Silicon valence electron density

Valence electron density for the diamond structures of carbon and silicon. (Figure redrawn from Cohen M L i. Predicting New Solids and Superconductors. Science 234 549-553.)... 

Chattopadhyay T. K. and von Schnering, H. G., Pyrite-type silicon diphosphide p-SiP2 Structural parameters and valence electron density distribution, Z f. Kristallographie 167 (1984) pp. 1-12. 

Figure Al.3.22. Spatial distributions or charge densities for carbon and silicon crystals in the diamond structure. The density is only for the valence electrons the core electrons are omitted. This charge density is from an ab initio pseudopotential calculation [27].

A similar explanation can be given for the larger Si-O-Si bond angles as compared to C-O-C. Electron density is given over from the oxygen atom into the valence shells of the silicon atoms, but not of the carbon atoms, in the sense of the resonance formulas ... 

Fig. 14.4. The valence electron charge density of silicon. Contour spacings are in units of electrons per unit cell volume. Shaded circles represent atomic cores.

The situation in covalent crystals is something in between that of insulators and metals. In covalent crystals, valence electrons are not sharply localized near the ion cores. However, the density of electrons is not uniform instead, it concentrates along certain preferred directions, leading to chemical bonds. It is interesting to note that conventional semiconductors (and especially silicon) are covalent crystals. 

The participation of the outer electrons in the chemical bonds, accompanied by a change in the spatial electron density distribution, has an effect on the position of the K level. As Karal nik [6] showed, the level displacement caused by a change in the screening of the nucleus by the valence electrons is as high as 1 eV. Our investigation of the position of the K line of silicon in the chromium silicides showed that, within the limits of experimental error, it coincides with its position in the pure element. 

The silicon which separates is pushed out into the interstitial positions, but the amount is clearly insufficient to form a second phase. This reaction is at least slightly exothermic. The presence of interstitial Frenkel defects is also a reason for the hi ly unusual properties of this defect phase. If all the valence electrons participate in forming chemical bonds in the Mn4Si7 lattice, the phase transition characterized by Eq. ( ) should cause interstitial silicon to act as an acceptor for some of the valence electrons, which may lead to the formation of holes and to the occurrence of p-type conductivity. This assumption is supported by the fact that the hole density obtained experimentally is approximately equal to the number of Frenkel defects (4.6 10 and 6.5 10 cm", respectively). For this reason, the crystal chemical formula MnSii 73 corresponds to the phase which exists in the temperature range up to about 1125 C. 

---