Importance of cluster distortions in the tetrahedral cluster compounds GaM4X8 (M=Mo,V,Nb,Ta; X=S,Se): Ab initio investigations

In this paper, we study the structural properties of selected representatives of the so-called molybdenum cluster compounds. Belonging to this family are the GaM4X8 compounds with M=Mo as a group VIB element and V, Nb, or Ta as a group VB element. X denotes either S or Se. These compounds are known to exhibit semiconducting behavior in the electrical resistivity, caused by hopping of electrons between well-separated metal clusters. The large separation of the tetrahedral metal (M4) clusters is believed to be the origin of strong correlations. We show that recent calculations neglected an important type of structural distortion, namely, those happening only within the M4 unit at a fixed angle α=60 deg. of the trigonal (fcc-like) cell. These internal distortions gain a significant amount of energy compared to the cubic cell and they are--to our knowledge--almost undetectable with powder x-ray diffraction experiments. However, they strongly influence the band-structure by opening up a gap at the Fermi-energy. This, however, puts into question whether all compounds of this family are really Mott insulators as stated elsewhere. In particular, ferromagnetic GaMo4S8 and GaV4S8 are well described within density functional theory. Only the Nb- and Ta-based representatives require a large effort due to the lack of magnetic long-range order caused by frustrated antiferromagnetic M-M interactions