Distortion of the crystal structure of MnO at ambient conditions

Abstract

High resolution synchrotron radiation diffraction studies of polycrystalline powder MnO show a systematic distortion from the standard NaCl-type cubic structure (space group Fm3¯m). Two models of the distortion of MnO crystallites are presented: (i) anisotropic Bragg peak broadening due to a distribution of internal microstrains, and (ii) a model with tetragonal symmetry (space group I4/mmm) with at≈ac/√2, ct≈ac. Both models show good agreement with the experimental data which is significantly better than the cubic model with isotropic Bragg peak broadening. Both models imply a distortion of the cubic symmetry of the MnO crystal structure. The size of the tetragonal distortion is x=ct/(at√2)=1±Δ, where Δ≈2×10−4 at ambient conditions. Both tetragonal structures with elongation along ct (x=1+Δ) and shrinking along ct (x=1−Δ) give the same fit quality estimators. The distortion from cubic symmetry increases with milling time for powder MnO samples milled in a planetary mill. The proposed tetragonal space group I4/mmm has a group-subgroup relation with the monoclinic space group C2/m observed in MnO below TN.