## 2 Materials and Methods
### Powder Analyses  
SFS experiments are conducted for 2 h, which yields \(\sim\)799 mg \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) for 0.1 M INN and \(\sim\)1597 mg for 0.2 M INN in theory. This was expected to be enough to conduct all powder analysis intended. For particle collection, the flame aerosols are extracted at a height of \(\sim\)20 cm HAB through a fume hood and subsequently conveyed through an ePTFE membrane (R+B Filter GmbH, Langenbretach, Germany) using a vacuum pump type Mink MM 1144 BV (Dr.-Ing. K. Busch GmbH, Maulburg, Germany). According to the volume flow rate of the pump (\(\sim\)60 m\({}^{3}\) h\({}^{-1}\)), extracted aerosol samples exhibit a convection time span until reaching the surface filtration device of \(\sim\)75 ms, which is enough time for sufficient cooling. By thermocouple, one measures temperatures in the range of \begin{table}
\begin{tabular}{c c c c c} \hline INN conc. & 0.1 M & 0.2 M & 0.1 M & 0.2 M \\ \hline Burner type & \multicolumn{2}{c}{SpraySyn1} & \multicolumn{2}{c}{SpraySyn2} \\ \hline \(H\)/kJ mol\({}^{-1}\) & 3.11 & 3.10 & 3.11 & 3.10 \\ \(\phi\)/- & 0.2624 & 0.2622 & 0.2624 & 0.2622 \\ \hline \end{tabular}
\end{table} 
Table 1: Comparison of \(\phi\) and \(H\) applied. Figure 2: Comparison of nozzle geometries of SpraySyn1 (**a**) and SpraySyn2 (**b**) incl. gas supplies and liquid feeds (grey arrows). 100-150 \({}^{\circ}\)C near the membrane. All four powder samples are subsequently stored and analyzed identically by means of the subsequent six measurement techniques. 1. Raman spectroscopy is performed using an inVia(tm) confocal Raman microscope (Renishaw GmbH, Pliezhausen, Germany). Therefore, a low laser intensity of 1% at 633 nm (0.175 mW) is used to avoid any phase transformations that could otherwise occur during Raman measurements [41]. Each measurement consisted of 50 repetitions, with each repetition lasting 10 s, resulting in a total sampling duration of 500 s.
2. TEM samples are analyzed using a JEOL JEM-ARM200F high-resolution microscope with Cs correction and field emission HRTEM-STEM capabilities sourced from JEOL (Germany) GmbH (Freising, Germany). Therefore, conventional carbon-coated Cu grids (type S160, PLANO GmbH, Wetzlar, Germany) are used. TEM samples are either obtained from thermophoretic sampling or dipping sample grids into the powder. No organic solvents are used for preparation, which can otherwise cause biases in TEM examinations [11]. 3. Thermogravimetric analysis (TGA) coupled with differential scanning calorimetry (DSC) and mass spectrometry (MS) is conducted using an STA 449 F1 Jupiter(r) device obtained from NETZSCH GmbH (Selb, Germany). Therefore, powders are pressed into pellets and heated from 25 to 800 \({}^{\circ}\)C in a silicon carbide oven, while the gas atmosphere was monitored by MS using a QMS 403 Aeolos(r) quadrupole device also sourced from NETZSCH GmbH (Selb, Germany). Mass-to-charge (\(m/z\)) ratios of 18 (H\({}_{2}\)O), 44 (CO\({}_{2}\)), and 32 (O\({}_{2}\)) are monitored while a rather low-temperature ramp of 7.5 K min\({}^{-1}\) is applied to mitigate possible superpositions of thermal events. Measurements are carried out in synthetic air and argon. Altering the atmosphere in this way is valuable for revealing the origin of thermal events, i.e., oxidative decomposition events are sensitive to the O\({}_{2}\) availability, while physical mechanisms, such as physical gas desorption and evaporation, are not. 4. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) measurements are conducted using a Vertex(r) 70 spectroscope manufactured by Bruker Corporation (Billerica, MA, USA). The absorbance is measured in the wavelength range of 370-4500 cm\({}^{-1}\) with a resolution of 4 cm\({}^{-1}\). A platinum holder is used for the ATR measurements. 5. Macroscopic magnetic properties are investigated with the vibrating sample magnetometer (VSM) option of a PPMS DynaCool obtained from Quantum Design GmbH (Pfungstadt, Germany).