W0096
Entropy Considerations in Polymorphic Phase Transitions of
Solid Acids to Disordered, High-Conductivity Phases. Sossina M. Haile and
Calum R. I. Chisholm, Materials Science, 138-78, California Institute of
Technology, Pasadena, CA 91125; smhaile@caltech.edu.
Several alkali acid sulfates, phosphates and selenates are
known to undergo “superprotonic” transitions in response to a change
in temperature, at which the conductivity jumps 3-5 orders of magnitude.
Examples include CsHSO4,
Cs5(HSO4)3(H2PO4)2,
and Rb3H(SeO4)3 with tetragonal, cubic and
trigonal high temperature phases, respectively. In contrast to ferroelectric
transitions, which such materials can also undergo, there are no widely accepted
guidelines for predicting high-temperature superprotonic behavior based on room
temperature structure or properties. As a first step towards addressing this
challenge, we present an analysis of the transition entropies of the
xCsHSO4 – (1-x)CsH2PO4 class of
compounds, interpreted in terms of Pauling’s model for the residual
entropy of ice. Each of the seven known compounds within this pseudo-binary
system crystallizes into a unique monoclinic structure at room temperature. At
high temperature, however, only one of two structures, one tetragonal and the
other cubic, is observed. Calculated transition entropies were found to
match the experimental values, indicating that the bulk of the entropic driving
force of the transition is accounted for in this model. Ultimately, such insight
will aid the effort to identify new compounds with high proton
conductivity.