Electronic structure of the Chevrel-phase compounds SnxMo6Se7.5: Photoemission spectroscopy and band-structure calculations

We have studied the electronic structure of two Chevrel-phase compounds, ${\mathrm{Mo}}_{6}{\mathrm{Se}}_{7.5}$ and ${\mathrm{Sn}}_{1.2}{\mathrm{Mo}}_{6}{\mathrm{Se}}_{7.5},$ by combining photoemission spectroscopy and band-structure calculations. Core-level spectra taken with x-ray photoemission spectroscopy show systematic core-level shifts, which do not obey a simple rigid-band model. The inverse photoemission spectra imply the existence of an energy gap located \ensuremath{\sim}1 eV above the Fermi level, which is a characteristic feature of the electronic structure of the Chevrel compounds. Quantitative comparisons between the photoemission spectra and the band-structure calculations have been made. While good agreement between theory and experiment in a wide energy range was obtained as already reported in previous studies, we found that the high density of states near the Fermi level predicted theoretically due to the Van Hove singularity is considerably reduced in the experimental spectra taken with higher energy resolution than in previous reports. Possible explanations for this observation are proposed.