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

TL;DR

This paper predicts a stable, single-atom-thick CrN4 monolayer with a square-planar lattice that exhibits high-temperature ferromagnetism, driven by unique π-d conjugation mechanisms, differing from typical 2D materials.

Contribution

It introduces a novel square-planar CrN4 monolayer with unique stability mechanisms and demonstrates its high-temperature ferromagnetism through theoretical modeling.

Findings

CrN4 monolayer is stable and free-standing.

The monolayer exhibits high-temperature ferromagnetism.

Unique π-d conjugation stabilizes the square-planar structure.

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.