Among the various approaches to realize white light-emitting diodes (WLEDs), phosphor-converted white LEDs (pc-WLEDs) have become the predominant technology owing to their superior color stability, uniform spectral distribution, straight forward fabrication process, and cost-effectiveness. Among the phosphors utilized in WLEDs, red phosphors play an indispensable role in numerous methods for obtaining high-quality white LEDs. They contribute to enhancing the color rendering index and lowering the correlated color temperature, thereby facilitating the development of healthier lighting sources. Therefore, the development of highly efficient red phosphors suitable for blue or near-ultraviolet excitation continues to be a research focus in this field. The 4f electron configuration of Eu³⁺ gives rise to a rich variety of metastable energy levels under the influence of the crystal field. The distinctive ⁵D₀→⁷F_J (J=1, 2, 3, 4) electronic transitions result in high-purity red light emission with the main peak positioned between 610 nm and 620 nm, rendering Eu³⁺ an exceptional red-emitting ion. Borate materials have attracted extensive research interest as promising luminescent hosts owing to their advantages, including low synthesis temperatures, broad ultraviolet-visible transparency ranges, and high luminescence efficiencies. Ba₃Sr₄B₃O₉F₅ exhibits a non-centrosymmetric structure belonging to the hexagonal crystal system with space group P6₃mc. This structure features one non-centrosymmetric Ba²⁺ site and two distinct non-centrosymmetric Sr²⁺ sites. Doping Eu³⁺ into the Ba₃Sr₄B₃O₉F₅ matrix is expected to yield highly efficient red-emitting phosphors. In this study, a series of Ba₃Sr₄B₃O₉F₅∶xEu³⁺ red-emitting phosphors were successfully synthesized. Systematic investigations were carried out on their phase structures, morphologies, luminescence properties, and thermal stabilities. Density functional theory (DFT) calculations were performed to determine the preferred lattice sites occupied by Eu³⁺ after doping. Additionally, the concentration quenching mechanism of Eu³⁺ was thoroughly analyzed.

Ba₃Sr₄B₃O₉F₅∶xEu³⁺ (0 ≤ x ≤ 0.42) fluorescent ceramics were synthesized in vacuum via high-temperature solid-state reaction. High purity SrCO₃、BaCO₃、BaF₂、H₃BO₃ and Eu₂O₃ powders were used as starting materials. These materials were weighed in stoichiometric ratios, and ground to obtain a powder mixture of finer granularity. The powder mixture was placed into the crucible, and heated at 400 ℃ for 2 h. After being cooled down to room temperature, the resulting item was grounded manually and transferred in sequence to another furnace to be annealed at 830 ℃ for 10 h in air to produce the final product.

Based on DFT calculations, it was found that when Eu³⁺ occupied the Ba²⁺ lattice sites in the Ba₃Sr₄B₃O₉F₅ crystal, the total energy E_d of the doping system reached its minimum value. Therefore, it is reasonable to speculate that upon doping into the Ba₃Sr₄B₃O₉F₅ lattice, Eu³⁺ preferentially occupy the Ba²⁺ lattice sites. The excitation spectrum of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ powder comprises a broad band peak and a set of sharp line-shaped peaks. The broad band peak in the range of 210-335 nm can be ascribed to the charge-transfer band (CTB) peak, which arises from the energy transfer between O₂^- and Eu³⁺. A series of sharp excitation peaks within the wavelength range of 300-500 nm originate from the 4f-4f transitions of Eu³⁺, with the strongest peak located at 394 nm. This is well-matched with the near-ultraviolet (NUV) chips currently employed in WLEDs. The emission spectrum of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ powder exhibits five distinct emission peaks located at 580.3, 593, 613, 653.8 nm, and 703.2 nm, which are attributed to the characteristic transitions of Eu³⁺ from the ⁵D₀ excited state to the ⁷F₀, ⁷F₁, ⁷F₂, ⁷F₃, and ⁷F₄ ground states, respectively. Among these, the intensity of the electric dipole transition (⁵D₀→⁷F₂) emission peak at 613 nm is significantly higher than that of the magnetic dipole transition (⁵D₀→⁷F₁) emission peak at 593 nm. The Eu³⁺ ions are situated at the non-centrosymmetric lattice sites within the matrix crystal structure, predominantly exhibiting allowed electric dipole transitions from ⁵D₀ to ⁷F₂. The emission peaks are primarily centered around 613 nm, corresponding to red luminescence.

When the doping concentration of Eu³⁺ (denoted as x) is below 0.38, the luminescence intensity of the phosphor exhibits a gradual increase with rising concentrations of Eu³⁺. Conversely, when x exceeds 0.38, there is a progressive decline in luminescence intensity as the doping concentration continues to rise. The fluorescence intensity reaches its maximum when the Eu³⁺ doping concentration x = 0.38. This concentration quenching phenomenon is attributed to the quadrupole-quadrupole interactions between the doped Eu³⁺ ions at high doping concentrations. Thermal stability is a key factor in evaluating luminescent materials. At the operating temperature of LED devices (150 ℃), the integrated emission intensity of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ phosphor remains above 70% of its initial value. Moreover, at 497 K, the emission intensity is retained at 49.3% of that at 297 K, indicating excellent luminescence thermal stability.

A series of red phosphors Ba₃Sr₄B₃O₉F₅∶xEu³⁺ were successfully synthesized using the traditional high-temperature solid-state reaction method. The Ba₃Sr₄B₃O₉F₅∶xEu³⁺ phosphor exhibits a non-centrosymmetric hexagonal crystal structure. Under 394 nm excitation, the Ba₃Sr₄B₃O₉F₅∶xEu³⁺ sample predominantly emits red luminescence centered at 613 nm. The optimal doping concentration of Eu³⁺ ions is determined to be 0.38. Through DFT theory calculations and R/O ratio analysis, it is proved that Eu³⁺ ions will preferentially occupy the Ba²⁺ lattice positions in the Ba₃Sr₄B₃O₉F₅ matrix and the surrounding environment exhibits high non-centrosymmetry. The main mechanism of nonradiative energy transfer in Ba₃Sr₄B₃O₉F₅∶xEu³⁺ phosphor is quadrupole-quadrupole interaction, which also explains the exchange interaction is the main mechanism of the concentration quenching process. The color purity of the Ba₃Sr₄B₃O₉F₅∶xEu³⁺ phosphor reaches 93.9%, and under the operating temperature of LED devices, the integral emission intensity of Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ remains above 70% of initial value at 297 K. In summary, Ba₃Sr₄B₃O₉F₅∶0.38Eu³⁺ phosphor can serve as a promising red-emitting fluorescent material for application in white LED technology.