Revealing unusual chemical bonding in planar hyper-coordinate Ni₂Ge and quasi-planar Ni₂Si two-dimensional crystals

Abstract

We discover unusual chemical bonding in a novel planar hyper-coordinate Ni2Ge free-standing 2D monolayer, and also in a nearly planar slightly buckled Ni2Si monolayer. This unusual bonding is revealed by Solid State Adaptive Natural Density Partitioning analysis. This analysis shows that a new type of 2c-2e Ni-Si σ and 3c-2e Ni-Ge-Ni σ bonds stabilize these 2D crystals. This is completely different from any previously known 2D crystals. Both of these free-standing monolayers are global minima in two-dimensional space. Although their exotic structure has unprecedented chemical bonding, they show extraordinary stability as single layers. The stabilities of these frameworks are confirmed by phonon dispersion calculations and ab initio molecular dynamics calculations. For Ni2Si, the framework was maintained during short 10 ps molecular dynamics annealing up to 1500 K, while Ni2Ge survived 10 ps runs up to 900 K. Both systems are predicted to be non-magnetic and metallic. As these new 2D crystals contain hypercoordinated Group 14 atoms, they are examples of a new class of 2D crystals with unconventional chemical bonding and potentially exciting new properties. Interestingly, we find that the stabilities of Ni2Si and Ni2Ge are much higher than that of silicene and germanene. Thus, this work provides a novel way to stabilize 2D sheets of Group 14 elements.

Adding a new dimension to the chemistry of phosphorus and arsenic

“…We compared the calculated values of our new materials to that of several commonly known 2D materials, including graphene, MoS 2 , silicene, germanene, and recently predicted X 2 Y (X = Cu/Ni; Y = Si/Ge) [34][35][36] and pure transition metal monolayers. The in-plane Young's modulus, (or in-plane stiffness), is commonly used to evaluate the mechanical stability of 2D layered materials.…”

Stability, electronic, and optical properties of two‐dimensional phosphoborane

“…To analyze the chemical bonding pattern of the 2D‐B 4 P 2 sheet, the solid‐state adaptive natural density partitioning (SSAdNDP) algorithm was used . Previously it was shown that SSAdNDP is a powerful tool for analyzing chemical bonding in two‐dimensional periodic systems . A plane wave (PW) calculation was performed using 450 eV energy cut off with convergence threshold 10 −6 eV for total energy.…”

Two-Dimensional Group IV Monochalcogenides: Anode Materials for Li-Ion Batteries

“…24,25 Such new 2D materials have unusual chemical bonding and stoichiometry, which could structurally mimic graphene monolayer. [26][27][28] Black phosphorous, the thermodynamically most stable allotrope of phosphorous 29 , is a layered material like graphite and is known to possess a large theoretical capacity of 2596 mAhg − 1 . 30,31 However poor cycle life and irreversible structural change limits its application as negative electrode in LIB.…”