Near-normal-incidence vacuum ultraviolet reflectivity spectra, in the photon energy range 3.7 to 14 eV (in some cases the range is extended to 30 eV) have been obtained from basal faces of freshly cleaved single crystals of the layered trisulphides, triselenides, and tritellurides of titanium, zirconium, and hafnium (with the exception of the hitherto unreported Ti${\mathrm{Se}}_3$ and Ti${\mathrm{Te}}_3$). The spectra are interpreted using physical and chemical arguments as well as simple molecular-orbital ideas. Chemical trends in the series S→Se→Te are manifest in the presented spectra. Using the information inferred from these spectra as well as the information on these materials available in the literature to date, band-structure models for these Zr${\mathrm{Se}}_3$-type compounds have been proposed and are presented in this paper. Unlike their sister compounds (the Zr${\mathrm{Se}}_2$-type layer compounds), the Zr${\mathrm{Se}}_3$-type materials are envisaged as having predominantly dimerized main valence and conduction bands.

• Received 14 March 1980

DOI:https://doi.org/10.1103/PhysRevB.22.2078