Up-conversion and energy migration in the holmium hexachloroelpasolites

摘要

Excitation into the $+5$/I$-8$/$YLD$+5$/F$-5$/ transition of Cs$-2$/NaHoCl$-6$/ near 15 633 cm$+$MIN@1$/ at 300 K results in strong, long lived emission from the $+5$/F$-5$/ state wit an almost exponential decay curve. The decay constant of this emission decreases slightly in Cs$-2$/NaY$-0.99$/Ho$- 0.01$/Cl$-6$/ the decay becoming exactly exponential, the difference between the decay constants is greater at lower temperatures. This suggests that there is migration in the host lattice followed by energy transfer to defect sites with short $+5$/F$-5$/ lifetimes in the pure Ho compound and this hypothesis is confirmed by measurements of the relaxation of the $+5$/F$-5$/ state in Cs$-2$/NaNd$- 0.01$/Ho$-0.99$/Cl$-6$/ where the decay constant is greatly increased but the decay curves are exponential. Under the same conditions and at temperatures from 1.2K to 300K there is emission in the ultraviolet, green and blue regions from Cs$-2$/NaY$-1$MIN@x$/Ho$-x$/Cl$-6$/. The decay curves are complex with more than one rise and decay processes reflecting a variety of up-conversion processes. The mechanisms of these processes are discussed.