Crystal Structure and Nontypical Deep-Red Luminescence of Ca3Mg[Li2Si2N6]:Eu2+

Article abstract of DOI:10.1021/acs.chemmater.7b00871

Rare-earth-doped nitridosilicates exhibit outstanding luminescence properties and have been intensively studied for solid-state lighting. Here, we describe the new nitridolithosilicate Ca₃Mg[Li₂Si₂N₆]:Eu²⁺ with extraordinary luminescence characteristics. The compound was synthesized by the solid-state metathesis reaction in sealed Ta ampules. The crystal structure was solved and refined on the basis of single-crystal X-ray diffraction data. Ca₃Mg[Li₂Si₂N₆]:Eu²⁺ crystallizes in the monoclinic space group C2/m (no. 12) [Z = 4, a = 5.966(1), b = 9.806(2), c = 11.721(2) ?, β = 99.67(3)°, V = 675.9(2) ?³] and exhibits a layered anionic network made up of edge- and corner-sharing LiN₄ tetrahedra and [Si₂N₆]^10- bow-tie units. The network charge is compensated by Ca²⁺ and Mg²⁺ ions. Upon irradiation with UV to blue light, red emission at exceptionally long wavelengths (λ_em = 734 nm, fwhm ≈2293 cm^-1) is observed. According to emission in the near-infrared, application in LEDs for horticultural lighting appears promising.

Full text of DOI:10.1021/acs.chemmater.7b00871

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Article
Crystal Structure and Nontypical Deep-
Red Luminescence of Ca3Mg[Li2Si2N6]:Eu2+
Christine Poesl, and Wolfgang Schnick
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.7b00871 • Publication Date (Web): 03 Apr 2017
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Chemistry of Materials
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Crystal Structure and Nontypical Deep-Red Luminescence of
Ca3Mg[Li2Si2N6]:Eu2+
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Christine Poesl and Wolfgang Schnick*
Department of Chemistry, University of Munich (LMU), Butenandtstr. 5-13 (D), 81377 Munich, Germany
ABSTRACT: Rare-earth doped nitridosilicates exhibit outstanding luminescence properties and have been intensively
studied for solid-state lighting. Here, we describe the new nitridolithosilicate Ca3Mg[Li2Si2N6]:Eu2+ with extraordinary
luminescence characteristics. The compound was synthesized by solid-state metathesis reaction in sealed Ta ampules.
The crystal structure was solved and refined on the basis of single-crystal X-ray diffraction data. Ca3Mg[Li2Si2N6]:Eu2+
crystallizes in the monoclinic space group C2/m (no. 12) [Z = 4, a = 5.966(1), b = 9.806(2), c = 11.721(2) Å, β = 99.67(3)°,
V = 675.9(2) Å3] and exhibits a layered anionic network made up of edge- and corner-sharing LiN4 tetrahedra and
[Si2N6]10‒ bow-tie units. The network charge is compensated by Ca2+ and Mg2+ ions. Upon irradiation with UV to blue
light, red emission at exceptionally long wavelengths (λem = 734 nm, fwhm ≈ 2293 cm−1) is observed. According to emission
in the near infrared, application in LEDs for horticultural lighting appears promising.
compounds Ba[Mg3SiN4], Sr[Mg3GeN4], and M[Mg2Ga2N4]
INTRODUCTION
with M = Sr,Ba).17,20-21
Nitridosilicates are characterized by its manifold structural
Nitridosilicates show a great diversity of interesting materi-
chemistry and intriguing materials properties. Due to the
als properties, e.g. high mechanical strength and high
high sensitivity to air and moisture of both starting mate-
thermal stability of silicon nitride ceramics, lithium ion
rials and products, synthesis of less condensed nitridosili-
conductivity (e.g. Li2SiN2) 22-23 or luminescence when
cates is quite challenging. Over the last years, different
doped with Eu2+ or Ce3+.22,24-28 (Sr,Ba)2Si5N8:Eu2+,29-31
synthetic approaches leading to nitridosilicates have been
CaAlSiN3:Eu2+,19
and
SrMg3SiN4:Eu2+ 16
represent
elaborated. These include high-temperature reactions,
precursor routes, ammonothermal syntheses and flux
methods by using, for example, liquid sodium.1-9 Solid-
state metathesis reactions have gained significant im-
portance as well, since a variety of ternary or higher ni-
trides have recently been synthesized by this approach.10-15
Thereby, key feature is the employment of reactive start-
ing materials by coproducing a metathesis salt, which on
one hand drives the reaction thermodynamically and on
the other hand concurrently acts as reactive flux.
