Thermoluminescence Study of Persistent Luminescence Materials:Eu~(2+)- and R~(3+)-Doped Calcium Aluminates,CaAl_2O_4:Eu~(2+),R~(3+)

摘要

Thermoluminescence properties of the Eu~(2+)-,R~(3+)-doped calcium aluminate materials,CaAl_2O_4:Eu~(2+),R~(3+),were studied above room temperature.The trap depths were estimated with the aid of the preheating and initial rise methods.The seemingly simple glow curve of CaAl_2O_4:Eu~(2+) peaking at ca.80 deg C was found to correspond to several traps.The Nd~(3+) and Tm~(3+) ions,which enhance most the intensity of the high-temperature TL peaks,form the most suitable traps for intense and long-lasting persistent luminescence,too.The location of the 4f and 5d ground levels of the R~(3+) and R~(2+) ions were deduced in relation to the band structure of CaAl_2O_4.No clear correlation was found between the trap depths and the R~(3+) or R~(2+) level locations.The traps may thus involve more complex mechanisms than the simple charge transfer to (or from) the R~(3+) ions.A new persistent luminescence mechanism presented is based on the photoionization of the electrons from Eu~(2+) to the conduction band followed by the electron trapping to an oxygen vacancy,which is aggregated with a calcium vacancy and a R~(3+) ion.The migration of the electron from one trap to another and also to the aggregated R~(3+) ion forming R~(2+) (or R~(3+)-e~-) is then occurring.The reverse process of a release of the electron from traps to Eu2+ will produce the persistent luminescence.The ability of the R~(3+) ions to trap electrons is probably based on the different reduction potentials and size of the R~(3+) ions.Hole trapping to a calcium vacancy and/or the R~(3+) ion may also occur.The mechanism presented can also explain why Na~+,Sm~(3+),and Yb~(3+) suppress the persistent luminescence.