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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 that is, thus, of first order. Neutron powder-diffraction data are compatible with a collinear magnetic structure with the moments ordered in the monoclinic ac plane.
Jauch et al. (Wed,) studied this question.