prominent examples of rare-earth doped nitrides, which
show strong broadband emission in the red spectral
region after irradiation with blue light, originating from
parity allowed 4f65d1 ꢀ 4f7 transitions in Eu2+ and spin and
parity allowed 4f05d1 ꢀ 4f1 transitions in Ce3+. Warm white
light can be generated by combination of red and green
emitting phosphors, both covered on a blue LED chip,
which is usually an (In,Ga)N semiconductor chip.32-33 The
so-called phosphor-converted light-emitting diodes (pc-
LEDs) are highly efficient and, over the last decade, devel-
oped into an energy-saving replacement for inefficient
incandescent light bulbs. Since most commercially availa-
ble warm white pc-LEDs still show a large portion of its
emission beyond the sensitivity of the human eye,34-35
research for narrow-band red emitting luminescent mate-
rials is of particular interest for general illumination pur-
poses.
Common structural motifs of nitridosilicates are SiN4
tetrahedra, which may be linked through vertices and/or
edges, forming more or less condensed anionic substruc-
tures ranging up to three-periodic anionic frameworks.
The resulting structural diversity of nitridosilicates can be
ascribed to the N atoms, which are able to connect up to
four adjacent tetrahedral centers. Cations like alkaline
earths (AE2+), Li+ or Mg2+ are distributed among the voids
of the anionic networks and balance their charges. As
some examples in literature have already shown, the
number of new nitrides can be further increased by partial
substitution of Si4+ by Li+, Mg2+ or Al3+.12-13,16-19 Complete
exchange of Si4+ by Ga3+ or Ge4+ is also possible and results
in the strongly related compound classes of nitridogallates
and nitridogermanates, respectively (see homeotypical
During our research for innovative red emitting com-
pounds, we came across Ca3Mg[Li2Si2N6]:Eu2+, a new ni-
tridolithosilicate with outstanding emission properties at
fairly long wavelengths. In this contribution, we report on
synthesis, structural elucidation, and optical properties of
Ca3Mg[Li2Si2N6]:Eu2+. Unprecedented luminescence prop-
erties differ essentially from previously described Eu2+-
doped nitridosilicates and are discussed in detail, also in
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comparison to recently reported narrow-band red emit-
Luminescence. Luminescence investigations on single
ting Eu2+-doped nitridosilicates.
crystals of Ca3Mg[Li2Si2N6]:Eu2+ were performed using a
luminescence microscope, consisting of a HORIBA Fluo-
rimax4 Spectrofluorimeter system, which is attached to an
Olympus BX51 microscope via fiber optics. The single
crystals were measured in sealed glass capillaries with an
excitation wavelength of 440 nm. The emission spectrum
was measured between 460 and 800 nm; the excitation
spectrum was measured between 380 and 680 nm, with a
step size of 2 nm, respectively. Since Ca3Mg[Li2Si2N6]:Eu2+
shows emission beyond 800 nm and the spectrometer is
only calibrated for wavelengths up to 800 nm, emission
peak in the near infrared was simulated using a Gauss-fit.
