Pressure-induced variation of persistent luminescence characteristics in Y₃Al_5-xGa_xO₁₂: Ce³⁺-M³⁺(M = Yb, and Cr) phosphors: opposite trend of trap depth for 4f and 3d metal ions

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 Y3Al5-xGaxO12:Ce(3+)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 Cr(3+)or Yb(3+)codoping into the Y3Al5-xGaxO12:Ce(3+)phosphors. The different trap depths formed by Cr(3+)and Yb(3+)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+-Yb(3+)phosphor, the slope of persistent luminescence decay curve becomes more gentle with increasing pressure, while for the Y3AlGa4O12:Ce3+-Cr(3+)phosphor the slope becomes steeper. These results indicate that the trap depth of Yb(3+)becomes deeper and that of Cr(3+)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+(4f(14)) or Cr2+(3d(4)) levels are discussed. The CB bottom energy is increased relative to the ground(1)S(0)state of Yb(2+)with increasing pressure, which results in deepening of the electron trap depth of the Yb(2+)state. The opposite tendency of the Cr(3+)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 Cr(2+)ions take the high spin state (t32ge1g) rather than the low-spin state (t42g).