SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study
We report the results of the comprehensive study of the structural, magnetic and optical properties of SrTiO₃ perovskite doped with Eu³⁺ ions. Polycrystalline powders were obtained by sol-gel process including high-temperature annealing at 1300 °C. The structural analysis showed that material is com...
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Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України
2016
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| Zitieren: | SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study / A.S. Pusenkova, О.N. Marchylo, L.V. Zavyalova, I.S. Golovina, S.V. Svechnikov, B.А. Snopok // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 4. — С. 343-351. — Бібліогр.: 45 назв. — англ. |
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irk-123456789-1216542017-06-16T03:02:34Z SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study Pusenkova, A.S. Marchylo, О.N. Zavyalova, L.V. Golovina, I.S. Svechnikov, S.V. Snopok, B.А. We report the results of the comprehensive study of the structural, magnetic and optical properties of SrTiO₃ perovskite doped with Eu³⁺ ions. Polycrystalline powders were obtained by sol-gel process including high-temperature annealing at 1300 °C. The structural analysis showed that material is composed of several phases with dominant SrТiO₃ and onsiderable quantity of titanium dioxide (rutile, 10…20%). Both the amount of Eu and ratio of Eu:Sr in the final product are considerably smaller as compared to the original solutions for synthesis. The elemental analysis reveals europium only in the phase of EuSrTi₂O₇ compound for equimolar ratio of Eu and Sr during the synthesis. The EPR analysis reports deficiency of Eu²⁺ in the samples under investigations. SrTiO₃:Eu³⁺ powders demonstrate weak photoluminescence, which intensity grows up with increasing the concentration of Eu and reaches its maximum at c.a. 8 mol.% of Eu in the original solutions. Addition of Al increases the intensity of photoluminescence (c.a. 2.2 for 10 mol.%). Emission spectra are typical for Eu³⁺ occupied sites with high symmetric environment (dominant ⁵D₀→⁷F₁ transition) with relatively low distortion (both ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄ as well present, but with a lower intensity) of coordination polyhedron. Low solid-solubility limit of Eu³⁺ in perovskite matrix (less than 1 mol.%), peculiarities of optical spectra, effect of Al, production/annealing temperature dependence etc. have suggested that structural effects are dominating in functional, in particular, optical properties of SrTiO₃:Eu³⁺ phosphors. 2016 Article SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study / A.S. Pusenkova, О.N. Marchylo, L.V. Zavyalova, I.S. Golovina, S.V. Svechnikov, B.А. Snopok // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 4. — С. 343-351. — Бібліогр.: 45 назв. — англ. 1560-8034 DOI: 10.15407/spqeo19.04.343 PACS 61.05.cp, 61.72.Hh, 78.55.Hx http://dspace.nbuv.gov.ua/handle/123456789/121654 en Semiconductor Physics Quantum Electronics & Optoelectronics Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
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We report the results of the comprehensive study of the structural, magnetic and optical properties of SrTiO₃ perovskite doped with Eu³⁺ ions. Polycrystalline powders were obtained by sol-gel process including high-temperature annealing at 1300 °C. The structural analysis showed that material is composed of several phases with dominant SrТiO₃ and onsiderable quantity of titanium dioxide (rutile, 10…20%). Both the amount of Eu and ratio of Eu:Sr in the final product are considerably smaller as compared to the original solutions for synthesis. The elemental analysis reveals europium only in the phase of EuSrTi₂O₇ compound for equimolar ratio of Eu and Sr during the synthesis. The EPR analysis reports deficiency of Eu²⁺ in the samples under investigations. SrTiO₃:Eu³⁺ powders demonstrate weak photoluminescence, which intensity grows up with increasing the concentration of Eu and reaches its maximum at c.a. 8 mol.% of Eu in the original solutions. Addition of Al increases the intensity of photoluminescence (c.a. 2.2 for 10 mol.%). Emission spectra are typical for Eu³⁺ occupied sites with high symmetric environment (dominant ⁵D₀→⁷F₁ transition) with relatively low distortion (both ⁵D₀→⁷F₂ and ⁵D₀→⁷F₄ as well present, but with a lower intensity) of coordination polyhedron. Low solid-solubility limit of Eu³⁺ in perovskite matrix (less than 1 mol.%), peculiarities of optical spectra, effect of Al, production/annealing temperature dependence etc. have suggested that structural effects are dominating in functional, in particular, optical properties of SrTiO₃:Eu³⁺ phosphors. |
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Pusenkova, A.S. Marchylo, О.N. Zavyalova, L.V. Golovina, I.S. Svechnikov, S.V. Snopok, B.А. |
