Quartz electron structure

Figures 8.7 and 8.8 show the local DOS for O and Si atoms in bulk quartz. In these calculations, the radii of O and Si were set to 1.09 and 1.0 A, respectively. The total DOS for this material was shown in Fig. 8.5. Each LDOS is split into contributions from s and p bands in the electronic structure. It can be seen that the band at the top of the valence band is dominated by p states from O atoms, with a small contribution from Si p states. The band located...

It is of interest that atomic hydrogen centres, which are unstable even at 4 K in quartz, were observed in both coesite and stishovite at 77K.66 The electron centre associated with Ti impurity was observed in stishovite, but not clearly in coesite. Electronic structures of these paramagnetic centres were calculated with the discrete variational (DV)-Xa method to establish a model for centres having substitutional impurities of Al, Ge and Ti.67... 

Let us now construct a representation of the electronic structure of SiOj. There have been many early studies of Si02, principally aimed at interpreting various experimental spectra these have been reviewed recently by Ruffa (1968, 1970). More recently, studios based upon calculations for large clusters of atoms have been made by Reilly (1970), Bennett and Roth (1971a,b), Gilbert ct al. (1973), and Yip and Fowler (1974). Most recently, a full, self-consistent pseudopotential calculation on quartz was made by trhelikowsky and Schliitcr (1977). Here, we shall... 

Further information on the electronic structure of SiOy and other silicates may be obtained from various spectroscopic studies. An excellent review of the various spectra and their interpretation can be found in Gris-com (1977). We will consider, in turn, the photoelectron, x-ray emission, x-ray absorption, uv absorption, Auger, NMR, and NQR spectra of SiOj, primarily in the quartz polymorph. A discussion of the spectra of other polymorphs of SiOj and their differences compared to quartz will be presented in Chapter 7. 

Additional information on electronic structure may be obtained from the x-ray emission spectra of the SiOj polymorphs. As explained in Chapter 2, x-ray emission spectra obey rather strict selection rules, and their intensities can therefore give information on the symmetry (atomic or molecular) of the valence states involved in the transition. In order to draw a correspondence between the various x-ray emission spectra and the photoelectron spectrum, the binding energies of core orbitals must be measured. In Fig. 4.12 (Fischer et al., 1977), the x-ray photoelectron and x-ray emission spectra of a-quartz are aligned on a common energy scale. All three x-ray emission spectra may be readily interpreted within the SiO/ cluster model. Indeed, the Si x-ray emission spectra of silicates are all similar to those of SiOj, no matter what their degree of polymerization. Some differences in detail exist between the spectra of a-quartz and other well-studied silicates, such as olivine, and such differences will be discussed later. 

Dovesi, R., C. Pisani, C. Roetti, and B. Silvi (1987). The electronic structure of a-quartz A periodic Hartree-Fock calculation. J. Chem. Phys. 86, 6967-71. 

NO2 is a stable paramagnetic gaseous molecule at normal temperatures. The ESR parameters of NO2 trapped in a solid matrix have been well established from single-crystal ESR measurements and have been related to the electronic structure by molecular orbital studies [39]. Thus, the NO2 molecule has potential as a spin probe for the study of molecular dynamics at the gas-solid interface by ESR. More than two decade ago temperature-dependent ESR spectra of NO2 adsorbed on porous Vycor quartz glass were observed [40] Vycor is the registered trademark of Coming, Inc. and more information is available at their website. The ESR spectral line-shapes were simulated using the slow-motional ESR theory for various rotational diffusion models developed by Freed and his collaborators [41]. The results show that the NO2 adsorbed on Vycor displays predominantly an axial symmetrical rotation about the axis parallel to the O—O inter-nuclear axis below 77 K, but above this temperature the motion becomes close to an isotropic rotation probably due to a translational diffusion mechanism. 

Different electron structure of two quartz dust samples taken from the same locality and indiscernible by mineralogical methods, were attributable to the incorporation of aluminium ions into the Si04 lattice (Beck et al. 1973). They also differed in their toxicities to macrophages studied with tri-phenyltetrazolium chloride and in the pulmonary hydroxyproline and phospholipid contents 4 to 12... 

Figure 8 shows UV-vis-NIR spectra of thin films of HCSA fully doped polyaniline emeraldine salt that were spin coated on quartz plates from solutions in chloroform and m-cresol, respectively. As discussed previously, different polymer conformations are responsible for these two totally different UV-vis-NlR spectra. Figure 8a indicates a random coil conformation for the polymer chains the three distinctive absorption peaks at 360,440, and 780 nm are consistent with an electronic structure... 

The electronic structure of non-stoichiometric a-quartz surfaces was studied by LEED, XANES and EELS (Bart et al., 1992 1994). Transitions at energies equal to 5.2 eV and 7.4 eV - lower than the bulk gap -have been observed. They have been related to models of vacancies in the quartz lattice. 

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