Computational screening of MOX2 transition metal oxydihalides with M = V, Nb, Ta, Mo, Ru, or Os, and X = Cl, Br, or I

Abstract

Several members of the van der Waals (vdW) layered family of transition metal oxydihalides MOX₂ have been shown to exhibit lattice distortions, leading to interesting electronic properties such as colossal magnetoresistance, ferroelectricity and strong second harmonic generation. Two of these structures have been reported as exfoliated into two-dimensional (2D) materials, out of which one, NbOCl₂, has been shown to have almost layer-independent electronic properties. We here report a computational study of the MOX₂ phases within the compositional space M = V, Nb, Ta, Mo, Ru, or Os, and X = Cl, Br, or I, chosen to cover all experimentally reported three-dimensional (3D) structures within the family. The thermodynamical stability with respect to competing phases and the dynamical stability are considered, and the electronic bandstructure and density of states are evaluated for each MOX₂ structure in both 3D and 2D form. Trends are identified between structures with the same type of distortions. For MOX₂ with M = V, Nb or Ta, a Peierls distortion opens up a wide bandgap at the Fermi level. MoOX₂ also display a Peierls distortion, and while the increased valence renders it conductive, the characteristic features of the bandstructure are still similar to the semiconducting phases. For M = Ru and Os, which are also found conducting, the Peierls distortion is not observed, and the bandstructure and DOS are significantly different from the other considered phases. All 3D phases are shown to be of vdW nature, with very similar properties between their 3D and 2D versions. Analysis of binding energies as well as delamination energies suggest that the investigated 3D phases can all be converted into 2D.