Octanuclear Oxothiomolybdate(V) Rings: Structure and Ionic-Conducting Properties
Résumé
A family of alkali salts of octanuclear oxothiomolybdate rings has been synthesized by crystallization of the [Mo8S8O8(OH)8{HMO5(H2O)}]3− (noted HMo8M3−; M=Mo, W) and [Mo8S8O8(OH)8(C2O4)]2− (noted Mo8ox2−) anions in an aqueous solution of ACl (A=Li, Na, K, Rb). Single‐crystal X‐ray diffraction experiments have been performed showing that the alkali salts exhibit a similar three‐dimensional structure. Disordered alkali ions form columns to which the anionic rings are anchored. Ionic‐conductivity measurements on pressed pellets have revealed two different behaviors. The lithium salts of HMo8M3− (M=Mo, W) are moderately good proton conductors at room temperature (σ=10−5 S cm−1) and the profile of conductivity as a function of relative humidity shows that the conductivity is due to surface‐proton motion (particle‐hydrate‐type mechanism). On the other hand, the lithium salt of Mo8ox2− competes with the best crystalline lithium conductors at room temperature (σ=10−3 S cm−1), and 7Li NMR experiments confirm the mobility of the lithium ions along the one‐dimensional channels of this material.