PREPARATION, CRYSTAL-STRUCTURES, EXPERIMENTAL AND THEORETICAL ELECTRONIC BAND STRUCTURES OF COBALT TELLURIDES IN THE COMPOSITION RANGE COTE1.3-COTE2

Cobalt tellurides in the composition range CoTe1.3-CoTe2 crystallize i n a CdI2-type structure with short intra-and interslab Te-Te contacts indicating a polymeric network with multiple Te-Te bonds explaining th e very low c/a values of 1.38 to 1.41 of the hexagonal cells. Single-c rystal x-ray investigations performed on CoTe2 confirm the marcasite-t ype structure in the centrosymmetric space group Pnnm. Experimental va lence band spectra (UPS) confirm that the Co tellurides in the composi tion range CoTex (1.3 < x < 2) are metals. The emission at the Fermi l evel E-F decreases with the Te content and is due to Co 3d and Te 5p s tates. This assignment is supported by the results of the calculated d ensity of states curve (DOS) which demonstrates that Te p states contr ibute about 50% in the CdI2-type and about 35% in the marcasite-type s tructure. The Te d states contribute about 15% to the total Te contrib utions. This behaviour cannot be understood on the basis of a simple t ight-binding description, ignoring d-valence states of Te. Core level spectra (XPS) suggest that all CoTex samples are best described as int ermetallic compounds. Small chemical shifts between the different samp les are mainly due to the different Madelung contributions rather than to changes of the electron density located on the Co atoms. An oxidat ion number for Te < -1 in all CoTex samples is deduced, in good agreem ent with the value of about -1.3 for the Te in CoTe2 that would be ded uced from the relation between the Te-Te distances versus oxidation st ates of the anion in (Te-2)(-11), Te-11. The three-dimensional charact er of the Co tellurides deduced from the crystal structure is further confirmed by the calculated energy dispersion E(k).