Identification and Quantification of Charge Transfer in CaAl 2 O 4 :Eu 2+ ,Nd 3+ Persistent Phosphor

Reversible charge transfer between lanthanide ions is identified as storage mechanism for the current workhorse persistent phosphors. Present evidence relies on sophisticated X‐ray absorption spectroscopy to detect the reversible charge transfer. Here, simple optical spectroscopy is used to study the charge transfer in one of the benchmark persistent phosphors CaAl 2 O 4 :Eu 2+ ,Nd 3+ . Based on the observation that both the trapping and de‐trapping processes in CaAl 2 O 4 :Eu 2+ ,Nd 3+ are thermally activated, forward charge transfer from Eu 2+ to Nd 3+ and backward charge transfer from Nd 2+ to Eu 3+ are identified. The percentages of Eu 2+ and Nd 3+ involved in the charge transfer are >10%, which exceeds the previous estimates in other persistent phosphors. Furthermore, this strategy offers additional advantage of site selectivity, which enables the identification of the distinct contributions of different Nd 3+ sites to the charge transfer. These findings underline the significance of reversible charge transfer in persistent phosphors and move towards a complete understanding of persistent luminescence mechanism.