Oxygen-impurity charge transfer in NaMgF3:Ln (Ln = Yb, Sm, or Eu): Establishing the lanthanide energy levels in NaMgF3

Abstract Photoluminescence (PL) features arising from intraconfigurational 4f – 4f transitions and also O 2− → Ln 3+ charge transfer (CT) where Ln 3+  = Yb 3+ , Sm 3+ or Eu 3+ are observed for Ln-doped NaMgF 3 with oxygen impurities. The peak CT stimulation energies are 4.54 eV, >5.1 eV, and 3.65 eV for Yb 3+ , Sm 3+ and Eu 3+ , respectively. Exciting into the CT bands produces intense 4f – 4f emissions characteristic of the Ln 3+ dopant in all cases. The CT peak excitation energies are used to construct a host-referred binding energy (HRBE) diagram for all Ln 3+ and Ln 2+ dopants, which can then be referenced to understand the luminescence and trapping properties of Ln 3+ and Ln 2+ ions in all NaMgF 3 :Ln compounds. We demonstrate that the HRBE accurately describes the electron trapping in NaMgF 3 :Yb where Yb 3+  + e − → Yb 2+ after exposure to X-ray irradiation that is stable under UV exposure to stimulation energies up to at least 5 eV. Furthermore, the first 5d levels of the studied Ln 2+ dopants are well described within the HRBE diagram. For NaMgF 3 :Eu, two unique Eu 3+ sites are observed where one is highly distorted and is primarily excited by CT, likely due to an adjacent O 2− impurity. For the distorted site, the peak intensity of the 5 D 0 → 7 F 0 emission is approximately 30 times greater than that of the 5 D 0 → 7 F 1 emission. Ultimately, the incorporation of oxygen impurities into the NaMgF 3 lattice is shown to enhance Ln 3+ luminescence, and the constructed HRBE diagram will be useful for future studies of the compound.