12 research outputs found
Tunable Red Luminescence in Nitridomagnesoaluminates α‑Sr<sub>2</sub>[MgAl<sub>5</sub>N<sub>7</sub>]:Eu<sup>2+</sup>, β‑Sr<sub>2</sub>[MgAl<sub>5</sub>N<sub>7</sub>]:Eu<sup>2+</sup>, and Sr<sub>8</sub>[LiMg<sub>2</sub>Al<sub>21</sub>N<sub>28</sub>]:Eu<sup>2+</sup>
Red-emitting
phosphors with the general sum formula Sr<sub>8</sub>[Li<sub>(2–0.5<i>x</i>)</sub>Mg<sub><i>x</i></sub>Al<sub>(22–0.5<i>x</i>)</sub>N<sub>28</sub>]:Eu<sup>2+</sup> 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<sup>–1</sup>, 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-% Eu<sup>2+</sup>). 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-Sr<sub>8</sub>[Li<sub>(2–0.5<i>x</i>)</sub>Mg<sub><i>x</i></sub>Al<sub>(22–0.5<i>x</i>)</sub>N<sub>28</sub>]:Eu<sup>2+</sup> (<i>x</i> = 2 and 4) crystallizes isotypically
with Sr<sub>4</sub>[LiAl<sub>11</sub>N<sub>14</sub>]:Eu<sup>2+</sup> 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-Sr<sub>2</sub>[MgAl<sub>5</sub>N<sub>7</sub>]:Eu<sup>2+</sup> (<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) Ã…,
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-Sr<sub>2</sub>[MgAl<sub>5</sub>N<sub>7</sub>]:Eu<sup>2+</sup> consists of a network of vertex- and edge-sharing AlÂ(/Mg)ÂN<sub>4</sub> tetrahedra with four- and five-membered ring channels along [010].
The Sr<sup>2+</sup> ions are located within the five-membered ring
channels and are coordinated cube-like by eight N atoms
Oxonitridosilicate Oxides <i>RE</i><sub>26</sub>Ba<sub>6</sub>[Si<sub>22</sub>O<sub>19</sub>N<sub>36</sub>]O<sub>16</sub>:Eu<sup>2+</sup> (<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><sub>26</sub>Ba<sub>6</sub>[Si<sub>22</sub>O<sub>19</sub>N<sub>36</sub>]ÂO<sub>16</sub>:Eu<sup>2+</sup> (<i>RE</i> = Y, Tb) were synthesized by high-temperature reaction
in a radiofrequency furnace starting from <i>RE</i>F<sub>3</sub>, <i>RE</i><sub>2</sub>O<sub>3</sub> (<i>RE</i> = Y, Tb), BaH<sub>2</sub>, SiÂ(NH)<sub>2</sub>, and EuF<sub>3</sub>. 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) Ã…, 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) Ã…, β = 119.103(2)° for <i>RE</i> = Tb]. The unique silicate layers are made up from Q<sup>1</sup>-, Q<sup>2</sup>-, and Q<sup>3</sup>-type SiÂ(O/N)<sub>4</sub>- as well as Q<sup>4</sup>-type SiN<sub>4</sub>-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, Y<sub>26</sub>Ba<sub>6</sub>[Si<sub>22</sub>O<sub>19</sub>N<sub>36</sub>]ÂO<sub>16</sub>:Eu<sup>2+</sup> shows
broad orange-red luminescence (λ<sub>em</sub> = 628 nm, fwhm
≈ 125 nm/3130 cm<sup>–1</sup>). An optical band gap
of 4.2 eV was determined for the doped compound by means of UV/vis
spectroscopy
Unprecedented Deep-Red Ce<sup>3+</sup> Luminescence of the Nitridolithosilicates Li<sub>38.7</sub><i>RE</i><sub>3.3</sub>Ca<sub>5.7</sub>[Li<sub>2</sub>Si<sub>30</sub>N<sub>59</sub>]O<sub>2</sub>F (<i>RE</i> = La, Ce, Y)
Ce<sup>3+</sup> 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
Ce<sup>3+</sup>, even in nitride host materials. Here, we report on
the nitridolithosilicates Li<sub>38.7</sub><i>RE</i><sub>3.3</sub>Ca<sub>5.7</sub>[Li<sub>2</sub>Si<sub>30</sub>N<sub>59</sub>]ÂO<sub>2</sub>F (<i>RE</i> = La, Ce, Y) with unique, red-shifted
Ce<sup>3+</sup> 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 SiN<sub>4</sub> and LiN<sub>4</sub> 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<sub>exc</sub> ≈ 540 nm. The
emission spectra peak is in the deep-red with λ<sub>em</sub> = 638–651 nm (fwhm ≈ 3600 cm<sup>–1</sup>).
