Tunable Red Luminescence in Nitridomagnesoaluminates α‑Sr₂[MgAl₅N₇]:Eu²⁺, β‑Sr₂[MgAl₅N₇]:Eu²⁺, and Sr₈[LiMg₂Al₂₁N₂₈]:Eu²⁺

Red-emitting phosphors with the general sum formula Sr₈[Li_{(2–0.5<i>x</i>)}Mg_<i>x</i>Al_{(22–0.5<i>x</i>)}N₂₈]:Eu²⁺ for application in warm-white illumination grade light emitting diodes were obtained by reacting the metals, hydrides, and nitrides in molybdenum crucibles in a hot isostatic press. Upon irradiation with blue light (440 nm), the materials exhibit red luminescence (emission maximum: 633–665 nm; full width at half-maximum (fwhm): 1736–1964 cm^–1, 78–82 nm) tunable by adjusting the compositional variable <i>x</i> (here, 2 and 4) as well as the activator concentration (here, 0.075–1 atom-% Eu²⁺). The materials show promising thermal behavior of the emission with relative quantum efficiencies, compared to room temperature, of 66% (<i>x</i> = 0) and 84% (<i>x</i> = 2) at 200 °C. α-Sr₈[Li_{(2–0.5<i>x</i>)}Mg_<i>x</i>Al_{(22–0.5<i>x</i>)}N₂₈]:Eu²⁺ (<i>x</i> = 2 and 4) crystallizes isotypically with Sr₄[LiAl₁₁N₁₄]:Eu²⁺ in the orthorhombic space group <i>Pnnm</i> (no. 58) with unit cell parameters <i>a</i> = 10.51818(5)–10.54878(6), <i>b</i> = 10.44513(5)–10.48253(6), and <i>c</i> = 3.25704(1)–3.272752(15) Å. β-Sr₂[MgAl₅N₇]:Eu²⁺ (<i>x</i> = 4) crystallizes in the monoclinic space group <i>C</i>2/<i>m</i> (no. 12) with unit cell parameters <i>a</i> = 11.12874(11), <i>b</i> = 3.27289(3), <i>c</i> = 10.54530(11) Å, and β = 109.8939(7)° and is obtained as side phase (≤40 wt %) in syntheses with <i>x</i> = 4. The crystal structure of β-Sr₂[MgAl₅N₇]:Eu²⁺ consists of a network of vertex- and edge-sharing Al­(/Mg)­N₄ tetrahedra with four- and five-membered ring channels along [010]. The Sr²⁺ ions are located within the five-membered ring channels and are coordinated cube-like by eight N atoms

Oxonitridosilicate Oxides <i>RE</i>₂₆Ba₆[Si₂₂O₁₉N₃₆]O₁₆:Eu²⁺ (<i>RE</i> = Y, Tb) with a Unique Layered Structure and Orange-Red Luminescence for <i>RE</i> = Y

The oxonitridosilicate oxides <i>RE</i>₂₆Ba₆[Si₂₂O₁₉N₃₆]­O₁₆:Eu²⁺ (<i>RE</i> = Y, Tb) were synthesized by high-temperature reaction in a radiofrequency furnace starting from <i>RE</i>F₃, <i>RE</i>₂O₃ (<i>RE</i> = Y, Tb), BaH₂, Si­(NH)₂, and EuF₃. The structure elucidation is based on single-crystal X-ray data. The isotypic materials crystallize in the monoclinic space group <i>Pm</i> (no. 6) [<i>Z</i> = 3, <i>a</i> = 16.4285(8), <i>b</i> = 20.8423(9), <i>c</i> = 16.9257(8) Å, β = 119.006(3)° for <i>RE</i> = Y and <i>a</i> = 16.5465(7), <i>b</i> = 20.9328(9), <i>c</i> = 17.0038(7) Å, β = 119.103(2)° for <i>RE</i> = Tb]. The unique silicate layers are made up from Q¹-, Q²-, and Q³-type Si­(O/N)₄- as well as Q⁴-type SiN₄-tetrahedra, forming three slightly differing types of cages. The corresponding 3-fold superstructure as well as pronounced hexagonal pseudosymmetry complicated the structure elucidation. Rietveld refinement on powder X-ray diffraction data, energy-dispersive X-ray spectroscopy and infrared spectroscopy support the findings from single-crystal X-ray data. When excited with UV to blue light, Y₂₆Ba₆[Si₂₂O₁₉N₃₆]­O₁₆:Eu²⁺ shows broad orange-red luminescence (λ_em = 628 nm, fwhm ≈ 125 nm/3130 cm^–1). An optical band gap of 4.2 eV was determined for the doped compound by means of UV/vis spectroscopy

