Electron-density studies. II. Further comments on the electron density in diamond

Abstract

X-ray powder diffraction data for diamond have been re-analysed, in terms of an 'at rest' deformation-density model of monopole, octupole, hexadecapole and hexacontatettarapole terms of exponential form. The values of the parameters describing the third and fourth-order terms, and the refinement indices, are no different from those of an 'at rest' valence-density model. Unlike earlier analyses, however, the model is consistent with experimental measurements of the absolute scale, and with independently determined values of the Debye-Waller parameter. The predicted values of physical properties such as the gradient of the electric-field gradient at the carbon nucleus are shown to be strongly model-dependent. It is noted that the fit to the data, and in particular to the observed structure factor of the overlapping 333-511 reflection, is poor for all published charge-density analyses. Recent theoretical (crystal 'Hartree-Fock') calculations indicate, within the convolution and harmonic approximations, an experimental B value of 0.172 ± 0.006 A2 and a scale factor of 1.004 ± 0.002. These theoretical results are, however, in poor agreement with the experimental structure factors.