Poisson’s ratios of two-dimensional (2D) all-inorganic perovskites Cs2PbX4 (X = Cl, Br, I) have been calculated by the first-principles calculations. The contribution of each geometric parameter (bond length [Formula: see text], bond angle [Formula: see text], rotation angle [Formula: see text], and tilt angle [Formula: see text]) to Poisson’s ratio is obtained analytically. Through a comprehensive analysis of the geometric deformations of the perovskite under the uniaxial strain, we find that Poisson’s ratios of the perovskites are sensitive to the change of the bond length [Formula: see text] and the bond angle [Formula: see text]. In addition, the value of the bond angle [Formula: see text] in the strain-free structure mainly determines the high in-plane anisotropy of Poisson’s ratios in Cs2PbX4.

中文翻译：

---

全无机钙钛矿单层中的泊松比

二维 (2D) 全无机钙钛矿 Cs 的泊松比2PBX4(X = Cl, Br, I) 已通过第一性原理计算得到。解析得到每个几何参数（键长[公式：见正文]、键角[公式：见正文]、旋转角[公式：见正文]和倾斜角[公式：见正文]）对泊松比的贡献. 通过对钙钛矿在单轴应变下的几何变形进行综合分析，我们发现钙钛矿的泊松比对键长[公式：见正文]和键角[公式：见正文]的变化很敏感。此外，无应变结构中键角[公式：见正文]的值主要决定了Cs中泊松比的高面内各向异性2PBX4.