Orange-Emitting Li4Sr4[Si4O4N6]O:Eu2+ - A Layered Lithium Oxonitridosilicate Oxide

Article abstract of DOI:10.1021/acs.inorgchem.8b02391

We report on the structure and properties of the lithium oxonitridosilicate oxide Li₄Sr₄[Si₄O₄N₆]O:Eu²⁺ obtained from solid-state metathesis. The crystal structure was solved and refined from single-crystal X-ray data in the space group P4₂/nmc (No. 137) [Z = 2, a = 7.4833(6), c = 9.8365(9) ?, and R1(obs) = 0.0477]. The structure of Li₄Sr₄[Si₄O₄N₆]O:Eu²⁺ is built up from a layered 2D network of SiN₃O tetrahedra and exhibits stacking disorder. The results are supported by transmission electron microscopy and energy-dispersive X-ray spectroscopy as well as lattice energy, charge distribution, and density functional theory (DFT) calculations. Optical measurements suggest an indirect band gap of about 3.6 eV, while DFT calculations on a model free of stacking faults yield a theoretical electronic band gap of 4.4 eV. Samples doped with Eu²⁺ exhibit luminescence in the orange spectral range (λ_em 625 nm; full width at half-maximum 4164 cm^-1 internal quantum efficiency at room temperature = 24%), extending the broad field of phosphor materials research toward the sparsely investigated materials class of lithium oxonitridosilicate oxides.

Full text of DOI:10.1021/acs.inorgchem.8b02391

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Orange-Emitting Li4Sr4[Si4O4N6]O:Eu2+a Layered Lithium
Oxonitridosilicate Oxide
Robin Niklaus,† Lukas Neudert,† Juliane Stahl,† Peter J. Schmidt,‡ and Wolfgang Schnick*,†
†Department of Chemistry, University of Munich (LMU), Butenandtstrasse 5−13, 81377 Munich, Germany
‡Lumileds Germany GmbH, Lumileds Phosphor Center Aachen, Philipsstrasse 8, 52068 Aachen, Germany
S
* Supporting Information
ABSTRACT: We report on the structure and properties of the lithium
oxonitridosilicate oxide Li4Sr4[Si4O4N6]O:Eu2+ obtained from solid-state
metathesis. The crystal structure was solved and refined from single-crystal X-
ray data in the space group P42/nmc (No. 137) [Z = 2, a = 7.4833(6), c =
9.8365(9) Å, and R1(obs) = 0.0477]. The structure of Li4Sr4[Si4O4N6]O:Eu2+
is built up from a layered 2D network of SiN3O tetrahedra and exhibits
stacking disorder. The results are supported by transmission electron
microscopy and energy-dispersive X-ray spectroscopy as well as lattice
energy, charge distribution, and density functional theory (DFT) calculations.
Optical measurements suggest an indirect band gap of about 3.6 eV, while
DFT calculations on a model free of stacking faults yield a theoretical
electronic band gap of 4.4 eV. Samples doped with Eu2+ exhibit luminescence
in the orange spectral range (λem ≈ 625 nm; full width at half-maximum ≈ 4164 cm−1; internal quantum efficiency at room
temperature = 24%), extending the broad field of phosphor materials research toward the sparsely investigated materials class of
lithium oxonitridosilicate oxides.
promising in order to increase the structural diversity, while
maintaining fundamental structure motifs.4−6 That is, these
compounds can all be considered to be Ca-substituted variants
of Ca5[Si2N6]. Similar analogies are known from Li-substituted
garnet-type materials, which have led to a number of suitable
solid electrolytes.7−9 This coincides with a general trend to
incorporate Li into phosphor host structures because it may
function both as a countercation like in Li2Ca2[Mg2Si2N6] and
Ca18.75Li10.5[Al39N55] and as part of the anionic network like in
Sr[LiAl3N4], Sr4[LiAl11N14], or Ca3Mg[Li2Si2N6].4,5,10−13 All
of which exhibit interesting luminescence properties upon Eu2+
or Ce3+ doping with auxiliary examples such as LiSi2N3:Eu2+,
Ba[Li2(Al2Si2)N6]:Eu2+, and Ba2LiSi7AlN12:Eu2+ at hand.14−16
Undeniably, Li-containing nitridosilicates have attracted a lot
of attention in recent years as novel phosphor materials.
With regard to the exploration of new and diverse anionic
networks and frameworks, the extensive study of oxonitrido-
silicates has further promoted the synthesis of Eu-doped
phosphor materials such as β-sialon, and the layered materials
(Sr,Ba)Si2O2N2 and RE26Ba6[Si22O19N36]O16 (RE = Y,
Tb).17−22 M(Si2O2N2) (M = Ca, Sr, Ba) represents an
excellent example as to how the choice of the cation may
further impact the structural properties. Alterations of layers
and metal coordination are introduced upon going from
CaSi2O2N2 (P21) to SrSi2O2N2 (P1) and BaSi2O2N2 (Pbcn),
impacting the luminescence properties upon Eu2+ doping.
