INFLUENCE OF IMPURITIES ON THE LUMINESCENCE QUANTUM EFFICIENCY OF (LA, CE, TB)PO4

The influence of impurities on the luminescence quantum efficiency of La0.75Ce0.25PO4 and La0.85Tb0.15PO4 has been studied. The transition metal ions Fe and Mn are not built into the LaPO4 lattice under a reducing firing atmosphere. Tetravalent Ce4+ is probably present in a second phase. Codoping with cations with different valencies has no influence on the Ce4+ amount nor on the Ce3+ quantum efficiency. The energy transfer rate from Ce3+ to a number of rare-earth ions (Pr3+, Nd3+, Dy3+, Ho3+, Er3+) is low. Therefore, low concentrations of these ions do not compete efficiently with the Tb3+ ion for the excitation energy on the Ce3+ ion. However, the Ce3+ quantum efficiency is reduced by Eu3+ due to an electron transfer process. Energy transfer between Tb3+ ions is restricted to Tb3+ clusters, so that the probability that the excitation energy reaches impurities is low. Therefore low concentrations of rare-earth ions have only a small effect on the Tb3+ quantum efficiency.