Structural and optical characteristics of novel rare‐earth‐free red-emitting BaSn(PO4)2:Mn4+ phosphor

Synthesis, Photoluminescent Characteristics and Eu3+-Induced Phase Transitions in Sr3Zr2O7:Eu3+ Red Phosphors

Designing phosphors that are excited by blue light is extraordinarily important for white light-emitting diodes (w-LEDs). In the present study, a new Ruddlesden-Popper type of SZO:xEu³⁺ (x = 0.01~0.10) phosphors was developed using solid-state reactions. Interestingly, a Eu³⁺ doping-induced phase transformation from the Sr₃Zr₂O₇ (cubic) to the SrZrO₃ (orthorhombic) phase was observed, and the impact of the occupied sites of Eu³⁺ ions on the lifetime of Sr₃Zr₂O₇:xEu³⁺ phosphors is discussed in detail. Diffuse reflectance spectroscopy results showed that the band gap of SZO:xEu³⁺ phosphors gradually increased from 3.48 eV for undoped Sr₃Zr₂O₇ hosts to 3.67 eV for SZO:0.10Eu³⁺ samples. The fluorescence excitation spectrum showed that ultraviolet (300 nm), near-ultraviolet (396 nm) and blue light (464 nm) were all effective excitation pump sources of Sr₃Zr₂O₇:xEu³⁺ phosphors, and the strongest emission at 615 nm originated from an electric dipole transition (⁵D₀→⁷F₂). CIE coordinates moved from orange (0.5969, 0.4267) to the reddish-orange region (0.6155, 0.3827), and the color purity also increased. The fabricated w-LED was placed on a 460 nm chip with a mixture of YAG:Ce³⁺ and SZO:0.1Eu³⁺ samples and showed "warm" white light with a color rendering index (CRI) of 81.8 and a correlation color temperature (CCT) of 5386 K, indicating great potential for application in blue chip white LEDs.

A new rare-earth oxyborate Ba3BiPbEuO(BO3)4 and the luminescence properties of the Ba3BiPbY1-xEuxO(BO3)4 phosphors

A new rare-earth oxyborate Ba₃BiPbEuO(BO₃)₄ was prepared, and its crystal structure was established by the single-crystal X-ray diffraction method with the following structural data: P6₃/mmc, a = 5.4305(1) Å, c = 26.3501(3) Å, V = 672.96(3) ų and Z = 2. This is a layered material containing two types of layers: one is formed by corner-sharing EuO₆ octahedra and BO₃ triangles and the other is composed of corner- or edge-sharing (Bi/Pb)O₇ hexagonal pyramids and BO₃ triangles. These layers and their equivalent partners are alternately stacked along the c-axis, with Ba²⁺ cations situated in between and within the layers. Furthermore, the phase composition, morphology, and IR, Raman, XPS, and UV-vis absorption spectra of this compound were studied. In addition, solid solutions of Ba₃BiPbY_1-xEu_xO(BO₃)₄ (0 ≤ x ≤ 1) were prepared via the solid-state reaction method and their photoluminescence properties were investigated. Ba₃BiPbYO(BO₃)₄ and Ba₃BiPbEuO(BO₃)₄ showed blue and reddish-orange luminescence when excited at 242 and 394 nm, respectively. By doping Eu³⁺ into the Ba₃BiPbYO(BO₃)₄ matrix and changing the Eu³⁺ doping concentration, luminescence color tunability from blue to orange-red was realized in Ba₃BiPbY_1-xEu_xO(BO₃)₄ phosphors under excitation at 331 nm. This kind of material with color variability may have potential application in field-emission displays.

