College Park, Maryland June 6 - 10 , 2004
M3-D3 (2:15 PM): The “Bucket Brigade” Mechanism of Proton Diffusion in Superprotonic Conductors
T. Yildirim, D. A. Neumann, T. J. Udovic (NIST Center for Neutron Research)
Proton diffusion plays a central role in many processes ranging from the regulation of biological functions to the production of electricity in fuel cells. The renewed emphasis on developing the next generation of portable sources of power requires the development of light weight protonic conducting materials for use as electrolytic membranes. Furthermore, understanding the atomic-scale mechanism of proton diffusion in candidate materials is necessary to optimize their performance.
First-principles computational techniques combined with neutron scattering measurements provide detailed atomistic information about proton incorporation and dynamics in these materials. Here we briefly describe inelastic (INS) and quasielastic neutron scattering measurements (QNS) on superprotonic conductors of the general formula MHXO4 (M=Rb, Cs, X=S, Se, etc) that reveal detailed information on the proton-conduction mechanism.
QNS measurements indicate that the hydrogen dynamics is dominated by two types of reorientational jumps of H-tetrahedra. Even though such dynamical orientational disorder brings hydrogen atoms from one molecule closer to the next one, it does not result in long-range proton conduction since the motions are localized on a single tetrahedron. However, when one combines these two motions with that of the dynamically disordered hydrogen bond inferred from INS, the mechanism of proton conduction can be surmised. Namely, the protons move between two neighboring tetrahedra via dynamically disordered hydrogen bonds. The tetrahedra then reorient, allowing the protons to move to a third tetrahedron, again via the dynamically disordered hydrogen bonds. Thus the tetrahedra play the role of a “bucket brigade” by accepting a proton from an adjacent tetrahedron then turning and handing it on to the tetrahedron on the other side. On the atomic scale, superprotonic conductivity occurs in the MHXO4 family compounds due to this efficient mechanism of transporting protons through the lattice.
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