Structure and magnetic properties of hollandite Ba_1.2Mn ₈O₁₆

The synthesis, crystal structure and magnetic properties of Ba _1.2Mn₈O₁₆, obtained using a KCl mineralizer, are reported. This material crystallizes in the monoclinic hollandite structure, containing Mn-O double chains (space group: I 2/m, a ≤ 10.052(1) Å, b ≤ 2.8579(2) Å, c ≤ 9.7627(10) Å, β ≤ 89.96(1)°), and undergoes a magnetic transition at 40 K with the magnetization having a small ferromagnetic component. The ordering temperature is nearly 10 times lower than expected from the Weiss temperature of -385 K, evidencing magnetic frustration. Preliminary neutron powder diffraction studies suggest a complex spin arrangement within the ac-plane (perpendicular to the chain axis). The resistivity is consistent with one-dimensional variable range hopping.