Chemical bonding effects on the diffraction intensities in amorphous silicon and carbon

Abstract

The X-ray and electron diffraction intensities from amorphous silicon and carbon cannot be accounted for solely from consideration of a model structure and the appropriate atomic scattering factor. Therefore the aim of the present work is to study the effects of chemical bonding on these diffraction intensities. The linear combination of atomic orbitals description of a covalent bond has been modelled as a superposition of two (identical) spherical charge distributions centred on the nuclei and a third (different) spherical distribution at the bond centre. Using this model of the covalent bond density, plus nuclear‐nuclear, nuclear-bond centre and bond centre–bond centre partial structure factors, the diffraction intensities are calculable without further approximation. The partial structure factors have been calculated from two models: (a) a continuous random network (CRN) model including odd membered rings; and (b) an ordered units (polytetrahedral) model. The principal conclusions are (i) that chemical bonding plays a significant role in determining the first two peaks in the X-ray and electron diffraction intensities of amorphous silicon and carbon and (ii) that CRN models with odd membered rings afford a better structural description of amorphous silicon than models based on ordered units.