论文标题
在混合有机无机perovskites中，热波动和振动熵的作用：FAPBI3案例
The role of thermal fluctuations and vibrational entropy for the delta-to-alpha transition in hybrid organic-inorganic perovskites: the FAPbI3 case
论文作者
论文摘要
FAPBI3作为典型的杂化有机甲孔酮，由于其适合可见光吸收和良好的热稳定性的带隙而引起了极大的兴趣。在商业，稳定设备中使用FAPBI3的障碍是其不需要的黑对黄色（非玻璃体到钙钛矿，通常称为Delta-to-alpha）相位过渡时，在大约300 k的情况下。这种相位过渡的内在机制远非清晰，这是详细的结构性描述，是对alpha-phase的详细结构描述。通过合并的密度功能理论（DFT）计算，晶格动力学分析和DFT分子动力学模拟，我们将α期分配给高度动态的四方相，高对称性立方结构在系统电势能环境中作为动态不稳定的最大值出现。我们从计算上证明了衍射观察的立方结构是由于增强的FA动力学而而不是立方对称性的静态系统，因此在实验检测时间尺度中采样了不同四方畸变的结果。进一步的有限温度吉布斯自由能计算证实，与先前的六边形到我们的六边形分配相反，增长tta-to-alpha转变应视为六边形到四边形的跃迁。更重要的是，模拟表明该过程的驱动力是振动熵的差异，而不是先前提出的旋转熵。这些结果指出了α期的动力学性质，被忽视的四方结构的重要性以及振动熵在钙钛矿相关相位转变中的关键作用，其利用对于成功摄取ABX3混合有机有机甲甲虫在商业应用中的成功至关重要。
FAPbI3, as a typical hybrid organic-inorganic perovskite, has attracted considerable interest due to its band gap suitable for visible light absorption and good thermal stability. A barrier to the use of FAPbI3 in commercial, stable devices is its unwanted black-to-yellow (non-perovskite to perovskite, commonly known as delta-to-alpha) phase transition at around 300 K. The intrinsic mechanisms of such phase transition are far from clear, being the detailed structural description for the alpha-phase still missing. By combined Density Functional Theory (DFT) calculations, lattice dynamics analysis and DFT molecular dynamics simulations, we assign the alpha-phase to the highly dynamic tetragonal phase, with the high-symmetry cubic structure emerging as a dynamically unstable maximum in the system potential energy landscape. We demonstrate computationally that the diffraction-observed cubic structure is the result of the averaging of different tetragonal distortions sampled in the experimental detection time scale as a result of the enhanced FA dynamics, instead of a static system of cubic symmetry. Further finite-temperature Gibbs free energy calculations confirm that the delta-to-alpha transition should be considered as a hexagonal-to-tetragonal transition in contrast to the previous hexagonal-to-cubic assignment. More importantly, the simulations indicate that the driving force of the process is the vibrational entropy difference rather than the rotational entropy as previously proposed. These results point out the dynamical nature of the alpha-phase, the importance of the overlooked tetragonal structure, and the key role of the vibrational entropy in perovskite-related phase transitions, the harnessing of which is critical for successful uptake of ABX3 hybrid organic-inorganic perovskites in commercial applications.