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<sub>3–<i>x</i></sub>Ca<sub>1.5<i>x</i></sub>Si<sub>6</sub>N<sub>11</sub>:Eu<sup>2+</sup> (<i>x</i> ≈ 0.77): A Promising Phosphor for Next-Generation Amber pcLEDs
The
nitridosilicate La<sub>3–<i>x</i></sub>Ca<sub>1.5<i>x</i></sub>Si<sub>6</sub>N<sub>11</sub>:Eu<sup>2+</sup> (<i>x</i> ≈ 0.77) was synthesized in a radiofrequency
furnace starting from LaF<sub>3</sub>, LaÂ(NH<sub>2</sub>)<sub>3</sub>, CaH<sub>2</sub>, SiÂ(NH)<sub>2</sub>, and EuF<sub>3</sub>. 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)
Ã…, and <i>Z</i> = 2. Thereby, the so far unknown charge
balance mechanism in the system (La,Ca)<sub>3</sub>Si<sub>6</sub>N<sub>11</sub>, which is necessary as bivalent Ca<sup>2+</sup> substitutes
trivalent La<sup>3+</sup>, was clarified. Accordingly, charge balance
is achieved by incorporation of Ca<sup>2+</sup> on three cation sites,
including an additional third site compared to the homeotypic La<sub>3</sub>Si<sub>6</sub>N<sub>11</sub> 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 Eu<sup>2+</sup> doped compound exhibits a remarkably
narrow emission in the yellow-orange spectral range (λ<sub>em</sub> ≈ 587 nm, fwhm ≈ 60 nm/1700 cm<sup>–1</sup>). Because of the intriguing yellow-orange luminescence, La<sub>3–<i>x</i></sub>Ca<sub>1.5<i>x</i></sub>Si<sub>6</sub>N<sub>11</sub>:Eu<sup>2+</sup> (<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 Li<sub>2</sub>Ca<sub>2</sub>[Mg<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup>, BaÂ[Li<sub>2</sub>(Al<sub>2</sub>Si<sub>2</sub>)ÂN<sub>6</sub>]:Eu<sup>2+</sup>, and SrÂ[LiAl<sub>3</sub>N<sub>4</sub>]:Eu<sup>2+</sup> 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 SrÂ[LiAl<sub>3</sub>N<sub>4</sub>]:Eu<sup>2+</sup> and SrÂ[Mg<sub>3</sub>SiN<sub>4</sub>]:Eu<sup>2+</sup>. These techniques are widely
applicable and create a comprehensive, experimental picture of the
Eu<sup>2+</sup> 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 Li<sub>2</sub>(Ca<sub>1–<i>x</i></sub>Sr<sub><i>x</i></sub>)<sub>2</sub>[Mg<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> (<i>x</i> = 0–0.06)
The
nitridomagnesosilicates Li<sub>2</sub>Ca<sub>2</sub>[Mg<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> and Li<sub>2</sub>(Ca<sub>1.88</sub>Sr<sub>0.12</sub>)Â[Mg<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]:Eu<sup>2+</sup> show narrow-band red emission at 638
and 634 nm, respectively, with an emission bandwidth of 62 nm (∼1513
cm<sup>–1</sup>) after excitation in the blue spectral region.
Ce<sup>3+</sup>-doped samples show luminescence in the green spectral
range (λ<sub>em</sub> = 540 nm). The compounds were synthesized
via solid-state metathesis reaction in Li melts. Refinement of single-crystal
X-ray diffraction data revealed that Li<sub>2</sub>(Ca<sub>1.88</sub>Sr<sub>0.12</sub>)Â[Mg<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>] crystallizes
isomorphic to Li<sub>2</sub>Ca<sub>2</sub>[Mg<sub>2</sub>Si<sub>2</sub>N<sub>6</sub>]: <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><sub>1</sub> = 0.021, <i>wR</i><sub>2</sub> =
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<sub>18.75</sub>Li<sub>10.5</sub>[Al<sub>39</sub>N<sub>55</sub>]:Eu<sup>2+</sup>î—¸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., SrÂ[LiAl<sub>3</sub>N<sub>4</sub>]:Eu<sup>2+</sup> or CaÂ[LiAl<sub>3</sub>N<sub>4</sub>]:Eu<sup>2+</sup>. Here, we describe the novel
material Ca<sub>18.75</sub>Li<sub>10.5</sub>[Al<sub>39</sub>N<sub>55</sub>]:Eu<sup>2+</sup> with highly efficient narrow-band red emission
(λ<sub>em</sub> ≈ 647 nm, full width at half-maximum,
fwhm ≈ 1280 cm<sup>–1</sup>). This compound features
a rather uncommon crystal structure, comprising sphalerite-like T<sub>5</sub> supertetrahedra that are composed of tetrahedral AlN<sub>4</sub> units that are interconnected by additional AlN<sub>4</sub> moieties. The network charge is compensated by Ca<sup>2+</sup> and
Li<sup>+</sup> 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>3Ì…<i>m</i> (No. 227) with <i>a</i> = 22.415(3) Ã… 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 Lu<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]O:Eu<sup>2+</sup> and Orange-Red Emitting Y<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]O:Eu<sup>2+</sup>: Two Oxonitridosilicate Oxides with Outstanding Structural Variety
The
