In hybrid materials, a high-symmetry lattice is decorated by low-symmetry building blocks. The result is an aperiodic solid that hosts many nearly-degenerate disordered configurations. Using the perovskite methylammonium lead iodide (MAPbI₃) as a prototype hybrid material, we show that the inherent disorder renders the conventional phonon picture of transport insufficient. Ab initio molecular dynamics and analysis of the spectral energy density reveal that vibrational carriers simultaneously exhibit features of both classical phonons and of carriers typically found in glasses. The low frequency modes retain elements of acoustic waves but exhibit extremely short lifetimes of only a few tens of picoseconds. For higher frequency modes, strong scattering due to rapid motion and reconfiguration of the organic cation molecules induces a loss of definition of the wave vector. Lattice dynamics shows that these carriers are more akin to diffusons – the nonwave carriers in vitreous materials – and are the dominant contributors to thermal conduction near room temperature. To unify the framework of glassy diffusons with that of phonons scattered at the ultimate limit, three-phonon interactions resolved from first-principles expose anharmonic effects two orders of magnitude higher than in silicon. The dominant anharmonic interactions occur within modes of the PbI₆ octahedral framework itself, as well as between modes of the octahedral framework and modes localized to the MA molecules. The former arises from long-range interactions due to resonant bonding, and the latter from polar rotor scattering of the MA molecules. This establishes a clear microscopic connection between symmetry-breaking, dynamical disorder, anharmonicity, and the loss of wave nature in MAPbI₃.

中文翻译：

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混合卤化钙钛矿中混合的声子和非声子传输：玻璃-晶体对偶，动力学无序和非谐性†

在混合材料中，高对称的晶格由低对称的构建块装饰。结果是一个非周期性固体，其中包含许多几乎简并的无序构型。使用钙钛矿甲基铵碘化铅（MAPbI ₃）作为原型杂化材料，我们显示出固有的无序性使得常规声子的运输图不充分。从头算分子动力学和对光谱能量密度的分析表明，振动载流子同时具有经典声子和通常在玻璃中发现的载流子的特征。低频模式保留了声波的元素，但寿命极短，仅为几十皮秒。对于更高频率的模式，由于快速运动和有机阳离子分子的重新配置而引起的强散射会导致波矢量清晰度的丧失。晶格动力学表明，这些载流子更类似于扩散子（玻璃质材料中的非波载流子），并且是室温附近热传导的主要贡献者。为了将玻璃质扩散子的框架与最终扩散的声子的框架统一起来，从第一性原理解决的三声子相互作用暴露出比硅高两个数量级的非谐效应。主要的非谐相互作用发生在PbI的模式内₆八面体框架本身，以及八面体框架的模式与局域在MA分子之间的模式之间。前者是由于共振键引起的长距离相互作用，后者是由于MA分子的极性转子散射而产生的。这在MAPbI _3中的对称性破坏，动力学失调，非谐和波性质的损失之间建立了清晰的微观联系。