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

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-Ca(2+) with temperature can be attributed to the thermally-assisted energy transfer from Eu-Sr(2+) to Eu-Ca(2+) and an increase of Eu-Ca(2+) 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.