Pressure-induced variation of persistent luminescence characteristics in Y3Al5−xGaxO12:Ce3+–M3+ (M = Yb, and Cr) phosphors: opposite trend of trap depth for 4f and 3d metal ions

Abstract

Changing the electronic structure of materials by pressure and the accompanying changes in optical properties have attracted scientific interest. We have reported that the energy position of the conduction band (CB) bottom and the crystal field splitting of the Ce³⁺:5d excited level in Y₃Al_5−xGa_xO₁₂:Ce³⁺ are changed by applying pressure, which results in the red shifting of the Ce³⁺:5d → 4f luminescence and the increase of the quenching temperature. We also reported dramatic improvement of the persistent luminescence performance by either Cr³⁺ or Yb³⁺ codoping into the Y₃Al_5−xGa_xO₁₂:Ce³⁺ phosphors. The different trap depths formed by Cr³⁺ and Yb³⁺ affect the initial persistent luminescence intensity and the persistent luminescence duration. In this study, the effect of pressure on the persistent luminescence performance was investigated. For the Y₃AlGa₄O₁₂:Ce³⁺–Yb³⁺ phosphor, the slope of persistent luminescence decay curve becomes more gentle with increasing pressure, while for the Y₃AlGa₄O₁₂:Ce³⁺–Cr³⁺ phosphor the slope becomes steeper. These results indicate that the trap depth of Yb³⁺ becomes deeper and that of Cr³⁺ becomes shallower with increasing pressure. Based on the pressure-dependence of the luminescence quenching and the trap depth change estimated from the decay slopes, the relative electronic energies of the CB bottom and the Yb²⁺ (4f¹⁴) or Cr²⁺ (3d⁴) levels are discussed. The CB bottom energy is increased relative to the ground ¹S₀ state of Yb²⁺ with increasing pressure, which results in deepening of the electron trap depth of the Yb²⁺ state. The opposite tendency of the Cr³⁺ codoped sample was described by a decreasing tendency of the energy gap between the CB bottom and the Cr²⁺:e_g level, the relative energy level of which is increased by the increase of the crystal field with increasing pressure in the garnet host material, where the electron-trapping Cr²⁺ ions take the high spin state (t³_2ge¹_g) rather than the low-spin state (t⁴_2g).