## 3 Results
### Phase of the Oxide  
Figure 3 shows the results of XRD analysis on all samples. In particular, the diffractograms of samples prepared via the SG method are displayed in Figure 3a, whereas Figure 3b,c report the XRD patterns of electrospun samples derived from PVP and PAN, respectively. Regardless of both methods and details of the preparation, reflections at ca. 18.3\({}^{\circ}\), 30.2\({}^{\circ}\), 35.6\({}^{\circ}\), 37.2\({}^{\circ}\), 43.3\({}^{\circ}\), 53.7\({}^{\circ}\), 57.3\({}^{\circ}\), 63.0\({}^{\circ}\), 71.5\({}^{\circ}\), 74.6\({}^{\circ}\) and 75.6\({}^{\circ}\) 20-angles are detected in all patterns. They are the reflections from 111, 220, 331, 222, 400, 422, 511, 440, 620, 533 and 622 planes of the face-centered cubic (fcc) spinel structure, _Fd-3m_ space group (JCPDS no. 22-1084) [45; 47; 48; 49; 50; 51; 52]. In most samples, no additional reflections are detected, which proves that the spinel phase is the unique crystalline phase formed. On the contrary, in samples 5 MA-sg, 5 MK-sg, 5 ML-sg (Figure 3a), 5 MKa, 5 ML-a (Figure 3b), 5 ML-b and 5 MD-b (Figure 3c), the detection of weaker additional reflections reveals the formation of secondary phases. Interestingly, this seems to be a common feature of the diffractograms of samples having the same nominal composition (e.g., 5 ML-sg, 5 ML-a and 5 ML-b). The reflections, detected in the XRD patterns of 5 ML-sg, 5 ML-a and 5 ML-b at 32.6\({}^{\circ}\), 38.6\({}^{\circ}\), 48.6\({}^{\circ}\), 61.6\({}^{\circ}\), 66.0\({}^{\circ}\) and 67.9\({}^{\circ}\) (owing to the scale, only the most intense of them are visible in Figure 3), are from  
100, 101, 102, 200, 112 and 201 planes of the monoclinic structure of CuO (JCPDS No. 45-0937) [53; 54]. The presence of Cu among the five selected metals promotes the formation  
Figure 3: XRD patterns of the samples produced via (**a**) sol-gel method and (**b**,**c**) electrospinning by the use of (**b**) PVP and (**c**) PAN as a polymer. Stars mark reflections not assignable to the spinel phase. of CuO as a secondary phase also in sample 5 MA-sg and 5 MH-sg. In 5 MI-sg and 5 MJ-sg, a weak reflection from the 112 planes of \(\alpha\)-Mg is detected at 66\({}^{\circ}\)[55; 56]. In sample 5 MK-sg, an additional reflection is visible at 32.8\({}^{\circ}\); in sample 5 MK-a, besides it, reflections at 23.8\({}^{\circ}\) and 49.1\({}^{\circ}\) are also detected. They are reflections from 211, 222 and 431 crystallographic planes of the body-centered cubic (bcc) structure of Mn\({}_{2}\)O\({}_{3}\) (JCPDS No. 89-4836) [57; 58]. The behavior of sample 5 MD-b differs from that of the samples with the same nominal composition. Reflections 222, 400 and 440 seem to be split owing to the appearance of very close signals at ca. 36.8\({}^{\circ}\), 42.7\({}^{\circ}\) and 62.2\({}^{\circ}\). Their detection might be indicative either of the existence of two spinel sublattices or of the segregation of some metal(s) in a secondary phase. In the latter case, the reflections might originate from 111, 200 and 220 planes of nickel oxide (NiO, JCPDS No. 04-0835) [59; 60] and/or cobalt oxide (CoO, JCPDS No. 78-0431) [61], both characterized by the same fcc crystalline structure. Rietveld refinements from XRD data (Figure S6) were carried out to assess the relative amount of the secondary phase(s). The results obtained are reported in Table S1. In samples produced via the SG method, secondary phases reach 12.4%, while in electrospun samples their percentage is 7.3-8.8%. For all samples, the mean sizes of the HESO crystallites are calculated from the most intense reflection through the Scherrer's equation [60]. The values obtained, reported in Figure 3, range from 19.2 to 38.7 nm for HESOs produced via the SG method. For electrospun samples, the mean sizes of the crystallites vary in the ranges 20.5-44.5 nm and 15.9-27.4 nm, for PVP- and PAN-derived HESOs, respectively. For a given metal combination, the observed variations of crystallite sizes do not follow any general trend or dependence on the preparation method.