Remarkable changes in the photoluminescent properties of Y2Ce2O7:Eu3+ red phosphors through modification of the cerium oxidation states and oxygen vacancy ordering

Abstract

A new series of red phosphors based on Eu³⁺-doped yttrium cerate [Y_1.9Ce₂O₇:0.1Eu³⁺, Y₂Ce_1.9O₇:0.1Eu³⁺ and Y₂Ce_2−xO₇:xEu³⁺ (x = 0.05, 0.10, 0.15, 0.20, 0.25 and 0.50)] was prepared via a conventional solid-state method. The influence of the substitution of Eu³⁺ at the aliovalent site on the photoluminescent properties was determined by powder X-ray diffraction, FT Raman spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, scanning electron microscopy with energy-dispersive spectroscopy, UV-visible absorption spectroscopy, photoluminescence spectroscopy and lifetime measurements. The substitution of Eu³⁺ at the Ce⁴⁺ site induces a structural transition from a defect fluorite to a C-type structure, which increases the oxygen vacancy ordering and the distortion of the Eu³⁺ environment, and decreases the formation of Ce³⁺ states. In contrast, phosphors with isovalent substitution at the Y³⁺ site exhibit the biphasic nature of defect fluorite and a C-type structure, thereby increasing the number of Ce³⁺ oxidation states. These modifications resulted in remarkable changes in the photoluminescent properties of Y₂Ce_1.9O₇:0.1Eu³⁺ red phosphors, with emission intensities 3.8 times greater than those of the Ce_0.9O₂:0.1Eu³⁺ and Y_1.9Ce₂O₇:0.1Eu³⁺. The photoluminescent properties of Y₂Ce_2−xO₇:xEu³⁺ were studied at different Eu³⁺ concentrations under excitation with blue light. These phosphors emit intense red light due to the ⁵D₀–⁷F₂ transition under excitation at 466 nm and no concentration quenching is observed with up to 50 mol% Eu³⁺. They show increased lifetimes in the range 0.62–0.72 ms at Eu³⁺ concentrations. The cation ordering linked to the oxygen vacancy ordering led to the uniform distribution of Eu³⁺ ions in the lattice, thus allowing higher doping concentrations without quenching and consequently increasing the lifetime of the ⁵D₀ states. Our results demonstrate that significant improvements in the photoluminescence properties can be achieved by the structural alteration of a fluorite CeO₂ to a C-type lattice.