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

The layered 2D van der Waals ferromagnetsCrX(3) (X =Cl, Br, I) show broad d-d photoluminescence (PL). Here we reportpreparation, structural characterization, and spectroscopic studiesof all three CrX3 compounds doped with the optical impurity,Yb3+. EXAFS measurements show very similar Cr K-edge andYb L-edge data for each doped compound, and good fits of the latterare obtained for structures having Yb3+ occupying substitutionaloctahedral sites. Yb-X bond lengths are systematically & SIM;0.25 & ANGS; larger than their Cr-X counterparts. 4 K PL measurementsshow efficient sensitization of Yb3+ luminescence uponphotoexcitation into lattice absorption bands [Cr3+ d-dand ligand-to-metal charge-transfer (LMCT)] for all three compounds,converting their nondescript broadband d-d PL into sharp f-femission. The PL of CrCl3:Yb3+ and CrBr3:Yb3+ occurs at energies typical for [YbX6](3-) with these halides, with PL decay times of0.5-1.0 ms at 4 K, but CrI3:Yb3+ displaysanomalously low-energy Yb3+ emission and an unusually shortPL decay time of only 8 & mu;s at 4 K. Data analysis and angularoverlap model (AOM) calculations show that Yb3+ in CrI3:Yb3+ has a lower spin-orbit splitting energythan reported for any other Yb3+ in any other compound.We attribute these observations to exceptionally high covalency ofthe Yb3+ f orbitals in CrI3:Yb3+ stemmingprimarily from the shallow valence-shell ionization potentials ofthe iodide anions.