Unprecedented Deep-Red Ce³⁺ Luminescence of the Nitridolithosilicates Li_38.7<i>RE</i>_3.3Ca_5.7[Li₂Si₃₀N₅₉]O₂F (<i>RE</i> = La, Ce, Y)

Ce³⁺ doped solids find broad application, e.g. in phosphor converted light emitting diodes, utilizing the usually broad blue to yellow-orange emission of the respective phosphors. The red to infrared spectral range was not yet accessible with the activator Ce³⁺, even in nitride host materials. Here, we report on the nitridolithosilicates Li_38.7<i>RE</i>_3.3Ca_5.7[Li₂Si₃₀N₅₉]­O₂F (<i>RE</i> = La, Ce, Y) with unique, red-shifted Ce³⁺ luminescence. The materials were synthesized by solid-state metathesis reactions in tantalum ampules. The isotypic crystal structures exhibit a highly condensed three-dimensional network made up of SiN₄ and LiN₄ tetrahedra. Crystal structures were refined from single-crystal and powder X-ray diffraction data. The results are supported by energy-dispersive X-ray spectroscopy as well as charge distribution, lattice energy, and bond valence sum calculations. Optical band gaps of ≈4 eV were determined from diffuse reflectance UV/vis data using Tauc plots. The nitridolithosilicates are highly excitable from the UV to the green-yellow spectral range with a preferred excitation wavelength of λ_exc ≈ 540 nm. The emission spectra peak is in the deep-red with λ_em = 638–651 nm (fwhm ≈ 3600 cm^–1). According to the unique absorption and emission properties, application as luminescent solar concentrators or in horticultural lighting appears promising

Narrow-Band Yellow-Orange Emitting La_3–<i>x</i>Ca_1.5<i>x</i>Si₆N₁₁:Eu²⁺ (<i>x</i> ≈ 0.77): A Promising Phosphor for Next-Generation Amber pcLEDs

The nitridosilicate La_3–<i>x</i>Ca_1.5<i>x</i>Si₆N₁₁:Eu²⁺ (<i>x</i> ≈ 0.77) was synthesized in a radiofrequency furnace starting from LaF₃, La­(NH₂)₃, CaH₂, Si­(NH)₂, and EuF₃. The crystal structure was solved and refined from single-crystal X-ray data in the tetragonal space group <i>P</i>4<i>bm</i> (no. 100) with <i>a</i> = 10.1142(6), <i>c</i> = 4.8988(3) Å, and <i>Z</i> = 2. Thereby, the so far unknown charge balance mechanism in the system (La,Ca)₃Si₆N₁₁, which is necessary as bivalent Ca²⁺ substitutes trivalent La³⁺, was clarified. Accordingly, charge balance is achieved by incorporation of Ca²⁺ on three cation sites, including an additional third site compared to the homeotypic La₃Si₆N₁₁ structure type. The results are supported by Rietveld refinement on powder X-ray diffraction data as well as energy-dispersive X-ray spectroscopy. Fourier transform infrared spectroscopy indicates absence of N–H bonds. An optical band gap of ≈ 4.0 eV was determined using UV/vis reflectance spectroscopy. The Eu²⁺ doped compound exhibits a remarkably narrow emission in the yellow-orange spectral range (λ_em ≈ 587 nm, fwhm ≈ 60 nm/1700 cm^–1). Because of the intriguing yellow-orange luminescence, La_3–<i>x</i>Ca_1.5<i>x</i>Si₆N₁₁:Eu²⁺ (<i>x</i> ≈ 0.77) is a promising candidate for application in next-generation amber phosphor-converted light emitting diodes