INTRODUCTION
■
The increase of the complexity with regard to the elemental
and structural compositions of nitride phosphor materials has
recently led to unexpected multinary phosphors like
Li38.7RE3.3Ca5.7[Li2Si30N59]O2F:Ce3+ (RE = La, Ce, Y).1 This
process forebodes the importance of extending fundamental
research toward similar complex systems by offering a number
of tunable set screws in terms of composition and structural
phenomena, which, in turn, can influence the materials
properties. However, because of the difficulty of targeted
synthesis toward phosphor materials from scratch, it ever so
often resembles a roll of the dice. This is partly because of a
number of competing reactions and possible thermodynamic
sinks. This makes it difficult to control the synthesis and
respective formation conditions of such multinary component
systems, which often leads to microcrystalline samples.
However, high-throughput screening methods like single-
particle diagnosis in combination with transmission electron
microscopy (TEM) can help to facilitate the identification of
novel compounds for which structure determination is not
straightforward, such as La24Sr14−7x[Si36N72](O1−xFx)14 (x =
0.489).2,3 The subsequent migration into conventional
synthetic optimization strategies, upon the identification of
promising materials, can then be used to optimize bulk
synthesis. With regard to maintaining at least a partially
directed synthesis approach, phosphor host structures such as
Li2Ca2[Mg2Si2N6], Ca3Mg[Li2Si2N6], and Ca2Mg[Li4Si2N6]
exemplify that formal substitution of alkaline-earth metals by
significantly smaller and lighter cations such as Li appears
Received: August 23, 2018
© XXXX American Chemical Society
A
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light. Doped and nondoped products are stable toward O2 yet
sensitive to the extended exposure of water.
Concurrently, additional intergrowth occurs upon different
mixed occupation ratios on the (Sr, Ba) site between the latter
two.21,23 Especially for such layered compounds, real structure
phenomena such as stacking disorder and intergrowth can pose
major obstructions upon structure determination in the
absence of suitable single crystals or even in the attempted
confirmation of bulk syntheses. Challenges can be successfully
tackled by advanced analytical methods such as aberration-
corrected TEM. Next to a detailed elucidation of the latter
compounds by various TEM methods, applications comprise
the elucidation of modulated structures and the differentiation
of heavy cations with a roentgenographically similar Z-
contrast.3,20,24 In this contribution, we present the synthesis
and characterization of the novel orange luminescent layered
lithium oxonitridosilicate oxide Li4Sr4[Si4O4N6]O:Eu2+, ex-
tending the sparse field of lithium oxonitridosilicate com-
pounds. The structure was solved by single-crystal X-ray
diffraction (XRD). TEM investigations confirm the elemental
composition and cell metrics while revealing low-level stacking
disorder in the ab plane in both single crystals and bulk
samples.
Single-Crystal XRD. Single crystals of Li4Sr4[Si4O4N6]O:Eu2+
were isolated and mounted on MicroMounts (MiTeGen) with an
aperture size of 20 μm. XRD data were measured with a Bruker D8
Venture diffractometer with a rotating anode (Mo Kα radiation).
Absorption correction was performed using SADABS.27 The crystal
structure was solved using direct methods (SHELXS) and refined by a
least-squares method (SHELXL).28−30 Details on the crystal structure
investigation can be obtained from the Cambridge Crystallographic
Data Centre (CCDC) upon quoting the CCDC code 1862968.
Powder XRD (PXRD). PXRD measurements were carried out on
ground samples, which were loaded into silica glass capillaries of 0.2
mm diameter with a wall thickness of 0.01 mm (Hilgenberg GmbH,
Malsfeld, Germany). Diffraction data were obtained using a STOE
STADI P diffractometer [Cu Kα1 radiation, Ge(111) monochroma-
tor, Mythen1K detector] in modified parafocusing Debye−Scherrer
geometry. Rietveld refinements were carried out with TOPAS
Academic V4.1 software by applying the fundamental parameters
approach (direct convolution of source emission profiles, axial
instrument contributions, crystallite size, and microstrain ef-
fects).31−34 Preferred orientation was taken into account by using
fourth-order spherical harmonics, and anisotropic broadening of the
reflection profiles was refined using the LeBail−Jouanneaux
algorithm.35 Absorption effects were corrected using the calculated
absorption coefficient from XRD refinements.
Electronic and mechanical properties were calculated by
subsidiary density functional theory (DFT) calculations. Our
study combines different interlocking analytical methods that
together allow for concluding discussions on the possible
impact of real-structure phenomena on the resulting
luminescence properties relevant to future materials design
considerations.
UV/Vis Spectroscopy. A diffuse-reflectance spectrum of
Li4Sr4[Si4O4N6]O was measured with a Jasco V-650 UV/vis
spectrophotometer in the range of 240−800 nm with a 5 nm step
size. The device is equipped with a deuterium/halogen lamp (Czerny-
Turner monochromator with 1200 lines mm−1 concave grating,
photomultiplier tube detector).
Fourier Transform Infrared (FTIR) Spectroscopy. FTIR
spectroscopy on Li4Sr4[Si4O4N6]O:Eu2+ was carried out with a
Perkin Elmer BXII spectrometer using the attenuated-total-reflectance
(ATR) method.
