Oxygen influence on luminescence properties of rare-earth doped NaLaF₄

Luminescence properties of erbium and europium doped NaLaF₄ with different oxygen content have been studied. Vacuum ultraviolet (VUV) excitation luminescence spectroscopy technique has been applied by using synchrotron radiation excitation. It was found that oxygen impurity leads to significant degradation of Er³⁺ or Eu³⁺ emission under VUV excitation. The intensive O²⁻–Er³⁺ charge transfer excitation band has been detected from oxygen abundant NaLaF₄ in the 150–165 nm spectral range. This band reveals a competing absorption mechanism in oxygen containing NaLaF₄. It is clearly demonstrated that one reason for the Er³⁺ emission degradation in oxygen abundant NaLaF₄ is strong suppression of 4f–5d transitions in Er³⁺ ion. The degradation of the Eu³⁺ emission under VUV excitation was explained by diminishing of F⁻–Eu³⁺ charge transfer absorption band as well as by competing relaxation centers in the oxygen abundant NaLaF₄.