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

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