A low-temperature synthesis route for CaAlSiN3 doped with Eu2+

Article abstract of DOI:10.1002/zaac.201200533

A new low-temperature synthesis route of the strongly red-emitting Eu ²⁺ activated nitride phosphor CaAlSiN₃ is presented. The fluorides CaSiF₆ and AlF₃ were used as a source of metal ions and Li₃N as a nitrogen source. A KCN/LiCl flux system was employed to lower the temperature of the reaction from 1100 to 750 °C. The course of the reaction was studied by differential thermal analysis, and the product of the reaction was inspected by X-ray powder diffraction and luminescence measurements of CaAlSiN₃:Eu. Copyright

Full text of DOI:10.1002/zaac.201200533

SHORT COMMUNICATION
DOI: 10.1002/zaac.201200533
A Low-Temperature Synthesis Route for CaAlSiN Doped with Eu2
+
3
[a]
[a]
Mariusz Kubus* and H.-Jürgen Meyer*
2
+
Keywords: CaAlSiN ; Eu doping; Photoluminescence; Europium; Synthesis route
3
Abstract. A new low-temperature synthesis route of the strongly red- lower the temperature of the reaction from 1100 to 750 °C. The course
emitting Eu2 activated nitride phosphor CaAlSiN
fluorides CaSiF and AlF were used as a source of metal ions and
Li N as a nitrogen source. A KCN/LiCl flux system was employed to
+
is presented. The
of the reaction was studied by differential thermal analysis, and the
product of the reaction was inspected by X-ray powder diffraction and
luminescence measurements of CaAlSiN :Eu.
3
3
6
3
3
Introduction
der atmospheric pressure can be applied for example by using
[5]
CaH (1300 °C) as a source of calcium ions instead of Ca N2
2
3
The preparation of red-emitting solid materials is a temp-
tation in inorganic chemistry and an important contribution to
the field of solid state lighting. These materials can be used in
warm white light-emitting diodes, activated with a blue GaN-
based LED chip. The most outstanding currently known red
emitting phosphors are (Sr,Ca)S:Eu, (Ca,Sr) Si N :Eu, CaAl-
[8,9]
(
1800 °C, 1 MP).
Pure nitrides without oxygen impurities
are difficult to make and special attention should be paid to
[10]
the choice of substrates.
A flux agent can be used to en-
hance the yield of nitridosilicates synthesis, e.g.: molten so-
dium for the synthesis of AESiN (AE = Ca, Sr, Ba) at 900–
2
[11]
2
5
8
1100 °C,
900 °C.
liquid lithium for Li Ca Si N and LiCa Si N at
4 3 2 6 3 2 5
The mixed compound (Ca,Sr)AlSiN can be syn-
3
[
1]
SiN :Eu, (Sr,Ba) SiO :Eu, and K SiF :Mn.
The main
[12]
3
3
5
2
6
requirements for luminescent materials are chemical and ther-
mal stabilities, high quantum yields, and appropriate excitation
thesized by solid-state reaction from nitrides at 1900 °C and
[13]
under a pressure of 190 MPa.
[2]
2+
and emission band positions. The employment of Eu and
4+
Mn as activator ions is limited by the availability of appro-
priate host lattices and by the stability of oxidation states in Results and Discussion
