Photoemission study of the electronic structure ofCrCl3andRuCl3compounds

X-ray and ultraviolet photoelectron spectra of the valence band and core levels of ${\mathrm{CrCl}}_3$ and α-${\mathrm{RuCl}}_3$ crystals have been performed in order to study their electronic structure. It is found that the 3d and 4d states of the transition metal and the 3p states of the chlorine contribute to the valence-band structure. The valence band consists of overlapping Cr 3d and Cl 3p states in ${\mathrm{CrCl}}_3$, while a narrow Ru 4d band is separated by 2–2.5 eV from the top of the Cl 3p valence states in ${\mathrm{RuCl}}_3$. Photoemission, photoconductivity, and optical spectra indicate that in ${\mathrm{CrCl}}_3$ and ${\mathrm{RuCl}}_3$ the energy gap involves d-d Coulomb and exchange interactions on the transition-metal atom. On this basis, the main 3d- and 4d-emission structures are assigned to unscreened 3${\mathit{d}}^2$ and 4${\mathit{d}}^4$ hole states within the framework of the Mott-Hubbard model.