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

Abstract

Abstract Photoluminescence (PL) features arising from intraconfigurational 4f – 4f transitions and also O2− → Ln3+ charge transfer (CT) where Ln3+ = Yb3+, Sm3+ or Eu3+ are observed for Ln-doped NaMgF3 with oxygen impurities. The peak CT stimulation energies are 4.54 eV, >5.1 eV, and 3.65 eV for Yb3+, Sm3+ and Eu3+, respectively. Exciting into the CT bands produces intense 4f – 4f emissions characteristic of the Ln3+ dopant in all cases. The CT peak excitation energies are used to construct a host-referred binding energy (HRBE) diagram for all Ln3+ and Ln2+ dopants, which can then be referenced to understand the luminescence and trapping properties of Ln3+ and Ln2+ ions in all NaMgF3:Ln compounds. We demonstrate that the HRBE accurately describes the electron trapping in NaMgF3:Yb where Yb3+ + e− → Yb2+ 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 Ln2+ dopants are well described within the HRBE diagram. For NaMgF3:Eu, two unique Eu3+ sites are observed where one is highly distorted and is primarily excited by CT, likely due to an adjacent O2− impurity. For the distorted site, the peak intensity of the 5D0 → 7F0 emission is approximately 30 times greater than that of the 5D0 → 7F1 emission. Ultimately, the incorporation of oxygen impurities into the NaMgF3 lattice is shown to enhance Ln3+ luminescence, and the constructed HRBE diagram will be useful for future studies of the compound.