Synthesis, X-ray and Neutron Diffraction Characterization, and Ionic Conduction Properties of a New Oxothiomolybdate Li3[Mo8S8O8(OH)8{HWO5(H2O)}]⋅18 H2O

Abstract

The new oxothiomolybdate anion [Mo8S8O8(OH)8{HWO5(H2O)}]3− (denoted HMo8W3−) has been synthesized in aqueous solution by an acido–basic condensation reaction. Four {MoV2S2O2} building blocks are connected through hydroxo bridges around a central {WVIO6} octahedron. X-ray and neutron diffraction studies have been performed on single crystals of the lithium salt Li3[Mo8S8O8(OH)8{HWO5(H2O)}]⋅18 H2O (Li3HMo8W⋅18 H2O) grown from HMo8W3− in an aqueous solution of LiCl (1 M). The neutron diffraction experiment enabled us to locate both the protons and the lithium ions. In the structure of Li3HMo8W⋅18 H2O, ring-shaped anions interleaved by a cluster of disordered hydrogen-bonded water molecules stack on top of each other along lithium pillars. The lithium columns are formed by alternating edge-sharing octahedra and tetrahedra, with one lithium site in four being totally vacant. Ionic conductivity measurements on pressed pellets have shown that Li3HMo8W⋅18 H2O is a good ionic conductor at room temperature (σ=10−5 S cm−1), but the ionic conductivity on single crystals is smaller by two orders of magnitude and is isotropic; this suggests the main path of conduction involves surface protons rather than lithium ions of the bulk.