EXPERIMENTAL SECTION
■
Synthesis. All synthesis steps were performed in an argon-filled
glovebox (Unilab, MBraun, Garching; O2 < 1 ppm; H2O < 1 ppm) or
in flame-dried glassware on a Schlenk line. The screening of different
product mixtures yielded a minority phase of single crystals of
Li4Sr3.62Eu0.38[Si4O4N6]O from reacting a composite mixture of SrF2
(0.17 mmol, 21.7 mg, Sigma-Aldrich, 99.99%), “Si(NH)2” (0.18
mmol, 10.6 mg), LiN3 (0.38 mmol, 19 mg), Mn (0.84 mmol, 46.4 mg,
Alfa Aesar, 99.95%), and EuF3 (0.009 mmol, 1.8 mg, Sigma-Aldrich,
99.99%).25,26 The ground starting materials were placed in a Ta
ampule, which was supposedly contaminated with traces of O during
welding. The ampule was placed in a tube furnace under vacuum,
heated to 950 °C at 5 K min−1, and kept at 950 °C for 32 h before the
furnace was turned off. The yellow-to-orange single crystals were
identified out of the heterogeneous product by UV illumination.
Following structure determination, the synthesis was optimized to
obtain bulk samples of Li4Sr4[Si4O4N6]O:Eu2+. Powder samples of
Li4Sr4[Si4O4N6]O were synthesized from SrF2 (0.18 mmol, 22.2 mg;
Sigma-Aldrich, 99.99%), SrH2 (0.18 mmol, 15.9 mg; Materion,
99.7%), LiN3 (0.363 mmol, 17.8 mg), Li(NH2) (0.206 mmol, 7.6 mg,
Sigma-Aldrich, 95%), Li2O (0.45 mmol, 13.56 mg, Schuchardt, 98%),
and “Si(NH)2” (0.36 mmol, 21.1 mg). CsI (0.2 mmol, 53 mg,
Chempur, 99.9%) was used as a flux. The starting materials were
thoroughly ground in an agate mortar and filled into a Nb ampule.
The ampule was subsequently welded shut and placed in a tube
Computational Details. The structural relaxation of
Li4Sr4[Si4O4N6]O was performed with the Vienna ab initio simulation
package (VASP).36−38 The total energy of the unit cell was converged
to 10−7 eV/atom with residual atomic forces below 5 × 10−3 eV Å−1.
The exchange correlation was treated within the generalized gradient
approximation (GGA) of Perdew, Burke, and Ernzerhof and the
projector-augmented-wave method.39−42 A plane-wave cutoff of 535
eV was chosen for calculations with a Brillouin zone sampling on a Γ-
centered k-mesh produced from the method of Monkhorst and Pack
of 7 × 7 × 6.43 Additional calculations were performed with the
modified Becke−Johnson formalism (GGA-mbj) to treat the
electronic band gap.44−46 In order to obtain elastic constants and
moduli, calculations of the elastic tensors were conducted by deriving
the stress−strain relationship from six finite lattice distortions of the
crystal utilizing displacements of 0.015 Å.47
Luminescence. Photoluminescence measurements were per-
formed on nanocrystalline powder samples in poly-
(tetrafluoroethylene) sample holders using an in-house-built system
based on a 5.3-in. integrating sphere and a spectrofluorimeter
equipped with a 150 W Xe lamp and two 500 mm Czerny−Turner
monochromators (1800 grooves mm−1 and 250/500 nm blaze
gratings) with a spectral range from 230 to 820 nm. The internal
quantum efficiency (IQE) of the samples was determined by
comparing integrated emission intensities and absorption at excitation
wavelength with standard materials (BaSO4, Merck, for white
standard DIN 5033; commercial (Sr,Ca)AlSiN3:Eu2+, Mitsubishi
Chemical, and Y3Al5O12:Ce3+, Philips) at room temperature. Thermal
quenching of the emission was investigated with an AvaSpec-2048
spectrometer and a stabilized light-emitting-diode (LED) light source
(450 nm) for sample excitation. Samples were measured in a
temperature range from room temperature to 330 °C with a step size
of ≈24 °C in Cu-plated sample holders that were temperature-
controlled with an IR lamp. The cryospectroscopy setup covers the
range from 300 to 6 K. The measurement was performed on a thick-
furnace (type AXIO 10/450, Huttinger Elektronik, Freiburg,
̈
Germany; maximal electrical output 10 kW), heated under vacuum
to 900 °C for 3 h, maintained at that temperature for 32 h, and finally
quenched to room temperature by switching off the furnace. The
resulting product was washed with dry ethanol to remove excess CsI.
The filter residue yielded a colorless mixture of variable amounts of
LiF, Li2SiN2, and nanocrystalline Li4Sr4[Si4O4N6]O as the majority
phase. The addition of EuF3 during synthesis with a nominal
concentration of 1 mol % Eu related to Sr yielded a powder of
Li4Sr4[Si4O4N6]O:Eu2+ with yellow body color. All such samples
show strong yellow-orange luminescence under irradiation with blue
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structure motifs of Li4Sr4[Si4O4N6]O:Eu2+ are depicted in
Figure 1a−d. The structure is composed of a layerlike network
of doubly bridging Q3-type SiN3O tetrahedra stacked along
[001].
bed powder layer of the specimen positioned in an evacuated cooling
chamber. Cooling was done via a liquid-He compressor system from
Advance Research System Inc. (ARS4HW).The samples were
measured by a fiber-coupled spectroscopy system containing a
thermally stabilized LED light source and a fiber-optic spectrometer
from Ocean Optics (HR2000+ES).
