The prediction of new materials with peculiar topological properties is always desirable to achieve new properties and applications. In this work, by means of density functional theory computations, we extend the rule-breaking chemical bonding of planar pentacoordinate silicon (ppSi) into a periodic system: a C_2v Ca₄Si₂²⁻ molecular building block containing a ppSi center is identified first, followed by the construction of an infinite CaSi monolayer, which is essentially a two-dimensional (2D) network of the Ca₄Si₂ motif. The moderate cohesive energy, absence of imaginary phonon modes, and good resistance to high temperature indicate that the CaSi monolayer is a thermodynamically and kinetically stable structure. In particular, a global minimum search reveals that the ppSi-containing CaSi monolayer is the lowest-energy structure in 2D space, indicating its great promise for experimental realization. The CaSi monolayer is a natural semiconductor with an indirect band gap of 0.5 eV, and it has rather strong optical absorption in the visible region of the solar spectrum. More interestingly, the unique atomic configuration endows the CaSi monolayer with an unusually negative Poisson's ratio. The rule-breaking geometric structure together with its exceptional properties makes the CaSi monolayer quite a promising candidate for applications in electronics, optoelectronics, and mechanics.

中文翻译：

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具有准平面五坐标硅的二维CaSi单层†

为了获得新的特性和应用，总是需要对具有特殊拓扑特性的新材料进行预测。在这项工作中，通过密度泛函理论计算，我们将平面五配位硅（ppSi）的打破规则的化学键扩展到周期系统中：识别出包含ppSi中心的C _2v Ca ₄ Si ₂ ^2-分子构建基块首先，接着构造无限的CaSi单层，该单层本质上是Ca ₄ Si ₂的二维（2D）网络主题。适度的内聚能，不存在虚构的声子模态以及对高温的良好耐受性表明，CaSi单层是热力学和动力学稳定的结构。尤其是，一项全球最小搜索显示，含ppSi的CaSi单层是2D空间中最低的能量结构，表明其在实验实现方面的巨大前景。CaSi单层是具有0.5 eV的间接带隙的天然半导体，并且在太阳光谱的可见光区域具有相当强的光吸收能力。更有趣的是，独特的原子构型赋予CaSi单层非同寻常的负泊松比。突破规则的几何结构及其卓越的性能使CaSi单层成为电子应用中很有希望的候选者，