Two new rare-earth borates Sr2Tb3B27-δO46 and Ba2Eu3B27-δO46 (δ = 2/3): syntheses, crystal structures, and luminescence properties

Two new alkali-earth and rare-earth metal borates, Sr₂Tb₃B_27-δO₄₆ and Ba₂Eu₃B_27-δO₄₆ (δ = 2/3), have been prepared by high-temperature solution reactions at 950 °C. The single-crystal XRD analysis showed that the borates crystallize in the rhombohedral space group R3 with the cell parameters a = 7.7468(1) Å, c = 46.3657(5) Å, V = 2409.75(3) ų, Z = 3 and a = 7.7964(1) Å, c = 46.5086(6) Å, V = 2448.23(7) ų, Z = 3, respectively, which further confirms that "LnMB₉O₁₆" has the right formula of M₂Ln₃B_27-δO₄₆ (δ = 2/3). Their crystal structures consist of 3D networks constructed by Tb(Eu)O₆ trigonal prisms, BO₃ planar triangles, and BO₄ tetrahedra, with large cavities accommodating Sr²⁺ (Ba²⁺) cations. Furthermore, the phase composition, morphology, IR, Raman, and XPS spectra as well as photoluminescence properties of these two compounds were studied. Upon excitation by ultraviolet radiation, Sr₂Tb₃B_27-δO₄₆ exhibits the characteristic emission lines originating from Tb³⁺ corresponding to ⁵D₄-⁷F_{6,5,4,3,2,1,0} transitions, whereas Ba₂Eu₃B_27-δO₄₆ displays bright red emission from Eu³⁺ with four groups of emission bands associated with ⁵D₀ → ⁷F_1,2,3,4 transitions. Both phosphors show stable luminescence and color purity at elevated temperatures. As it may be expected, Sr₂Tb₃B_27-δO₄₆ and Ba₂Eu₃B_27-δO₄₆ (δ = 2/3) are promising green (Tb) and red (Eu) components for use in WLEDs.

Site Preference-Driven Mn4+ Stabilization in Double Perovskite Phosphor Regulating Quantum Efficiency from Zero to Champion

The tetravalent-state stability of manganese is of primary importance for Mn⁴⁺ luminescence. Double perovskite-structured A₂B'B″O₆:Mn⁴⁺ has been recently prevalent, and the manganese ions are assumed to substitute for the B″(IV-VI)O₆ site to stabilize at the tetravalent charge state to generate far-red emissions. However, some Mn-doped A₂B'B″O₆-type materials show no or weak luminescence such as typical Ca₂MgWO₆:Mn. In this work, a cation-pair co-substitution strategy is proposed to replace 2Ca²⁺ by Na⁺-La³⁺ to form Ca_2-2xNa_xLa_xMgWO₆:Mn. The significant structural distortion appears in the solid solution lattices with the contraction of [MgO₆] but enlargement of [WO₆] octahedron. We hypothesize that the site occupancy preference of Mn migrates from Mg²⁺ to W⁶⁺ sites. As a result, the effective Mn⁴⁺/Mn²⁺ concentration enhances remarkably to regulate nonluminescence to highly efficient Mn⁴⁺-related far-red emission. The optimal CaNa_0.5La_0.5MgWO₆:0.9%Mn⁴⁺ shows an internal quantum efficiency of 94% and external quantum efficiency of 82%, reaching up to the top values in Mn⁴⁺-doped oxide phosphors. This work may provide a new perspective for the rational design of Mn⁴⁺-activated red phosphors, primarily considering the site occupancy modification and tetravalent-state stability of Mn.

Charge compensation engineering for high-performance dolomite-type BaTi(BO3)2:Eu3+ red-emitting phosphor for backlight display applications with a wide color gamut

Exploiting high color purity phosphors is a core problem in the development of phosphor conversion light-emitting diodes (pc-LEDs) for display devices. Eu³⁺-activated BaTi(BO₃)₂ (BTB) red-emitting phosphors were first synthesized via a solid-state reaction at low temperature and alkali metal ions Na⁺ were co-doped in BTB:xEu³⁺ to improve the luminescence properties. The occupation of the Eu³⁺ ions and the enhancement principles of the Na⁺ ions and their effect on the photoluminescence properties of the BTB:xEu³⁺ phosphors are discussed in detail. The BTB:xEu³⁺,Na⁺ system demonstrated a strong thermal stability (68.4% at 150 °C), low color temperature (about 1940-1950 K) and high color purity (almost 90%). Furthermore, the prototype LED device can emit a bright white light, has a stable luminous efficiency and color rendering index, and the color gamut reaches 115.5% of the NTSC standard. Therefore, the BTB:xEu³⁺,Na⁺ series phosphors provide a better choice for the development of lighting and display devices with a wide color gamut.