Persistent luminescence — Quo vadis?

The present status of the persistent luminescence mechanisms is reviewed and the remaining unsolved details are discussed. These details include the identification and role of defects in the Eu²⁺-doped and R³⁺ co-doped alkaline earth aluminates (MAl₂O₄) and disilicates (M₂MgSi₂O₇; M:Ca, Sr, Ba) which can be partly resolved by the thermoluminescence (TL) measurements. The use of the synchrotron radiation - presently only sparsely used in the studies of persistent luminescence - is introduced: the oxidation state of the presumed R²⁺/R³⁺/R^IV species occurring in the persistent luminescence materials during the luminescence processes were examined with synchrotron radiation XANES (and EXAFS) methods. The band gap energies (E_g), the defect-related luminescence as well as the 4f⁷→4f⁶5d¹ transition energies were derived from the synchrotron radiation excitation spectra of the materials. Subsequently, the early steps of the density functional theory (DFT) calculations involving the solution of the persistent luminescence mechanisms (band gap energies, position of the Eu²⁺ levels) are discussed. Some remaining challenges are eventually highlighted.