THE STRUCTURE OF Cs5(HSO4)3(H2PO4)2. G. Staneff, P. Calkins, S. Fu and S.M. Haile, Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195
The new compound Cs5(HSO4)3(H2PO4)2 was synthesized as part of an ongoing study to examine the relationship between hydrogen bonding and phase transitions in solid acid compounds. The compound crystallizes in space group C2/c and has lattice constants a = 34.07(2), b = 7.661(4), c = 9.158(6) and b = 90.44(2)(. The structure of Cs5(HSO4)3(H2PO4)2 contains both layers and chains of hydrogen-bonded XO4 groups (where X = P or S). Phosphate groups form corrugated layers that are perpendicular to b, whereas sulfate groups form branched chains, located between the phosphate layers. These chains extend along c. The compound is unusual in that the two crystallographically distinct SO4 sites have different numbers of hydrogen-bonded oxygen atoms: S(1) has only one oxygen nearest neighbor that is hydrogen bonded whereas all 4 oxygen atoms bonded to S(2) are additionally hydrogen-bonded to neighboring oxygen atoms. Similarly, all oxygen atoms participating the PO4 tetrahedra are hydrogen bonded.
The presence of both hydrogen-bonded and non-hydrogen-bonded oxygen atoms suggests that the material will undergo a superprotonic phase transition at elevated temperatures. Indeed, a transition is observed at 391K by differential scanning calorimetry. Furthermore, the presence of locally disordered hydrogen bonds suggest the material will undergo a ferroelectric material at low temperatures.