New MOF based on lithium tetrahydrofuran-2,3,4,5-tetracarboxylate: Its structure and conductivity behavior

A novel metal-organic framework, [Li₆(HTFTA)₂(H ₂O)₃]·3H₂O (LiTFTA, H₄TFTA is tetrahydrofuran-2,3,4,5-tetracarboxylic acid), has been synthesized under solvothermal conditions and its structure was determined by single-crystal X-ray diffraction studies. It has six different LiO₄ tetrahedra which forms two kinds of tetranuclear clusters that are coordinated to the ligands and form an overall three-dimensional structure with channels running along the crystallographic a axis. Some of the carboxylate groups of the ligand moiety jut out into the channels and the channels are occupied by water molecules. The proton conductivity behavior of LiTFTA with respect to temperature and relative humidity was analyzed by our newly developed impedance data analysis method called a random-walk approach. The results were compared with the data obtained by an analysis using an equivalent electric circuit model. Based on the physical parameters obtained by the random-walk method and the known structure we suggested the mechanism of the proton conductivity in this material. The proton conduction proceeds most probably by a vehicle conduction mechanism which employs mainly water molecules of crystallization present in the channels of LiTFTA. The value of the diffusion coefficient increases with temperature whereas the number of charge carriers remains constant. On the other hand, both the number of charge carriers and diffusion coefficient increase with increasing relative humidity.