## 3 Results and Discussion
### Physicochemical Properties of the NFs
The Fourier filtered signal and the fit are shown in Figure S8c,d. The best fit was obtained starting from a GeO\({}_{4}\) structural model; the Ge-O distance was found to be 1.771 (6) A, in good agreement with the crystallographic distance in quartz-like GeO\({}_{2}\). The parameters of the EXAFS fit are reported in Table S3. In summary, all the XAS evidences pointed towards the presence of Ge(IV) in fourfold coordination, strongly suggesting that Ge is hosted in the tetrahedral sites of the maghemite spinel structure. PND data were analyzed on the base of the phases identified by XRD, XAS and MRS; hence, to properly fit the pattern, the hematite and maghemite structures were introduced. The final Rietveld refinement is presented in Figure 5. With respect to the XRD pattern, the neutron data showed strong extra reflections (e.g., 17\({}^{\circ}\)-26\({}^{\circ}\) range, 51\({}^{\circ}\) in Figure 5) associated to the magnetic structure of hematite [90]; this magnetic structure was introduced and considered in the refinement as an independent magnetic phase [91]. As the maghemite phase, cubic (S.G. Fd3m and P4\({}_{3}\)32) and tetragonal (S.G. P4\({}_{1}\)2\({}_{1}\)2) polymorphs had been reported in order to account for the ordering of the Fe vacancies [92]. A preliminary Le Bail fit, considering the different maghemite polymorphs, led to identification of the Fd3m phase as the one able to properly account for most of the observed reflections. On this base, Rietveld refinements were performed using the hexagonal \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and the cubic \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) structures. The attempts to refine the scale of GeO\({}_{2}\) as a possible impurity led to very small amount of the oxide, in good agreement with the results of MRS analysis. This finding indicated that most of germanium was successfully dispersed in the iron oxide phase(s) as a dopant. Results obtained from XAS clearly indicated the presence of Ge in tetrahedral coordination. Based on these indications, for the refinement of neutron data, germanium was considered only in the maghemite structure (the only one where FeO\({}_{4}\) sites are present). However, this scenario was not compatible with the nominal Fe:Ge molar ratio (11.7:1), as all performed refinements led to \(>\)85% \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and \(<\)15% \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), which should imply  
Figure 5: Rietveld refinement of the PND pattern of Fe\({}_{2}\)O\({}_{3}\):Ge NFs (\(\chi^{2}\) = 22.0, \(R_{\rm p}\) = 1.97, \(R_{\rm wp}\) = 2.39). Hematite 91.1%, Maghemite 8.6%, GeO\({}_{2}\) 0.3%. Structural parameters: Hematite \(a\) = 5.031 (1) Å, \(c\) = 13.7416 (4) Å; Maghemite \(a\) = 8.33041 (41) Å.  
a quantitative Ge substitution in the maghemite structure. Nevertheless, the maghemite phase is characterized by a magnetic structure at RT, but the quality of the experimental data did not allow for the determination of such structure and for its subsequent refinement. Due to the complexity of the system, the results obtained from the neutron diffraction gave only a partial vision of the actual phases and composition. It was concluded that the Fe\({}_{2}\)O\({}_{3}\):Ge NFs were composed by \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\), as dominant phases, and of an amorphous component with similar chemical composition. Most of germanium was accommodated in the tetrahedral sites of the maghemite structure, even if also the hematite phase could accept Ge dopant, probably on the defective surface sites as no GeO\({}_{6}\) species (that should be associated to the bulk \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\)) were revealed. These results were consistent with previous report on the possibility to obtain \(\alpha\)- and \(\gamma\)-phases of iron oxides, respectively oxidized and fully oxidized, upon calcination in air above 500 \({}^{\circ}\)C of a hydrated precursor [93].