Luminescence and Covalency in Ytterbium-Doped CrX₃ (X = Cl, Br, I) van der Waals Compounds

The layered 2D van der Waals ferromagnets CrX₃ (X = Cl, Br, I) show broad d-d photoluminescence (PL). Here we report preparation, structural characterization, and spectroscopic studies of all three CrX₃ compounds doped with the optical impurity, Yb³⁺. EXAFS measurements show very similar Cr K-edge and Yb L-edge data for each doped compound, and good fits of the latter are obtained for structures having Yb³⁺ occupying substitutional octahedral sites. Yb-X bond lengths are systematically ∼0.25 Å larger than their Cr-X counterparts. 4 K PL measurements show efficient sensitization of Yb³⁺ luminescence upon photoexcitation into lattice absorption bands [Cr³⁺ d-d and ligand-to-metal charge-transfer (LMCT)] for all three compounds, converting their nondescript broadband d-d PL into sharp f-f emission. The PL of CrCl₃:Yb³⁺ and CrBr₃:Yb³⁺ occurs 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 of only 8 μs 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⁺ f orbitals in CrI₃:Yb³⁺ stemming primarily from the shallow valence-shell ionization potentials of the iodide anions.