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Condens. Matter 2021, 6, 27 5 of 11 2.2. X-ray Absorption Spectroscopy The strong selectivity of the XAS technique for the atomic species allows monitoring the local structure of both Cu and Fe in the studied samples, thus revealing complementary structural information with respect to the longer order provided by the XRPD. The Cu and Fe K-edges XANES spectra of Figure S6 relative to the hydrated and the anhydrous copper nitroprusside are similar, suggesting a similar local structure in both phases, as reported previously [16]. More quantitative local structural information can be gained by analyzing the extended portion of the XAS spectra. Materials based on metal hexacyanoferrates XAS spectra are characterized by a strong multiple scattering (MS) signal at the metal K-edges due to the high degeneracy of the linear atomic chains with angles close to planar, which involves high atomic scatterers. This fact, known for a couple of decades [17], has been reported and summarized more recently [18]. Briefly, (i) the MS signals arise from both three- and four-body contributions, and the full EXAFS spectrum can be interpreted with a few MS paths only, (ii) the strong MS signal extends up to more than 5 Å in the atoms probed by the XAS technique, which is unusual in EXAFS spectroscopy, (iii) the analysis would imply the multiple edge refinement which allows us not only to constrain common structural variables of the two metal edges but also to account for the oscillation of the strong MS contributions at the Fe K-edge that may overlay the Cu K-edge, overall enhancing the fitting reliability. Figure 3 displays the comparison of the experimental and theoretical k2-EXAFS signals at Fe (Panel a) and Cu K-edges (Panel b) for both powders and their relative FT plots (Panels c, d), whereas Table 1 reports the best fit results in terms of fitted and released structural and non-structural parameters. The theoretical signals of Figure 3 match well within the experimental curves, signifying the accuracy of the present data analysis. It is also relevant to underline the large contribution of the MS signals in the FT plots, where the third peaks in both Fe and Cu K-edge correspond to the -Fe-C-N-Cu- Condens. Matter 2021, 6, x FOR PEER REVIEW 6 of 11 chain (and therefore to the four body MS term), which is enhanced due to the strong MS contribution. Figure 3. EXAFS best fit result, in terms of (a,b) k2-extr2acted EXAFS and relative (c,d) FTs, for hy- Figure 3. EXAFS best fit result, in terms of (a,b) k -extracted EXAFS and relative (c,d) FTs, for drated and anhydrous CuNP, at both Fe and Cu K-edges. (e,f) Contour plots of the highly corre- hydrated and anhydrous CuNP, at both Fe and Cu K-edges. (e,f) Contour plots of the highly lated variables Cu-N first-shell bond distance and E0 values. correlated variables Cu-N first-shell bond distance and E0 values. Table 1. EXAFS fitting results. The Fe-C bond lengths match, within their accuracy, in the two samples, giving a value of 1.916(3) and 1.916(4) Å, respectively. The same holds true for the C≡N and Cu-N Anhydrous CuNP Hydrated CuNP interactions, thus confirming a very close structure for hydrated and anhydrous material. Fe-C (x5)/Å 1.916(3) 1.913(4) A statistical analysis has been computed on the bond lengths quotation. Examples are σ2 Fe-C/Å2 0.0021(8) 0.0023(7) displayed in Figure 3 panels e and f for the highly correlated variables first shell bond C≡N (x5)/Å 1.154(3) 1.150(5) distance and E0 values, providing a clear ground of the parameter’s reliability. More σ2 C≡N/Å2 0.012(1) 0.008(1) contour plots are available in Figure S7. From the table, it is also seen that the -Cu-N-C-Fe- Cu-N (x4)/Å σ2 Cu-N/Å2 Cu-N/O (x1/x2)/Å σ2 Cu-N (O)/Å2 1.973(5) 0.0038(6) 2.13(2) 0.018(3) 1.969(7) 0.004(1) 2.48(2) * 0.034(3) Fe-N (x1)/Å 1.68 ** 1.68 **PDF Image | Cross-Investigation on Copper Nitroprusside: Combining XRD and XAS
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