Luminescence and Covalency in Yb³⁺-Doped CrX₃ (X = Cl, Br, I) Van der Waals Compounds

The layered 2D van der Waals ferromagnets CrX₃ (X = Cl, Br, I) are some of the most promising and well-studied materials for spintronics and related quantum systems. Despite potential applications in quantum sensing and as single-photon sources, investigations on the optical properties of CrX₃, characterized by broad <i>d-d</i> photoluminescence (PL), are severely lacking. The incorporation of carefully designed atomic defects such as lanthanides opens up a pathway to modulate the overall electronic and magneto-optical properties. Here we report preparation, structural characterization, and spectroscopic studies of all three CrX₃ compounds doped with the optical impurity Yb³⁺. 4 K PL measurements show efficient sensitization of Yb³⁺ luminescence upon photoexcitation into lattice absorption bands for all three compounds, converting their nondescript broadband <i>d-d</i> PL into sharp <i>f-f</i> emission. The PL of CrCl₃:Yb³⁺ and CrBr₃:Yb³⁺ occur at energies typical for [YbX₆]^3- with these halides, with PL decay times of 0.5 – 1.0 ms at 4 K, but CrI₃:Yb³⁺ displays anomalously low-energy Yb³⁺ emission and an unusually short PL decay time at 4 K. Data analysis and angular overlap model (AOM) calculations show that Yb³⁺ in CrI₃:Yb³⁺ has a lower spin-orbit splitting energy than reported for any other Yb³⁺ in any other compound. We attribute these observations to exceptionally high covalency of the Yb³⁺ <i>f</i> orbitals in CrI₃:Yb³⁺ stemming primarily from the shallow valence-shell ionization potentials of the iodide anions. These results provide rare fundamental insights into the electronic structure and luminescence of the remarkably underexplored [YbI₆]^3- motif.