Lattice defect sites

The observation of most of the lines reported in Table IV is not correlated with the doping of the material. At least two possibilities exist to explain them they are either due to the neutralization by hydrogen of accidental impurities or to the local mode of vibration of hydrogen at a lattice defect site. 

Semiconductor particles typically contain a high density of lattice defect sites (Te, Th), most of which are concentrated at the particle surface. The nature of these defect sites depends strongly on the constituent material of the particle and the method of particle synthesis [61]. Thus, charge carriers photogenerated in accordance with equation (9.1) may subsequently either recombine directly, re-emitting the absorbed energy as heat (A) or light (hv),... 

Absorbance Spectra In Figures 3 and 4, spectra of the freshly activated, unloaded zeolite sample A, as well as of the otherwise unchanged sample after admitting a partial pressure of 3.1 mbar of n-hexane are given. The experimental resolution was 2 cm. Although 25 spectra were accumulated, the signal-to-noise ratio is, due to the small sample area of about 20 x 20 pm rather low. The band at 2350 cm is attributed to COj present in the beam path within the IR microscope. The spectrum of the freshly activated sample exhibits IR bands at 2007, 1882 and 1644 cm which may be attributed to overtones of zeolite lattice vibrations [11]. The broad structure at 3500 cm is due to SiOH groups of lattice defect sites [12]. After equilibration of the sample with 3.1 mbar of n-hexane, the positions and relative intensities of the IR... 

In addition to surface redox (external) processes, there is formation of defects and recombination events occurring in the bulk lattice that can also be considered as redox processes, albeit internal processes. For instance, formation of electron colour centres (e.g. Ti " in Ti02, Zr in Zr02, and F -type centres) is formally a reduction of lattice defect sites, whereas hole trapping by a lattice defect centre is formally an oxidative event (see e.g. eq. 5.72). 

As for other multidithiadiazolyl radicals (Sections XXII.F and XXIII.F), 922 is essentially diamagnetic in the solid state with a residual paramagnetic response (estimated at 3% unpaired spin per molecule) arising from lattice defect sites [93JCS(D)142lj. 

It is also possible for water to bond in dissociated form. In this case, the driving force is the formation of metal-oxygen or metal-hydroxyl bonds. The end-products formed as a result of the water adsorption are adsorbed hydroxyl, atomic oxygen, and atomic hydrogen. When metal oxides are present, water may adsorb in either dissociative or molecular form. Lattice defect sites seem to facilitate dissociation, as observed, for instance, on monocrystalline 1102, NiO, and a-Fc203. The dissociation of water forms a mono-molecular thick film of surface hydroxyl groups that is relatively protective and reduces the subsequent reaction rate of water. The first monolayer of water adsorbed to the hydroxylated oxide surface is highly immobile, whereas the second and third layers are more randomly oriented and less immobile. 

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