Crystal structures of Na2SeO4·1.5H2O and Na2SeO4·10H2O

Abstract

The crystal structures of Na2SeO4·1.5H2O (sodium selenate sesquihydrate) and Na2SeO4·10H2O (sodium selenate decahydrate) are isotypic with those of Na2CrO4·1.5H2O and Na2XSeO4·10H2O (X= S, Cr), respectively. The asymmetric unit of the sesquihydrate contains two Na+cations, one SeO4tetrahedron and one and a half water molecules, the other half being generated by twofold rotation symmetry. The coordination polyhedra of the cations are a distorted monocapped octahedron and a square pyramid; these [NaOx] polyhedra are linked through common edges and corners into a three-dimensional framework structure, the voids of which are filled with the Se atoms of the SeO4tetrahedra. The structure is consolidated by O—H...O hydrogen bonds between coordinating water molecules and framework O atoms. The asymmetric unit of the decahydrate consists of two Na+cations, one SeO4tetrahedron and ten water molecules. Both Na+cations are octahedrally surrounded by water molecules and by edge-sharing condensed into zigzag chains extending parallel to [001]. The SeO4tetrahedra and two uncoordinating water molecules are situated between the chains and are connected to the chains through an intricate network of medium-strength O—H...O hydrogen bonds.