Trapped hole centers

The structural and spectral complexity of clay minerals is sufficient to consider a single mineral as a multicomponent mixture in itself. Detectible by near infrared spectroscopy are adsorbed water and structural hydroxyls (25.) exchangeable and structural transition metal cations (2fL and this work), adsorbed species including atmospheric gases (22), organic materials (2) accessory minerals (2SL) and, possibly, trapped hole centers (0 -centers). Thus it is of interest to adapt NIRA to studies of mineral surface activity. We have done this by examination of a small set of highly homologous clays in which laboratory control of only one variable at a time could be accurately achieved and independently confirmed. 

The inclusion of impurity atoms in MgO is much more interesting from a chemical point of view when alkali metals are used to replace Mg ions. In fact, this results in trapped-hole centers. The MVO pairs have been extensively studied in the bulk of alkaline-earth oxides by optical studies, EPR and ENDOR measurements [185,186] as well as by embedded cluster calculations [187]. The LiVO ions create an effective dipole which polarizes the surrounding lattice, with the two ions moving toward each other. The presence of an O radical, however, is most interesting when one is dealing with surface properties. This center in fact is very reactive and is the subject of the next paragraph. 

The STH has a high cross-section for recombination. Its short lifetime precludes its detection in pure AgCl by conventional EPR methods, except at temperatures close to 1.2 K [169]. In samples doped with deep electron traps, recombination is suppressed and an EPR spectrum from (AgCl6)4 can be observed up to about 50 K, at which point the hole becomes mobile and is annihilated. Although ENDOR data from the self-trapped hole center have... 

The stability of the self-trapped hole center in AgCl is enhanced by its association with bromide ion impurities and its recombination cross-section is reduced. As a consequence, the lifetimes of electron states are increased. For example, EPR signals from both shallowly trapped electrons and STHs can be detected during bandgap exposures at temperatures well above 50 K, a situation that does not occur in the pure material [177]. 

Zuc J, Pandey R and Kunz A B 1991 Embedded-cluster study of the lithium trapped-hole center in magnesium oxide Phys. Rev. B 44 7187-91... 

The second example concerns trapped hole centers in alkaline earth oxides (BeO, MgO, CaO, and SrO). These neutral defects essentially consist of the substitution of a monovalent cation (H/D, Li, Na, K) for one of the bivalent cations (Be, Mg, Ca, Sr). Thus, one electron is missing, so that an electron hole is expected to be localized and trapped at the substitutional cation. In both cases, the defect is paramagnetic, and in the second one, part of the original lattice symmetry is lost. 

When other Hamiltonians are considered, the quahtative picture of the defect remains essentially unaltered (we will see that this is not the case for the trapped hole centers in alkaline earth oxides), as shown in Table 24, where Mulliken charge q and spin density p P at the vacancy site obtained with five different Hamiltonians is reported. 

Trapped Hole Centers in Alkaline Earth Oxides... 

The Cubic Oxides Li in MgO. Ionizing radiation produces a variety of trapped hole centers in alkaline earth oxides at low temperature. In the cubic... 

Figure 50 Schematic picture of a Sis supercell model of the trapped hole center in cubic alkaline earth oxide MO [X]° (where M = Mg, Ca, Sr and X = H, Li, Na). Oi is the oxygen ion at which the hole is trapped.

Figure 53 Schematic view of a Sis supercell model of the two different trapped hole centers in BeO. 0 indicates the oxygen ion where the electron hole is localized. Axial oxygen in light-gray, and equatorial oxygen in black.

The cost of a bulk calculation is primarily a function of the unit cell size. Figure A2.2 shows the total time required for the calculation of the total energy with the MgO supercells that we used in the study of trapped-hole centers. We are considering the supercells before creating the defect. Therefore, the system possesses the full symmetry of the perfect crystal (48 point-symmetry operations). Calculations refer to the S4, Sg, Sie, S32, Sg4, S128, and S256 supercells nine AOs are used for every ion, so that the largest cell contains 4608 basis functions. 

Y. Chen and M. M. Abraham,/. Phys. Chem. Solids, 51,747 (1990). Trapped-Hole Centers in... 

Stapelbroek, M., Griscom, D. L, Friebele, E. J., and Sigel, Jr., G. M. 1979. Oxygen-associated trapped-hole centers in high-purity fused silicas. J. Non-Qyst. Solids 32 313-326. 

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