The influence of impurities on the luminescence quantum efficiency of La0v~Ce0.~PO4 and La0.35Tb0 ~PO4 has been studied. The transition metal ions Fe and Mn are not built into the LaPO~ lattice under a reducing firing atmosphere. 4+Tetravalent Ce is probably present in a second phase. Codoping with cations with different valencies has no influence onthe Ce amount nor on the Ce quantum efficiency. The energy transfer rate from Ce to a number of rare-earth ions (Pr , Nd ~+, Dy ~+, Ho ~+, Er 3+) is low. Therefore, low concentrations of ~hese ions do not compete efficiently with the Tb ~+ ion for the excitation energy on the Ce ~ § ion. However, the Ce ~* quantum efficiency is reduced by Eu 3+ due to an electron transfer process. Energy transfer between Tb ~+ ions is restricted to Tb 3+ 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 Tb 3 § quantum efficiency.

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