Luminescence and Covalency in Ytterbium-Doped CrX 3 (X = Cl, Br, I) van der Waals Compounds.

The layered 2D van der Waals ferromagnets CrX ₃ (X = Cl, Br, I) show broad d-d photoluminescence (PL). Here we report preparation, structural characterization, and spectroscopic studies of all three CrX ₃ compounds doped with the optical impurity, Yb ³⁺ . 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 Yb ³⁺ 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 Yb ³⁺ luminescence upon photoexcitation into lattice absorption bands [Cr ³⁺ 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 CrCl ₃ :Yb ³⁺ and CrBr ₃ :Yb ³⁺ occurs at energies typical for [YbX ₆ ] ^3- with these halides, with PL decay times of 0.5-1.0 ms at 4 K, but CrI ₃ :Yb ³⁺ displays anomalously low-energy Yb ³⁺ 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 Yb ³⁺ in CrI ₃ :Yb ³⁺ has a lower spin-orbit splitting energy than reported for any other Yb ³⁺ in any other compound. We attribute these observations to exceptionally high covalency of the Yb ³⁺ f orbitals in CrI ₃ :Yb ³⁺ stemming primarily from the shallow valence-shell ionization potentials of the iodide anions.