THE STRUCTURE OF (-Cs₃(HSO₄)₂(H₂PO₄). S. Faulk, P. Calkins and S.M. Haile, Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195 USA

Solid solution studies in the CsHSO₄-CsH₂PO₄ system carried out in a search for compounds with unique hydrogen-bonding schemes and related phase transitions, yielded the compound [[beta]]-Cs₃(HSO₄)₂(H₂PO₄. The compound appears to be a disordered form of [[alpha]]-Cs₃(HSO₄)₂(H₂PO₄₁/n and has the following lattice parameters: a = 19.546(3), b = 7.8798(10), c = 9.1854(17) and [[beta]] = 100.536(14)(. The arrangement of Cs⁺, SO₄^-, and PO₄⁼ ions are virtually identical in the two forms Cs₃(HSO₄)₂(H₂PO₄): both exhibit zig-zag chains of hydrogen-bonded XO₄ groups (where X = P or S), alternating with chains of Cs ions. The XO₄ chains are, in turn, cross-linked to form a three-dimensional framework. The difference between the two polymorphs lies in the absence of the c-centering symmetry in the [[alpha]] form that is present in the [[beta]] form. Accordingly, the number of atoms in the asymmetric unit of the [[alpha]] form is almost double that of the latter. In particular, in the C2/c space group, two pairs of oxygen atoms in the vicinity of a single hydrogen bond are related by a center of symmetry. The result is a rather diffusely located proton, distributed over four oxygen atoms in the high symmetry space group, whereas it is localized to the vicinity of two oxygen atoms in the low symmetry space group. The consequences of such a distributed proton are evident in the rather long O-O distances. The influence of this local hydrogen-bonding geometry on phase transitions is discussed.