Delocalization, thermal ionization, and energy transfer in singly doped and codoped CaAl4O7 and Y2O3 -: art. no. 235113

Ce3+ and Tb3+ singly doped and codoped CaAl4O7 and Y2O3 single crystal fibers were grown using the laser heated pedestal growth method. Energy transfer from Ce3+ to Tb3+ was studied in these samples. No energy transfer between Ce3+ and Tb3+ was observed in codoped Y2O3. This is because electrons excited into the Ce3+ 5d states delocalize into the conduction band at a rate that is faster than the interionic energy transfer rate, whereas the converse is the case for CaAl4O7 doped with Ce3+ (1 at %) and Tb3+ (1 at %). The energy transfer rate was determined to be on the order of 10(9) s(-1). The transfer occurs before the excited electrons can be thermally promoted into the conduction band. From these results, it can be inferred that the thermal ionization rate (W-Th) from the lowest 5d excited Ce3+ state at 355 nm is less than 10(9) s(-1) at room temperature, whereas the delocalization rate (W-D) into the conduction band is faster than this rate. The location of the Tb3+ and Ce3+ states relative to the valence and conduction band of these two materials has been determined through photoconductivity measurements.