Structure, long persistent luminescent properties and mechanism of a novel efficient red emitting Ca2Ga2GeO7:Pr3+ phosphor

A novel red emitting long persistent luminescent phosphor Ca2Ga2GeO7:Pr3+ (2217) was developed by solid state method at 1300 °C in air. The refined crystal structure of Ca2Ga2GeO7 host was solved. The intense red long persistent luminescence originates from the f-f transitions of Pr3+ centers at highly disordered Ca2+ sites in CaO8 polyhedrons. It can be properly recorded for 1.5 h by the definition of 0.32 mcd/m2 and is actually visible for more than 5 h by dark-adapted vision in darkness. At least one type of intrinsic lattice defects of the Ca2Ga2GeO7 host as proper shallow traps exists in Ca2Ga2GeO7:Pr3+ material and the depth of the traps is calculated to be 0.59 eV, which is very suitable for long persistent luminescence. It reveals that the Pr3+ dopants not only act as emitters but also increase the density of these significant intrinsic traps, resulting in the efficient persistent luminescence of Pr3+. The retrapping and tunneling effects are demonstrated to be responsible for the nonexponential and much slower decay behaviors in the late slow-decay stage (t > 300 s). A new parameter of threshold depth is firstly defined for the evaluation of the retrapping and tunneling effects and it is calculated to be 0.76 eV in this case by a classic multi-peak fitting method based on general-order kinetics. Finally, the long persistent luminescence mechanism of this Ca2Ga2GeO7:Pr3+ phosphor is proposed according to these investigation results.