these structures, especially for manganese ions.
The preparation of CaAlSiN was carried out according to
reaction (1) in a LiCl/KCN flux at 750 °C for two days.
3
2
+
CaAlSiN :Eu is a successfully commercialized nitride
3
phosphor used in LEDs and is receiving great attention in sci-
ence.[3] Preparations of nitridosilicates with a high degree of
CaSiF + AlF + 3Li N Ǟ CaAlSiN + 9LiF
(1)
6
3
3
3
condensation of nitridosilicate networks are performed by
The obtained reaction product contained CaAlSiN besides
LiF and CaO. To our surprise the sample did not contain any
3
[4]
high-temperature syntheses. The temperature and pressure
2
+
conditions for the production of the CaAlSiN :Eu phosphor
3
AlN, which is often present as a side-product when following
are crucial because the starting materials behave chemically
inert in reactions.[ Multinary nitrides can be synthesized by
many methods originating from nitrides, pure metals, or oxides
at temperatures between 1400 and 1800 °C or even at lower
temperatures (800 °C) but with high pressure (100 MP) and
the use of sodium azide.[ The reaction can be also conducted
at temperatures between 500 and 800 °C in supercritical am-
monia, where the addition of sodium amid can help to improve
the quality of the product.[ Lower reaction temperatures un-
[6,7,14]
other preparations of CaAlSiN3.
The oxygen contami-
5]
nation in our reaction represented as CaO side-phase may re-
sult from impurities in the starting material.
The shape of diffraction peaks of CaAlSiN in the recorded
3
XRD pattern indicated that the sample is not highly crystalline.
6]
The crystal structure of CaAlSiN (Cmc2 ) is isotypic to the
3
1
structure of LiSi N and it is built from corner sharing silicon
2
3
and aluminum nitride tetrahedra, creating a three-dimensional
7]
[15,16]
framework.
The lattice parameters of CaAlSiN (powder)
3
doped with Eu2+ ions were refined by orthorhombic indexing
with the program WinPLOTR
*
*
Dr. M. Kubus
[17]
[18]
of the FullProf suite
E-Mail: mariusz.kubus@anorg.uni-tuebingen.de
Prof. Dr. H.-J. Meyer
[
a = 9.699(3) Å, b = 5.648(1) Å, and c = 5.0267(9) Å]. The
obtained values are similar to the literature data obtained from
Fax: +49-7071-29-5702
E-Mail: Juergen.Meyer@uni-tuebingen.de
the refined crystal structure of CaAlSiN [a = 9.7558(2) Å,
3
16]
[
a] Abteilung für Festkörperchemie und theoretische Anorganische
b = 5.6473(1) Å, and c = 5.0524(1) Å].[
Chemie
Institut für Anorganische Chemie
Eberhard-Karls-Universität Tübingen
Auf der Morgenstelle 18
A differential thermal analysis of the reaction mixture was
conducted between room temperature and 750 °C (Figure 1).
Multilateral solid-state metathesis reactions with more than
72076 Tübingen, Germany
Z. Anorg. Allg. Chem. 2013, 639, (5), 669–671
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
669