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EXPERIMENTAL SECTION
Synthesis. Due to the sensitivity of the starting materials
and the product towards air and moisture, all manipula-
tions were performed under inert-gas conditions using
argon filled glove boxes (Unilab, MBraun, Garching,
O2 < 1 ppm, H2O < 1 ppm) and combined Schlenk/vacuum
lines. Ar was dried and purified by passing through glass
tubes filled with silica gel (Merck), molecular sieve (Fluka,
4 Å), P4O10 (Roth, ≥99%) and titanium sponge (Johnsen
Matthey, 99.5%). Ca3Mg[Li2Si2N6]:Eu2+ was synthesized by
solid-state metathesis reaction using Ca(NH2)2 (17.9 mg,
0.25 mmol, synthesized according to Brokamp),36 CaH2
(8.3 mg, 0.20 mmol, Cerac, 99.5%), MgF2 (15.5 mg,
0.25 mmol, ABCR, 99.99%), Si3N4 (6.9 mg, 0.05 mmol,
UBE, 99.9%), 1.7 mol% EuF3 (0.9 mg, 4.3 ∙ 10‒3 mmol, Sig-
ma-Aldrich, 99.99%), and Li (9.8 mg, 1.41 mmol, Alfa Ae-
ser, 99.9%) as starting materials. The powdery reactants
were finely ground and placed with metallic Li in a tanta-
lum ampule, which was weld shut in an arc furnace. The
reaction mixture was heated up to 950 °C with a rate of
300 °C∙h‒1, maintained for 24 h, subsequently cooled down
to 500 °C with a rate of 5 °C∙h‒1 and finally quenched to
room temperature by switching off the furnace. Orange
colored single crystals of Ca3Mg[Li2Si2N6]:Eu2+ were ob-
tained embedded in an amorphous powder. Additionally,
red rod-shaped crystals of Ca[Mg3SiN4]:Eu2+, colorless
crystals of LiF, and some metallic residues were found as
side phases of the reaction. The sample is sensitive to-
wards air and moisture and partially shows intensive red
luminescence under irradiation with ultraviolet (UV) to
green light.
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RESULTS AND DISCUSSION
Synthesis and Chemical Analysis. Solid-state metathe-
sis reaction as described above yielded a heterogeneous
reaction
mixture
containing
single-crystals
of
Ca3Mg[Li2Si2N6]:Eu2+. The crystals show orange body color
and undergo rapid hydrolysis when exposed to air. The
elemental composition was determined by energy-
dispersive X-ray spectroscopy (EDX). Due to the method,
light elements like N cannot be determined with a rea-
sonable degree of accuracy, also Li is not determinable.
The atomic ratio Ca/Mg/Si of 3.0/1.1/2.0 (six measure-
ments on different crystals), normalized to the Ca con-
tent, is in good agreement with the sum formula obtained
from single-crystal structure analysis. Besides a small
amount of oxygen, which can be attributed to superficial
hydrolysis of the product, no further elements were de-
tected.
Single-Crystal Structure Analysis. The crystal structure
of Ca3Mg[Li2Si2N6]:Eu2+ was solved and refined in the
monoclinic space group C2/m (no. 12) with a = 5.966(1),
b = 9.806(2), c = 11.721(2) Å, and β = 99.67(3)°. The crystal-
lographic data are summarized in Table 1. Atomic coordi-
nates, Wyckoff positions, and isotropic displacement
parameters are listed in Table 2. Selected bond lengths
and angles as well as anisotropic displacement parameters
are given in the Supporting Information (Tables S1-S3).
Due to the small dopant concentration and, therefore, the
insignificant scattering intensity, Eu2+ was neglected dur-
ing structure refinement.
Single-Crystal X-ray Diffraction. Single crystals of
Ca3Mg[Li2Si2N6]:Eu2+ were isolated from the reaction
product, washed with dry paraffin oil and enclosed in
glass capillaries, which were sealed under argon to avoid
hydrolysis. X-ray diffraction data were collected on a Stoe
IPDS I diffractometer (Mo-Kα radiation, graphite mono-
chromator). Structure solution was done in SHELXS-97,37
by using direct methods. The crystal structure was refined
by full-matrix least-squares calculation on |F|2 in
SHELXL-97 38-39 with anisotropic displacement parameters
for all atoms. Further details of the crystal-structure re-
finement may be obtained from the Fachinfor-
mationszentrum
Karlsruhe,
76344
Eggenstein-
Leopoldshafen, Germany (Fax: +49-7247-808-666; E-mail:
crysdata@fiz-karlsruhe.de), on quoting the depository
number CSD-432605.
