Mn~24Mo6O9: First Example of Empty Twin Chains of Edge-Sharing M6 Octahedra in Transition Metal Cluster Chemistry

The novel ternary reduced molybdenum oxide Mn~Mo6O9 has been synthesized by solid-state reaction at 1400 "C for 96 h in sealed molybdenum crucibles. Electron diffraction studies showed that Mn~Mo6O9 presents a complex crystal structure with a 3d incommensurate modulation. The average crystal structure was determined on a single-crystal by X-ray diffraction in the orthorhombic space group Pnma with the following lattice parameters: a = 16.4824(2) A, b = 2.8273(2) A, c = 17.3283(2) A, Z= 4. The Mo network consists of empty twin chains of trans-edge-sharing octahedra that occur for the first time in a solid-state compound. The Mo-Mo distances within the chains range from 2.62 to 2.92 A, and the Mo-O distances from 1.99 to 2.17 A as usually observed in the reduced molybdenum oxides. Single-crystal resistivity measurements show that Mn~Mo6O9 is metallic between 4.2 and 300 K. The magnetic susceptibility data indicate paramagnetic behavior due to the Mn moment at high temperatures with a weak ferromagnetic behavior below 80 K.