Luminescence and Covalency in Ytterbium-Doped CrX<sub>3</sub>(X = Cl, Br, I) van der Waals Compounds
Thom J. Snoeren, Kimo Pressler, Kyle T. Kluherz, Kelly M. Walsh, James J. De Yoreo, Daniel R. Gamelin
Abstract
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.