Nd 2 (SeO 3 ) 2 (SeO 4 ) · 2H 2 O – a Mixed‐Valence Compound containing Selenium in the Oxidation States +IV and +VI

Pale pink crystals of Nd 2 (SeO 3 ) 2 (SeO 4 ) · 2H 2 O were synthesized under hydrothermal conditions from H 2 SeO 3 and Nd 2 O 3 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 Nd 2 (SeO 3 ) 2 (SeO 4 ) · 2H 2 O 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 IV O 3 ) 2− groups, (Se VI O 4 ) 2− tetrahedra and water molecules. The [NdO 8 ] 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 Nd 2 (SeO 3 ) 2 (SeO 4 ) · 2H 2 O starts at 260 °C. The thermal decomposition into Nd 2 SeO 5 , SeO 2 and O 2 at 680 °C is followed by further loss of SeO 2 leaving cubic Nd 2 O 3 .