## 3 Results and Discussion
### After Aging: Evaluation of Paint Samples
#### 3.3.2 Inorganic Fraction
[35], the shift of this peak could depend on how the phosphate group is conjugated to the other elements. In addition, as for Oil_V_COMM, the main peaks detected before aging are still visible, and there is a strong increase in the band at 1027 cm\({}^{-1}\). Regarding the acrylic sample (Figure 8c), two new bands are visible at 577 and 976 cm\({}^{-1}\), and they could be related to the out-of-plane _bending_ and the symmetric _stretching_ of the sulfate group, respectively [62, 63, 70]. Moreover, a shift from 307 cm\({}^{-1}\) to 297 cm\({}^{-1}\) of the vibration of the phosphate group can be noticed. As for the previous samples, new peaks are visible in the Sty_V sample (Supplementary Figure S10b), in particular, a peak at 300 cm\({}^{-1}\) that could be related to the vibration of the phosphate group and a peak at 982 cm\({}^{-1}\) attributable to the symmetrical _stretching_ of the SO\({}_{4}\)\({}^{2-}\) group [62, 64]. To gain more information on the crystals formed through the accelerated weathering, XRPD analysis was additionally carried out. For this purpose, the aged styrene-acrylic-based sample was considered since it was the most representative in terms of degradation products, such as crystals, observed and detected by SEM and \(\mu\)-Raman. In particular, the  
Figure 8: \(\mu\)-Raman spectra acquired on the paint samples after the accelerated aging and obtained with a 785 nm laser: (**a**) Oil_V_COMM, (**b**) Alk_V, and (**c**) Acr_V. analysis was performed on crystals visible to the naked eye and the grey area. As can be seen in Supplementary Figure S11, the crystals were identified as (NH4)2(HSO4)(H2 PO4) (ICSD#100795). On the other hand, the investigation of the XRPD pattern of the grey area was quite complex. As evidenced in Figure 9, in addition to a fraction of unmodified \(\alpha\)-NH4 MnP2O7 phase, the formation of (NH4)2(HSO4)(H2 PO4), the presence of Al2(SO4)3 related to the minority phase of the kaolinite, and the formation of the (NH4)2 Mn2(SO4)3 compound (ICSD#170052) were detected. Combining the SEM-EDX, ATR-FTIR, and \(\mu\)-Raman results with the XRPD results, it can be concluded that crystal CRY_1 corresponds to (NH4)2 Mn2(SO4)3 and crystal CRY_2 to (NH4)2(HSO4)(H2 PO4), whereas crystal CRY_3 could not be identified from these initial analyses. For this reason, further investigations are needed. A further step was taken on the Sty_V sample to evaluate the effect of the accelerated weathering on pigment color using FORS. In particular, the Kubelka-Munk function was applied to the reflectance spectra acquired before and after aging. In Supplementary Figure S12a, the origin of the violet color of the \(\alpha\)-NH4 MnP2O7 pigment can be attributed to the Mn3+ ion (3d4 electron configuration) in the octahedral site dominated by the main absorption band centered on 545 nm due to the spin-allowed \({}^{5}\)Eg \(\rightarrow\)\({}^{5}\)T2g transition. It is also well known that the optical properties of Mn3+ ions are influenced by the Jahn-Teller effect, which results in the splitting of the ground state \({}^{5}\)E [71; 72] and the \({}^{5}\)T2 and \({}^{1}\)T2 excited states [73]. From the results described previously, it appears that one of the main reactions that took place during the artificial aging was the transformation of \(\alpha\)-NH4 MnP2O7 to (NH4)2 Mn2(SO4)3, suggesting the reduction of Mn3+ (3d4) to Mn2+ (3d5). Considering that in (NH4)2 Mn2(SO4)3 the two Mn2+O6 octahedrons are characterized by very similar Mn-O bond lengths, the absorption bands visible after the accelerated aging at 460 nm and 565 nm (Supplementary Figure S12b) can be assigned to the transitions from the \({}^{6}\)A1g ground state to the \({}^{4}\)T2g and \({}^{4}\)T1g excited states, respectively [74].