Aluminum oxygen hole centers

Although vitreous silica usually remains colorless following irradiation to very high doses, doped silicas can become colored through the formation of defects associated with impurities. Purple samples, for example, are formed if the glass contains a small amount of aluminum, due to the formation of aluminum-oxygen hole centers (AlOHC). Other impurities, such as germanium or titanium, can also produce colored vitreous silica by formation of defect centers. 

Apart from other pubhshed experiments [4.57] related to EURECA, mission radiation and recording of EPR spectra have been done at room temperature (see spectrum 3 in Fig. 4.66). Spectra 1 and 2 result from samples irradiated at 77 K, recorded at 170 K and irradiated at 77 K, annealed at 440K, and recorded at 170K, respectively (published in [4.57]). The main, quite complicated signal around consists basically of a variety of the so-called oxygen hole centers (OHCs) close to aluminum or silicon, aluminum has a natural abundance of 100% Al with I = 5/2, yielding a hyperfine coupling. In accordance with other publications it has been shown that all... 

As discussed in Sec. 3.1.3 the presence of defects and impurities can promote loss in piezoelectric crystals. Defects are divided into two main groups, point and extended defects. The point defects include the aluminum-related centers as well as oxygen-vacancy centers. Aluminum ions can easily substitute for silicon in quartz however, charge compensation is required. Aluminum has a - -3 charge whereas the silicon valence is +4. An additional positive charge is required that can be supplied by H-I-, Li- -, Na- -, or holes at interstitial sites in the crystal lattice. Iron-related defects are also possible since iron is also a trivalent (-1-3) ion. 

Acid-treated clays were the first catalysts used in catalytic cracking processes, but have been replaced by synthetic amorphous silica-alumina, which is more active and stable. Incorporating zeolites (crystalline alumina-silica) with the silica/alumina catalyst improves selectivity towards aromatics. These catalysts have both Fewis and Bronsted acid sites that promote carbonium ion formation. An important structural feature of zeolites is the presence of holes in the crystal lattice, which are formed by the silica-alumina tetrahedra. Each tetrahedron is made of four oxygen anions with either an aluminum or a silicon cation in the center. Each oxygen anion with a -2 oxidation state is shared between either two silicon, two aluminum, or an aluminum and a silicon cation. 

A second type of defect is associated with boron or aluminum impurities that are present in SiCh- In porous glass Muha (129) observed a rather complex spectrum which results from hyperfine interaction with 10B and UB isotopes. The spectrum is characterized by g = 2.0100, g = 2.0023, an = 15 and a a. = 13 G for nB. The paramagnetic defect is apparently a hole trapped on an oxygen atom which is bonded to a trigonally coordinated boron atom. This center is irreversibly destroyed upon adsorption of hydrogen. 

Different results are obtained when NH4Y is activated at higher temperatures (600°C). Vedrine et al. (266) showed that y irradiation in vacuo of such an activated zeolite leads to two types of signals. The signal with g = 2.0125, g = 2.0030, and a 12-line hyperfine structure with A aiso = 10.0 G was attributed to a positive hole (V center) trapped on an oxygen bridging two aluminum atoms ... 

Color." The most obvious effect produced in silica gel by radiation is a grayish-purple color (64), which can be almost surely attributed to the same type of center as that responsible for the similar color in irradiated quartz, namely, a positive hole trapped at an oxygen ion adjacent to a substitutional Al + impurity ion (65-67). The attribution rests on the similarity in optical absorption between irradiated gel and irradiated quartz (66), on the dependence of the intensity of the color on the aluminum content (69), and on the observation of a hyperfine interaction characteristic of the spin of the 2 a1 nucleus (I = 5/2) in the ESR spectrum of the irradiated gel (70). Furthermore, the ESR sextet and the color are annealed at comparable rates above 200° (70) and are both destroyed by adsorption of H2 at room temperature (64, 70). Their intensities increase in parallel as the aluminum content, the severity of preirradiation heat treatment, or the length of irradiation is increased (70). The concentration of the center does not increase indefinitely. After some lO i ev/gm, it approaches a limiting value which depends on the impurity content, for typical gels around lO H2/gm (69). 

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