## 3 Results and Discussion
### Phase of the Oxide  
Figure 2 displays the results of the XRD analysis. In the diffractograms of CrMnFeCoNi-HEO NPs and NFs (Figure 2a), only the reflections from the (111), (220), (331), (222), (400), (422), (511), (440), (620), (533), and (622) planes of the face-centered cubic (fcc) spinel structure (JCPDS no. 22-1084) [41; 42; 43] are detected. This proves that (Cr\({}_{0.2}\)Mn\({}_{0.2}\)Fe\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\))\({}_{3}\)O\({}_{4}\) exclusively forms. The mean sizes of the sHEO crystallites, calculated through Scherrer's equation from the strongest reflection, are 30.1 and 23.6 nm for NPs and NFs, respectively.  
In the XRD pattern of C/CrMnFeCoNi-HEO NFs (Figure 2a), the peaks arising from the reflections from the spinel lattice planes are much broader. This finding points to smaller size and/or lower crystallinity of the C-embedded sHEO NPs. Moreover, two extra-peaks are clearly visible (at 44.3\({}^{\circ}\) and 51.6\({}^{\circ}\)), which hints at the metal segregation in a secondary phase.  
In the diffractogram of C/MgCoNiCuZn-HEO NFs (Figure 2b), the sharpness of the reflections from the (111), (200), (220), (311), and (222) crystallographic planes of the rock-salt (Mg\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.2}\))O structure (JCPDS no. 47-1049) [44; 45] is consistent with  
Figure 2: XRD patterns of (**a**) sHEO NPs, sHEO NFs, and C/sHEO composite NFs and (**b**) C/rHEO composite NFs (the pattern of rHEO NFs is also reported for comparison purposes).  
the larger size of the rHEO NPs incorporated in the C NFs. Extra-peaks are detected also in C/rHEO NFs; the most intense of them are located at 20-angles very close to those of extra-peaks observed in C/sHEO NFs. The preparations of C/sHEO and sHEO NFs (and analogously of C/rHEO and rHEO NFs [29]) differ from each other only for the heat treatment. Since no secondary phase forms in either sHEO or rHEO NFs [29] (i.e., in the absence of carbon), it is argued that some metals react with carbon (preserved by the polymer stabilization) and form carbide during carbonization [46]. Indeed, the formation of metal carbides in electrospun composite NFs obtained from a spinnable solution containing PAN and metal salts, as in this case, has been also reported by other groups [46; 47; 48]. The similarity of the 20-angular positions of the extra-peaks detected in C/CrMnFeCoNi- and C/MgCoNiCuZn-HEO NFs hints at the possible involvement of Co and/or Ni. However, the clarification of this point is beyond the purposes of the present paper.  
Figure 3 shows the micro-Raman spectra of all samples, as obtained by averaging on several random positions in each specimen to obtain a reliable picture of the sample bulk. The HEO-modes dominate the lower frequency region (below 1200 cm\({}^{-1}\)) of all spectra; the modes associated with the amorphous Csp\({}^{2}\) component of the composite NFs are detected above 1200 cm\({}^{-1}\)[36; 39; 40]. The spinel and rock-salt structures belong to _Fd-3m_ and _Fm-3m_ space groups, respectively. An in-depth discussion of the vibrational modes in sHEO NPs/NFs and rHEO NFs can be found in refs. [29; 31]. The narrower features in the spectrum of sHEO NPs are consistent with the larger average size of the (Cr\({}_{0.2}\)Mn\({}_{0.2}\)Fe\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\))\({}_{3}\)O\({}_{4}\) crystallites. In C/(Mg\({}_{0.2}\)Co\({}_{0.2}\)Ni\({}_{0.2}\)Cu\({}_{0.2}\)Zn\({}_{0.2}\))O NFs, the detection of the one-phonon longitudinal optical at 540 cm\({}^{-1}\) proves the presence of structural disorder and lattice distortion [29; 49; 50].