Delocalization, thermal ionization, and energy transfer in singly doped and codoped CaAl4O7 and Y2O3

Abstract

${\mathrm{Ce}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ singly doped and codoped $\mathrm{Ca}{\mathrm{Al}}_4{\mathrm{O}}_7$ and ${\mathrm{Y}}_2{\mathrm{O}}_3$ single crystal fibers were grown using the laser heated pedestal growth method. Energy transfer from ${\mathrm{Ce}}^{3+}$ to ${\mathrm{Tb}}^{3+}$ was studied in these samples. No energy transfer between ${\mathrm{Ce}}^{3+}$ and ${\mathrm{Tb}}^{3+}$ was observed in codoped ${\mathrm{Y}}_2{\mathrm{O}}_3$. This is because electrons excited into the ${\mathrm{Ce}}^{3+}5d$ 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 $\mathrm{Ca}{\mathrm{Al}}_4{\mathrm{O}}_7$ doped with ${\mathrm{Ce}}^{3+}(1\mathrm{at}\%)$ and ${\mathrm{Tb}}^{3+}(1\mathrm{at}\%)$. The energy transfer rate was determined to be on the order of ${10}^9{\mathrm{s}}^{-1}$. 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 $\left(W_{\mathit{Th}}\right)$ from the lowest $5d$ excited ${\mathrm{Ce}}^{3+}$ state at $355\mathrm{nm}$ is less than ${10}^9{\mathrm{s}}^{-1}$ at room temperature, whereas the delocalization rate $\left(W_D\right)$ into the conduction band is faster than this rate. The location of the ${\mathrm{Tb}}^{3+}$ and ${\mathrm{Ce}}^{3+}$ states relative to the valence and conduction band of these two materials has been determined through photoconductivity measurements.