Scanning Electron Microscopy. The chemical composi-
tion of Ca3Mg[Li2Si2N6]:Eu2+ was confirmed by energy
dispersive X-ray spectroscopy (FEI Helios G3 UC scanning
electron microscope equipped with an EDX detector,
scanning transmission detector and focused ion beam).
The reaction product was placed on an adhesive conduc-
tive carbon pad and coated with a conductive carbon film
(BAL-TEC MED 020, Bal Tec AG).
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Chemistry of Materials
tahedral coordination by N and with distances varying
Figure 1. Details of layers in Ca3Mg[Li2Si2N6]:Eu2+. Left:
[Si2N6]10‒ bow-tie units (turquoise) and achter rings of
LiN4 tetrahedra (violet). Right: Layer made up of bow-tie
units and achter rings forming two sublayers of con-
densed dreier rings, only one of them shown for reasons
of clarity.
from 2.310(4)-2.744(5) Å. Mg2+ exhibits fourfold rectangu-
lar surroundings by N. Mg−N bond lengths are in a range
of 2.159(5)-2.199(5) Å. Additionally, there is one further N
atom at a distance of 2.693(7) Å, its coordination to Mg1 is
illustrated in Figure 3 by a dashed line. Ca−N as well as
Mg−N distances correspond to the sums of the ionic radii
(2.55 and 2.10 Å, respectively)45 as well as to distances
known from other nitridosilicates.1,12,16 The Madelung part
of the lattice energy has been calculated to confirm the
crystal structure of Ca3Mg[Li2Si2N6]:Eu2+.45,47-49 Therefore,
the charge, distance, and coordination spheres of consti-
tuting ions were taken into account. MAPLE values for
Ca3Mg[Li2Si2N6]:Eu2+ and its constituting binary and ter-
nary nitrides are given in Table 3. The resulting deviation
of 0.31% verifies the electrostatic consistency of the re-
fined structure.
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The crystal structure of Ca3Mg[Li2Si2N6]:Eu2+ is closely
related to that of Ca2Mg[Li4Si2N6],12 which has already
been reported in literature. Both structures contain iden-
tical layers of LiN4 and SiN4 tetrahedra as well as Ca and
Mg ions, which are located between the layers. Since Li as
well as Si are part of the tetrahedral network, the com-
pounds can be more precisely classified as nitridolithosili-
cates. The arrangement of tetrahedra within one layer is
shown in Figure 1. It consists of vertex- and corner-
sharing LiN4 tetrahedra forming achter rings,40 intercalat-
ed by edge-sharing [Si2N6]10‒ “bow-tie” units. This results
in layers of condensed dreier rings in the ab plane. The
differences between the structurally related compounds
Ca2Mg[Li4Si2N6] and Ca3Mg[Li2Si2N6]:Eu2+ are additional
[Si2N6]10‒ bow-tie units in the crystal structure of
Ca3Mg[Li2Si2N6]:Eu2+. Together with the Ca2+ and Mg2+
ions, these bow-tie units are located between layers of
dreier rings. A direct comparison of both crystal struc-
tures is illustrated in Figure 2.
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Figure 3. Coordination spheres of counter ions Ca2+ and
Mg2+.
Table 1. Crystallographic data of the single-crystal
structure determination of Ca3Mg[Li2Si2N6]:Eu2+.
formula
Ca3Mg[Li2Si2N6]:Eu2+
monoclinic
C2/m (no. 12)
a = 5.966(1)
b = 9.806(2)
c = 11.721(2)
β = 99.67(3)
675.9(2)
crystal system
space group
lattice parameters / Å,°
Figure 2. Crystal structures of Ca3Mg[Li2Si2N6]:Eu2+ (a) and
Ca2Mg[Li4Si2N6] (b).
cell volume / Å3
formula units / unit cell
density / g∙cm−3
µ / mm−1
4
2.935
[Si2N6]10‒ bow-tie units were first described in Ba5Si2N6,2
and are also known in other nitridosilicates, e.g.
