K 5 Mn 3 O 6 and Rb 8 Mn 5 O 10 , New Charge Ordered Quasi One‐Dimensional Oxomanganates (II, III)

K 5 Mn 3 O 6 and Rb 8 Mn 5 O 10 have been synthesized via the azide/nitrate route from stoichiometric mixtures of MnO and KNO 3 /KN 3 , or RbNO 3 /RbN 3 , respectively. The two new members of manganese (II/III) mixed‐valent oxides have been studied by single‐crystal X‐ray diffraction, magnetic susceptibility measurements and density functional theory calculations (DFT). Both crystal structures [K 5 Mn 3 O 6 : Pna 2 1 , Z = 4, a = 1632.43(7), b = 970.07(4), c = 616.48(3) pm, R 1 = 0.051; Rb 8 Mn 5 O 10 : P 2 1 / c , Z = 8, a = 1223(2), b = 2717(3), c = 1220(2) pm, β = 118.25(2)°, R 1 = 0.086] display quasi one‐dimensional [MnO 2 ] n – chains, composed of edge sharing MnO 4 tetrahedra. Complete charge ordering of Mn 2+ and Mn 3+ is observed along the chain direction with repetition units [‐Mn 3+ ―Mn 2+ ―Mn 2+ ‐] resulting for K 5 Mn 3 O 6 , and [‐Mn 3+ ―Mn 2+ ―Mn 2+ ―Mn 3+ ―Mn 2+ ‐] for Rb 8 Mn 5 O 10 . Magnetic susceptibility data as well as DFT calculations indicate strong anti ‐ferromagnetic intra‐chain coupling to be present already at ambient temperature, and above. Rb 8 Mn 5 O 10 features three‐dimensional spin ordering below T N = 10.9 K, mediated by ferromagnetic coupling between the chains, while such a 3D coupling appears to be frustrated in K 5 Mn 3 O 6 .