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

The crystal structures of Na2SeO4·1.5H2O (sodium selenate sesquihydrate) and Na2SeO4·10H2O (sodium selenate deca-hydrate) are isotypic with those of Na2CrO4·1.5H2O and Na2 XSeO4·10H2O (X = S, Cr), respectively. The asymmetric unit of the sesquihydrate contains two Na(+) cations, one SeO4 tetra-hedron and one and a half water mol-ecules, the other half being generated by twofold rotation symmetry. The coordination polyhedra of the cations are a distorted monocapped octa-hedron and a square pyramid; these [NaO x ] 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 SeO4 tetra-hedra. The structure is consolidated by O-H⋯O hydrogen bonds between coordinating water mol-ecules and framework O atoms. The asymmetric unit of the deca-hydrate consists of two Na(+) cations, one SeO4 tetra-hedron and ten water mol-ecules. Both Na(+) cations are octa-hedrally surrounded by water mol-ecules and by edge-sharing condensed into zigzag chains extending parallel to [001]. The SeO4 tetra-hedra and two uncoordinating water mol-ecules are situated between the chains and are connected to the chains through an intricate network of medium-strength O-H⋯O hydrogen bonds.