Ca2Mg[Li4Si2N6], Li2Ca2[Mg2Si2N6],12 and Ca3[Li4Si2N6].41-42
2.827
T / K
293(2)
diffractometer
radiation
Stoe IPDS I
Si−N
bond
lengths
in
these
bow-tie
units
Mo-Kα
(λ = 0.71073 Å),
(Ca3Mg[Li2Si2N6]:Eu2+: 1.719(5)-1.843(7) Å) are slightly
larger than distances found in other nitridosilicates.1,5,16,43-
graphite monochroma-
tor
44
This can be attributed to the repulsion of the Si atoms
F(000)
592
in the pairs of edge-sharing SiN4 tetrahedra (Si−Si:
2.363(4)-2.477(4) Å), which in turn is related to a reduced
angle N-Si-N. Li−N distances range from 2.088(11) to
2.225(11) Å and are in good agreement with distances
known from other nitrides and with the sum of the ionic
radii.11,44-46
profile range
index ranges
4.04 ≤ θ ≤ 29.90
−8 ≤ h ≤ 8
−13 ≤ k ≤ 13
−16 ≤ l ≤ 16
1028 [R(int) = 0.0722]
72
independent reflections
refined parameters
Goodness of fit
Ca2+ and Mg2+ ions balance the charges of the anionic
framework. Its coordination spheres are displayed in Fig-
ure 3. Ca2+ occupies two different sites with distorted oc-
1.323
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Eu2+-doped compound exhibits a maximum absorption
R1 (all data); R1 (F2> 2σ(F2))
wR2 (all data); wR2 (F2> 2σ(F2))
Δρmax, Δρmin / e∙Å–3
0.0770, 0.0618
0.1118, 0.1080
0.826, −1.176
1
2
3
4
around 450 nm and is therefore efficiently excitable with
UV to blue light, as provided, for example, by (In,Ga)N-
LEDs. Excitation and emission spectra are illustrated in
Figure 4.
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Table 2. Atomic coordinates, Wyckoff positions and
equivalent isotropic displacement parameters of
Ca3Mg[Li2Si2N6]:Eu2+.
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atom Wyck. x
y
z
Ueq/Å2
0.0500(2) 0.32384(11) 0.24678(9) 0.0068(2)
Ca1 8j
Ca2 4h
0
0.1840(2)
0
½
0.0085(3)
Si1
4i
0.3521(3)
0.4192(2) 0.0050(4)
0.0632(2) 0.0048(4)
0.2220(2) 0.0094(5)
Si2 4i
Mg1 4i
0.6885(3) 0
0.0623(5) 0
Li1
N1
N2
N3
8j
8j
8j
4i
0.1731(18) 0.1767(11) 0.0594(8) 0.015(2)
0.2335(7) 0.1447(4) 0.3513(4) 0.0070(8)
0.3413(7) 0.3566(5) 0.1179(4) 0.0064(8)
Figure 4. Excitation (blue) and emission (red) spectra of
0.3418(10) 0
0.3943(11) 0
0.5703(5) 0.0084(12)
0.0921(5) 0.0068(11)
Ca3Mg[Li2Si2N6]:Eu2+.
