PHASE TRANSITIONS IN HYDROGEN-BONDED INORGANIC ACIDS. S.M. Haile, P. Calkins, S. Fu, S. Faulk and G Staneff, Department of Materials Science and Engineering, University of Washington, Seattle, WA 98195
A number of solid acid sulfates and phosphates exhibit ferroelectric transitions at low temperatures and superprotonic transitions at elevated temperatures. In particular, CsH2PO4 undergoes a ferroelectric transition at 170K, whereas CsHSO4 undergoes a superprotonic transition at 417K. While the structural features that lead to ferroelectricity in CsH2PO4 and not in CsHSO4 are fairly well-understood, the same cannot be said of superprotonic phase transitions. In this work we have investigated phase transitions in a number of CsHSO4-CsH2PO4 compounds. Selected models describing ferroelectric transitions are examined with respect to the new compounds, Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2, both of which exhibit ferroelectric transitions. In both, low temperature ferroelectricity is expected based on the presence of locally disordered hydrogen bonds in the room temperature structures. With respect to high temperature transformations, we propose that, in contrast, superprotonic transitions are driven by global features of the hydrogen bonded network. Specifically, the compounds CsHSO4, Cs3(HSO4)2(H2PO4) and Cs5(HSO4)3(H2PO4)2, all of which undergo superprotonic transitions, all contain some oxygen atoms that do not participate in hydrogen bonding. At elevated temperatures, entropy considerations drive the compounds to a state in which all oxygen atoms are chemically, if not crystallographically equivalent. As the number of protons is not sufficient to satisfy all the potential hydrogen bonds, the only way in which all oxygen atoms can participate in hydrogen bonding is via the introduction of partial occupancies, either on oxygen sites or proton sites or both. It is precisely this state of disorder that leads to a highly conducting state. Relevant details of the structures of these new compounds supporting the present hypothesis are described.