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Condens. Matter 2019, 4, 5 5 of 12 First, most material scientists prefer mRIXS images with excitation energies along the horizontal axis. An example is shown in the top of Figure 2a with mRIXS data collected from a fully charged LR-NMC electrode (1st cycle, 4.6 V). The benefit of such plots is the direct correspondence to the conventional XAS spectra upon the same excitation energies (horizontal axis in Figure 2a). Because mRIXS is technically further resolved XAS fluorescence signals along the emission energy (vertical axis in Figure 2a) [8], the vertical integration of the mRIXS intensity corresponds to XAS-type signal channels. Different features could be emphasized depending on the area of integration. This could be seen directly through the comparison between the mRIXS image and the extracted spectra in Figure 2a. For example, the integration of intensities along the whole emission energy range generates the conventional XAS spectrum in the total fluorescence yield (TFY) mode. The O-K XAS TFY peaks in the interested 528–534 eV pre-edge range are known to be of TM character from the strong TM(3d)-O(2p) hybridization, as explicitly concluded in the seminal study by de Groot et al. in 1989 [11], and later confirmed by extensive theoretical and experimental studies. Indeed, direct comparison between the mRIXS and XAS shows that the pre-edge peaks are from the broad features around the 525 eV emission energy (mRIXS Feature-1), which is also known as the typical (nominally O2−) emission energy value for TM oxides [20]. Along the same emission energy range, the broad mRIXS feature above the 535 eV excitation energy (Feature-2) corresponds to the broad hump in XAS at high energies, stemming from the weakly structured TM 4s and 4p states mixed with oxygen 2p bands [11]. The decay from the broad O-2p valence band electrons to the excited core holes leads to the so-called X-ray emission spectroscopy (XES), sometimes called a fluorescence feature in RIXS experiments [8], which dominates the featureless signals above 545 eV excitation energy around the same 525 eV emission energy. A super-partial fluorescence yield (sPFY) by integrating mRIXS intensity only around the 525 eV emission energy (blue frame in Figure 2a) shows that the overall XAS (TFY) line shape is dominated by features that are standard to TM oxides. Second, the characteristic behavior of an XES type of signal is the constant emission energy defined by the energy difference between the valence-band electrons and the core holes. Figure 2b is a more typical mRIXS plot than Figure 2a, with emission energy as the horizontal axis (Figure 2b). In such a plot, the emission energy of the broad vertical features is clearly shown at 525 eV and are extended to high excitation energies beyond the absorption edges. As explained above, depending on the excitation energy (vertical axis here) range, broad mRIXS features around the 525 eV emission energy are of a TM 3d character (Feature-1), TM 4s/p character (Feature-2), and XES signals with O-2p character at very high excitation energies. Third, other than the decays of electrons to the core holes, the excited system after photon absorption triggers various excitations before the core holes are filled, especially when the excitation energy is close to the absorption threshold [8]. Such excitations are critical for understanding the chemistry and physics of a material with specific electronic configurations. For example, for TM 3d states, excitations between the occupied and unoccupied 3d states manifest themselves in RIXS results with a cost of energy, called “energy loss”. Such a d-d excitation has typical energy loss values of several eVs, which is far more sensitive to the chemical states than conventional XAS results [21]. More importantly, it has long been found that TM d-d excitations could be observed in O-K RIXS experiments in many oxide systems with a couple eVs of energy loss [22]. Figure 3 displays the mRIXS images of the pristine (discharged) LR-NMC electrode with the horizontal axis as the emission energy (Figure 3a) and energy loss values (Figure 3b). The d-d excitation feature could be seen through the weak intensity parallel to the elastic line. This is also better seen through the individual energy distribution curves at different excitation energies (Figure 3c), where an energy loss of about 2.4 eV could be seen within the excitation energy range for the strongest TM-O feature. Note such a d-d excitation feature cannot be observed clearly in charged electrodes, e.g., Figure 2c, which will be discussed below. Fourth, two associated mRIXS features emerged in the charged electrodes if oxygen redox reactions were involved [8,17–19]. One is a sharp feature at a 523.7 eV emission and 531 eV excitation energy (Feature-5), while the other is a weak feature close to the elastic line at the same excitationPDF Image | Oxygen Redox Reactions in Batteries Resonant Inelastic X-ray Scattering
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