Crystal Structure and Ferrimagnetism in NiMnO3and CoMnO3

X-ray diffraction studies of individual crystals of NiMn${\mathrm{O}}_{3}$ and CoMn${\mathrm{O}}_{3}$ show that these compounds have the structure of ilmenite (space group $R\overline{3}$). The crystals have a plate-like crystal habit with the threefold symmetry axis perpendicular to the faces of the platelet. Positions of atoms within the unit cell have been determined by comparing calculated structure factors with those obtained from x-ray powder patterns. There are two Ni-O-Mn configurations that appear to be favorable to a magnetic superchange interaction, and these are sufficient to extend magnetic ordering in three dimensions.