N4 4i
The broad emission band of Ca3Mg[Li2Si2N6]:Eu2+ is as-
signed to parity allowed 4f65d1 ꢀ 4f7 transitions in
Eu2+.33,52-53 Since Eu2+ is expected to occupy the Ca site, we
assume that luminescence originates from octahedrally
coordinated Eu2+ ions. The presence of two crystallo-
graphically different Ca sites with slightly differing distor-
tion of the octahedra and, therefore, different interatomic
distances, influences the width of the emission band by
broadening. A comparable 6-fold coordination can also be
found in Ba3Si6O12N2 54 and its solid-solution series Ba1‒xSr-
xSi6O12N2 with x≈0.4 and 1. These compounds doped with
2 mol% Eu2+ exhibit broad emission bands with maxima
between 523 and 549 nm (green luminescence) when
irradiated with near-UV to blue light. In general, both the
spectral position and the band width of the emission
strongly depend on the energy level of 5d states of the
activator ion, which in turn is influenced by the host lat-
tice. The stronger the covalent interactions between Eu2+
and its surroundings, the smaller is the transition energy
between 4f and 5d orbitals of the activator ion (nephe-
lauxetic effect). For this reason and because of the higher
formal charge of N3‒ compared to O2‒, the 5d states of Eu2+
in Ca3Mg[Li2Si2N6]:Eu2+ are at lower energies than those in
Ba3Si6O12N2:Eu2+ and Ba1‒xSrxSi6O12N2:Eu2+, respectively,
and emission of Ca3Mg[Li2Si2N6]:Eu2+ occurs at longer
wavelengths.
Table 3. MAPLE values and MAPLE sums /kJ∙mol−1 for
Ca3Mg[Li2Si2N6]:Eu2+. Δ is the deviation between the
MAPLE sum of Ca3Mg[Li2Si2N6]:Eu2+ and the MAPLE
sum of constituting binary and ternary nitrides.[a]
atom
Ca12+
Ca22+
Si14+
MAPLE
2083
1812
atom
Li1+
N13−
N23−
N33−
N43−
MAPLE
717
∑, Δ
5056
4951
9201
Si24+
Mg12+
9202
2241
5440
5439
∑ = 58949
Δ = 0.31%
Total MAPLE: Ca3Mg[Li2Si2N6] = Ca3N2 + ⅓ꢀMg3N2
⅔ꢀLi2SiN2 + ⅔ꢀLiSi2N3 = 59131 kJ∙mol−1
[a] Typical partial MAPLE values /kJ∙mol−1: Ca2+: 1700-2200;;
Mg2+: 2100-2500; Li+: 550-860; Si4+: 9000-10200; N3−: 4300-
6200.9,16,50-51
+
Luminescence. Addition of EuF3 to the starting materials
yielded a gray microcrystalline sample containing orange
colored crystals of Ca3Mg[Li2Si2N6]:Eu2+. Under blue irra-
diation, the sample shows luminescence in the red spec-
tral region. Due to inhomogeneity of the sample, lumines-
cence investigations have been performed on single crys-
tals of Ca3Mg[Li2Si2N6]:Eu2+ in sealed silica glass capillar-
ies. All crystals show comparable red emission under blue
irradiation. Excitation of the title compound with a nomi-
nal dopant concentration of 1.7 mol% at 440 nm yields an
emission band with a maximum at 734 nm and a full
width at half maximum (fwhm) of 2293 cm−1 (124 nm). The
Typical Eu2+ emission of other compound classes are in a
range of 356-461 nm (fluorides), 360-540 nm (oxosili-
cates), 420-555 nm (aluminates and gallates), 470-660 nm
(sulfides) and 529-670 nm (nitrides).55-56 The here pre-
sented nitridolithosilicate Ca3Mg[Li2Si2N6]:Eu2+ is, to the
best of our knowledge and except for CaO:Eu2+
(λem = 733 nm),57-58 and MH2:Eu2+ (M = Ca,Sr,Ba; λem = 728-
764 nm)59 the first and so far the only example of a Eu2+-
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Chemistry of Materials
(1170 cm‒1), and Sr[LiAl3N4]:Eu2+ (1180 cm‒1).16,34 Both com-
doped compound that emits at fairly long wavelengths of
1
2
>700 nm.
pounds show a highly symmetric cube-like coordination
of the alkaline earth ion by N.
