Synthesis of rare-earth selenate and selenite materials under “sol-gel” hydrothermal conditions: crystal structures and characterizations of La(HSeO3)(SeO4) and KNd(SeO4)2

Two rare-earth compounds containing selenium atoms, La(HSeO₃)(SeO₄) with a new open framework structure and KNd(SeO₄)₂ with a layered structure, have been synthesized under “sol-gel” hydrothermal conditions for the first time. Single-crystals of La(HSeO₃)(SeO₄) crystallize in the monoclinic system (P2₁, , , , β=104.91(3)°, Z=2, R_All=0.032). The structure contains puckered polyhedral layers made of LaO_x (x=9,10) and SeO₄ groups, which are connected via SeO₃-uints to the 3D structure. The crytal structure of KNd(SeO₄)₂ (monoclinc, P2₁/c, , , , β=91.38(3)°, Z=4, R_All=0.051) contains honeycomb-like six-ring NdO₉ polyhedra forming layers which are further decorated with SeO₄ tetrahedra. The K⁺ ions occupy the interspaces of these layers and provide the charge balance.