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Condens. Matter 2018, 3, 36 9 of 13 XRD data were collected using a monochromatic X-ray beam of 1 Å. The XRD pattern of the FeCo powder sample was recorded in a capillary geometry, setting the spinner at 300 revolutions per minute, and acquiring the diffractogram consecutively in the 5◦ < 2θ < 70◦ range, with steps of 0.01◦ and an acquisition time of 1 s per step. Operando data were instead collected in the flat plate mode consecutively from 10◦ to 30◦ 2θ range with a 0.01◦ step and 0.5 s/point acquisition time. XAFS data were collected at Fe and Co K-edges in transmission mode using ionization chambers filled with a mixture of Ar, N2, and He to have 10%, 70%, and 95% of absorption in the I0, I1, and I2 chambers, respectively. An internal reference of iron and cobalt foil was used for energy calibration in each scan. This allowed a continuous monitoring of the energy during consecutive scans. No energy drifts of the monochromator were observed during the experiments. Spectra at Fe and Co K-edges were collected with a constant k-step of 0.03 Å−1 with 2 s/point acquisition time. Data were collected from 6900 eV to 8320 eV at the Fe and Co K-edges. The energies were calibrated by assigning the first inflection point of the spectra of the metallic iron and cobalt to 7112 eV and 7709 eV, respectively. The white beam was monochromatized using a fixed-exit monochromator equipped with a pair of Si(111) crystals. Harmonics were rejected by using the cutoff of the reflectivity of the platinum mirror placed at 3 mrad with respect to the beam upstream of the monochromator and by detuning the second crystal of the monochromator by 30% of the maximum. In situ data were acquired during the first charge to 4.0 V vs. Li+/Li and subsequent discharge to 1.8 V vs. Li+/Li at C/34 and C/31 current rates for operando XRD and XAFS, respectively, by considering 1C rate equals to the current needed to insert one equivalent of Li-ion per formula unit (in the charged state) in one hour, thus a theoretical specific capacity of 88 mAh g−1. The equivalents of reacted ions (or better, exchanged electrons) have been calculated from the imposed value of current and the elapsed time in the operando measurement. Rietveld refinement was carried out on XRD patterns using FullProf Suite software [46] and assuming as structural model the one reported by Mullica et al. [33]. A pseudo-Voigt function was adopted for peak shape. Peaks corresponding to PTFE (contained in the electrode formulation) were not refined, excluding the corresponding regions, that is, 11.60–11.95◦ and 16.78–17.31◦ [47]. Also, the 29.31–29.43◦ region was not considered, due to the presence of the beryllium peak arising from the in situ cell window. Graphical representation of structures was exploited by means of VESTA software [48]. XAFS spectra were pretreated and calibrated using the Athena program [49]. The pre-edge background was removed by subtracting a linear function extrapolated from the pre-edge region, and the XANES spectra were normalized at the unity by extrapolation of the atomic background. The EXAFS analysis was performed using the GNXAS package [50,51] which is based on the MS theory. The method uses the decomposition of the EXAFS signals into a sum of several contributions, namely the n-body terms. The theoretical signal is calculated ab initio and contains the relevant two-body γ(2), three-body γ(3), and four-body γ(4) MS terms [52]. The two-body terms are associated with pairs of atoms, and probe their distances and variances. The three-body terms are associated with triplets of atoms and probe angles, and bond–bond and bond–angle correlations. The four-body terms are associated to chains of four atoms, and probe distances and angles in-between, and bond–bond and bond–angle correlations. However, because of the linearity of the Fe–N–C–Co chains, all the angles were set to be 180◦, hence the actual number of parameters used to define the γ(3) or the γ(4) peak was reduced by symmetry. More details on the use of parameters correlation in the four-body term is out of the aim of the present work and can be found in the references [43,53]. Data analysis was performed by minimizing an χ2-like residual function that compares the theoretical (model) signal, μmod(E), to the experimental one, μexp(E). The phase shifts for the photoabsorber and backscatterer atoms were calculated starting from the structure reported by Mullica et al. [33] according to the muffin-tin approximation and allowing 10% overlap between the muffin-tin spheres. The Hedin–Lundqvist complex potential [54] was used for the exchange-correlation potential of the excited state. The core-hole lifetime, Γc, was fixed to the tabulated value [55] and was included inPDF Image | XAFS and XRD Study of a Prussian Blue Analogue Cathode Iron Hexacyanocobaltate
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