Clarifying concentration quenching mechanisms by lattice site-occupation and luminescence kinetics of Eu3+-activated Cs2Mg2Mo3O12

Abstract

The Eu³⁺ ion is not only an efficient red-emitting activator, but also a well-known spectral probe. In this work, a series of Cs₂Mg₂Mo₃O₁₂:2xEu³⁺ (x = 0–0.1) phosphors are successfully synthesized using the solid-state reaction. Rietveld refinements confirmed the cubic structure of Eu³⁺-doped Cs₂Mg₂Mo₃O₁₂ with the P2₁3 space group (no: 198), in which the activators substituted for the Cs⁺ lattice positions. The phosphor exhibited a sharp luminescence line at 613 nm (⁵D₀ to ⁷F₂ of Eu³⁺) with a half-bandwidth of approximately 1 nm. The remarkable CIE chromaticity coordinates (x = 0.66, y = 0.33) are close to the ideal red chromaticity specified by the NTSC. Unfortunately, the optimal luminescence of Cs₂Mg₂Mo₃O₁₂:2xEu³⁺ was quenched at a low concentration of 5.0 at% (x = 0.05). The quenching mechanism was analyzed through the Eu³⁺-site occupations and the corresponding dynamic spectra were obtained using the laser site-selective spectroscopy technique. In the samples with low doping levels of Cs₂Mg₂Mo₃O₁₂:2xEu³⁺ (x ≦ 0.05), only one Eu³⁺ center was regularly arranged on Cs(2) sites. Beyond this level of doping in Cs₂Mg₂Mo₃O₁₂:2xEu³⁺ (x = 0.07, 0.1), Eu³⁺ began to occupy Cs(1) sites in the lattices in addition to Cs(2). The site-selective luminescence and decay characteristics confirmed that the two Eu³⁺ centers occupying Cs(1) and Cs(2) sites had sufficient energy transfer (ET) between them. In this case, the excitation energy was easily lost during the excitation process. This study demonstrates that the substitution position of the Eu³⁺ activator in the lattice can greatly affect the quenching performance of red emission.