Nd2(SeO3)2(SeO4) · 2H2O – a Mixed‐Valence Compound containing Selenium in the Oxidation States +IV and +VI

Pale pink crystals of Nd₂(SeO₃)₂(SeO₄) · 2H₂O were synthesized under hydrothermal conditions from H₂SeO₃ and Nd₂O₃ at about 200 °C. X‐ray diffraction on powder and single‐crystals revealed that the compound crystallizes with the monoclinic space group C 2/c (a = 12.276(1) Å, b = 7.0783(5) Å, c = 13.329(1) Å, β = 104.276(7)°). The crystal structure of Nd₂(SeO₃)₂(SeO₄) · 2H₂O is an ordered variant of the corresponding erbium compound. Eight oxygen atoms coordinate the Nd^III atom in the shape of a bi‐capped trigonal prism. The oxygen atoms are part of pyramidal (Se^IVO₃)^2? groups, (Se^VIO₄)^2? tetrahedra and water molecules. The NdO₈] polyhedra share edges to form chains oriented along 010]. The selenate ions link these chains into layers parallel to (001). The layers are interconnected by the selenite ions into a three‐dimensional framework. The dehydration of Nd₂(SeO₃)₂(SeO₄) · 2H₂O starts at 260 °C. The thermal decomposition into Nd₂SeO₅, SeO₂ and O₂ at 680 °C is followed by further loss of SeO₂ leaving cubic Nd₂O₃.