Preparation, crystal structures, experimental and theoretical electronic band structures of cobalt tellurides in the composition range CoTe1.3-CoTe2

摘要

Cobalt tellurides in the composition range CoTe1.3-CoTe2 crystallize in a CdI2-type structure with short intra-and interslab Te-Te contacts indicating a polymeric network with multiple Te-Te bonds explaining the very low c/a values of 1.38 to 1.41 of the hexagonal cells. Single-crystal x-ray investigations performed on CoTe2 confirm the marcasite-type structure in the centrosymmetric space group Pnnm. Experimental valence band spectra (UPS) confirm that the Co tellurides in the composition range CoTex (1.3 < x < 2) are metals. The emission at the Fermi level E-F decreases with the Te content and is due to Co 3d and Te 5p states. This assignment is supported by the results of the calculated density of states curve (DOS) which demonstrates that Te p states contribute about 50% in the CdI2-type and about 35% in the marcasite-type structure. The Te d states contribute about 15% to the total Te contributions. This behaviour cannot be understood on the basis of a simple tight-binding description, ignoring d-valence states of Te. Core level spectra (XPS) suggest that all CoTex samples are best described as intermetallic compounds. Small chemical shifts between the different samples are mainly due to the different Madelung contributions rather than to changes of the electron density located on the Co atoms. An oxidation number for Te < -1 in all CoTex samples is deduced, in good agreement with the value of about -1.3 for the Te in CoTe2 that would be deduced from the relation between the Te-Te distances versus oxidation states of the anion in (Te-2)(-11), Te-11. The three-dimensional character of the Co tellurides deduced from the crystal structure is further confirmed by the calculated energy dispersion E(k). [References: 51]