Prediction of single-atom-thick transition metal nitride CrN${}_4$ with a square-planar network and high-temperature ferromagnetism

Abstract

Single-atom-thick two-dimensional materials such as graphene usually have a hexagonal lattice while the square-planar lattice is uncommon in the family of two-dimensional materials. Here, we demonstrate that single-atom-thick transition metal nitride CrN${}_4$ monolayer is a stable free-standing layer with a square-planar network. The stability of square-planar geometry is ascribed to the combination of N=N double bond, Cr-N coordination bond, and $\pi$-d conjugation, in which the double $\pi$-d conjugation is rarely reported in previous studies. This mechanism is entirely different from that of the reported two-dimensional materials, leading to lower formation energy and more robust stability compared to the synthesized g-C${}_3$N${}_4$ monolayer. On the other hand, CrN${}_4$ layer has a ferromagnetic ground state, in which the ferromagnetic coupling between two Cr atoms is mediated by electrons of the half-filled large $\pi$ orbitals from $\pi$-d conjugation. The high-temperature ferromagnetism in CrN${}_4$ monolayer is confirmed by solving the Heisenberg model with Monte Carlo method.

arXiv commenthttps://link.aps.org/doi/10.1103/PhysRevB.106.125421

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