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 s phere approximation (LMTO-ASA) and the Amsterdam density functional for ban d (ADF-band) programs. The first code (LMTO) provides band structures, dens ity of states (DOS), and crystal orbitals Hamiltonian populations (COHP) wh ile the second one allows accurate atomic charge calculations by means of a powerful electron density numerical integration. All those analyses were u sed to rationalize the electronic structures of the three 1T-TaX2 phases, i n particular to enlighten the root 13 x root 13 structural modulations obse rved in TaS2 and TaSe2, and to put forward the influence of the local chemi cal Ta-Te bonds on the relative stability of the 1T-TaTe2 phase vs the dist orted monoclinical one. The indirect overlap between the two bands responsi ble for the metallic properties of TaS2 and TaSe2 has been shown to signifi cantly 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 elec tronegative character of Te compared to S and Se, the direct overlap occurr ing at the Fermi level results in an electron transfer from local Ta-Te bon ding states to local Ta-Te antibonding ones yielding a destabilization of t he metal-chalcogen bonds. (C) 2000 American Institute of Physics. [S0021- 9 606(00)32237-1].