Tuning the electronic structure of the trichloride honeycomb lattice by transition metal substitution

Transition metal trichlorides show peculiar and versatile magnetic properties. Whereas ${\mathrm{CrCl}}_{3}$ is a layered antiferromagnet with potential applications as an ultrathin two-dimensional magnet, $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ may host a spin-liquid state driven by Kitaev interactions. The interest to control their material properties by chemical modifications is immense, both from an application related and from a fundamental point of view. Here, by studying ${\mathrm{CrCl}}_{3}$, ${\mathrm{Cr}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$, and $\ensuremath{\alpha}\text{\ensuremath{-}}{\mathrm{RuCl}}_{3}$ by photoemission and electron energy-loss spectroscopy, we find that transition metal substitution changes the optical properties of the host without compromising its underlying electronic structure. It does so by a Cr--Ru related charge transfer process across the Mott gap effectively opening up a new absorption channel below the principal gap edge of ${\mathrm{CrCl}}_{3}$. The Cr and Ru valencies as well as the respective valence band density of states remain stable for the mixed ${\mathrm{Cr}}_{0.5}{\mathrm{Ru}}_{0.5}{\mathrm{Cl}}_{3}$ compound. Our study underlines the potential of transition metal substitution as a means of material engineering of trichlorides.