Crystallographic symmetry and magnetic structure of CoO

The crystallographic symmetry of antiferromagnetic CoO was studied using high resolution synchrotron powder diffraction in the temperature range 10 300 K. The high quality powder patterns unambiguously revealed a monoclinic symmetry space group C2 m and allowed the extraction of accurate values for the lattice constants. The temperature dependence of the monoclinic deformation scales with the much stronger tetragonal distortion as determined from laboratory x ray diffraction. Magnetic ordering is associated with a cubic to monoclinic transition which is thus of first order type. The low crystal symmetry implies a collinear magnetic structure with the moments ordered in the monoclinic ac plane. From neutron powder diffraction follows an angle of 60 1 to the a axi