Nd₂(SeO₃)₂(SeO₄) . 2H₂O - A mixed-valence compound containing selenium in the oxidation states +IV and +VI

Pale pink crystals of Nd-2(SeO3)(2)(SeO4) (.) 2H(2)O were synthesized under hydrothermal conditions from H2SeO3 and Nd2O3 at about 200 degreesC. 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) Angstrom, b = 7.0783(5) Angstrom, c = 13.329(1) Angstrom, beta = 104.276(7)degrees). The crystal structure of Nd-2(SeO3)(2)(SeO4) (.) 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 ((SeO3)-O-IV)(2-) groups, ((SeO4)-O-VI)(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 Nd-2(SeO3)(2)(SeO4) (.) 2H(2)O starts at 260 degreesC. The thermal decomposition into Nd2SeO5, SeO2 and O-2 at 680 degreesC is followed by further loss of SeO2 leaving cubic Nd2O2.