X-Ray Diffraction Study of Cs_5(HSO_4)_3(H_2PO_4)_2, a New Solid Acid with a Unique Hydrogen-Bond Network

Abstract

Solid solution investigations in the CsHSO₄-CsH₂PO₄ system, carried out as part of an ongoing effort to elucidate the relationship between proton conduction, hydrogen bonding, and phase transitions, yielded the new compound Cs₅(HSO₄)₃(H₂PO₄)₂. Single-crystal X-ray diffraction methods revealed that Cs₅(HSO₄)₃(H₂PO₄)₂ crystallizes in space group C2/c (or possibly Cc), has lattice parameters a=34.066(19)Å, b=7.661(4)Å, c=9.158(6)Å, and β=90.44(6)°, a unit cell volume of 2389.9(24)ų, a density of 3.198 Mg m⁻³, and four formula units in the unit cell. Sixteen non-hydrogen atoms and five hydrogen sites were located in the asymmetric unit, the latter on the basis of geometric considerations rather than from Fourier difference maps. Refinement using anisotropic temperature factors for all non-hydrogen atoms and fixed isotropic temperature factors for all hydrogen atoms yielded residuals based on F² (weighted) and F values, respectively, of 0.0767 and 0.0340 for observed reflections F²>2σ(F²). The structure contains layers of (CsH₂XO₄)₂ that alternate with layers of (CsHXO₄)₃, where X is P or S. The arrangement of Cs, H, and XO₄ groups within the two types of layers is almost identical to that in the end-member compounds, CsH₂PO₄ and CsHSO₄-II, respectively. Although P and S each reside on two of the three X atom sites in Cs₅(HSO₄)₃(H₂PO₄)₂, the number of protons in the structure appears fixed, In addition, the correlation of S-O and S-OH bond distances with O ... O distances, where the latter represents the distance between two hydrogen-bonded oxygen atoms, was determined from a review of literature data.

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