Crystal structures of two hydrated sodium chromates: Na₂CrO₄ × 10 H₂O and Na₂CrO₄ × 1.5 H₂O

Abstract

Single crystals of Na₂CrO₄ × 10 H₂O (I) and Na₂CrO₄ × 1.5 H₂O (II) were synthesized from an aqueous solution of sodium chromate by isothermal evaporation of the solvent. The crystal structures of both compounds were determined from X-ray single crystal diffraction experiments performed at –100 °C. Basic crystallographic data of the phases are as follows: (I) monoclinic, space group P2₁/c, a = 11.5356(12) Å, b = 10.4989(9) Å, c = 12.9454(13) Å, β = 107.489(8)°, V = 1495.4(2) ų, Z = 4; (II) orthorhombic, space group F2dd, a = 6.7797(8) Å, b = 8.6548(10) Å, c = 35.078(5) Å, V = 2058.3(5) ų, Z = 16. The decahydrate is isostructural with the corresponding sulfate compound (Glauber’s salt or mirabilite). The two crystallographically independent sodium atoms in (I) are coordinated by six water molecules each in form of distorted octahedra. The octahedra are linked together by edge-sharing in chains parallel to the c-axis. The periodicity of a single chain is four. The [CrO₄]-tetrahedra are disordered among two positions. The sesquihydrate (II) represents a new structure type. The sodium cations are coordinated in form of distorted [NaO₄(H₂O)₂]-octahedra as well as [NaO₄(H₂O)]-pyramids. The octahedra share common edges. The resulting zig-zag chains represent the backbone of the structure and run along [110] and [1-10], respectively. The chains have a width of about 3.2 Å and are concentrated in layers parallel to (001). Within a single layer neighbouring chains are connected by [CrO₄]-tetrahedra and additional hydrogen bonds. Linkage between different layers is provided by sheets containing corner sharing tetragonal pyramids, with four oxygen atoms defining the basal plane and an additional water molecule forming the apex. The present investigation confirms the existence of a sesquihydrate in the system Na₂CrO₄—H₂O which has been a matter of debate in the literature.