2.1.1. Structural Properties Evolution upon Grinding

X-ray diffractograms obtained for LaMnO3.15 and LaFeO3 materials, after each step of the synthesis, are shown in Figure 1. In the case of LaMnO3.15, after solid state synthesis step (LaMn\_SSR), intense and narrow diffraction peaks that match with the LaMnO3.15 reference pattern (PDF#50-0298) of rhombohedral structure are observed. In addition to the main perovskite phase, weak remaining peaks (2θ = 26.2◦, 27.9◦, 29.2◦, 29.9◦, 36.1◦, 39.6◦, 46.1◦, 55.5 or 59.9◦) are also observed. These reflections are associated to unreacted oxide precursors and derived phases: La2O3 (PDF#74-2430), La(OH)3 (PDF#36-1481) and Mn3O4 (PDF#18-0803). However, these reflections, weak in intensity, suggest that most of the material consists in crystalline perovskite. After the high energy ball milling step, diffractogram recorded for LaMn\_HEBM shows the comparable reflections, nonetheless the peaks are less intense and broadened. In addition, the peaks associated to the unreacted phases disappeared. This kind of evolution was already reported in previous works [22,25], and corresponds to a significant decrease of the coherent crystal domain. After the second step of grinding, namely low energy ball milling (LEBM performed in wet conditions), the obtained diffractograms are very similar to that obtained for the LaMn\_HEBM sample. No significant evolution, neither for the phase detected nor the intensity/width of the reflection, can be observed. Comparable evolution of the Fe-containing perovskite diffractograms with the synthesis step is observed. Initially, LaFe\_SSR displays LaFeO3

perovskite-type phase (PDF#37-1493, Orthorhombic crystal structure) in addition to La(OH)3 (peaks at 2θ = 27.4◦, 28◦, PDF#36-1481) and Fe2O3 (peaks at 2θ = 33.2◦, 35.6◦, PDF#33-1481) impurity phases. After the HEBM step, perovskite reflection broadening is observed, in addition to the disappearance of reflections ascribed to unreacted phases, and finally, after LEBM step, a comparable diffractogram to that obtained previously is obtained.

**Figure 1.** Diffractograms obtained for LaMnO3.15 (**A**) and LaFeO3 (**B**) after each synthesis step. SSR: solid state reaction, HEBM: high-energy ball milling, LEBM: low-energy ball milling. Bottom of the figure: vertical bars are for cited JCPDS reference. #, La2O3; ‡, Mn3O4; -, La(OH)3; ♦, Fe2O3.

Crystal domain sizes were estimated using the Scherrer equation after correction for instrumental broadening. The obtained values are summarized in Table 1. Starting with micrometric crystals for the SSR-derived materials the crystal size drops down to a value in the size range of 14 to 22 nm depending on the perovskite composition and milling step. In that respect, the HEBM step induced a significant decrease of the crystal size attesting in that way that the energy transferred by the grinding induces a particle breakage. On the contrary, no significant modification of the crystal domain size could be observed after LEBM showing that this low-energetic wet milling process precludes additional decrease of the crystals. Interestingly, the B-metal nature did not have a significantly impact on the final crystal domain size which ranged in the 15–20 nm interval in line with a previous work reported in the case of Mn-containing perovskites [22] and slightly above the values reported by Gashdi et al. [25] for La1−xCexCoO3 (11–13 nm). However, in this last reference, the authors did not calcine their materials for characterization and application purpose that can explain the slightly lower reported values.

**Table 1.** Structural, textural properties and elemental composition obtained for LaMnO3.15 and LaFeO3 after each synthesis step.


P: perovskite, TM: transition metal; <sup>1</sup> estimated by SEM; <sup>2</sup> SSA: specific surface area; <sup>3</sup> measured by X-ray fluorescence spectroscopy.