Direct Measurements of Energy Levels and Correlation with Thermal Quenching Behavior in Nitride Phosphors

Highly efficient narrow-band red emitting (RE) phosphors are the most desired and requested materials for developing illumination grade phosphor-converted light emitting diodes (pcLEDs). This study presents direct measurements of RE energy levels, critical to the color and efficiency of LED phosphors. For the first time, we experimentally determine the energetic separation of the Eu 5d state and the conduction band, which is the key indicator of quantum efficiency. This was achieved for the next-generation pcLED phosphors Li₂Ca₂[Mg₂Si₂N₆]:Eu²⁺, Ba­[Li₂(Al₂Si₂)­N₆]:Eu²⁺, and Sr­[LiAl₃N₄]:Eu²⁺ using resonant inelastic X-ray scattering. Band to band and 4f to valence band transitions are directly observed in X-ray excited optical luminescence spectra of Sr­[LiAl₃N₄]:Eu²⁺ and Sr­[Mg₃SiN₄]:Eu²⁺. These techniques are widely applicable and create a comprehensive, experimental picture of the Eu²⁺ energy levels in these compounds, leading to a complete understanding of all pertinent electronic processes. This study forms the base needed for a detailed discussion of the structure–property relationships, such as specific atoms, coordination and density of states, underpinning phosphor color and efficiency

Luminescence of the Narrow-Band Red Emitting Nitridomagnesosilicate Li₂(Ca_1–<i>x</i>Sr_<i>x</i>)₂[Mg₂Si₂N₆]:Eu²⁺ (<i>x</i> = 0–0.06)

The nitridomagnesosilicates Li₂Ca₂[Mg₂Si₂N₆]:Eu²⁺ and Li₂(Ca_1.88Sr_0.12)­[Mg₂Si₂N₆]:Eu²⁺ show narrow-band red emission at 638 and 634 nm, respectively, with an emission bandwidth of 62 nm (∼1513 cm^–1) after excitation in the blue spectral region. Ce³⁺-doped samples show luminescence in the green spectral range (λ_em = 540 nm). The compounds were synthesized via solid-state metathesis reaction in Li melts. Refinement of single-crystal X-ray diffraction data revealed that Li₂(Ca_1.88Sr_0.12)­[Mg₂Si₂N₆] crystallizes isomorphic to Li₂Ca₂[Mg₂Si₂N₆]: <i>C</i>2/<i>m</i> [<i>Z</i> = 2, <i>a</i> = 5.5744(2), <i>b</i> = 9.8439(3), <i>c</i> = 6.0170(2) Å, β = 97.2520(10)°, <i>R</i>₁ = 0.021, <i>wR</i>₂ = 0.047]. Crystal composition was checked by EDS and ICP-OES measurements and luminescence properties are compared to state of the art narrow-band red emitting luminophors. On the basis of its narrow-band emission, application of the novel red luminophor in high CRI white pcLEDs is promising

