npj Computational Materials

Marios Zacharias, George Volonakis, Feliciano Giustino & Jacky Even

npj Computational Materials 9: 153 (2023). Published: 24 August 2023

doi.org/10.1038/s41524-023-01089-2

Abstract Anharmonicity and local disorder (polymorphism) are ubiquitous in perovskite physics, inducing various phenomena observed in scattering and spectroscopy experiments. Several of these phenomena still lack interpretation from first principles since, hitherto, no approach is available to account for anharmonicity and disorder in electron–phonon couplings. Here, relying on the special displacement method, we develop a unified treatment of both and demonstrate that electron–phonon coupling is strongly influenced when we employ polymorphous perovskite networks. We uncover that polymorphism in halide perovskites leads to vibrational dynamics far from the ideal noninteracting phonon picture and drives the gradual change in their band gap around phase transition temperatures. We also clarify that combined band gap corrections arising from disorder, spin-orbit coupling, exchange–correlation functionals of high accuracy, and electron–phonon coupling are all essential. Our findings agree with experiments, suggesting that polymorphism is the key to address pending questions on perovskites’ technological applications.

摘要在钙钛矿材料中，非谐性和局域无序（多态性）普遍存在，这导致了在散射和光谱实验中观察到的各种现象。迄今为止，这些现象仍然缺乏第一性原理的解释，因为目前还没有方法能够解释电子-声子耦合的非谐和无序。在本文中，通过特殊的位移方法，我们发展了统一处理两者的方法，并证明当我们采用多态钙钛矿网络时，电声耦合会受到强烈的影响。我们发现，卤化物钙钛矿的多态性导致了振动动力学远离理想的非相互作用声子图像，并驱动它们在相变温度附近带隙的逐渐变化。我们还阐明了由无序、自旋轨道耦合、高精度的交换相关泛函和电声耦合综合引起的带隙修正是必不可少的。我们的发现与实验结果一致，这表明多态性是解决钙钛矿技术应用中未解决问题的关键。

编辑概述

钙钛矿的神秘面纱：非谐性与局部无序

氧化物钙钛矿由于其固有的铁电、反铁电和压电性质，是一类具有广泛应用前景的迷人材料。卤化物钙钛矿因其在太阳能电池中的卓越效率，以及在光电子学、电催化和热电材料中的应用前景而备受关注。典型的四方或立方钙钛矿的密度泛函理论（DFT）计算依赖于高对称网络的假设，而不考虑局部无序的基态构型。这一假设忽略了对其电子结构的重要修正，并要求在非谐声子动力学上加强晶体的对称性，从而使它们被理想化的、定义明确的色散所表征。这种假设与对过阻尼光学振动、结构无序以及结构相变临界点附近复杂的前转变动力学测量不一致。这些问题的根本还在于目前还没有方法能够解释电子-声子耦合的非谐和无序。在本工作中，来自法国雷恩大学的Marios Zacharias等人，证明了非谐性和局部无序在氧化物和卤化物钙钛矿（包括SrTiO₃, CsPbBr₃, CsPbI₃, 和CsSnI₃）的电子结构、声子动力学和电声耦合中的重要作用。该研究通过第一性原理，证明了(i)局部无序和非谐性是过阻尼和强耦合声子的来源；(ii)局部无序和非谐性对于描述电子-声子耦合是至关重要的；(iii)低能量非谐光振动主导热带隙重正化；(iv)局部无序是解释带隙在相变温度附近随温度平稳演化的关键；(v)对带隙和有效质量的完整描述需要将无序与完全相对论效应结合起来。作者采用了通过特殊位移法的非谐性方法，使其能够统一处理非谐电声耦合。该研究提出了一种处理钙钛矿中晶格动力学的方法，并建立了一个通用的框架来精确模拟其载流子迁移率、电导率、激子谱、非平衡动力学和极化子物理, 为氧化物和卤化物钙钛矿中的晶格动力学和电声耦合提供了见解。

Editorial Summary

Mysteries of Perovskites：Anharmonicity and Local Disorder

Oxide perovskites are fascinating materials with extensive applications owing to their intrinsic ferroelectric, antiferroelectric, and piezoelectric properties. Halide perovskites are of high interest due to their impressive efficiencies in solar cells, and attractive applications in optoelectronics, electrocatalysis, and thermoelectrics. Typical density functional theory (DFT) calculations of tetragonal or cubic perovskites rely on the assumption of a high-symmetry network, disregarding the locally disordered ground state configurations. This assumption misses important corrections to the electronic structure and requires enforcing the crystal’s symmetries on anharmonic phonon dynamics, thus, represented by idealized well-defined dispersions. Such behavior is disconnected from measurements of overdamped optical vibrations, structural disorder, and complex pretransitional dynamics close to structural phase transitions. The key issue lies in the fact that so far no approach is available to account for anharmonicity and disorder in electron–phonon couplings. In this work, Marios Zacharias et al. from the University of Rennes, France, demonstrated the important role of anharmonicity and local disorder in the electronic structure, phonon dynamics, and electron–phonon coupling of oxide and halide perovskites (SrTiO3, CsPbBr3, CsPbI3, and CsSnI3). This work shows from first-principles that (i) local disorder and anharmonicity are at the origin of overdamped and strongly coupled phonons; (ii) local disorder and anharmonicity are essential to describe electron–phonon coupling; (iii) low-energy anharmonic optical vibrations dominate thermal band gap renormalization; (iv) local disorder is the key to explain the smooth evolution of the band gap with temperature around phase transitions; (v) a full description of band gaps and effective masses requires combining disorder with fully relativistic effects. The authors employed an approach based the special displacement method to treat , namely anharmonicity, which allows the unified treatment of anharmonic electron–phonon coupling. This work proposes a radically different way of conceptualize the lattice dynamics in perovskites, sets up a universal framework for accurate simulations of their carrier mobilities, conductivities, excitonic spectra, non-equilibrium dynamics, and polaron physics, and provides insights on the lattice dynamics and electron–phonon couplings in oxide and halide perovskites.