Photo-electronic properties and persistent luminescence in Pr3+ doped (Ca,Sr)TiO3 ceramics

We obtained direct evidence of intervalence charge transfer (IVCT) in Ca1−xSrxTiO3:Pr3+ (x=0, 0.3, 0.5, 0.8, and 1) by a photocurrent excitation (PCE) measurement, and proposed a mechanism of the persistent luminescence based on the results of the PCE and persistent luminescence measurements. The PCE spectra in all samples showed two bands in the UV range due to the band-to-band and IVCT transitions. With increasing Sr content, x, the host bandgap energy decreased monotonically from 3.59 to 3.23 eV, and the IVCT energy increased from 3.20 to 3.38 eV. PCE bands of the Pr3+:3H4→3PJ transitions in the blue region were observed only in the samples with x=0, 0.3, and 0.5 and the relative intensity decreased with increasing Sr content, x. Based on temperature dependence of the PCE results in the sample with x=0, a thermally assisted photoionization process was indicated under the 3H4→3PJ excitation. Red persistent luminescence was observed in the samples with x=0, 0.3 and 0.5, showing trap depth in the range of 0.48-0.35 eV, which was estimated from the thermoluminescence measurement. The afterglow process by the electron moving through the conduction band was evidenced by the similar quenching behavior of the 3H4→3PJ band both in the PCE and afterglow excitation spectra. The difference in the processes of persistent luminescence among the band-to-band, IVCT and 3H4→3PJ excitations was discussed.