# Near-Surface Material Phases and Microstructure of Scandate Cathodes
## 3 Results
### Elemental Analysis of Phases in Cross-Section Samples
Moreover, as shown in Figure 3b, a selected area electron diffraction pattern was obtained from the oxide particle, and it matched the known hexagonal crystal structure of BaAl\({}_{2}\)O\({}_{4}\). Indexing of this diffraction pattern indicated that it corresponds to a [100] zone axis orientation, and it matches the theoretical pattern expected for BaAl\({}_{2}\)O\({}_{4}\) in this orientation. A separate EDS measurement from a neighboring grain, in the region marked by the yellow box of Figure 3a, yielded a composition of W (78.5 at.%), Ba (0.1 at.%), Al (0.8 at.%), O (7.3 at.%) and C (13.3 at.%), confirming that the neighboring grain is W. The measured oxygen content of ~7 at.% and carbon content of ~13 at.% in the W area are assumed to arise from surface contamination, which is typical in such samples; it is noted that this would imply the EDS measurement in the blue box region more closely matches the expected stoichiometric composition of BaAl\({}_{2}\)O\({}_{4}\). Figure 3c-f present the elemental maps acquired by EELS in the region outlined by the green box in Figure 3a. These maps indicate highly correlated signals of O, Al and Ba (but with an absence of W), in agreement with the EDS analysis. To explore the relationship between Ba-Al oxide and embedded Sc-oxide, the region denoted by red arrows in Figure 2 was studied in more detail and these results are presented in Figure 4. The HAADF images, where Figure 4b corresponds to the red box in Figure 4a, shows the morphology of the Ba-Al oxide region, where it is seen that Sc-oxide is surrounded by Ba-Al oxide. Figure 4c-h are composite and elemental EDS maps showing the distribution of W, Ba, Al, O and Sc. It is noted that the Sc signal is distinct and confined to one particle, as seen in Figure 4h. EDS measurement of composition in the region outlined by the red box in Figure 4h indicates the following: Ba (12.6 at.%), Al (20.5 at.%),  
Figure 3: Structural and elemental analysis of a Ba–Al–O particle at the cathode surface. (**a**) HAADF image, with green square showing the area for EELS mapping, as well as blue and yellow boxes showing sites for EDS analysis of the Ba–Al–O particle and neighboring W grain, respectively; (**b**) selected area diffraction pattern acquired from the Ba–Al–O particle; (**c–f**) EELS elemental maps of O, Al, Ba and W; these four maps share the same scale bar. O (56.3 at.%), Sc (10 at.%), W (0.5 at.%), and Ga (0.2 at.%). This suggests that the measured area includes both BaAl2O4 and Sc2O3, which likely exist as distinct but overlapping oxide particles. In order to further investigate the relation between BaAl2O4 and Sc2O3, including possible physical mixing or chemical interactions between the oxides, a small W particle with attached oxides was selected for detailed visualization via a 3D EDS tomogram; this region is indicated by the red box in Figure 5a. It is noted that this experiment was conducted with a separate TEM lamella than the one imaged in Figures 1-4, but both lamellae were cut from cathode #1. The higher magnification image in Figure 5b shows a BaAl2O4 particle atop the larger W particle. 2D slices from the 3D EDS tomogram are presented in Figure 5c-e, showing progressively more elements in each EDS map. This figure sequence conveys the relative distribution of Ba, Sc and W, and confirms that Sc is distinct from both the W particle and the BaAl2O4 particle that encases it, i.e., Sc does not appear to be physically or chemically mixed with W or BaAl2O4. Video Supplementary Video S1 shows the 3D tomogram in imaging mode and in EDS mapping mode, and also rotated about the vertical axis, is available in the online Supplemental Files. The EDS composite elemental mapping in this 3D tomography movie corroborates the observations described above and, along with the observations portrayed in Figures 1-4, indicates that Sc exists in distinct Sc2O3 oxide particles that decorate the surfaces of W grains.