the presence of and groups in the same crystal. Differential scanning calorimetry traces show two phase transitions at 382 and 665 K in this material.