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The 5D emissions of Eu3+ in Y2O2S, La2O2S, and LaOCl quench sequentially with increasing temperature in the simple order 5D3, 5D2, 5D1, 5D0. These 5D quenchings are attributed to thermally activated resonance crossovers from the 5D states to the charge-transfer states (CTS). The activation energies for the 5D quenching place the CTS relative to the 5D states. The 7F→CTS absorption bands also place the CTS relative to the 7F states. The agreement between these two independent CTS placements supports the model of 5D−CTS resonance transitions. Thermally activated 5D→CTS crossovers also account for the two-step 5D2 quenching observed in LaOCl and La2O2S.

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