Large ternary-metal contributions to the wave functions at the Fermi level inTlMo6Se8

Abstract

Previous crystallographic studies of Chevrel phases containing the ternary metals thallium or indium suggested that these metals were of ‘‘mixed valence.’’ Here, we conclusively show that the Tl s band lies at the Fermi level, since large Knight shifts (∼2.4%) of the ${}_{}{}^{205}{}_{}{}^{}\mathrm{Tl}$ NMR spectra in ${\mathrm{TlMo}}_6$ ${\mathrm{Se}}_8$ are observed. This observation may help explain the high superconducting critical temperature of ${\mathrm{TlMo}}_6$ ${\mathrm{Se}}_8$ (12.2 K) and the unique rhombohedral-to-rhombohedral structural phase transitions observed in ${\mathrm{InMo}}_6$ ${\mathrm{S}}_{8\mathrm{-}\mathrm{x}}$ ${\mathrm{Se}}_{\mathrm{x}}$ compounds.