Ca_18.75Li_10.5[Al₃₉N₅₅]:Eu²⁺Supertetrahedron Phosphor for Solid-State Lighting

Highly efficient red-emitting luminescent materials deliver the foundation for next-generation illumination-grade white light-emitting diodes (LEDs). Recent studies demonstrate that the hardly explored class of nitridoaluminates comprises intriguing phosphor materials, e.g., Sr­[LiAl₃N₄]:Eu²⁺ or Ca­[LiAl₃N₄]:Eu²⁺. Here, we describe the novel material Ca_18.75Li_10.5[Al₃₉N₅₅]:Eu²⁺ with highly efficient narrow-band red emission (λ_em ≈ 647 nm, full width at half-maximum, fwhm ≈ 1280 cm^–1). This compound features a rather uncommon crystal structure, comprising sphalerite-like T₅ supertetrahedra that are composed of tetrahedral AlN₄ units that are interconnected by additional AlN₄ moieties. The network charge is compensated by Ca²⁺ and Li⁺ ions located between the supertetrahedra. The crystal structure was solved and refined from single-crystal and powder X-ray diffraction data in the cubic space group <i>Fd</i>3̅<i>m</i> (No. 227) with <i>a</i> = 22.415(3) Å and <i>Z</i> = 8. To verify the presence of Li, transmission electron microscopy (TEM) investigations including electron energy-loss spectroscopy (EELS) were performed. Based on the intriguing luminescence properties, we proclaim high potential for application in high-power phosphor-converted white LEDs

Efficient Yellow-Orange Phosphor Lu₄Ba₂[Si₉ON₁₆]O:Eu²⁺ and Orange-Red Emitting Y₄Ba₂[Si₉ON₁₆]O:Eu²⁺: Two Oxonitridosilicate Oxides with Outstanding Structural Variety

The oxonitridosilicate oxides Y₄Ba₂[Si₉ON₁₆]­O:Eu²⁺ and Lu₄Ba₂[Si₉ON₁₆]­O:Eu²⁺ have been synthesized starting from <i>RE</i>F₃, <i>RE</i>₂O₃ (<i>RE</i> = Y, Lu), BaH₂, Si­(NH)₂, and EuF₃ in a radiofrequency furnace at 1550 °C. The crystal structures were solved and refined from single-crystal X-ray data supported with Rietveld refinement on X-ray powder diffraction data. Both compounds are isotypic and crystallize in monoclinic space group <i>P</i>2₁/<i>c</i> (no. 14) with <i>Z</i> = 4 and <i>a</i> = 6.0756(2), <i>b</i> = 27.0606(9), <i>c</i> = 9.9471(3) Å, and β = 91.0008(8)° for <i>RE</i> = Y and <i>a</i> = 6.0290(3), <i>b</i> = 26.7385(12), <i>c</i> = 9.8503(5) Å, and β = 90.7270(30)° for <i>RE</i> = Lu. The unique crystal structure exhibits a three-dimensional network made up from Q⁴-type SiN₄ and Q³-type SiON₃ tetrahedra. Containing 4-fold bridging N^[4] atoms in star-shaped units [N^[4](SiN₃)₄] next to N^[3], N^[2], O^[1], and noncondensed oxide ions, the title compounds illustrate the vast structural variety in (oxo)­nitridosilicates. Under excitation with UV to blue light, Y₄Ba₂[Si₉ON₁₆]­O:Eu²⁺ shows emission in the orange-red spectral range (λ_max = 622 nm, full width at half-maximum (fwhm) ≈ 2875 cm^–1). Yellow-orange emitting Lu₄Ba₂[Si₉ON₁₆]­O:Eu²⁺ (λ_max = 586 nm, fwhm ≈ 2530 cm^–1) exhibits high internal quantum efficiency (IQE) ≈ 85%. This makes Lu₄Ba₂[Si₉ON₁₆]­O:Eu²⁺ a promising phosphor for low color rendering index (CRI) warm white phosphor converted light emitting diodes (pcLEDs)

Narrow-Band Green Emitting Nitridolithoalumosilicate Ba[Li₂(Al₂Si₂)N₆]:Eu²⁺ with Framework Topology <i>whj</i> for LED/LCD-Backlighting Applications

