Table 1.
EXAFS curve fitting parameters for the NifX:NifB-co spectrum compared to those measured for NafY:FeMo-co. N = number of backscattering atoms. R = distance. σ2 = Debye-Waller factor. ΔE0 = threshold energy. F = fit quality defined as √[Σ(χo − χc)2 k6/Σχo2 k6] (χo = observed EXAFS; χc = calculated EXAFS). The backscatter atom numbers (N) were fixed to the values expected for the models while R, σ2 and ΔE0 were floated. Threshold energies (ΔE0) were constrained to be the same for all components. The estimated uncertainties in R, σ2 and ΔE0 were less than ± 0.01 Å, ± 0.0001 Å2 and ± 0.4 eV respectively. Phase and amplitude functions were calculated using FEFF 7.0. The scale factor used was 0.85. The Fe-X interaction was simulated assuming X = N; assuming X = C or O gave similar results but with different Debye-Waller factors 30.
| NafY:FeMo-co |
6Fe Model |
7Fe Model |
8Fe Model |
|||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| N | R (Å) | σ2 (Å2) | N | R (Å) | σ2 (Å2) | N | R (Å) | σ2 (Å2) | N | R (Å) | σ2 (Å2) | |
| Fe-S | 3.10 | 2.251 | 0.00328 | 3.00 | 2.263 | 0.00424 | 3.00 | 2.264 | 0.00404 | 3.00 | 2.263 | 0.00383 |
| Fe-Fe (short) | 3.40 | 2.627 | 0.00456 | 3.00 | 2.655 | 0.00323 | 3.43 | 2.655 | 0.00385 | 3.75 | 2.654 | 0.00431 |
| Fe-Fe (long) | 1.70 | 3.726 | 0.00562 | 2.00 | 3.740 | 0.00646 | 1.71 | 3.742 | 0.00533 | 1.50 | 3.742 | 0.00436 |
| Fe-X(N) | 0.85 | 2.000 | 0.00199 | 1.00 | 2.046 | 0.00272 | 1.00 | 2.043 | 0.00240 | 1.00 | 2.037 | 0.00200 |
| Fe-Mo | 0.40 | 2.717 | 0.00350 | |||||||||
| ΔE0 (eV) | −12.66 | −11.99 | −11.81 | −12.00 | ||||||||
| F | 0.242 | 0.214 | 0.237 | 0.256 | ||||||||