The nature of the electronic ground state of M2C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes

Abstract

A systematic computational study is presented aimed at accurately describing the electronic ground state nature and properties of M₂C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes. Electronic band structure calculations in the framework of density functional theory (DFT), carried out with different types of basis sets and employing the generalized gradient approach (GGA) and hybrid functionals, provide strong evidence that Ti₂C, Zr₂C, Hf₂C, and Cr₂C MXenes exhibit an open-shell conducting ground state with localized spins on the metal atoms, while V₂C, Nb₂C, Mo₂C, Ta₂C, and W₂C MXenes exhibit a diamagnetic conducting ground state. For Ti₂C, Zr₂C, Hf₂C, and Cr₂C, the analysis of the low-lying spin polarized solutions with different spin orderings indicates that their ground states are antiferromagnetic (AFM), consisting of two ferromagnetic (FM) metal layers coupled antiferromagnetically. For the diamagnetic MXenes, the converged spin polarized solutions are significantly less stable than the closed shell solution except for the case of V₂C and Mo₂C where those excited open shell solutions can be thermally accessible (less than 300 meV per formula unit). The analysis of charge and spin density distributions of the ground state of the MXenes reveals that, in all cases, the metal atoms have a net charge close to +1 e and C atoms close to −2 e. In the case of diamagnetic MXenes, the electronic structure of V₂C, Nb₂C, and Ta₂C is consistent with metal atoms exhibiting a closed-shell s²d² configuration whereas for Mo₂C, and W₂C is consistent with a low-spin s¹d⁴ configuration although the FM solution is close in energy for V₂C and Mo₂C suggesting that they may play a role in their chemistry at high temperature. For the open shell MXenes, the spin density primarily located at the metal atoms showing one unpaired electron per Ti⁺, Zr⁺, and Hf⁺ magnetic center, consistent with s²d¹ configuration of the metal atom, and of ∼3.5 unpaired electrons per Cr⁺ magnetic center interpreted as a mixture of s²d³ and high-spin s¹d⁴ configuration. Finally, the analysis of the density of states reveals the metallic character of all these bare MXenes, irrespective of the nature of the ground state, with significant covalent contributions for Mo₂C and W₂C.