Recently, we reported on luminescence investigations of
the nitridomagnesosilicate Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+
(x = 0 and 0.6),60 with a nominal dopant concentration of
1 mol%. Just as Ca3Mg[Li2Si2N6]:Eu2+, the crystal structure
of Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+ contains [Si2N6]10‒ bow-tie
units. In contrast to the here presented nitridolithosilicate
with LiN4 and SiN4 tetrahedra as network-building struc-
tural motifs and Ca2+ and Mg2+ as counter ions, Li+ and
Ca2+ compensate the negative charge of the network,
which, in Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+, is made up of MgN4
and SiN4 tetrahedra. In the latter compound, chains of
edge-sharing MgN4 tetrahedra are interconnected by bow-
tie units, forming vierer and sechser ring channels along
[100]. Whereas Li+ ions are located within vierer ring
channels, Ca2+ (Sr2+) ions and, therefore, also Eu2+ ions,
show distorted octahedral coordination by N within sech-
ser ring channels. When irradiated with blue light, nar-
row-band red emission also occurs at rather long wave-
lengths of 638 nm (x = 0) and 634 nm (x = 0.6) with
fwhm = 1513-1532 cm‒1, respectively.
Both Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+ and Ca3Mg[Li2Si2N6]:Eu2+
exhibit similar body colors and show similarities in their
absorption bands. The main difference between both
nitridosilicates is the Stokes shift, which is larger for
Ca3Mg[Li2Si2N6]:Eu2+. This can be attributed to differences
in the chemical surroundings of Eu2+, which in each case
is expected to occupy the Ca2+ (Sr2+) site, and also to dif-
ferences in the phonon frequencies of respective host
lattices. Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+ exhibits only one
crystallographic Ca site (Wyckoff position 4g of space
group C2/m) with three different Ca‒N distances between
2.49 and 2.75 Å. In Ca3Mg[Li2Si2N6]:Eu2+, nine interatomic
distances Ca‒N of two Ca sites (Wyckoff positions 8j and
4h of space group C2/m) vary between 2.31 and 2.74 Å.
This is, in comparison, a significantly larger spread and
entails chemical differentiation of both sites, which even-
tually leads to a composed emission band of Eu2+ on either
of the crystallographic sites. Moreover, the shorter bond
lengths Ca‒N in Ca3Mg[Li2Si2N6]:Eu2+, especially occurring
on the lowest symmetry Ca1 site, may entail efficient en-
ergy transfer from Eu2+ incorporated on the larger higher
symmetry Ca2 site and, thus, also lead to a more pro-
nounced red shift of the emission band for the Eu doping
concentration investigated in this report. Whereas in
Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+, infinite chains of edge-
sharing MgN4 tetrahedra are interconnected by [Si2N6]10‒
bow-tie units leading to a three-periodic network,
Ca3Mg[Li2Si2N6]:Eu2+ consists of layers made up of LiN4
and SiN4 tetrahedra and isolated bow-tie units. Conse-
quently, due to the lower degree of condensation and also
due to lower symmetry of the Ca1 site with its short Ca-N
contact lengths, local lattice relaxation of Eu2+ in its excit-
ed state in Ca3Mg[Li2Si2N6]:Eu2+ is expected to be larger
than in Li2(Ca1‒xSrx)2[Mg2Si2N6]:Eu2+,61 resulting in a larger
Stokes shift of the title compound.
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CONCLUSION
In this contribution, we report on Ca3Mg[Li2Si2N6]:Eu2+, a
new nitridolithosilicate with exceptional luminescence
properties. The compound was synthesized by solid-state
metathesis reaction and could be obtained as orange col-
ored single crystals within an amorphous powder. The
crystal structure of Ca3Mg[Li2Si2N6]:Eu2+ is related to that
of Ca2Mg[Li4Si2N6]. It consists of corner- and edge-sharing
LiN4 tetrahedra, intercalated by [Si2N6]10‒ bow-tie units
forming anionic layers, which are separated by isolated
[Si2N6]10‒ bow-tie units and Ca2+ and Mg2+ ions. The latter
two act as counter ions and compensate the negative
charge of the anionic framework. Up to now, only a small
number of nitridosilicates containing Li+ and Mg2+ ions
are known from literature, namely Ca2Mg[Li4Si2N6],
Li2Ca2[Mg2Si2N6], and Li2(Ca1.86Sr0.14)[Mg2Si2N6].12,60 There-
by, Li+ and Mg2+ ions may either be part of the tetrahedral
network or act as counter ions. This behavior may signifi-
cantly increase the variety of possible structures within
the class of nitridosilicates.