TEM. The TEM investigations were done on crystallites originating
from several samples of Li4Sr4[Si4O4N6]O:Eu2+. The crystals were
crushed and suspended in ethanol and then drop-casted onto a Cu
grid covered with a holey C film (PLANO GmbH, Wetzlar,
Germany). TEM experiments were performed on a Titan Themis
60-300 microscope (FEI, Hillsboro, OR) operated at a 300 kV
acceleration voltage and equipped with an X-FEG, monochromator,
Cs corrector, and windowless four-quadrant Super-X EDX detector
(acquisition time 45 s). The TEM images were recorded using a 4K ×
4K Ceta CMOS camera (FEI, Hillsboro, OR). For data evaluation,
the following software were used: Digital Micrograph and ProcessDif-
fraction7 [geometric calculations for selected-area electron diffraction
(SAED)], JEMS (SAED simulations), and ES Vision (evaluation of the
EDX spectra).48−51
RESULTS AND DISCUSSION
■
Synthesis and Chemical Analysis. For the synthesis of
Li4Sr4[Si4O4N6]O:Eu2+, highly reactive starting materials such
as SrF2, “Si(NH)2”, SrH2, and EuF3 were used. The synthesis
route yields typically nanocrystalline aggregates with single-
crystal sizes of approximately 1 μm on average. A few crystals
of larger size (≈10 μm) could be isolated for single-crystal
analysis from a single reaction batch. The composition could
be corroborated by TEM−EDX measurements on representa-
tive crystals of different batches (Table S1). ATR-IR
spectroscopy further indicates the absence of N−H groups
(Figure S1).
Figure 1. Different projections of the crystal structure of
Li4Sr4[Si4O4N6]O:Eu2+ with SiN3O tetrahedra (gray/black), N
atoms (blue), O atoms (red), Li atoms (pink), and mixed Sr/Eu
atoms (orange). (a) Projection along nearly [001] with the unit cell
outlined in black. (b) Representation of a single layer forming vierer
and achter rings enclosing trigonal-planar-coordinated Li. (c and d)
Representation of SiN3O layers along [100] (c) and [001] (d)
illustrating the layered composition.
Crystal Structure. The crystal structure of
Li4Sr4[Si4O4N6]O:Eu2+ was solved and refined in the space
group P42/nmc (No. 137). All atoms aside from Li were
refined anisotropically. The crystallographic details are given in
Table 1.
Additional crystallographic information is given in Tables
S2−S4. The crystal structure within the unit cell and different
The atomic ratio Si:(O/N) within the network is 2:5, which
corresponds to the regular value found in single-layer
silicates.52 The crystal structure of Li4Sr4[Si4O4N6]O:Eu2+
exhibits motifs very similar to those of the previously reported
Table 1. Crystallographic Data of the Single-Crystal
Structure Determination of Li4Sr3.62(3)Eu0.38(3)[Si4O4N6]O
nitridosilicate nitride Li2Sr4[Si2N5]N (space group I4m2).53 Its
̅
refined composition
formula mass [g mol−1
cryst syst
Li4Sr3.62(3)Eu0.38(3)[Si4O4N6]O
679(2)
tetragonal
structural relationship can be formally understood by the
removal of half of the Sr atoms in the layered interspace of
Li2Sr4[Si2N5]N combined with a 180° twist of every second
layer. In Li4Sr4[Si4O4N6]O:Eu2+, an additional isolated O atom
is located centric of each achter ring. The cation coordination
polyhedra are depicted in Figure 2. The Sr/Eu site is
coordinated in the interspace of the layered network by O
and N atoms of the SiN3O tetrahedra, forming a distorted
Johnson polyhedron (J49). Li is coordinated trigonal-planar by
the isolated O atom, and the N and terminal O atoms of two
independent SiN3O tetrahedra. The prolate nature of the O
displacement ellipsoid of the isolated O2 is likely to originate
from its higher degree of vibrational freedom compared to
anions of the rigid and covalent oxonitridosilicate network. A
possible split position and partial occupancy were further ruled
out from difference Fourier mapping (Figure S2). CHARDI
analysis further corroborates the cation assignment (Table S5).
The values for the tetrahedral bond lengths range from
1.692(3) Å (Si−O) to 1.708(2)−1.712(8) Å (Si−N), in good
agreement with values found in comparable oxonitridosilicate
]
space group
P42/nmc (No. 137)
a = 7.4833(6), c = 9.8365(9)
550.84(10)
lattice param [Å]
cell volume [Å3]
formula units/unit cell
2
X-ray density [g cm−3
]
4.096
abs coeff [μ mm−1
abs corrn
]
20.033
multiscan
diffractometer
Mo Kα radiation [Å]
F(000)
Bruker D8 Venture
λ = 0.71073
623
θ range [deg]
indep reflns
refined parameters/restraints
GoF
3.421 ≤ θ ≤ 30.497
475 [Rint = 0.0430]
35/0
1.094
R1 [all data/for I > 2σ(I)]
wR2 [all data/for I > 2σ(I)]
0.0477/0.0282
0.0633/0.0576
0.59/−0.75
Δρmax/Δρmin (e Å−3
)
C
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Figure 3. Rietveld refinement for a washed sample of
Li4Sr4(Si4O4N6)O:Eu2+. Experimental data are in black, the Rietveld
fit is the red line, and the difference plot is the green line. Minor
unknown reflections (purple) are highlighted as triangles in the inset.
