Calculated Al/Si dependence

In order to improve the accuracy of the calculated acceptor levels in silicon and germanium, particularly for the even-parity ones, Lipari et al. [38] have used a screened point-charge impurity potential based on the wave-vector-dependent dielectric function calculated for Si, Ge, GaAs and ZnSe [65]. They make use of a phenomenological parameter a, adjusted to fit the calculated q-dependent dielectric function e(q), in this potential. The resulting potential in real space is  

In Figure 2 the dependence of the line widths of the 23Na resonance on the Si-Al ratio is shown. As mentioned, the results for low Si-Al ratios can be described by a model which uses calculations for occupancy of the S3 sites. Only the nuclei of the ions in the Si sites can be detected. 

In diamond, Sahoo et al. (1983) investigated the hyperfine interaction using an unrestricted Hartree-Fock cluster method. The spin density of the muon was calculated as a function of its position in a potential well around the T site. Their value was within 10% of the experimental number. However, the energy profiles and spin densities calculated in this study were later shown to be cluster-size dependent (Estreicher et al., 1985). Estreicher et al., in their Hartree-Fock approach to the study of normal muonium in diamond (1986) and in Si (1987), found an enhancement of the spin density at the impurity over its vacuum value, in contradiction with experiment this overestimation was attributed to the neglect of correlation in the HF method. 

The adsorption of NO and CO has been used to characterize the properhes of Co in Co-exchanged zeoHtes [148-151]. NO is a selective probe for Co and CO is selective for Co species. Datka and coworkers used the combination of CO and NO adsorption IR to quantitahvely determine the concentration of Co as an oxide and the Co present as in exchange positions, oxide-like clusters, and cobalt oxide in a series of Co-exchanged ZSM-5 and ferrierite (PER) zeolites [151]. They established conditions under which the CO and NO would react selectively with the various types of sites and estabhshed absorphon coefficients for the quanhtative calculations. Differences in the distributions of the various forms of Co species were found to be dependent on both the structure and framework Si/Al of the zeolite. 

Among other factors, the strength of the protons in zeolite depends on the framework A1 content, and should go toward a maximum around Si/Al = 10 (ref.20). Indeed, a volcano-shaped dependency between the rate and m = Al/(A1+Si) was reported for odCB isomerisation on HMOR, HBETA and HOFF (ref.7). Moreover, the nature of the zeolite influences the proton strength too. Table 3 reports the reaction rates and intrinsic activities, expressed as turnover number (TON), on various zeolites at a Si/Al content close to the optimum. The TON was calculated by dividing the rate by the proton concentration in the zeolite. 

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