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Recently, the quest for narrow-band red emitting phos-
phors has been triggered by attempts to improve lumines-
cence properties of commercially available pc-LEDs for
general illumination and signaling purposes.16,34,62 Mean-
while, LED technology for other specific applications is on
the advance and will increasingly be pursued. This in-
cludes, for example, LED technology for plants, in order to
replace currently used lamps to reduce costs and, seen in
the long term, improve productivity by optimizing growth
environments.63 Similar to requirements for LEDs applied
for general illumination, LEDs with regard to application
in horticulture have to be cheap, environmentally friend-
ly, highly efficient and reliable. Luminescence investiga-
tions on Ca3Mg[Li2Si2N6]:Eu2+ show red emission peaking
at 734 nm and a fwhm of 2293 cm−1 (124 nm). This repre-
sents an unexpected emission at fairly long wavelengths,
not yet known for Eu2+-doped nitrides. Despite emission
>700 nm and, therefore, too high energy loss beyond the
sensitivity of the human eye when applied in pc-LEDs,
Ca3Mg[Li2Si2N6]:Eu2+ may find application in more special-
ized fields like horticultural lighting.
ASSOCIATED CONTENT
Supporting Information. Selected bond lengths in
Ca3Mg[Li2Si2N6]:Eu2+ (Table S1), selected bond angles in
Ca3Mg[Li2Si2N6]:Eu2+ (Table S2), and anisotropic dis-
placement parameters for Ca3Mg[Li2Si2N6]:Eu2+ (Table S3).
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
Red emission and extremely narrow band widths with
fwhm <1200 cm‒1 have been reported for Sr[Mg3SiN4]:Eu2+
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Page 6 of 8
(15)
Meyer, H.-J. Solid state metathesis reactions as a
* E-mail: wolfgang.schnick@uni-muenchen.de
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conceptional tool in the synthesis of new materials. Dalton
Trans. 2010, 39, 5973-5982.
Notes
(16)
Schmiechen, S.; Schneider, H.; Wagatha, P.; Hecht, C.;
The authors declare no competing financial interest.
Schmidt, P. J.; Schnick, W. Towards New Phosphors for
Application in Illumination-Grade White pc-LEDs: The
Nitridomagnesosilicates Ca[Mg3SiN4]:Ce3+, Sr[Mg3SiN4]:Eu2+, and
Eu[Mg3SiN4]. Chem. Mater. 2014, 26, 2712-2719.
ACKNOWLEDGMENT
The authors thank Philipp Bielec for collecting single-crystal
X-ray data as well as Christian Minke (both at Department of
Chemistry, University of Munich) for EDX measurements.
Our thanks also go to Petra Huppertz (Lumileds Develop-
ment Center Aachen) for luminescence measurements as
well as Volker Weiler and Peter Schmidt (Lumileds Devel-
opment Center Aachen) for discussions. Financial support
from the Fonds der Chemischen Industrie (FCI) is gratefully
acknowledged.
(17)
Schmiechen, S.; Strobel, P.; Hecht, C.; Reith, T.;
Siegert, M.; Schmidt, P. J.; Huppertz, P.; Wiechert, D.; Schnick,
W. Nitridomagnesosilicate Ba[Mg3SiN4]:Eu2+ and Structure-
Property Relations of Similar Narrow-Band Red Nitride
Phosphors. Chem. Mater. 2015, 27, 1780-1785.
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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.
(19)
Uheda, K.; Hirosaki, N.; Yamamoto, H. Host lattice
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