Intensity misfits can be explained by stacking faults as observed by
HRTEM investigations. Reflection positions for Li4Sr4(Si4O4N6)-
O:Eu2+ (red bars, 84 wt %), Li2SiN2 (blue bars, 5 wt %), and LiF
(green bars, 11 wt %).
Figure 2. Coordination spheres of the Sr, Si, and Li atom sites in
Li4Sr4[Si4O4N6]O:Eu2+. Displacement ellipsoids shown with 90%
probability. Color code: Sr/Eu, orange; Si, turquoise; Li, ivory; O,
red; N, blue.
Additional diffuse intensity along [001]* was observed in
zone axes perpendicular to c like [100], [110], and [120]. It is
likely that the diffuse intensity is caused by structural
differences in the ab planes stacked along c. Figure 5 depicts
HRTEM investigations on thicker crystallites of the sample
from which the single crystal was taken and where the diffuse
intensity is more visible. It could be shown that areas of
approximately 100 nm2 are enough to cause diffuse intensity in
the corresponding Fourier transforms similar to SAED (see its
magnification in Figure S3). This is in agreement with the fact
that no diffuse intensities could be observed for such
crystallites with very thin crystal areas (Figure S4). The
observance of diffuse scattering can be in line with the
broadening of reflections in the PXRD patterns and
corroborates an assumed stacking disorder. Subsequently,
this effect could be approximated by DIFFaX simulations by
the introduction of stacking faults of about 2.5% impacting the
reflections in question, as shown in Figure 6.56 The chosen
stacking variant along with the matrix−vector relationship to
the original layer is found in Figure S5.
MAPLE. The assignment of O and N from single-crystal
refinement was further corroborated by calculations of the
lattice energies by the MAPLE concept.57−60 Results are
shown in Table 2. The total MAPLE value is in good
agreement (ΔE = 0.36%) with the one obtained by formation
from binary compounds. Furthermore, partial MAPLE values
of the different atoms are in the typical range of comparable
compounds in the literature, confirming the chosen N, O, and
Li assignments.53,61 For O2[0], there is no corresponding range
of MAPLE values, but lowered values with respect to O atoms
of higher bridging degree have been observed for other
oxonitridosilicate oxides before.52,62,63 We therefore conclude
that the lowered MAPLE value for O2 derives first from it not
being incorporated into the rigid nitridosilicate network and
second from its prolate displacement ellipsoid not being
described accurately by point-charge approximations.
compounds like Ce4[Si4O4N6]O or SrSi2O2N2.52,54 The same
applies for the Sr−O [2.580(2)−2.717(2) Å], Sr−N
[2.833(4)−2.846(2) Å], Li−O [1.955(8)−1.967(8) Å], and
Li−N [2.142(8) Å] interatomic distances.53,55 In order to
determine the composition of the samples, a Rietveld
refinement has been carried out based on a PXRD pattern
from an optimized synthesis bulk sample. A representative
refinement is seen in Figure 3. It indicates Li4Sr4[Si4O4N6]-
O:Eu2+ as the majority phase (≈84%) next to small amounts of
side phases like Li2SiN2 and LiF. A few minor unknown
reflections were observed that could not be assigned or
indexed. Furthermore, a broadening and intensity misfit for the
reflections (011), (012), and (211) was observed. Hence, in
order to corroborate the chemical composition, single-crystal
unit-cell parameters, ruling out unidentified superstructures
and in order to reveal further possible real-structure
phenomena, TEM investigations were conducted. Investiga-
tions comprise different bulk samples along with the single
crystals used for X-ray structure determination.
TEM and EDX. From a representative SAED tilt series of
crystallites, as depicted in Figure 4, it was possible to determine
the unit-cell parameters (tetragonal with a = 7.4 Å and c = 9.7
Å). They agree with the unit-cell parameters from X-ray
structure determination. In accordance with Laue class 4/
mmm, the SAED patterns along the zone axes [100], [110],
and [011] show mm2 symmetry, and the reflection intensities
approximately match those of the corresponding kinematical
simulations (Figure 4, bottom). EDX measurements on several
crystallites of Li4Sr4[Si4O4N6]O:Eu2+ further corroborate the
chemical composition (Table S1).
D
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Figure 4. SAED tilt series of a representative crystallite of Li4Sr4[Si4O4N6]O:Eu2+ with experimental patterns (top) and tilt angles (blue) as well as
simulated patterns (bottom) and tilt angles (red) based on the refined structure model from single-crystal X-ray data. Selected reflections are
labeled with indices. Diffuse intensities are observed along [001].
