雅恩-泰勒效应
衍射
从头算
对分布函数
单斜晶系
相变
材料科学
粘结长度
结晶学
相(物质)
凝聚态物理
分子动力学
X射线晶体学
分子物理学
化学
离子
晶体结构
计算化学
物理
光学
量子力学
有机化学
作者
Annalena R. Genreith‐Schriever,Alexandra Alexiu,George Searle Phillips,Chloe S. Coates,Liam A. V. Nagle‐Cocco,Joshua D. Bocarsly,Farheen N. Sayed,Siân E. Dutton,Clare P. Grey
标识
DOI:10.1021/acs.chemmater.3c02413
摘要
The atomistic structure of lithium nickelate (LiNiO2), the parent compound of Ni-rich layered oxide cathodes for Li-ion batteries, continues to elude a comprehensive understanding. The common consensus is that the material exhibits local Jahn–Teller distortions that dynamically reorient, resulting in a time-averaged undistorted R3̅m structure. Through a combination of ab initio molecular dynamics (AIMD) simulations and variable-temperature X-ray diffraction (VT-XRD), we explore Jahn–Teller distortions in LiNiO2 as a function of temperature. Static Jahn–Teller distortions are observed at low temperatures (T < 250 K) via AIMD simulations, followed by a broad phase transition that occurs between 250 and 350 K, leading to a highly dynamic, displacive phase at high temperatures (T > 350 K), which does not show the four short and two long bonds characteristic of local Jahn–Teller distortions. These transitions are followed in the AIMD simulations via abrupt changes in the calculated pair distribution function and the bond-length distortion index and in X-ray diffraction via the monoclinic lattice parameter ratio, amon/bmon, and δ angle, the fit quality of an R3̅m-based structural refinement, and a peak sharpening of the diffraction peaks on heating, consistent with the loss of distorted domains. Between 250 and 350 K, a mixed-phase regime is found via the AIMD simulations where distorted and undistorted domains coexist. The repeated change between the distorted and undistorted states in this mixed-phase regime allows the Jahn–Teller long axes to change direction. These pseudorotations of the Ni–O long axes are a side effect of the onset of the displacive phase transition. Antisite defects, involving Li ions in the Ni layer and Ni ions in the Li layer, are found to pin the undistorted domains at low temperatures, impeding cooperative ordering at a longer length scale.
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