Demonstration of Eu³⁺ → Eu²⁺ Energy Transfer in NIR Emitting CaO:Eu²⁺,Eu³⁺ LED Phosphor and its Implication for the Role of Eu³⁺ as a Killer Center for Long Wavelength Eu²⁺ Emission

Abstract While Eu 2+ → Eu 3+ energy transfer is well known, in this study the energy transfer from Eu 3+ to Eu 2+ is reported for the first time. The predominant condition for Eu 3+ → Eu 2+ energy transfer is a Eu 2+ 4f 5 5d band at lower energy than the position of the Eu 3+ 4f 6 [ 5 D 0 ] level, which is fulfilled in Eu‐doped CaO. X‐ray powder diffraction, Eu Mössbauer spectroscopy and optical absorption measurements are employed to determine the Eu 3+ and Eu 2+ concentrations in the prepared CaO:1at.%Eu samples. Synthesis in an H 2 /N 2 atmosphere and addition of graphite powder as a reducing agent to the starting mixture are found to result in respective Eu 3+ and Eu 2+ concentrations of 0.6–0.7% and 0.3–0.4%. For this sample, the Eu 3+ → Eu 2+ energy transfer efficiency is estimated to be high (> 90%). This is explained by the high oscillator strength of the 4f 7 → 4f 6 5d excitation transition of the Eu 2+ ion to which energy is transferred. As the Eu 2+ 4f 5 5d band lies below the Eu 3+ 4f 6 [ 5 D 0 ] level, Eu 3+ does not act as a killer center for the near‐infrared (NIR) Eu 2+ emission at about 720 nm. Therefore, a full reduction of Eu 3+ is not required to attain a high quantum efficiency. Implications of the demonstrated Eu 3+ → Eu 2+ energy transfer for application of long wavelength Eu 2+ phosphors are discussed.