Calorimetric and Quasielastic Neutron Scattering Studies on a Mg-Ion Conductor Using a Metal-Organic Framework...

A metal-organic framework (MOF) MIL-101 containing Mg(TFSI)2 (TFSI: bis(trifluoromethanesulfonyl)imide) and guest molecules within pores exhibits high Mg2+ conductivity (>10-3 S cm-1) at room temperature and is attracting attention as a hopeful solid Mg conductor. To investigate the phase behavior and the dynamics of acetonitrile (AN) molecules in the pores, heat capacity (Cp) and quasielastic neutron scattering (QENS) experiments were performed for MIL-101/Mg(TFSI)2/AN, MIL-101/AN, bulk AN/Mg(TFSI)2, bulk AN, and dry MIL-101. The Cp measurement of MIL-101/AN revealed a broad phase transition around 110-250 K, corresponding to some sort of order/disorder process of AN molecules within the pores. A similar Cp peak accompanied by a glass transition at the Cp rising part appeared in MIL-101/Mg(TFSI)2/AN. This glass transition corresponds to the freezing of the AN motion in the pores. The samples containing AN exhibited QENS broadening above the phase transitions. The QENS spectra of the samples with Mg(TFSI)2 were well explained by the jump diffusion model, while those without Mg(TFSI)2 by the continuous diffusion model. The jump length was about 1.6 Å, which is thought to be related to the connection/disconnection processes of AN molecules to Mg2+ ions. The diffusion coefficient of AN molecules decreased in the order of bulk AN > bulk AN/Mg(TFSI)2 > MIL-101/AN > MIL-101/Mg(TFSI)2/AN. Only MIL-101/Mg(TFSI)2/AN exhibited non-Arrhenius temperature dependence. These results are consistent with our proposed model that the coordination of AN to Mg2+ shields interionic electrostatic interactions and enhances the Mg2+ mobility, leading to the high Mg2+ conductivity in MIL-101/Mg(TFSI)2/AN.