Tunable color emitting of Ba_1-x-ySiO₃:xEu,yBi³⁺ phosphors with the self-reduction of Eu³⁺ ions calcined in air

A series of Ba_1-xSiO₃:xEu and Ba_0.92-ySiO₃:0.08Eu, yBi³⁺ phosphors have been synthesized using one-step calcination of a precursor prepared by chemical co-precipitation. The self-reduction phenomenon of Eu³⁺ is observed and verified by photoluminescence (PL), excitation (PLE) spectra and X-ray photoelectron spectroscopy (XPS). Under 353 nm excitation, the Ba_1-x-ySiO₃:xEu,yBi³⁺ phosphors exhibit a broad band ranging from 390 nm to 560 nm with a peak centered at 499 nm originating from the 4f⁶5d→4f⁷ transition of Eu²⁺ ions and several narrow band emissions peaked at 464 nm, 537 nm, 581 nm, 590 nm, 615 nm, 660 nm and 705 nm can be attributed to ⁵D₂→⁷F₀, ⁵D₁→⁷F₁, ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D₀→⁷F₂, ⁵D₀→⁷F₃ and ⁵D₀→⁷F₄ transition of Eu³⁺, respectively. The self-reduction of Eu³⁺→Eu²⁺ in BaSiO₃:Eu calcined in air has been explained from the following aspects: the charge compensation mechanism and the structures of 3-D networks composed by SiO₄ tetrahedra, helpful for maintaining of the reduction (Eu³⁺→Eu²⁺) at a high temperature. Co-doping Bi³⁺ ions can both enhance the emissions from Eu²⁺ and Eu³⁺ because the doped Bi³⁺ ions not only can make vacancy for Eu²⁺ but also can enhance the emitting for Eu³⁺ in BaSiO₃. Moreover, when excited by 370 nm, the CIE chromaticity coordinates of Ba_0.92SiO₃:0.08Eu and Ba_0.912SiO₃:0.08Eu,0.008Bi³⁺ are (0.3521, 0.3141) and (0.3558, 0.3408), meanwhile their correlated color temperature (CCT) are 4478 K and 4523 K, which belongs to the warm white light region. Furthermore, the decay curves are measured and can be well fitted with double exponential decay equation. It shows that both the average lifetimes of Eu²⁺ and Eu³⁺ are extended with the concentration of Bi³⁺ ions increasing. Therefore, the Ba_1-x-ySiO₃:xEu,yBi³⁺ phosphor is a promising warm white‐light emitting diodes (w-LEDs) integrated with NUV chips.