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 Eu2+ → Eu3+ energy transfer is well known, in this study the energy transfer from Eu3+ to Eu2+ is reported for the first time. The predominant condition for Eu3+ → Eu2+ energy transfer is a Eu2+ 4f55d band at lower energy than the position of the Eu3+ 4f6[5D0] level, which is fulfilled in Eu‐doped CaO. X‐ray powder diffraction, Eu Mössbauer spectroscopy and optical absorption measurements are employed to determine the Eu3+ and Eu2+ concentrations in the prepared CaO:1at.%Eu samples. Synthesis in an H2/N2 atmosphere and addition of graphite powder as a reducing agent to the starting mixture are found to result in respective Eu3+ and Eu2+ concentrations of 0.6–0.7% and 0.3–0.4%. For this sample, the Eu3+ → Eu2+ energy transfer efficiency is estimated to be high (> 90%). This is explained by the high oscillator strength of the 4f7 → 4f65d excitation transition of the Eu2+ ion to which energy is transferred. As the Eu2+ 4f55d band lies below the Eu3+ 4f6[5D0] level, Eu3+ does not act as a killer center for the near‐infrared (NIR) Eu2+ emission at about 720 nm. Therefore, a full reduction of Eu3+ is not required to attain a high quantum efficiency. Implications of the demonstrated Eu3+ → Eu2+ energy transfer for application of long wavelength Eu2+ phosphors are discussed.