Electronic properties of 3d transition metal dihalide monolayers predicted by DFT methods: Is there a pattern or are the results random?

We use periodic DFT calculations at LDA and PBE level to investigate 3d transition metal dihalide (TMDH) monolayers in H- and T-phase. By analyzing the phonon dispersion, we have obtained a rough overview which combinations may form stable structures. We have focused on identifying and explaining trends in the predicted electronic properties. Although their geometric structures are simple, the associated electronic and magnetic properties are not as easy to understand. At first glance, it seems that there is no clear trend, as even isovalence-electronic TMDH monolayers formed from the same metal but different halides can feature different magnetic moments. The identification of potential trends is further complicated by the fact that for a significant number of species, LDA results and PBE results predict different ground-state electronic structures. By rigorously analyzing the potential energy surfaces associated with different magnetic moments, we could show that the apparent inconsistencies can be easily understood as a result of the differences in the relative energy between electronic states of different magnetic moments. We further show that the trends in the band gaps can be easily rationalized by an electron counting rule based on simple symmetry arguments.

Automated summary

We use periodic DFT calculations to study 3d transition metal dihalide monolayers in H- and T-phase. By analyzing the phonon dispersion, we identify possible stable structures and explain trends in the predicted electronic properties. Despite their simple geometric structures, the associated electronic and magnetic properties are not easily understood due to inconsistent results and differences in relative energy between electronic states.