Rock salt vs. wurtzite phases of Co1−xMnxO: control of crystal lattice and morphology at the nanoscaleElectronic supplementary information (ESI) available: Additional powder X-ray diffraction analyses, details on the analysis of the rod and branched rod structures, and effect of growth time on relative metal content, 6 pages. See DOI: 10.1039/c2ce26440d

Mixed Mn-Co oxide nanoparticles have been prepared by decomposition of metal carboxylate precursors in a mixture of oleic acid (OA) and tri- n -octylamine (TOA). In addition to the stable MO rock salt structures, wurtzite-type Co 1− x Mn x O (0.55 < x < 0.65) nanoparticles have been synthesized for the first. Molar ratios of OA to precursor in excess of 5 were required to obtain the metastable phase. Varying the surfactant, water and precursor ratios allows control of particle size, morphology and stoichiometry: diamond cuboid, rhombohedra and hexagonal-shaped nanocrystals as well as branched rods of rock salt Co 1− x Mn x O (0.01 ≤ x ≤ 0.9) have also been synthesized. The rhombohedra and hexagons exhibited dimensions