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 CrN4 monolayer is a stable freestanding 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 π-d conjugation, in which the double π-d conjugation situation is reported for the first time. This mechanism is entirely different from that of the reported two-dimensional materials, leading to ultralow formation energy and robust stability. On the other hand, CrN4 layer has a ferromagnetic ground state, in which the ferromagnetic coupling between two Cr atoms is mediated by electrons of the half-filled large π orbitals from π-d conjugation. The high-temperature ferromagnetism with the Curie temperature above 1000 K is confirmed from the GGA+U, SCAN, and HSE calculations.