## 3 Results
### Structural and Morphological Results
For all the samples, XRPD patterns after 7, 14, and 30 days of maintenance in air after the synthesis were collected: the results are shown in Figures S2-S4 in the Supplementary Materials. NMO (Figure S2A) was slightly degraded: the main P2 phase maintained good crystallinity, but after only 7 days and up to 30 days, Na\({}^{+}\)/H\({}_{3}\)O\({}^{+}\) exchange or H\({}_{2}\)O insertion takes place (as suggested by the presence of two peaks at about 12 and 25\({}^{\circ}\)) [27]. The NMO-Q sample showed a marked decrease in peak intensities with time and only after 30 days, a small peak at about 12\({}^{\circ}\) was seen (Figure S2B). We should, however, \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **NMO** & **Cu02** & **Fe02** & **Cu02-Q** & **Fe02-Q** & **NMO-Q** \\ \hline \multicolumn{7}{c}{**P2**} & \multicolumn{7}{c}{_Cmcm_} \\ \hline Lattice parameters (Å) & 2.8700 (1) & 2.8857 (1) & 2.8960 (1) & 2.8980 (1) & 2.9117 (1) & 2.8358 (3) \\  & 11.1306 (7) & 11.1536 (5) & 11.1844 (10) & 11.1806 (8) & 11.2487 (10) & 5.2561 (5) \\  & & & & & & \\   Volume (Å\({}^{3}\)) & 79.4 & 80.4 & 81.2 & 81.3 & 82.6 & 167.2 \\ \hline Cry. size (nm) & 78 (1) & 105 (2) & 84 (1) & 101 (2) & 87 (2) & 70 (2) \\ \hline \multicolumn{7}{c}{**P’2**} & \multicolumn{7}{c}{_P’2_} & \multicolumn{7}{c}{_P’2_} \\ \hline Lattice parameters (Å) & 2.7853 (11) & 2.8810 (4) & 2.8882 (6) & 2.8892 (4) & 2.9056 (6) & 2.8864 (5) \\  & 4.9738 (21) & 5.0115 (8) & 5.0233 (9) & 5.0346 (6) & 5.0765 (8) & 4.9743 (12) \\  & 11.2651 (30) & 11.1648 (13) & 11.2101 (19) & 11.2005 (12) & 11.2383 (18) & 11.1984 (22) \\  & 91.61 (3) & 90.58 (1) & 90.55 (1) & 90.39 (1) & 90.42 (2) & 90.76 (2) \\ \hline Volume (Å\({}^{3}\)) & 156 & 161.2 & 162.6 & 162.9 & 165.8 & 160.8 \\ \hline Crystallite size (nm) & 40 (2) & 70 (1) & 40 (2) & 59 (3) & 43 (2) & 38 (2) \\ \hline Main phases ratio & 6.5 & 2.8 & 2.4 & 1.6 & 1.7 & 2.8 \\ \hline \multicolumn{7}{c}{O2} & \multicolumn{7}{c}{O1} & \multicolumn{7}{c}{O1} & \multicolumn{7}{c}{O1} & \multicolumn{7}{c}{O1} \\ Secondary phases (types and total weight \%) & \multicolumn{7}{c}{-} & \multicolumn{7}{c}{O3} & \multicolumn{7}{c}{O3} \\ \multicolumn{7}{c}{5 wt \%} & \multicolumn{7}{c}{4 wt\%} & \multicolumn{7}{c}{4 wt \%} & \multicolumn{7}{c}{8 wt \%} & \multicolumn{7}{c}{20 wt \%} \\ \hline R\({}_{\rm wp}\)/GoF & 11.08/1.53 & 8.07/1.53 & 12.40/1.62 & 8.01/1.39 & 11.9/1.54 & 13.57/1.65 \\ \hline \end{tabular}
\end{table} 
Table 1: Main structural parameters, secondary phase amounts, and agreement indices (R\({}_{\rm wp}\) and GoF) obtained from Rietveld refinements on undoped and doped sample patterns from both the slow-cooling and quenching processes. take into consideration that its main phase is represented by the _Cmcm_ polymorph, which is different from NMO. The Cu-doped samples (Figure S3A,B) were stable: only in the case of Cu02, a very small peak at about 12\({}^{\circ}\) was evidenced starting from 7 days, while Cu02-Q did not show hydrated phases in all the analyzed range of time. The peak intensities of the P2 phase did not change with time in Cu02, while they slightly decreased in Cu02-Q. It is different in the case of Fe-doped samples (Figure S4): in Fe02, the hydrated phase started to form after 14 days and increased after 30 days (Figure S4A), while the intensities of the main phase slightly decreased. Fe02-Q heavily degraded only after 30 days, with a loss of crystallinity of the main P2 phase and the formation of a large amount of hydrated phase (Figure S4B). The samples' morphology was analyzed via SEM measurements (Figure 2). NMO (Figure 2A) and Fe02 (Figure 2C) share a similar form of grains: aggregates of particles with clear edges and sizes up to 3-4 \(\upmu\)m can be identified. Some flat particles, with plate-like forms, were also present. The Cu-doped sample morphology (Figure 2B) is different, showing larger particles that are bigger than 5 \(\upmu\)m, with blunt edges. The quenched samples were morphologically different (Figure 2D-F).