The poor stability of organometallic halide perovskites, especially CH₃NH₃PbI₃ (MAPbI₃), in high-temperature environments is one of the challenges retarding its wider applicability. A better understanding of how MAPbI₃ perovskite degrades thermally will allow us to develop strategies to improve its “intrinsic” stability. In this study, we employed first-principles density functional theory to obtain detailed thermal degradation energy landscapes for MAPbI₃. We focused on studying two reaction pathways: P1, for proton abstraction from methylammonium (MA) to iodine, and P2, for substitution (S_N2) of an iodide anion (I^–, as nucleophile) at the carbon atom of MA to yield iodomethane (CH₃I) and ammonia (NH₃). Our calculations provided kinetic data, allowing us to assess the “intrinsic” (in)stability of MAPbI₃ under heat stress and to make sense of the seemingly contradictory experimental results. The I^– species is more reactive for reaction P1 but not for P2. The energy of the reaction along pathway P1 increases monotonically with respect to proton abstraction. The reaction along pathway P2 is a two-step process, with a CH₃–I–Pb–X intermediate formed in the crystal in conjunction with the release of NH₃ gas. Our results suggest that the method of preparation (e.g., iodine-deficient conditions) and the morphology (e.g., improved crystallinity) of MAPbI₃ could both influence its thermal stability.

中文翻译：

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甲基铵三碘化铅钙钛矿热降解机理的第一性原理研究

有机金属卤化物钙钛矿的稳定性差，特别是CH ₃ NH ₃碘化铅₃（MAPbI ₃），在高温环境下是延迟它的更广泛的应用的挑战之一。更好地了解MAPbI ₃钙钛矿如何热降解，将使我们能够制定改善其“固有”稳定性的策略。在这项研究中，我们采用第一原理密度泛函理论来获得MAPbI _3的详细热降解能分布图。我们专注于研究两种反应途径：P1，用于从甲基铵（MA）到碘的质子提取；P2，用于替代（S _N2）碘阴离子（第I ^- ，作为亲核试剂）的MA的碳原子以产生碘甲烷（CH ₃ NH I）和氨（₃）。我们的计算提供了动力学数据，使我们能够评估热应激下MAPbI ₃的“内在”（不稳定性），并理解似乎矛盾的实验结果。I ^–对反应P1更具反应性，但对P2则没有。沿着路径P1的反应能量相对于质子提取单调增加。沿着途径P2的反应是两步过程，其中CH ₃与NH ₃气体的释放一起在晶体中形成了–I–Pb–X中间体。我们的结果表明，MAPbI ₃的制备方法（例如，缺碘条件）和形态（例如，改善的结晶度）都可能影响其热稳定性。