Proton Conduction Mechanisms in Superionic Phases of Phosphate-Based Solid Acids Cristian E. Botez Department of Physics, The University of Texas at El Paso El Paso, TX 79968 Phosphate-based solid acids of the form MH2PO4 (M = Cs, Rb) exhibit several-orders-of-magnitude increases in proton conductivity upon heating above a certain temperature threshold. This superprotonic behavior makes these materials good candidates for use as fuel cell electrolytes. However, the underlying crystal structures and mechanisms which lead to this increase in proton conductivity remain in question. Our study is aimed at determining the crystal structures and microscopic mechanisms that govern the superprotonic behavior of these compounds. Using synchrotron x-ray diffraction: we have clarified the structural modifications associated with the superprotonic transition in CsH2PO4, including the details of the crystal structure of the high-pressure high-temperature cubic phase. we have discovered a new intermediate-temperature monoclinic RbH2PO4 phase that is isomorphic with room-temperature CsH2PO4. Phase transition to a new intermediate-temperature RbH2PO4 monoclinic polymorph. The next step of our investigation will be to propose scenarios for the microscopic mechanisms of proton diffusion through the crystal structures of the superprotonic phases, and test these scenarios via neutron spectroscopy methods.