Chemical and Structural Evolution in the Th–SeO₃^2–/SeO₄^2– System: from Simple Selenites to Cluster-Based Selenate Compounds

While extensive success has been gained in the structural chemistry of the U–Se system, the synthesis and characterization of Th-based Se structures are widely unexplored. Here, four new Th–Se compounds, α-Th­(SeO₃)₂, β-Th­(SeO₃)₂, Th­(Se₂O₅)₂, and Th₃O₂(OH)₂(SeO₄)₃, have been obtained from mild hydrothermal or low-temperature (180–220 °C) flux conditions and were subsequently structurally and spectroscopically characterized. The crystal structures of α-Th­(SeO₃)₂ and β-Th­(SeO₃)₂ are based on ThO₈ and SeO₃ polyhedra, respectively, featuring a three-dimensional (3D) network with selenite anions filling in the Th channels along the <i>a</i> axis. Th­(Se₂O₅)₂ is a 3D framework composed of isolated ThO₈ polyhedra interconnected by [Se₂O₅]^2– dimers. Th₃O₂(OH)₂(SeO₄)₃ is also a 3D framework constructed by octahedral hexathorium clusters [Th₆(μ₃-O)₄(μ₃-OH)₄]¹²⁺, which are interlinked by selenate groups SeO₄^2–. The positions of the vibrational modes associated with both Se^IVO₃^2– and Se^VIO₄^2– units, respectively, were determined for four compounds, and the Raman spectra of α- and β-Th­(SeO₃)₂ are compared and discussed in detail