Chemical and Structural Evolution in the Th–SeO32–/SeO42– 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 a 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 SeO42–. The positions of the vibrational modes associated with both SeIVO32– and SeVIO42– units, respectively, were determined for four compounds, and the Raman spectra of α- and β-Th­(SeO3)2 are compared and discussed in detail.