Insight into the electron transfer and anti-thermal quenching of europium doped Li4SrCa(SiO4)2

Abstract

The understanding of lanthanide-related electron transfer processes is crucial for designing efficient and functional luminescent phosphors. However, the current knowledge on those processes between Eu ions with different oxidation states is rather scarce, which limits the progress of developing new efficient phosphors with good stability. In this work, we report on Eu²⁺-doped Li₄SrCa(SiO₄)₂ and its solid solutions, in which optically-induced reversible electron transfer between Eu²⁺ and Eu³⁺ is demonstrated under illumination at different wavelengths. It is confirmed that the reversible electron transfer between Eu²⁺ and Eu³⁺ in inequivalent sites can be initiated by exciting the corresponding Eu²⁺. Additionally, it is shown that the strong increase in orange emission intensity of Eu_Ca²⁺ with temperature can be attributed to the thermally-assisted energy transfer from Eu_Sr²⁺ to Eu_Ca²⁺ and an increase of Eu_Ca²⁺ centers due to photoinduced electron transfer. This phosphor was previously reported as a potential candidate for luminescence thermometry but, as shown here, the temperature-dependent, reversible electron transfer processes in combination with temperature-dependent energy transfer between Eu²⁺ ions on two different crystallographic sites would strongly compromise any attempt of measuring temperatures with this phosphor. The results of this work advance our understanding of electron transfer between Eu²⁺ and Eu³⁺ in general, which is very relevant for other europium doped phosphors.