Dynamics of water and other small molecules confined in nanoporous materials is one of the current topics in condensed matter physics. One popular host material is a benzenedicarboxylate-bridging metal (III) complex abbreviated to MIL-53, whose chemical formula is M(OH)[C6H2(CO2)2R2] where M = Cr, Al, Fe and R = H, OH, NH2, COOH. These materials absorb not only water but also ammonia molecules. We have measured the quasi-elastic neutron scattering of MIL-53(Fe)-(COOH)2·2H2O and MIL-53(Fe)-(COOH)2·3NH3 which have full guest occupancy and exhibit the highest proton conductivity in the MIL-53 family. In a wide relaxation time region (τ = 10-12-10-8 s), two relaxations with Arrhenius temperature dependence were found in each sample. It is of interest that their activation energies are smaller than those of bulk H2O and NH3 liquids. The momentum transfer dependence of the relaxation time and the temperature dependence of the relaxation intensity suggest that the proton conduction is due to the Grotthuss mechanism with thermally excited H2O and NH3 molecules.
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