Quantitative Determination of the Effective Mn4+ Concentration in a Li2TiO3:Mn4+ Phosphor and Its Effect on the Photoluminescence Efficiency of Deep Red Emission

Dazzling Red-Emitting Europium(III) Ion-Doped Ca2LaHf2Al3O12 Garnet-Type Phosphor Materials with Potential Application in Solid-State White Lighting

Bright red-emitting phosphors with high color purity and high photoluminescence quantum yield (PLQY) are highly demanded for the fabrication of high-performance warm-white light-emitting diodes (LEDs). Herein, we demonstrated a novel efficient Eu³⁺-activated Ca₂LaHf₂Al₃O₁₂ garnet phosphor with excellent luminescence properties for near-ultraviolet (near-UV) excited warm-white LEDs. The Ca₂LaHf₂Al₃O₁₂:Eu³⁺ phosphors exhibited an intense excitation spectrum in the near-UV region with a maximum around 394 nm, and they produced dazzling red luminescence peaking at 592, 614, 659, and 711 nm due to the ⁵D₀ → ⁷F_J (J = 1-4) transitions of Eu³⁺ ions when the excitation wavelength was set at 394 nm. Luminescent properties have been studied as a function of Eu³⁺ doping concentration, and the highest emission intensity was achieved at 50 mol % Eu³⁺, while the dipole-dipole interaction brought the concentration quenching effect. The Ca₂LaHf₂Al₃O₁₂:50%Eu³⁺ sample exhibited CIE chromaticity coordinates of (0.6419, 0.3575) with a color purity of 92.7%, and its PLQY was measured to be 64%. The thermal stability and activation energy of Ca₂LaHf₂Al₃O₁₂:50%Eu³⁺ phosphors were also discussed and analyzed. Finally, we made a near-UV chip-based white LED device in which the Ca₂LaHf₂Al₃O₁₂:50%Eu³⁺ phosphor was utilized as a red ingredient. A bright warm-white light emission was realized from this LED device under 80 mA driving current, accompanied by a high color rendering index (CRI) of 88.3, a low correlation color temperature of 3853 K, and good CIE chromaticity coordinates of (0.3909, 0.3934). These results revealed that these red-emitting Ca₂LaHf₂Al₃O₁₂:Eu³⁺ phosphors have promising application prospect in near-UV-excited warm-white LEDs with high a CRI.

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.

Novel Orange Color Pigments Based on La3LiMnO7

La₃LiMn_1−xTi_xO₇ (0 ≤ x ≤ 0.05) samples were synthesized by a solid-state reaction method, and a single-phase form was observed for the samples in the range of x ≤ 0.03. Crystal structure, optical properties, and color of the La₃LiMn_1−xTi_xO₇ (0 ≤ x ≤ 0.03) samples were characterized. Strong optical absorption was observed at a wavelength between 400 and 550 nm, and a shoulder absorption peak also appeared around 690 nm in all samples; orange colors were also exhibited. Among the samples synthesized, the most brilliant orange color was obtained at La₃LiMn_0.97Ti_0.03O₇. The redness (a*) and yellowness (b*) values of this pigment were higher than those of the commercially available orange pigments. Therefore, the orange color of this pigment is brighter than those of the commercial products. Since the La₃LiMn_0.97Ti_0.03O pigment is composed of non-toxic elements, it could be a new environmentally friendly inorganic orange pigment.

Fedorite from Murun Alkaline Complex (Russia): Spectroscopy and Crystal Chemical Features

Fedorite is a rare phyllosilicate, having a crystal structure characterized by SiO4-tetrahedral double layers located between continuous layers formed by edge-sharing (Ca,Na)-octahedra, and containing interlayer K, Na atoms and H2O molecules. A mineralogical-petrographic and detailed crystal-chemical study of fedorite specimens from three districts of the Murun alkaline complex was performed. The sequence of the crystallization of minerals in association with fedorite was established. The studied fedorite samples differ in the content of interlayer potassium and water molecules. A comparative analysis based on polyhedral characteristics and deformation parameters was carried out. For the first time, EPR, optical absorption and emission spectra were obtained for fedorite. The raspberry-red coloration of the mineral specimens could be attributed to the presence of Mn4+ ions.

Effect of [MnO6] Octahedra to the Coloring Mechanism of (Li1-x Na x )2MnO3

(Li_1-x Na _x )₂MnO₃ (0 ≤ x ≤ 0.10) solid solutions were synthesized by a conventional solid-state reaction technique to investigate the relationship between the steric structure of the [MnO₆] octahedra and coloration mechanisms. The color, optical properties, and crystal structure of the solid solutions were characterized. The (Li_1-x Na _x )₂MnO₃ (0 ≤ x ≤ 0.10) solid solutions absorbed the visible light at wavelengths shorter than 550 nm and around 680 nm. The former and latter optical absorption bands were attributed to the spin-allowed (⁴A_2g → ⁴T_1g, ⁴T_2g) and spin-forbidden (⁴A_2g → ²E_g, ²T_1g) d-d transitions of tetravalent manganese ions, respectively. The absorption band assigned to the ⁴A_2g → ⁴T_2g transition shifted toward longer wavelengths with the enlargement of the average [Mn(2)O₆] bond distance by doping Na⁺. In contrast, the latter absorption bands did not shift but the absorption intensities increased due to the distortion of the [Mn(2)O₆] octahedra. Consequently, the red color purity of the sample gradually increased with the increase in the Na⁺ concentration. Among the (Li_1-x Na _x )₂MnO₃ (0 ≤ x ≤ 0.10) samples synthesized in this study, the highest red color purity was obtained in the (Li_0.93Na_0.07)₂MnO₃ (hue angle: h° = 39.1) sample. The results of this study provide important insights for the development of environment-friendly inorganic red pigments containing Mn⁴⁺ ions as a coloring source.