At present, the concept of planar hypercoordination in chemistry meets the fast development of two-dimensional (2D) nanomaterials, leading to considerable interest in searching for 2D materials with planar hypercoordinate atoms. In this work, by means of the swarm-intelligence structure search method and first-principles calculations, we predict a hitherto unknown 2D TiB₄ monolayer with a planar octacoordinate Ti moiety, in which each Ti atom binds to eight B atoms with equal distances in a perfect plane, and has the highest coordination of Ti known for 2D materials thus far. Systematic ab initio calculations demonstrate the superior thermodynamic and dynamic stabilities of the predicted TiB₄ monolayer, indicating the high feasibility for experimental synthesis. The stabilization of this perfect planar structure originates from the geometric fit between the atomic radius of Ti and the size of the 8-membered B ring, as well as the electron transfer from Ti to B atoms which compensates the electron deficiency of the full sp² hybridized B network. Motivated by the unforeseen geometry of the TiB₄ monolayer, a series of other 2D transition metal borides (TMB₄, TM = V, Cr, Mo, W and Os) with quasi-planar octacoordinate TM atoms are further designed and discussed. The present work provides a useful roadmap for the discovery of 2D hypercoordinate materials.

中文翻译：

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二维TiB ₄单层具有平面八坐标Ti

目前，化学中的平面超配位的概念满足了二维（2D）纳米材料的快速发展，从而引起了人们对寻找具有平面超配位原子的2D材料的浓厚兴趣。在这项工作中，借助群智能结构搜索方法和第一性原理计算，我们预测了迄今未知的具有平面八坐标Ti部分的2D TiB ₄单层，其中每个Ti原子与八个等距的B原子结合。完美的平面，迄今为止，Ti具有最高的2D材料协调性。系统的从头算计算证明了预测的TiB ₄具有出色的热力学和动态稳定性单层，表明实验合成的高度可行性。这种完美的平面结构的稳定性源于Ti的原子半径与8元B环的大小之间的几何拟合，以及电子从Ti原子转移到B原子，这补偿了整个sp ²的电子缺陷。混合B网络。由于TiB ₄单层的不可预见的几何形状，进一步设计和讨论了一系列其他具有准平面八坐标TM原子的其他2D过渡金属硼化物（TMB ₄，TM = V，Cr，Mo，W和Os）。本工作为发现2D超坐标材料提供了有用的路线图。