Cannelli Oliviero, Wiktor Julia, Colonna Nicola, Leroy Ludmila, Puppin Michele, Bacellar Camila, Sadykov Ilia, Krieg Franziska, Smolentsev Grigory, Kovalenko Maksym V, Pasquarello Alfredo, Chergui Majed, Mancini Giulia F
Laboratory of Ultrafast Spectroscopy (LSU) and Lausanne Centre for Ultrafast Science (LACUS), École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland.
Department of Physics, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden.
J Phys Chem Lett. 2022 Apr 21;13(15):3382-3391. doi: 10.1021/acs.jpclett.2c00281. Epub 2022 Apr 11.
A comprehensive microscopic description of thermally induced distortions in lead halide perovskites is crucial for their realistic applications, yet still unclear. Here, we quantify the effects of thermal activation in CsPbBr nanocrystals across length scales with atomic-level precision, and we provide a framework for the description of phase transitions therein, beyond the simplistic picture of unit-cell symmetry increase upon heating. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. Our findings allow to reconcile temperature-dependent modifications of physical properties, such as changes in the optical band gap, that are incompatible with the perovskite time- and space-average structures.
对卤化铅钙钛矿中热致畸变进行全面的微观描述对其实际应用至关重要，但目前仍不清楚。在此，我们以原子级精度量化了CsPbBr纳米晶体中热激活在不同长度尺度上的影响，并提供了一个描述其中相变的框架，超越了加热时晶胞对称性增加的简单图景。由于声子非谐性，温度升高显著增强了卤化铅框架的短程结构畸变，这导致多余自由能表面随温度变化。结果，相变可以通过软模模型进行合理解释，该模型也描述了氧化物钙钛矿中的位移热相变。我们的发现有助于协调与钙钛矿时间和空间平均结构不相容的物理性质的温度依赖性变化，如光学带隙的变化。
