Tri-α-PbO₂-Type Fe–Sb Tungstate by Topotactic Ion Exchange of LiSbWO₆

A tri-α-PbO2-type oxide, Li0.08Fe0.46SbWO6, is synthesized for the first time by topotactic ion exchange of LiSbWO6 in an aqueous Fe(II) solution at 60 °C for 4 days under an argon atmosphere. The structure of the resulting compound is ascertained by Rietveld refinement of PXRD data, and the topotactic nature of exchange is established. The Fe–Sb tungstate showed a considerable reduction in the band gap to 2.06 from 3.05 eV of the parent LiSbWO6. The compound is metastable in nature; it undergoes exothermic structural transitions or decompositions at around 790 and 950 °C and cannot be stabilized by conventional solid-state reactions. The compound shows an antiferromagnetic transition at ∼20 K, which is ascribed to the presence of Fe(II) dimers in the edge-shared chains of Li/FeO6 octahedra running parallel to the c-direction in the tri-α-PbO2 structure. The applicability of the Fe-exchanged compound for visible-light-driven semiconductor photocatalysis is investigated by degradation of RhB dye. The topotactic ion exchange in the tri-α-PbO2 structure is unique and rare owing to its three-dimensional quasi-ordered structure with corner- and edge-connected moieties; however, plenty of them are reported in other structure types, including layered, tunnel, and framework structures. The strategy will be useful in constructing ordered or pseudo-ordered transition-metal arrays in three-dimensional structures with intriguing electronic properties.