Electron density study of spinels: Magnesium chromium oxide

The electron density distribution in magnesium chromium oxide spinel [composition MgCr2O4; for general formula TX2O4, tetrahedral site T = MgxCr1 − x, octahedral site X = Cr1 + xMg1 − x, x = 0.984(6)] has been studied with single-crystal X-ray diffraction methods through scattering factor refinement procedures. In the refinement with harmonic parameters, there are electron density residuals around the X-site; six residuals of −1.0e/Å3 height along, eight of +0.7~0.8e/Å3 height avoiding the directions toward the coordinated oxygens. Also, a +0.6e/Å3 height residual is seen near oxygen along the T-O direction. These residuals almost completely disappear with anharmonic refinement. Another residual (in harmonic refinement, +1.27e /Å3; anharmonic, +0.90e/Å3 in height) is situated at the center of the X4O4 cuboidal unit in the structure. [230 equivalent-averaged reflexions, R = 0.0091 (harmonic refinement), R =0.0064 (anharmonic refinement)]