The influence of impurities on the luminescence quantum efficiency of has been studied. Low concentrations of Pr 3+ , Nd 3+ , Dy 3+ , Ho 3+ , and Er 3+ do not compete efficiently with Gd 3+ and Tb 3+ for the excitation energy on the Ce 3+ ions. However, the luminescence efficiency of Ce 3+ is strongly decreased by Sm 3+ and Eu 3+ due to an electron transfer process. The luminescence efficiency of Ce 3+ is also decreased by Cr 2+ , Mn 2+ , Fe 2+ , and Ni 2+ due to energy transfer to these ions. In the case of Cr 3+ the luminescence efficiency of Ce 3+ is quenched due to competitive absorption. The luminescence efficiency of Gd 3+ is strongly decreased by Cr 2+ , Mn 2+ , and Fe 2+ due to energy transfer to these ions. The excitation energy migrates over the Gd 3+ ions. Therefore, low concentrations of impurities have a considerable effect on the luminescence efficiency of Gd 3+ . Since there is no energy migration over the Tb 3+ ions, small concentrations of impurities have a limited effect on the luminescence efficiency of Tb 3+ .
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Schaik et al. (1994) studied this question.