Berlin 2008 – scientific programme
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MM: Fachverband Metall- und Materialphysik
MM 23: Poster session
MM 23.9: Poster
Tuesday, February 26, 2008, 14:45–18:00, Poster B
The importance of cluster-distortions in the tetrahedral cluster compounds: Ab initio investigations — Martin Sieberer, Stefan Turnovszky, Peter Mohn, and •Josef Redinger — Center for Computational Materials Science, Vienna University of Technology, Vienna, Austria
We study the electronic and structural properties of selected representatives of the so-called Molybdenum cluster compounds such as GaM4X8 with M=Mo as a group VIB element and V, Nb, 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 distortions, namely those happening only within the M4 unit upon a fixed angle φ = 60o 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 within powder x-ray diffraction. However, they strongly influence the band-structure by opening up a gap at the Fermi-energy, which puts into question whether all compounds of this family are really called Mott insulators as stated elsewhere. In particular ferromagnetic GaMo4S8 and GaV4S8 are well described within DFT. Only the Nb and Ta-based representatives require a large effort due to the lack of magnetic long range order caused by frustrated AF M-M interactions.