Table 2. Results of the MAPLE Calculations (kJ mol−1) for
a
Li4Sr4[Si4O4N6]O
Li4Sr4[Si4O4N6]O
Sr2+
1666.653
9719.430
864.558
Si4+
Li+
N1[2]
N2[2]
O1[1]
O2[0]
total MAPLE
5464.986
5401.430
2469.910
1550.042
93139 kJ mol−1
93479 kJ mol−1
Figure 5. Bright-field image (left) and HRTEM images viewed along
the [100] and [110] zone axes (middle) with corresponding Fourier
transforms (right). Thin area 1 (highlighted by the solid line) shows
no diffuse intensity in contrast to the thicker area 2 (highlighted by
the dotted line). Contrast variations perpendicular to c possibly
caused by defect layers are highlighted in yellow.
2
4/3Si3N4 + 4SrO + Li2O + /3Li3N
a
Typical partial MAPLE values (kJ mol−1): Sr2+, 1500−2300; Si4+,
9000−10200; N[2]3−, 4600−6000; Li+, 550−860; O[1]2−, 2000−2800.
7. Li4Sr4[Si4O4N6]O exhibits insulating properties with an
electronic band gap of about 4.4 eV. This increase compared to
Figure 6. DIFFaX simulation of a possible stacking variant of
Li4Sr4[Si4O4N6]O:Eu2+ indicating a broadening of selected reflections
upon the introduction of low levels of stacking faults. Simulation
based on a structure from single-crystal data.
Figure 7. Density of states of Li4Sr4(Si4O4N6)O as calculated with the
mBJ functional.
DFT Calculations. In order to calculate the electronic band
gap and mechanical properties of Li4Sr4[Si4O4N6]O, we
performed additional DFT calculations.
Electronic Structure. Calculations of the electronic
density of states for Li4Sr4[Si4O4N6]O are depicted in Figure
that of Li2Sr4(Si2N5)N, which was estimated to have an Eg of
about 2.4 eV, can possibly be attributed to the removal of one
Sr layer within Li4Sr4[Si4O4N6]O. The elastic constants,
resulting moduli, and Debye temperature (θD) are depicted
in Table 3 and are obtained from common relations.64−69 The
E
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Table 3. Calculated Elastic Constants Cnm, Bulk (B), Shear
(G), and Young’s (Y) Moduli (GPa), Debye Temperature
a
(θD, K), and Poisson’s Ratio (v) for Li4Sr4[Si4O4N6]O
elastic moduli
elastic constants
230.4
Bv
BR
BVRH
Gv
GR
GVRH
YVRH
v
114.8
107.4
111.1
70.0
62.0
65.9
165.1
0.25
592.2
C11
C22
C33
C44
C55
C66
C12
C13
C23
230.4
174.4
47.8
47.8
108.8
117.5
40.6
θD
40.6
a
Bulk and shear moduli according to the Voigt (BV and GV), Reuss
(BR and GR), and Voigt−Reuss−Hill (BVRH and GVRH) approaches.
The subscripts in Cnm were obtained from Cijkl according to the Voigt
notation xx → 1, yy → 2, zz → 3, yz → 4, zx → 5, and xy → 6.
calculations of elastic constants are in line with the layered
structure of Li4Sr4[Si4O4N6]O because C11 and C22 (a and b
directions) are significantly increased and less compressible
with respect to C33 (c direction). The Debye temperature (θD)
is a prominent proxy for phosphor materials, denoting the
rigidity of the crystal.70−72 An increase in θD formally equates
to an increase in rigidity and concomitantly energetically
elevated lattice vibrations, which are harder to excite. Dopant
atoms such as Eu2+ exhibiting transitions to protruding Eu 5d
orbitals upon excitation should, therefore, be less likely affected
by electron−phonon coupling effects with the host structure if
embedded in a highly rigid network of anions. Compared to
nitride compounds with higher condensation degrees such as
SrLiAl3N4 (θD = 716 K) or Sr2Si5N8 (θD = 702 K),72 the lower
θD value of 592 K for Li4Sr4[Si4O4N6]O:Eu2+, while reflecting
both the low bulk and shear moduli in line with the layered
nature of Li4Sr4[Si4O4N6]O:Eu2+, can possibly indicate an
extenuated thermal quenching.
Figure 8. Diffuse-reflectance spectra of undoped Li4Sr4[Si4O4N6]O
with insets of the respective Tauc plots [F(R) hν]1/n for (a) indirect
(n = 2) and (b) direct (n = /2) band gaps. Linear fit in order to
1
determine Eg in blue.
calculations because of the possible introduction of defect
states.
UV/Vis Spectroscopy. The optical band gap was
determined by UV/vis spectroscopy on a nondoped sample
of Li4Sr4[Si4O4N6]O. The diffuse-reflectance spectrum is
shown in Figure 8 together with insets of a pseudoabsorption
spectrum obtained by transformation via the Kubelka−Munk
formalism.73−76 The diffuse-reflectance spectra show broad
absorption ranging from 260 to 380 nm. The slight indent
starting from 280 nm is attributed to natural defect states in
the band gap possibly arising from the observed stacking faults.
