Drużbicki Kacper, Pinna Roberto Simone, Rudić Svemir, Jura Marek, Gorini Giuseppe, Fernandez-Alonso Felix
Faculty of Physics, Adam Mickiewicz University , Umultowska 85, 61-614 Poznan, Poland.
Frank Laboratory of Neutron Physics, Joint Institute for Nuclear Research , 141980, Dubna, Russia.
J Phys Chem Lett. 2016 Nov 17;7(22):4701-4709. doi: 10.1021/acs.jpclett.6b01822. Epub 2016 Nov 8.
High-resolution inelastic neutron scattering and extensive first-principles calculations have been used to explore the low-temperature phase of the hybrid solar-cell material methylammonium lead iodide up to the well-known phase transition to the tetragonal phase at ca. 160 K. Contrary to original expectation, we find that the Pnma structure for this phase can only provide a qualitative description of the geometry and underlying motions of the organic cation. A substantial lowering of the local symmetry inside the perovskite cage leads to an improved atomistic model that can account for all available spectroscopic and thermodynamic data, both at low temperatures and in the vicinity of the aforementioned phase transition. Further and detailed analysis of the first-principles calculations reveals that large-amplitude distortions of the inorganic framework are driven by both zero-point-energy fluctuations and thermally activated cation motions. These effects are significant down to liquid-helium temperatures. For this important class of technological materials, this work brings to the fore the pressing need to bridge the gap between the long-range order seen by crystallographic methods and the local environment around the organic cation probed by neutron spectroscopy.
高分辨率非弹性中子散射和广泛的第一性原理计算已被用于探索混合太阳能电池材料甲基碘化铅的低温相，直至约160 K时向四方相的著名相变。与最初的预期相反，我们发现该相的Pnma结构只能对有机阳离子的几何形状和潜在运动提供定性描述。钙钛矿笼内局部对称性的大幅降低导致了一个改进的原子模型，该模型可以解释低温下以及上述相变附近所有可用的光谱和热力学数据。对第一性原理计算的进一步详细分析表明，无机框架的大幅度畸变是由零点能涨落和热激活的阳离子运动共同驱动的。这些效应在液氦温度下仍然显著。对于这类重要的技术材料，这项工作凸显了弥合晶体学方法所观察到的长程有序与中子光谱所探测的有机阳离子周围局部环境之间差距的迫切需求。