## 2.1.2. Textural Properties Evolution upon Grinding

N2-adsorption/desorption isotherms over LaFe based samples are presented in Figure 2. The curves exhibit the same pattern of a type II isotherm, regardless of the material composition and this isotherm shape is characteristic of materials displaying no significant porosity and the sharp N2 adsorption occurring at P/P0 above 0.8 is characteristic of external aggregate/particle porosity [26]. The amount of adsorbed N2 for the SSR material (shown for LaFeO3, Figure 2) is very low, demonstrating a limited pore volume. After the high-energy ball milling step, an increase of adsorbed N2 was observed but total volume adsorbed remained relatively low. The impact of the LEBM step on the total N2 volume adsorbed was more noticeable, as observed in Figure 2, showing that this step will impact the global porosity of the material, even if it was previously observed to have a very limited effect on the crystal domain size (Figure 1 and Table 1). As expected, the SSR materials, obtained at high crystallization temperature, displayed very limited surface areas (Table 1, ranging from 1.0 to 1.6 m2·g<sup>−</sup>1). This low surface area was directly related to the large crystal domain size displayed by the material (Table 1). After the HEBM step, leading to a significant decrease of the crystal domain size, the surface area slightly increased to 3.5 m2·g−<sup>1</sup> <sup>±</sup> 0.1 m2·g−<sup>1</sup> for LaTM\_HEBM, samples. Interestingly, the significant decrease of the crystal size did not promote the SSA of the samples as expected for an increase of the external surface due to non-interacting low size crystallites. Indeed, taking into account a LaFeO3 having a mean crystallite size of spherical-shape amounting to 17 nm and a density of 6.65 a theoretical SSA of 53.1 m2·g−<sup>1</sup> was expected. The large discrepancy between the experimental surface area (3.4 m2·g−<sup>1</sup> of accessible surface to N2) and the theoretical one indicated the formation of nanoparticle agglomerates displaying very limited porosity. After LEBM, a significant increase of the SSA was observed despite the absence of crystal domain size evolution during the milling. Then, LaFe\_LEBM displayed a surface area of 18.8 m2·g−<sup>1</sup> five time larger than the parent one. Such evolution demonstrated that the LEBM is acting mostly on the morphology of the aggregates present in the material. Two phenomena can be the origin of the SSA increase: (i) the decrease of the agglomerate size (deagglomeration process), (ii) formation/stabilization of a new porosity in the formed agglomerates [21,27]. However, it is very difficult to conclude on the impact of each phenomenon on the total surface increase, based only on N2 physisorption results.

**Figure 2.** N2-Physisorption isotherms recorded for LaFeO3 samples after each step of synthesis.

#### 2.1.3. Morphology Evolution upon Grinding

Evolution of the morphology of the LaTM perovskite after each synthesis step was observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It should be noted that similar observations hold whatever TM is concerned. As an example, the corresponding SEM images for LaMn based samples are given in Figure 3a–c. The SSR sample, synthesized at high temperature (1100 ◦C), exhibited large hexagonal particles of different sizes, most of them being above 500 nm large and agglomerated. After HEBM, the morphology of the material consisted in small particles aggregated into large objects of about one micron in size. Then, the HEBM step allowed us to reduce the elementary crystallite size, which was consistent with the observed evolution from X-ray diffraction (XRD) analysis.

**Figure 3.** Scanning electron microscope (SEM) images obtained for LaMnO3.15 (left) and transmission electron microscope (TEM) images obtained for LaFeO3 (right), after each synthesis steps. (**a,d**) SSR: solid state reaction, (**b,e**) HEBM: high-energy ball milling, (**c,f**) LEBM: low-energy ball milling.

TEM analysis, presented in Figure 3d–f for LaFe-based samples, allowed us to observe the elementary particles (Figure 3e) which ranged from 10 to 40 nm. These sizes were in accordance with the XRD results, while the low surface area determined after the HEBM can be explained by the formation of dense, poorly porous, aggregates. When the material is subjected to LEBM, no significant modification of the large-scale morphology could be observed by SEM. Indeed, only aggregates of small nanocrystals were detected (Figure 3c). However, observation of the sample by TEM evidenced the formation of lower size aggregate/elementary particles despite the elementary particles remaining of comparable size than for the HEBM-derived material. As already concluded from XRD analysis, the LEBM step did not allow crystal size (as confirmed by TEM analysis, Figure 3f) to decrease, but seemed to allow the deagglomeration of the particles, leading in fine to the production of a material with higher SSA than after the HEBM step (Table 1). The Scheme 1 summarizes the role of the individual process steps, with the main characteristics of the perovskite obtained.

**Scheme 1.** Simplified view of the material evolution with the synthesis steps of the reactive grinding process. SSR: solid state reaction; HEBM: high energy ball milling; LEBM: low energy ball milling.
