SRC model

Best-Fit Parameters Obtained from Simulations of the Data in Figure 1.15 with the SRC Model [Equation (1.32)]... 

Parameters Extracted from Fitting SRC Model to Data Obtained from MbCO as well as from tbe Protobeme Alone Embedded in Various Solvents... 

Nearly identical results were obtained also for Y202S Eu3+ (Liu et al., 1998a) and Gd2C>2S Eu3+ (Chi et al., 1998a, 1998b). In both cases the reduction of the free-ion parameters was about -0.40% for F2 and -0.20% for f up to pressures of 13 GPa and 14.8 GPa, respectively. Considering the two different covalency models, the authors concluded that the SRC model describes the nephelauxetic effect more reasonably than the CFC model. 

The rare-earth trichlorides belong to the best studied systems at ambient as well as at high pressure. Voloshin et al. (1975) studied the effect of pressure on the free-ion parameters of NdCl3-6H20 up to 2.3 GPa. They found reductions of -0.26%, -0.29%, -0.12%, and -0.25% for F2, F4, F6, and f, respectively. Such variations can be explained neither by the CFC nor by the SRC model alone. 

The last point is not very surprising because it proves that pressure effects are stronger the more extended the wavefunction is. Thus, the order Nd3+ < Pr3+ < u3+ simply reflects an increasing extension of the f-wavefunction. Point (1) can be explained if the SRC model is valid in these cases, because it predicts a stronger variation of the Fk compared to f However, following the discussion by Gregorian et al. (1989), the SRC model has some drawbacks, for example it cannot explain the different shifts for different k parameters as noted in point (2). 

The first parts are derived from the SRC model, the second parts from the CFC model. A useful quantity to distinguish both models is the covalency ratio CR (Bungenstock et al., 2000b) ... 

Relative decrease of the ffee-ion parameters F2 and f, the renormalisation factor N and the effective nuclear charge Z for Pr3+, Nd3+, Eu3+, Sm2+, and U3+ in different host lattices under pressures up to 8 GPa. The covalency ratio CR is a measure of the relative importance of the CFC or SRC models (see text). Rows 0-5 GPa, 5-10 GPa, and 10-15 GPa give the CR value of LaOCl Pr3+ for different pressure regions... 

If only the CFC model is responsible for the decrease of the free-ion parameters, CR should be equal to 0.33, if only the SRC model is important, CR should be equal to 2. An intermediate value between 0.33 and 2 would indicate that both types of contributions are essential. 

According to table 5, in all cases except for some Eu3+ compounds, the decrease for F2 is larger than for . Thus, the SRC model plays the dominant role. In the case of some Eu3+ compounds on the contrary, the spin-orbit coupling parameter decrease more strongly than the Slater parameter, which leads to the conclusion that the CFC model is dominating here. 

The Eu3+ ion is, compared with Pr3+, Nd3+, Sm2+, and U3+, the ion with the least expanded 4f wavefunction. Therefore, one can conclude that the SRC model becomes increasingly more important when the wavefunction is expanded. This view is supported very strikingly by the data for U3+, whose 5f wavefunction is much more expanded than the 4f wave-functions of the other ions in table 5. For U3+, the decrease of F2 with respect to is very large indicating that the classical CFC model does not play any significant role anymore. 

On the other hand, if the effective nuclear charge does not change at all at high pressures, it would mean that the SRC model cannot describe the situation at strong compression or for strongly expanded wave functions. 

An interesting improvement of the SRC model has been discussed by Wang and Bulou (1995). They considered simplified expansion factors used in the Hartree-Fock radial wave-functions for the f-electrons. With these factors it was possible to introduce a k dependence for the pressure-induced change of different Slater parameters. This procedure would remove the weak point of the model which could not account for the observed -dependence of the parameters under pressure. 

In summary, the analysis of the free-ion parameters of different materials under pressure have led to a coherent picture. The nephelauxetic effect can be described by a combination of the CFC and SRC models and depends on the expansion of the f-electron wavefunctions. Only at very high pressures or in the case of strongly expanded wavefunctions the SRC and CFC models could not provide a completely adequate description of the observed variations. 

Reversible decoding is of great importance, since once a SRC model is established optimal values of the response can be chosen and values of the model molecular descriptors calculated by using the estimated SRC model. Then the possible molecular structures corresponding to the optimized descriptor values can be designed (and synthesized). This last operation is a troublesome task as the model molecular descriptors are not simple and easily interpretable. 

Equations 4-67 to 4-69 are then solved in a similar manner as presented in Section 4-10. Matsouk pointed out that his approach was different than the models of Shook and Roco (1991) (the SRC model), Lazarus (1989), and Lazarus and Cook (1993), which did not include the pipe axis component of the submerged weight (due to buoyancy) y. The... 

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