Eu²⁺- as well as Ce³⁺-doped Ba­[Li₂(Al₂Si₂)­N₆] and its related Mg-substituted compounds Ba­[(Mg_2–<i>x</i>Li_<i>x</i>) (Al_4–<i>x</i>Si_<i>x</i>)­N₆]:Eu²⁺ (<i>x</i> = 0–2) with <i>x</i> = 1.6, 1.8 have been synthesized by metathesis reactions in tantalum ampules. Crystal structures were solved and refined from single-crystal X-ray diffraction data. All three compounds crystallize in tetragonal space group <i>P</i>4/<i>ncc</i> (no. 130) (<i>Z</i> = 4, Ba­[Li₂(Al₂Si₂)­N₆]:Eu²⁺: <i>a</i> = 7.8282(4), <i>c</i> = 9.9557(5) Å, <i>R</i>1 = 0.0144, <i>wR</i>2 = 0.0366). Their crystal structures, exhibiting the novel framework topology <i>whj</i>, consist of a highly condensed anionic tetrahedra network of disordered (Li/Mg)­N₄ and (Al/Si)­N₄ units connected to each other by common edges and corners. The degree of condensation (i.e., atomic ratio (Al,Li,Mg,Si):N) is κ = 1. The Ba²⁺-position is coordinated eight-fold by N^3– in form of a truncated square pyramid. Upon doping with Eu²⁺, narrow-band emission in the green to yellow spectral range is observed (λ_em = 532–562 nm, fwhm ≈ 1962 cm^–1). Ce³⁺-doped crystals of Ba­[Li₂(Al₂Si₂)­N₆] show blue emission (λ_em = 468; 507 nm). According to the tunability of the narrow-band green emission, application in LED-backlight liquid crystal displays appears promising

Luminescence of an Oxonitridoberyllate: A Study of Narrow-Band Cyan-Emitting Sr[Be₆ON₄]:Eu²⁺

Oxo- and (oxo)­nitridoberyllates show exceptional potential as host lattices for application in illumination grade phosphor converted (pc)­LEDs due to their remarkable electronic and structural characteristics, allowing highly efficient narrow-band emission upon doping with Eu²⁺. Sr­[Be₆ON₄]:Eu²⁺, the first example of an oxonitridoberyllate phosphor, exhibits narrow-band cyan emission (λ_em = 495 nm; full width at half-maximum, fwhm = 35 nm; ≈1400 cm^–1), comparable to the emission of the oxonitridosilicate BaSi₂O₂N₂:Eu²⁺ (fwhm = 35 nm) or a cyan-emitting primary LED (fwhm = 27 nm). Sr­[Be₆ON₄]:Eu²⁺ reveals a highly condensed rigid 3D network with a remarkably large degree of condensation [i.e., atomic ratio Be:(O,N)] of κ = 1.2 that is achieved by interconnection of highly condensed layers of BeN₄ tetrahedra by Be₂ON₆ units via common edges. The crystal structure of Sr­[Be₆ON₄]:Eu²⁺ was solved on the basis of single-crystal and powder XRD data (<i>C</i>2/<i>c</i>, no. 15, <i>a</i> = 13.9283(14), <i>b</i> = 5.7582(6), <i>c</i> = 4.9908(5) Å, β = 90.195(1)°, <i>Z</i> = 4, <i>R</i>₁ = 0.033, <i>wR</i>₂ = 0.065, GoF = 1.046). Sr­[Be₆ON₄]:Eu²⁺ shows a close structural relationship to other nitride as well as oxide compounds, and therefore closes a structural gap helping to understand relations in Be-containing solid-state materials. The electronic structure of Sr­[Be₆ON₄]:Eu²⁺ was characterized by X-ray spectroscopy measurements, supported by density functional theory (DFT) calculations. Due to its excellent emission properties, large band gap, rigid 3D network, as well as chemical and thermal stability, Sr­[Be₆ON₄]:Eu²⁺ is a promising phosphor to close the cyan gap in efficient high-CRI pcLEDs (CRI, color rendering index)