Crystal Chemistry of the M 1 1+,2+ – M 2 2+,3+ –(H)‐Arsenites: the First Cadmium(II) Arsenite, Na 4 Cd 7 (AsO 3 ) 6

The crystal structures among M 1– M 2–(H)‐arsenites ( M 1 = Li + , Na + , K + , Rb + , Cs + , Ca 2+ , Sr 2+ , Ba 2+ , Cd 2+ , Pb 2+ ; M 2 = Mg 2+ , Mn 2+,3+ , Fe 2+,3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ ) are less investigated. Up to now, only the structure of Pb 3 Mn(AsO 3 ) 2 (AsO 2 OH) was described. The crystal structure of hydrothermally synthesized Na 4 Cd 7 (AsO 3 ) 6 was solved from the single‐crystal X‐ray diffraction data. Its trigonal crystal structure [space group R $\bar{3}$ , a = 9.5229(13), c = 19.258(4) Å, γ = 120°, V = 1512.5(5) Å 3 , Z = 3] represents a new structure type. The As atoms are arranged in monomeric (AsO 3 ) 3– units. The surroundings of the two crystallographically unique sodium atoms show trigonal antiprismatic coordination, and two mixed Cd/Na sites are remarkably unequal showing tetrahedral and octahedral coordinations. Despite the 3D connection of the AsO 3 pyramids, (Cd,Na)O x polyhedra and NaO 6 antiprisms, a layer‐like arrangement of the Na atoms positioned in the hexagonal channels formed by CdO 4 deformed tetrahedra and AsO 3 pyramids in z = 0, 1/3, 2/3 is to be mentioned. These pseudo layers are interconnected to the 3D network by (Cd,Na)O 6 octahedra. Raman spectra confirmed the presence of isolated AsO 3 pyramids.