Identification and Quantification of Charge Transfer in CaAl₂O₄:Eu²⁺,Nd³⁺ 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 CaAl₂O₄:Eu²⁺,Nd³⁺. Based on the observation that both the trapping and de-trapping processes in CaAl₂O₄:Eu²⁺,Nd³⁺ are thermally activated, forward charge transfer from Eu²⁺ to Nd³⁺ and backward charge transfer from Nd²⁺ to Eu³⁺ are identified. The percentages of Eu²⁺ and Nd³⁺ 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 Nd³⁺ 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.