Positive and negative monoclinic deformation of corundum-type trigonal crystal structures of M_2O_3 metal oxides

摘要

The crystal structures of several transition metal oxides, Ti_2O_3, V_2O_3, Cr_2O_3, Al_2O_3 and α-Fe_2O_3, 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. Ti_2O_3, V_2O_3 and Cr_2O_3 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 Al_2O_3 and α-Fe_2O_3 show a ‘negative’ distortion which leads to relatively longer metal-metal distances.