Photoemission study of chain- and layer-type TlS crystals

Thallium monosulphide is a mixed-valence compound containing monovalent and trivalent Tl ions. The compound crystallizes in two different structures, the tetragonal TlSe type and the monoclinic TlGaSe2 type. Photoemission studies were carried out, using a synchrotron photon source, on TIS crystals with both the TISe- and TlGaSe2-type structures. Comparison of the spectra shows that the line shape of the T1 5d core level is more asymmetric in the TlSe type than the TlGaSe2 type. The asymmetry reflects the density of states near the Fermi level of the crystals. It was also found that as the incident photon energy increases the intensities of the valence band peaks increase relative to those of the T1 5d core levels. Comparing the intensity ratios with the calculated photoionization cross section ratios, the characters of the valence bands were discussed.