Atomic-level description of thermal fluctuations in inorganic lead halide perovskites

The potential of lead-halide perovskites for realistic applications is currently hindered by their limited long-term stability under functional activation. While the role of lattice flexibility in the thermal response of perovskites has become increasingly evident, the description of thermally-induced distortions is still unclear. In this work, we provide a unified picture of thermal activation in CsPbBr₃ across length scales, showing that lattice symmetry does not increase at high temperatures. We combine temperature-dependent XRD, Br K-edge XANES, ab initio MD simulations, and calculations of the XANES spectra by first-principles, accounting for both thermal fluctuations and core hole final state effects. We find that the octahedral tilting of the Pb-Br inorganic framework statistically adopts multiple local configurations over time - in the short-range. In turn, the stochastic nature of the local thermal fluctuations uplifts the longer-range periodic octahedral tilting characterizing the low temperature structure, with the statistical mean of the local configurations resulting in a cubic-like time-averaged lattice. These observations can be rationalized in terms of displacive thermal phase transitions through the soft mode model, in which the phonon anharmonicity of the flexible inorganic framework causes the excess free energy surface to change as a function of temperature. Our work demonstrates that the effect of thermal dynamics on the XANES spectra can be effectively described for largely anharmonic systems, provided ab initio MD simulations are performed to determine the dynamically fluctuating structures, and core hole final state effects are included in order to retrieve an accurate XANES line shape. Moreover, it shows that the soft mode model, previously invoked to describe displacive thermal phase transitions in oxide perovskites, carries a more general validity.

当前，铅卤化物钙钛矿在现实应用中的潜力受到其功能性激活下长期稳定性的限制。尽管钙钛矿在热响应中的晶格柔韧性所扮演的角色已日益凸显，但热诱导畸变的描述仍显模糊。在本研究中，我们为CsPbBr₃在不同尺度上的热激活提供了一幅统一的图景，表明在高温下晶格对称性不会增加。我们结合了温度依赖性XRD、Br K边XANES、基于第一原理的分子动力学模拟以及通过第一原理计算XANES光谱，考虑了热波动和核心空穴最终状态效应。我们发现，在短程范围内，Pb-Br无机框架的八面体倾斜在统计上采取多种局部配置。反过来，局部热波动的随机性提升了表征低温结构的远程周期性八面体倾斜特征，局部配置的统计平均值导致时间平均后的晶格呈现出立方状。这些观察结果可以通过通过软模模型来合理化位移性热相变，其中柔性无机框架的声子非简谐性导致过剩自由能表面随温度变化。我们的研究证明了对于大部分非简谐系统，可以通过进行基于第一原理的分子动力学模拟来确定动态波动结构，并包括核心空穴最终状态效应以准确获取XANES谱线形状，从而有效地描述热动力学对XANES光谱的影响。此外，它还表明，之前用于描述氧化物钙钛矿位移性热相变的软模模型具有更广泛的适用性。