This is in line with the fact that dopant levels of about 1 mol %
Eu in Li4Sr4[Si4O4N6]O, resulting in distinct 4f and 5d states
within the band gap, lead to a more pronounced effect in the
same range of the spectrum (Figure S6). The optical band gap
was subsequently derived by fitting the first ascent of the
pseudoabsorption [F(R) hν]1/n and taking its inflection point
with the energy scale. The resulting band gap of
Li4Sr4[Si4O4N6]O is approximately 3.6 eV for indirect and
4.25 eV for direct transitions, agreeing well with band structure
calculations favoring indirect transitions by about 0.6 eV
(Figure S7). The value for direct transitions is in better
correspondence with the obtained electronic band gap by DFT
of approximately 4.4 eV. A higher value for the electronic band
gap is to be expected because of the core−hole effect. Further,
the stacking disorder in Li4Sr4[Si4O4N6]O might lower the
band gap with regard to the fully ordered model from DFT
Luminescence. Luminescence measurements were per-
formed on thick-bed powder samples of Eu2+-doped
Li4Sr4[Si4O4N6]O in air. Excitation of the samples with UV-
to-blue light leads to a broad-band emission in the orange
spectral range. The excitation and emission spectra of a sample
with a nominal doping content of 1 mol % referred to Sr are
shown in Figure 9. The excitation spectrum exhibits a broad
band with a maximum at around 410−425 nm, enabling the
phosphor to be efficiently excited by blue-emitting (In,Ga)N-
LEDs. The emission spectrum peaks at 625 nm and exhibits a
full width at half-maximum (fwhm) of 4164 cm−1 (160 nm),
surpassing that of YAG:Ce3+ (fwhm ≈ 3700 cm−1; 120 nm).77
A broad-band emission is observed for Li4Sr4[Si4O4N6]-
O:Eu2+, as seen from Figure 9, despite featuring only one
crystallographic Sr/Eu site. A possible impact of side phases
could be ruled out from single-crystal luminescence measure-
ments showing identical emission behavior, as seen from
Figure S8. A nearly Gaussian-shaped emission band on the
energy scale is observed, which indicates a large Huang−Rhys
parameter, corresponding to strong electron−phonon cou-
pling.78
Figure 10 shows the rather asymmetric coordination of Sr by
O and N ions paired with the relatively unidimensional Sr
displacement ellipsoid toward the noncoordinating O2 (dSr−O2
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Figure 9. Excitation (blue) and emission (orange; λexc = 440 nm)
spectra of bulk phase Li4Sr4[Si4O4N6]O:Eu2+ with a nominal dopant
concentration of 1 mol %.
Figure 11. CIE 1931 (top) and CIE 1960 (bottom) color spaces
showing the CIE coordinates of Li4Sr4[Si4O4N6]O:Eu2+ along with
YAG:Ce3+, a pure blue LED, and the those of an ideal blackbody
radiator at different temperatures (K). CCT of a single-phosphor LED
indicated as ☆ on the yellow conjugation line alongside a
construction to determine the CCT of the phosphor material itself
(dotted gray intersection of the planckian locus in CIE 1960).79
Figure 10. Coordination spheres of the Sr/Eu site (orange) in
Li4Sr4[Si4O4N6]O with schematic displacement vectors (green
arrows). Displacement ellipsoids are shown with 90% probability.
Color code: SiN3O tetrahedra, black; Sr/Eu coordination polyhedra,
gray; Li, ivory; O, red.
temperature (CCT) of the phosphor to be around 2150 K
from the CIE 1960 diagram according to
Ä
É
−1
Å
Å
Å
Å
Å
Å
Å
Å
Å
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
Ñ
i
j
j
j
j
j
y
z
z
z
z
z
θ1
θ2
1
1
1
CCT =
+
−
T
T
T
i
i+1
i
Å
Ç
Ñ
Ö
k
{
= 3.11 A) position. The observed stacking disorder, observed
from PXRD and TEM investigations, is likely to impact the
local environment even further. Because the crystal field of the
spacious 5d orbitals, involved in the electronic 4f7−4f65d1
transitions in Eu2+, is prone to strong local distortions, an
explanation for the observed broadening of emission, further
enhanced by strong electron−phonon coupling, can hence be
inferred. The color coordinates according to the CIE 1931
convention are x = 0.534 and y = 0.450 and are depicted in
Figure 11.
where Ti and Ti+1 correspond to the isothermals of the
planckian CCT of the higher and lower isothermals and θ1 to
the angle between those isothermals. θ2 corresponds to the
angle between the higher CCT isothermal and the line drawn
from the color point to the isothermal interception point.79
The combination of a blue LED (λem = 450 nm) with
Li4Sr4[Si4O4N6]O:Eu2+ would result in a CCT of ≈2200 K
(candle light) and a CRI of 77.2 suitable for, e.g., decorative
lighting applications.
From Figure 11, it can be seen that the color point of
Li4Sr4[Si4O4N6]O:Eu2+ lies in the orange spectral region close
to the planckian locus. We estimated the correlated color
Thermal Quenching. In order to determine the
luminescence behavior of Li4Sr4[Si4O4N6]O:Eu2+, thermal
quenching experiments were performed at low temperatures
G
DOI: 10.1021/acs.inorgchem.8b02391
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Inorganic Chemistry
Article
(6−300 K) and up to typical LED operating temperatures
(around 100−200 °C) by measuring the luminescence
intensity and performing integration at each step. The thermal
quenching behavior is shown in Figure 12, together with the
occurrence of broadened reflections with low intensity. TEM
investigations prove to be a suitable tool in overcoming such
challenges, revealing stacking disorder as a real structure
phenomenon of the layered compound while corroborating the
cell metrics. As a result, the broadening of some reflections
observed in the PXRD patterns was successfully modeled
through the introduction of stacking faults. Li4Sr4[Si4O4N6]-
O:Eu2+ contributes to the sparsely investigated class of lithium
oxonitridosilicate(oxides), exemplifying paths to the discovery
of novel phosphor materials, highlighting the importance of
continuous research for compounds in this materials class.
