NaIn(CrO4)2.2H2O, the first indium(III) member of the kröhnkite family.

Sodium indium(III) chromate(VI) dihydrate, NaIn(CrO4)2.2H2O, synthesized from an aqueous solution at room temperature, is the first indium(III) member of the large family of compounds with kröhnkite [Na2Cu(II)(S(VI)O4)2.2H2O]-type chains. The crystal structure is based on infinite octahedral-tetrahedral [In(CrO4)2(H2O)2]- chains along [010], linked via charge-balancing Na+ cations. The slightly distorted InO4(H2O)2 octahedra are characterized by a mean In-O distance of 2.125 A. The CrO4 tetrahedra are strongly distorted (mean Cr-O = 1.641 A). The Na atom shows an octahedral coordination, unprecedented among compounds with kröhnkite-type chains. The NaO6 octahedra share opposite edges with the InO4(H2O)2 octahedra to form infinite [001] chains. The hydrogen bonds are of medium strength. NaIn(CrO4)2.2H2O belongs to the structural type F2 in the classification of Fleck, Kolitsch & Hertweck [Z. Kristallogr. (2002), 217, 435-443], and is isotypic with KAl(CrO4)2.2H2O and MFe(CrO4)(2).2H2O (M = K, Tl or NH4). All atoms are in special positions except one O atom.