Deep-red photoluminescence from enhanced ⁵D₀-⁷F₄ transition in Eu³⁺ doped Ca₂Ga₂GeO₇ phosphors

In order to explore diverse luminescence properties of Eu³⁺ in inorganic phosphors, a series of novel deep-red-emitting Ca₂Ga₂GeO₇ phosphors were successfully synthesized by solid-state reactions. The phase formation was verified by the X-ray diffraction patterns and Rietveld refinement and there is only one kind of Ca²⁺ site in Ca₂Ga₂GeO₇. The excitation spectrum shows typical excitation peaks of Eu³⁺ and the broad band ranging from 200 nm to 300 nm was composed of the charge transfer band of O^2--Eu³⁺ and Ga³⁺-O^2- transition band. The emission spectrum depicts that the intensity of the peak located at 704 nm (⁵D₀ → ⁷F₄) is higher than that of 618 nm (⁵D₀ → ⁷F₂), which finally leads to the deep-red emission of the phosphor with high color saturation. The coordination dodecahedron of EuO8 distorted from a cubic geometry to the square antiprism is responsible for the unusual emission of Eu³⁺. The research of the concentration quenching behavior, lifetime and luminescence decay curves imply that the energy transfer between Eu³⁺ ions finally leads to the concentration quenching, and the interaction type is d-d interaction. The charge compensation, quantum efficiency and the thermal stability of Ca₂Ga₂GeO₇:Eu³⁺ were also studied.