Positive and negative monoclinic deformation of corundum-type trigonal crystal structures of M2O3 metal oxides

The crystal structures of several transition metal oxides, Ti2O3, V2O3, Cr2O3, Al2O3 and αlpha-Fe2O3, are studied using synchrotron radiation X-ray powder diffraction. The observed angular dependence of the integral breadths is described by two models: (i) the distorted corundum-type structure model described by the space group C2/c and (ii) the Stephens model of anisotropic Bragg peak broadening. These two models are shown to be equivalent. Ti2O3, V2O3 and Cr2O3 show a positive distortion which is related to the possible metal--metal bond suggested by Goodenough in the literature (the deformation leads to shorter metal--metal distances) whereas Al2O3 and αlpha-Fe2O3 show a negative distortion which leads to relatively longer metal--metal distances.