oxonitridosilicate oxides Y<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]ÂO:Eu<sup>2+</sup> and Lu<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]ÂO:Eu<sup>2+</sup> have been
synthesized starting from <i>RE</i>F<sub>3</sub>, <i>RE</i><sub>2</sub>O<sub>3</sub> (<i>RE</i> = Y, Lu),
BaH<sub>2</sub>, SiÂ(NH)<sub>2</sub>, and EuF<sub>3</sub> 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>2<sub>1</sub>/<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 Q<sup>4</sup>-type SiN<sub>4</sub> and Q<sup>3</sup>-type SiON<sub>3</sub> tetrahedra. Containing 4-fold bridging N<sup>[4]</sup> atoms in
star-shaped units [N<sup>[4]</sup>(SiN<sub>3</sub>)<sub>4</sub>] next
to N<sup>[3]</sup>, N<sup>[2]</sup>, O<sup>[1]</sup>, and noncondensed
oxide ions, the title compounds illustrate the vast structural variety
in (oxo)Ânitridosilicates. Under excitation with UV to blue light,
Y<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]ÂO:Eu<sup>2+</sup> shows emission in the orange-red spectral range (λ<sub>max</sub> = 622 nm, full width at half-maximum (fwhm) ≈ 2875
cm<sup>–1</sup>). Yellow-orange emitting Lu<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]ÂO:Eu<sup>2+</sup> (λ<sub>max</sub> = 586 nm, fwhm ≈ 2530 cm<sup>–1</sup>) exhibits
high internal quantum efficiency (IQE) ≈ 85%. This makes Lu<sub>4</sub>Ba<sub>2</sub>[Si<sub>9</sub>ON<sub>16</sub>]ÂO:Eu<sup>2+</sup> a promising phosphor for low color rendering index (CRI) warm white
phosphor converted light emitting diodes (pcLEDs)
Narrow-Band Green Emitting Nitridolithoalumosilicate Ba[Li<sub>2</sub>(Al<sub>2</sub>Si<sub>2</sub>)N<sub>6</sub>]:Eu<sup>2+</sup> with Framework Topology <i>whj</i> for LED/LCD-Backlighting Applications
Eu<sup>2+</sup>- as well as Ce<sup>3+</sup>-doped BaÂ[Li<sub>2</sub>(Al<sub>2</sub>Si<sub>2</sub>)ÂN<sub>6</sub>] and its related Mg-substituted
compounds BaÂ[(Mg<sub>2–<i>x</i></sub>Li<sub><i>x</i></sub>) (Al<sub>4–<i>x</i></sub>Si<sub><i>x</i></sub>)ÂN<sub>6</sub>]:Eu<sup>2+</sup> (<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, BaÂ[Li<sub>2</sub>(Al<sub>2</sub>Si<sub>2</sub>)ÂN<sub>6</sub>]:Eu<sup>2+</sup>: <i>a</i> = 7.8282(4), <i>c</i> = 9.9557(5)
Ã…, <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)ÂN<sub>4</sub> and (Al/Si)ÂN<sub>4</sub> 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 Ba<sup>2+</sup>-position is coordinated eight-fold by N<sup>3–</sup> in form of a truncated square pyramid. Upon doping with Eu<sup>2+</sup>, narrow-band emission in the green to yellow spectral range is observed
(λ<sub>em</sub> = 532–562 nm, fwhm ≈ 1962 cm<sup>–1</sup>). Ce<sup>3+</sup>-doped crystals of BaÂ[Li<sub>2</sub>(Al<sub>2</sub>Si<sub>2</sub>)ÂN<sub>6</sub>] show blue emission (λ<sub>em</sub> = 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[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup>
Oxo-
and (oxo)Ânitridoberyllates show exceptional potential as host
lattices for application in illumination grade phosphor converted
(pc)ÂLEDs due to their remarkable electronic and structural characteristics,
allowing highly efficient narrow-band emission upon doping with Eu<sup>2+</sup>. SrÂ[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup>, the first
example of an oxonitridoberyllate phosphor, exhibits narrow-band cyan
emission (λ<sub>em</sub> = 495 nm; full width at half-maximum,
fwhm = 35 nm; ≈1400 cm<sup>–1</sup>), comparable to
the emission of the oxonitridosilicate BaSi<sub>2</sub>O<sub>2</sub>N<sub>2</sub>:Eu<sup>2+</sup> (fwhm = 35 nm) or a cyan-emitting primary
LED (fwhm = 27 nm). SrÂ[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup> 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 BeN<sub>4</sub> tetrahedra by Be<sub>2</sub>ON<sub>6</sub> units
via common edges. The crystal structure of SrÂ[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup> 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) Ã…, β = 90.195(1)°, <i>Z</i> = 4, <i>R</i><sub>1</sub> = 0.033, <i>wR</i><sub>2</sub> = 0.065, GoF = 1.046). SrÂ[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup> 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 SrÂ[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup> 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, SrÂ[Be<sub>6</sub>ON<sub>4</sub>]:Eu<sup>2+</sup> is a promising phosphor to close the cyan gap in efficient
high-CRI pcLEDs (CRI, color rendering index)