Li4Sr4[Si4O4N6]O:Eu2+ shows emission in the yellow-to-
orange spectral range featuring an indirect optical band gap
of 3.6 eV. DFT calculations suggest an electronic band gap of
about 4.4 eV for a model free from stacking faults. Calculations
of the Debye temperature (592 K) reveal the highest
mechanical compressibility along the [001] direction, in line
with the layered structure of Li4Sr4[Si4O4N6]O. The emission
of Li4Sr4[Si4O4N6]O:Eu2+ is likely broadened because of large
electron−phonon coupling and concomitant stacking faults
between layers, as seen from the broadened Gaussian-shaped
emission profile. This, in turn, corroborates the use of θD as a
preliminary proxy when estimating the efficiency of a possible
phosphor host material beforehand. The synthesis and
subsidiary analytical approaches like TEM investigations of
Li4Sr4[Si4O4N6]O:Eu2+ further exemplify how optimization,
characterization, and in-depth illumination of real-structure
phenomena can interlock and help to push the understanding
of materials property relationships and phosphor materials
research approaches in general.
Figure 12. Thermal quenching measurements of Li4Sr4[Si4O4N6]-
O:Eu2+. The inset shows an Arrhenius plot of the thermal quenching
data at elevated temperatures in order to determine the thermal
activation energy Ea.
calculated thermal activation energy Ea obtained from an
Arrhenius fit according to the equation
ASSOCIATED CONTENT
■
S
* Supporting Information
I0
I =
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02391.
1 + eE/kT
Here I0 is the intensity at the beginning of measurement, k the
Boltzmann constant (eV), T the temperature (K), and I the
intensity at every following point of measurement.80,81 Ea is
then obtained as the slope of the plot ln[(I0/I) − 1] versus 1/
kT.
IR, further TEM and EDX results, atomic coordinates,
anisotropic displacement parameters, CHARDI results,
modeled stacking faults, band structure, and single-
crystal luminescence (PDF)
While the room temperature IQE was determined to be 24%
(38% at 6 K), Li4Sr4[Si4O4N6]O:Eu2+ shows significant
thermal quenching behavior at typical LED operating temper-
atures with Ea = 0.268 eV. This is subsequently significantly
lower than that of commercial phosphors like YAG:Ce3+ (0.81
eV).82 The observed behavior is in line with the strong
electron−phonon coupling, which could correspond to the 4f−
5d crossing model with increasing nonradiative emissions
possibly paired with thermal ionization to the conduction
band.81 The phosphor material is thus limited to a small
application space of solid-state lighting, despite its attractive
distribution of emission.
Accession Codes
CCDC 1862968 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: wolfgang.schnick@uni-muenchen.de (W.S.).
CONCLUSION
■
ORCID
A suitable method for the discovery of novel materials is the
screening of multicomponent mixtures. In this manner, the
novel layered lithium oxonitridosilicate oxide Li4Sr4[Si4O4N6]-
O:Eu2+ has been obtained. Its structure formally derives from
nonluminescent Li2Sr4[Si2N5]N by means of O substitution
and incorporation. The crystal structure was solved and refined
from single-crystal X-ray data. Reproduction of the compound
from optimized bulk syntheses was initially hindered by the
Wolfgang Schnick: 0000-0003-4571-8035
Author Contributions
All authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
H
DOI: 10.1021/acs.inorgchem.8b02391
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Inorganic Chemistry
Article
(16) Takeda, T.; Hirosaki, N.; Funahshi, S.; Xie, R.-J. Narrow-Band
Green-Emitting Phosphor Ba2LiSi7AlN12:Eu2+ with High Thermal
Stability Discovered by a Single Particle Diagnosis Approach. Chem.
Mater. 2015, 27, 5892−5898.
ACKNOWLEDGMENTS
■
We thank Dr. Peter Mayer for collecting single-crystal data
(LMU). Dr. Constantin Hoch is thanked for valuable
discussions regarding single-crystal analysis along with
Christian Maak and Philipp Bielec (all at LMU) for
constructive discussions. Detlef Wiechert and Volker Weiler
(both at Lumileds Phosphor Center Aachen) are thanked for
performing temperature-dependent luminescence measure-
ments and data processing. Prof. Dr. Oliver Oeckler (Institute
for Mineralogy, Crystallography and Materials Science, Leipzig
University) is thanked for contributory annotations. Financial
support by the Fonds der Chemischen Industrie is gratefully
acknowledged.
(17) Xie, R.-J.; Hirosaki, N.; Li, H.-L.; Li, Y. Q.; Mitomo, M.
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DOI: 10.1021/acs.inorgchem.8b02391
Inorg. Chem. XXXX, XXX, XXX−XXX