X-ray diffraction study of bond character of rutile-type SiO2, GeO2 and SnO2

The charge density and bond character of the rutile-type structures of SiO2, GeO2 and SnO2 were studied by X-ray diffraction using monopole refinement. The refinement of the charge density was performed by least-squares refinement including the kappa-parameter and the electron population in the X-ray atomic scattering parameters. The oxygen's ii-parameters of three oxides are 0.91 (SiO2), 1.06 (GeO2), 1.14 (SnO2). The oxygen valence electron of each sample is 1.06(8)e for SiO2, 1.10(16)e for GeO2 and 1.13(15)e for SnO2. These values indicate that the electron distributions an more localized with an increase in the cation atomic number. The difference Fourier map of each sample shows the deformation density of the valence electrons of cations. The overlapping electron orbitals cause the deformation of MO6 octahedra of rutile-type structures. The ratio between O-O shared and unshared edge and the ratio between the apical and equatorial M-O interatomic distances decrease with the cation ionicity and the repulsive force between the two cations in adjacent octahedra.