M. Kubus, H.-J. Meyer
SHORT COMMUNICATION
three reagents can be challenging with respect to the interpret- out that the formation of MAlSiN can succeed under ambient
3
ation of thermal effects that are monitored in course of a reac- pressure conditions by time-consuming, exhaustive nitridation.
[
19]
tion (Figure 1, line 2). In such cases a decomposition of a In addition it may be speculated that highly reactive LiCN
given reaction in separate reactions may help. In order to get can prevent the formation of a protective nitride layer on the
a better understanding of the reaction, separate measurements aluminum surface, since the formation of AlN was not ob-
were performed for the LiCl/KCN flux (Figure 1, line 1 and served in our reaction.[
21]
5) and for the reaction mixture without flux (Figure 1. line 3),
The excitation and emission spectra of CaAlSiN :Eu (2 mol-
3
before all solids were combined into one reaction.
%) synthesized according to the given flux route of reaction
(
1) are presented in Figure 2. The excitation spectrum moni-
7
6
2+
tored at 610 nm is represented by two 4f Ǟ 4f 5d Eu transi-
tion bands at 333 and 460 nm. The presence of two broad exci-
tation bands is a consequence of crystal field effecting on the
Eu ions, which are situated in a slightly distorted T sym-
metry environment.[ On excitation with blue light at λEx
2+
d
22]
=
4
60 nm an intense red-orange emission is obtained, which cor-
6
7
2+
responds to 4f 5d Ǟ 4f transitions in Eu ions. The emission
band position (610 nm) is shifted towards higher energies in
comparison to other results presented in literature (ca.
50 nm).[
8,14,20,22]
The obtained blueshift of the emission band
6
is typical for a substitution of nitride ions by oxygen ions,
2+
which reduces the nephelauxetic effect of Eu and changes
the crystal field energy.[
8,23]
The total charge of the system can
be compensated by an increased Al:Si ratio, as both ions oc-
cupy one and the same crystallographic site, or by introduction
of Ca2+ vacancies. Depending on the way of synthesis (degree
Figure 1. DTA for the reaction of CaSiF
6
, AlF
3
, Li
3
N, EuCl
2
, and the of oxygen contamination) blueshift values down to 610 nm
flux (LiCl:KCN) (solid line 2 and 4), flux (dotted line 1 and 5), and have been reported.[
22,24,25]
In addition, constituents of the flux
the reaction mixture without flux (dash-dot line 3). Black lines (1),
2), and (3) are for heating. Gray lines (4) and (5) are for cooling.
could also be incorporated in the structure. All these factors
can influence the crystal field of Eu2+ ions and affect the posi-
tion of the emission band, while the crystal structure of
(
The complete flux metathesis reaction (1) as monitored by
CaAlSiN is retained.
DTA may be characterized as follows: The first weak endo-
thermic effect observed at 287 °C can be ascribed to the melt-
ing point of eutectic LiCl:KCN mixture[ as can be assigned
by the separate measurement of the flux (line 1). The second
endothermic and consequent exothermic effect at 345 and
3
19]
359 °C, respectively, is a consequence of melting and reaction
of the substrates. The same effects are observed at lower tem-
peratures for the reaction without flux (line 3). The endother-
mic effect centered at 435 °C corresponds possibly to the for-
mation of nitrogen gas. The broad exothermic peak with maxi-
mum at about 555 °C can be attributed to the slow reaction
of the substrates under nitrogen pressure and the subsequent
formation process of CaAlSiN and side-products. The exo-
3
thermic effect during cooling process with the maximum at
7
00 °C is attributed to the slow recrystallization of the side-
products of the reaction (line 4). The last exothermic effect at
78 °C (line 4) was also observed for the measurements of the
2
flux (line 5). XRD measurements of the product collected from
the container after the DTA measurement revealed diffraction
Figure 2. Excitation and emission spectra of CaAlSiN
3
:Eu2+ synthe-
sized by flux metathesis.
patterns of CaAlSiN besides reflections of LiF and CaO.
3
The preparation of this nitride phosphor has been reported
(
under nitrogen gas pressure) in closed systems so far, which
The full-width-half-maximum (FWHM) obtained for
2+
suggests that the pressure generated during the reaction in the CaAlSiN :Eu is about 82 nm. This is smaller than the value
metal container was essential for the formation of this highly of 90 nm reported for CaAlSiN :Eu prepared by solid state
condensed nitride network. High pressure of nitrogen gas was reaction at high temperature (1800 °C),[ and larger than the
3
2+
3
20]
already mentioned as a crucial point in the synthesis of value of 75 nm reported for the compound prepared by self-
[20]
[22]
MAlSiN (M = Ca or Sr).
It can be, however, not ruled propagation high-temperature method (1550 °C).
3
6
70
www.zaac.wiley-vch.de © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Z. Anorg. Allg. Chem. 2013, 669–671

Synthesis Route for CaAlSiN
3
Doped with Eu2+
The photoluminescence excitation and emission spectra were recorded
with a Perkin-Elmer LS 50B spectrofluorimeter equipped with a pow-
Conclusions
The europium activated nitride phosphor CaAlSiN was suc- der sample holder.
3
cessfully prepared at low temperature (750 °C) in a closed sys-
tem from fluorides. The use of flux allows to lowering the
reaction temperature, which can be of great significance for
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[
1
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Received: December 5, 2012
Published Online: February 18, 2013
Z. Anorg. Allg. Chem. 2013, 669–671
© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
671