Silica and Germania

1 Silica and germania. Because of the special place occupied by Si02 as the archetypal network glass former, there have been extensive efforts using many experimental techniques to understand its structural details. The crystalline polymorphs used to calibrate these experimental probes have been studied in detail by O NMR. Their parameters are summarised in Table 6.5. 

Variable temperature O NMR has been used to examine the phase transition 

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For the light guiding effect of optical fibers it is important to have a core with a refractive index somewhat larger than that of the airrounding cladding. Therefore the core normally consists of silica doped with other oxides to increase the refractive index. In the following we therefore discuss the simultaneous deposition of silica and germania in the silica tube. 

It is interesting to note that the actual shape of the germanium concentration shows a maximum and a minimum, but this has not been fully investigated up to now. Of course, as for the profile of silica and germania alone, the concentration profile also depends on the experimental conditions. 

Ab Initio Calculations of Electronic Transitions and Photoabsorption and Photoluminescence Spectra of Silica and Germania Nanoparticles... 

Ab initio and density functional calculations of potential energy surfaces for the ground and excited electronic states of model clusters simulating various point defects, impurities, and their combinations in nanosized silica and germania materials are reported. The accurate geometric and electronic structures of these clusters, calculated photoabsorption and photoluminescence (PL) energies, and predicted absorption and PL spectra are obtained. Our calculations reproduced the experimental excitation energy (1.9-2.0 eV)... 

The particular property we are concerned with in this chapter is the ability of silica and germania nanostructures to absorb UV or visible light giving rise to visible or IR photoluminescence (PL). In bulk, Si02 and Ge02 are... 

The quantum chemical modeling is a very useful supplement to spectroscopic experimental methods for investigation of properties of point defects, however, until recently it was used mainly for calculations of vertical excitation energies. The modeling of structural transformation in excited electronic states is still a rather complicated task, which requires state-of-the-art quantum chemical calculations. In this chapter, we first describe theoretical methods applied in ab initio and vibronic theory calculations and then demonstrate their applications in theoretical studies of various point defects in silica and germania. 

In this section, we discuss theoretical methods, which can be applied for calculations of photoabsorption and PL spectra of silica and germania nanoparticles. We start with the choice of model cluster simulating these materials and point defects in them and consider methods for geometry optimization in the ground and excited electronic states (Subsection 2.1). This is followed by the description of more advanced quantum chemical methods for accurate calculations of excitation energies (Subsection 2.2) and the section is completed by the discussion on the theoretical procedure used for predicting vibronic spectra associated with point defects (Subsection 2.3). 

What is the role of point defects in optical properties of silica and germania nanomaterials ... 

Why the properties of point defects in silica and germania can be simulated using quantum chemical calculations of model clusters of a finite size Describe the requirements for model clusters to be suitable for such simulations. 

Discuss common features and differences in optical properties of analogous point defects in silica and germania nanomaterials. 

We note here that the ordinary condensed solid phase of CO2 is a molecular solid. At variance with silica and germania, a nonmolecular CO2 crystalline form of carbon dioxide in which carbon atoms are tetrahedrally coordinated to oxygen atoms only exists at high pressure [16,17], In addition, an amorphous phase of CO2, formed by a disordered arrangement of CO4 tetrahedra, has recently been obtained at very high pressure [18]. 

To our knowledge this is the first attempt demonstrating the use of a genetically engineered synthetic protein for mineralisation of silica and germania. The results obtained for each respective system are presented and discussed below. 

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