# XPS on Co\({}_{0.95}\)R\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) Nanoparticles with R = Gd or Ho
## 3 Results and Discussions  
The XRD patterns of the studied samples are presented in Figure 1a. The CoFe\({}_{2}\)O\({}_{4}\) compound patterns are also shown and confirm that there are no extra peaks present in the doped samples. All the diffraction peaks indexed in the XRD pattern were analogous to the JCPDS card number 22-1086. The recorded patterns were analyzed using the FullProf Suite Software by employing the Rietveld refinement technique and assuming a cubic _Fd-3m_ crystal structure for the Co\({}_{1-x}\)R\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\) compounds. The calculated profiles are shown in Figure 1b together with the experimental data. One can see that the differences between the calculated and experimental patterns are small, confirming the samples' quality. The recorded patterns showed that the prepared samples are single phase, without any impurities, with characteristic peaks confirming the phase purity of the studied compounds. The maximum intensity was obtained for the (311) peak, which is characteristic of the spinel ferrite structure. The lattice parameters, determined by Rietveld analysis, are 0.838(3) nm for the pristine sample, and 0.838(5) nm for R = Gd, respectively, and 0.838(9) nm for R = Ho and x = 0.05, in excellent agreement with the reported values for doped cobalt ferrites [35; 36; 37; 38; 39; 40]. The average crystallite sizes were calculated using the Debye-Scherrer formula, considering a shape factor of 0.94, which is typical for spherical crystallites with cubic symmetry. The instrumental peak broadening was subtracted. In the case of the pristine sample, the average crystallite size is 18.2 nm, while for the doped samples (x = 0.05), values of 20.3 nm for R = Gd, respectively 19.3 nm for R = Ho were obtained. TEM images of the Co1-xRxFFe2O4 samples have shown that the nanoparticles are well defined, presenting a small tendency to agglomerate, probably due to the higher magnetic interaction that is in competition with the much weaker electrostatic repulsion. As examples, in Figure 2, the TEM images are shown for the doped samples. One can see that the particles consist of some regular polygonal shapes with a tendency to adopt spherical structures in some cases. Figure 1: XRD patterns at room temperature of Co1-xRxFFe2O4 nanoparticles (**a**); Rietveld refinement results (**b**). The TEM images analysis reveals that the prepared compound consists of nanoparticles with sizes between 14 nm and 20 nm for the R = Gd sample, respectively between 15 nm and 24 nm for R = Ho, with a mean diameter of 17.24 \(\pm\) 1.19 nm for the sample with Gd and 18.84 \(\pm\) 1.81 nm for the Ho-doped compound. The obtained values are close to those obtained from XRD, within the calculation error for the Ho sample, indicating that most of the Co\({}_{1-x}\)R\({}_{x}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles have high crystallinity, like single crystals. The measured sizes are consistent with those obtained from XRD analysis. The small difference between the average values of the nanoparticles determined from TEM and XRD in the case of Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) is probably due to the considered shape factor in the Debye-Scherrer formula. Elemental analysis using EDS confirmed the presence of Fe, Co, Gd/Ho, and O in the synthesized sample. As an example, Figure 3a,b presents the SEM image and EDS spectrum of the Co\({}_{0.95}\)Gd\({}_{0.05}\)Fe\({}_{2}\)O\({}_{4}\) nanoparticles, respectively. The EDS data indicate that the nanoparticles are free from impurities; the observed Cu lines are from the TEM grid. Elemental mapping shown in Figure 3c-f demonstrates that Co, Gd, Fe, and O are uniformly distributed across the selected area of the sample. Similar results were observed for the compounds with Ho. The elemental ratios indicate that the concentrations of R, Co, and Fe in the nanoparticles closely match the expected stoichiometric values.