Extended Data Fig. 7: The EXAFS fitting of the Se K-edge spectra of Principal Components A and C.

a, Component A is fit very well using Se-Se correlations, as in crystalline Se⁰ (P 3₁ 2 1). b-e, The fit for component C using different models and summary of the fitting parameters (f). The top of each panel is the magnitude and real-part of Fourier transforms (FT) of k³χ(k), and the bottom is the corresponding k³χ(k) oscillation. The window in the FT panels is the fit range used and the window in the EXAFS panels is the k-space data range used for the FT. The fitting R-range is from 1.3 to 2.5 Å, except that when the longer Se-Na path is included, the R-range is increased from 1.3 to 3 Å. All fits were carried out in R-space. Evaluation of the various fits shows that the Component C is best modelled by using selenium-aluminium correlation exclusively (fit b). g-i, The attempted EXAFS fitting for component B using just one path: Se-Se (a) or Se-Al (b), and summary of the fitting parameters (c). Evaluation of all attempted fits shows that Component B is best modelled by using both Se-Al and Se-Se correlations. Importantly, we note that the R of Se-Al in Component B is different from that in Component C (2.494 vs. 2.397 Å), proving that Component B is distinguished from Component C and is thus indeed a necessary component.