The nature of the electronic ground state of M 2 C (M = Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W) MXenes.

A systematic computational study is presented aimed at accurately describing the electronic ground state nature and properties of M 2 C (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 Ti 2 C, Zr 2 C, Hf 2 C, and Cr 2 C MXenes exhibit an open-shell conducting ground state with localized spins on the metal atoms, while V 2 C, Nb 2 C, Mo 2 C, Ta 2 C, and W 2 C MXenes exhibit a diamagnetic conducting ground state. For Ti 2 C, Zr 2 C, Hf 2 C, and Cr 2 C, 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 V 2 C and Mo 2 C 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 V 2 C, Nb 2 C, and Ta 2 C is consistent with metal atoms exhibiting a closed-shell s 2 d 2 configuration whereas for Mo 2 C, and W 2 C is consistent with a low-spin s 1 d 4 configuration although the FM solution is close in energy for V 2 C and Mo 2 C 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 s 2 d 1 configuration of the metal atom, and of ∼3.5 unpaired electrons per Cr + magnetic center interpreted as a mixture of s 2 d 3 and high-spin s 1 d 4 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 Mo 2 C and W 2 C.