Ab initio density functional studies of transition-metal sulphides: I. Crystal structure and cohesive properties
Abstract
The structural and cohesive properties of more than thirty transition-metal sulphides of various stoichiometries and crystal structures have been investigated using density functional theory, with the aim of establishing a correlation between the strength of the metal - sulphur bond and the catalytic activities of these materials. It is shown that the local density approximation has a tendency to overestimate the strength of the bonding. The overbinding manifests itself in the prediction of too small atomic volumes and too large cohesive energies. Non-local corrections to the local exchange - correlation functional in the form of a generalized-gradient approximation correct the overbinding (albeit with a certain tendency to overcorrect, especially for the sulphides of the heavy transition metals) and result in accurate structural prediction and cohesive energies. A correlation between the sulphur - metal bond strength and the catalytic activities is established.
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Ab initio density functional studies of transition-metal sulphides: II. Electronic structure
Abstract
A study of the electronic structure of about thirty transition-metal sulphides (TMS) of various stoichiometries and crystal structures is presented, supplementing recent studies of their structural and cohesive properties (P Raybaud, G Kresse, J Hafner and H Toulhoat, preceding paper). The electronic structure of the TMS is found to be determined by short-range interactions in the S 3p - TM d band complex, with the ligand-field splitting of the TM d states in the environment of the S atoms determining the structure of the d band. For the layered group VI disulphides, for 
and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for 
and 
. We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).
and for the group VIII pyrites this leads to the formation of a gap at the Fermi surface. Semiconducting properties are predicted also for the monosulphides PtS and PdS and for and . We show that the semiconducting TMS have a higher catalytic activity for hydro-desulphurization than the metallic sulphides. We suggest a correlation between the catalytic activity and the characters of the highest occupied states (the frontier orbitals).
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