ELECTRON DENSITY STUDY OF SPINELS: ZINC CHROMIUM OXIDE

The electron density distribution in zinc chromium oxide spinel [composition ZnCr2O4; for general formula TX2O4, tetrahedral site T = ZnxCr1−x, octahedral site X2 = Cr1+xZn1−x = 0.96 [1]] has been studied with single-crystal X-ray diffraction methods through scattering factor refinement procedures. Smaller cell dimensions (a = 8.3267(3) Å), compared to those of MgCr2O4, along with electron density residuals 0.2∼0.3 e/Å3 in height situated between the X-site and oxygen atoms suggestive of bonding electrons imply stronger covalency in this compound. In the refinement with harmonic parameters, six residuals −0.9 e/Å3 in height are arranged around the X-site toward the coordinated oxygen, and eight residuals 0.6 e/Å3 in height are arranged around the X-site toward directions avoiding the oxygens. These residuals almost completely disappear with anharmonic refinement. Another residual 1.7∼1.8 e/Å3 in height is situated at the center of the cubical X4O4 unit in the structure [222 equivalent-averaged reflections, R = 0.0091 (harmonic refinement), R = 0.0071 (anharmonic refinement)]. © 1997 Elsevier Science Ltd