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The thermal stability and decomposition pathway of formamidinium iodide (FAI, HC(NH 2 ) 2 I) in contact with NiO and TiO 2 are investigated by combined experimental studies and density functional theory (DFT) calculations. Based on the decomposition temperature, we find that the stability decreases as FAI ∼ FAI + TiO 2 > FAI + NiO. Moreover, FAPbI 3 in contact with NiO and TiO 2 shows similar thermal stability behaviour to FAI. The bulk decomposition of FAI occurs via the formation of sym -triazine, and can also produce HCN, and NH 4 I at ∼280 °C, which further decomposes to NH 3 and HI above 300 °C. When FAI comes into contact with NiO, the interfacial reaction triggers decomposition at a much lower temperature (∼200 °C), resulting in the formation of NiI 2 as the solid product while releasing NH 3 and H 2 O into the gas phase; sym -triazine and HCN are observed near the FAI bulk decomposition temperature. In contrast, when FAI comes into contact with TiO 2 , the decomposition temperature is similar to bulk FAI; however, HCN is released at a lower temperature (∼260 °C) compared to sym -triazine. The difference in the degradation behavior of FAI with NiO and TiO 2 is elucidated using DFT calculations. Our results show that the interfacial reaction between the organic component of perovskite material and NiO occurs similarly for MA and FA, which thereby can induce device instability. 

Combining experimental and theoretical studies, we investigate the role of R-site (R = Y, Sm, Bi) element on the phase formation and thermal stability of R 2 (Mn 1−x Fe x ) 4 O 10−δ ( x = 0, 0.5, 1) mullite-type oxides. Our results show a distinct R-site dependent phase behavior for mullite-type oxides as Fe is substituted for Mn: 100% mullite-type phase was formed in (Y, Sm, Bi) 2 Mn 4 O 10 ; 55% and 18% of (Y, Sm) 2 Mn 2 Fe 2 O 10−δ was found when R = Y and Sm, respectively, for equal Fe and Mn molar concentrations in the reactants, whereas Bi formed 54% O10- and 42% O9-mixed mullite-type phases. Furthermore, when the reactants contain 100% Fe, no mullite-type phase was formed for R = Y and Sm, but a sub-group transition to Bi 2 Fe 4 O 9 O9-phase was found for R = Bi. Thermogravimetric analysis and density functional theory (DFT) calculation results show a decreasing thermal stability in O10-type structure with increasing Fe incorporation; for example, the decomposition temperature is 1142 K for Bi 2 Mn 2 Fe 2 O 10−δ vs. 1217 K for Bi 2 Mn 4 O 10 . On the other hand, Bi 2 Fe 4 O 9 O9-type structure is found to be thermally stable up to 1227 K. These findings are explained by electronic structure calculations: (1) as Fe concentration increases, Jahn–Teller distortion results in mid band-gap empty states from unstable Fe 4+ occupied octahedra, which is responsible for the decrease in O10 structure stability; (2) the directional sp orbital hybridization unique to Bi effectively stabilizes the mullite-type structure as Fe replaces Mn.