Direct evidence for excitation transfer from the ⁵D₄ manifold of Tb³⁺ to the ⁵D₁ manifold of Eu³⁺ in Tb_0.66Eu_0.33P₅O₁₄

⁵D₁→⁷F₁ emission lines of Eu³⁺ are clearly seen when the ⁵D₄ manifold of Tb³⁺ is excited. The ⁵D₁ lifetime at 45 K is 7 μs when excited through the ⁵D₂ manifold of Eu³⁺ but is equal to the ⁵D₄ (Tb³⁺) lifetime when excited through that manifold. These results together with the temperature dependence of the Tb³⁺→Eu³⁺ transfer rate are interpreted as direct evidence for a nonradiative transfer from the ⁵D₄ (Tb³⁺) manifold to the ⁵D₁ (Eu³⁺) manifold. The transfer rate at high temperatures supports the conjecture that the cross relaxation transfer at these temperatures is mainly via the ⁷F₁ (Eu³⁺) manifold.