Density functional theory analysis of the local chemical bonds in the periodic tantalum dichalcogenides TaX2 (X=S, Se, Te)

The electronic structure of layered tantalum dichalcogenides 1T-TaX2 (X=S, Se, Te) have been studied both with the linear muffin tin orbitals-atomic sphere approximation (LMTO-ASA) and the Amsterdam density functional for band (ADF-band) programs. The first code (LMTO) provides band structures, density of states (DOS), and crystal orbitals Hamiltonian populations (COHP) while the second one allows accurate atomic charge calculations by means of a powerful electron density numerical integration. All those analyses were used to rationalize the electronic structures of the three 1T-TaX2 phases, in particular to enlighten the root 13 x root 13 structural modulations observed in TaS2 and TaSe2, and to put forward the influence of the local chemical Ta-Te bonds on the relative stability of the 1T-TaTe2 phase vs the distorted monoclinical one. The indirect overlap between the two bands responsible for the metallic properties of TaS2 and TaSe2 has been shown to significantly increase the tantalum d electron count compared to its formal value (d(1)) leading to a more realistic occupation of the threefolded t(2g)-like bands involved in the root 13 x root 13 instability. Owing to the low electronegative character of Te compared to S and Se, the direct overlap occurring at the Fermi level results in an electron transfer from local Ta-Te bonding states to local Ta-Te antibonding ones yielding a destabilization of the metal-chalcogen bonds. (C) 2000 American Institute of Physics. [S0021- 9606(00)32237-1].