| spellingShingle |
Pusenkova, A.S. Marchylo, О.N. Zavyalova, L.V. Golovina, I.S. Svechnikov, S.V. Snopok, B.А. SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study Semiconductor Physics Quantum Electronics & Optoelectronics |
| author_facet |
Pusenkova, A.S. Marchylo, О.N. Zavyalova, L.V. Golovina, I.S. Svechnikov, S.V. Snopok, B.А. |
| author_sort |
Pusenkova, A.S. |
| title |
SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study |
| title_short |
SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study |
| title_full |
SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study |
| title_fullStr |
SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study |
| title_full_unstemmed |
SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study |
| title_sort |
srtio₃:eu³⁺ phosphors prepared by sol-gel synthesis: structural characterization, magnetic properties and luminescence spectroscopy study |
| publisher |
Інститут фізики напівпровідників імені В.Є. Лашкарьова НАН України |
| publishDate |
2016 |
| url |
http://dspace.nbuv.gov.ua/handle/123456789/121654 |
| citation_txt |
SrTiO₃:Eu³⁺ phosphors prepared by sol-gel synthesis: Structural characterization, magnetic properties and luminescence spectroscopy study / A.S. Pusenkova, О.N. Marchylo, L.V. Zavyalova, I.S. Golovina, S.V. Svechnikov, B.А. Snopok // Semiconductor Physics Quantum Electronics & Optoelectronics. — 2016. — Т. 19, № 4. — С. 343-351. — Бібліогр.: 45 назв. — англ. |
| series |
Semiconductor Physics Quantum Electronics & Optoelectronics |
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Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
343
PACS 61.05.cp, 61.72.Hh, 78.55.Hx
SrTiO3:Eu3+ phosphors prepared by sol-gel synthesis:
Structural characterization, magnetic properties
and luminescence spectroscopy study
A.S. Pusenkova, О.N. Marchylo, L.V. Zavyalova, I.S. Golovina, S.V. Svechnikov, B.А. Snopok
V. Lashkaryov Institute of Semiconductor Physics, NAS of Ukraine
41, prospect Nauky, 03680 Kyiv, Ukraine
Abstract. We report the results of the comprehensive study of the structural, magnetic
and optical properties of SrTiO3 perovskite doped with Eu3+ ions. Polycrystalline
powders were obtained by sol-gel process including high-temperature annealing at
1300 °C. The structural analysis showed that material is composed of several phases with
dominant SrТiO3 and considerable quantity of titanium dioxide (rutile, 10…20%). Both
the amount of Eu and ratio of Eu:Sr in the final product are considerably smaller as
compared to the original solutions for synthesis. The elemental analysis reveals europium
only in the phase of EuSrTi2O7 compound for equimolar ratio of Eu and Sr during the
synthesis. The EPR analysis reports deficiency of Eu2+ in the samples under
investigations. SrTiO3:Eu3+ powders demonstrate weak photoluminescence, which
intensity grows up with increasing the concentration of Eu and reaches its maximum at
c.a. 8 mol.% of Eu in the original solutions. Addition of Al increases the intensity of
photoluminescence (c.a. 2.2 for 10 mol.%). Emission spectra are typical for Eu3+
occupied sites with high symmetric environment (dominant 5D0→7F1 transition) with
relatively low distortion (both 5D0→7F2 and 5D0→7F4 as well present, but with a lower
intensity) of coordination polyhedron. Low solid-solubility limit of Eu3+ in perovskite
matrix (less than 1 mol.%), peculiarities of optical spectra, effect of Al,
production/annealing temperature dependence etc. have suggested that structural effects
are dominating in functional, in particular, optical properties of SrTiO3:Eu3+ phosphors.
Keywords: sol-gel synthesis, photoluminescence, SrTiO3:Eu3+ phosphors.
Manuscript received 07.06.16; revised version received 18.08.16; accepted for
publication 16.11.16; published online 05.12.16.
1. Introduction
Inorganic and especially ceramic phosphors are among
the promising materials for the next generation of solid-
state lighting, such as device indicators, backlights,
automobile headlights and conventional household
illumination. In particular, rare earth elements doped
phosphors have attracted significant attention due to
their potential application for various kinds of
fluorescent emitters (including those with conversion of
UV radiation based on photon cutting), X-ray detectors
and multi-color visualization tools. Researches in this
area stimulate progress in using the phosphors in
vacuum ultraviolet radiation excited plasma display
panels, mercury-free fluorescent lamps and in the
development of efficient solar cells [1-3].
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
344
0.0 0.2 0.4 0.6 0.8 1.0
50
100
150
200
250
300
Para phase
Tetragonal
“tilted”
phase Te
m
pe
ra
tu
re
(K
)
Composition x (a)
0.0 0.2 0.4 0.6 0.8 1.0
50
100
150
200
250
300
Para phase
Φ1
Te
m
pe
ra
tu
re
(K
)
Composition x
Φ3
−0.01%
Φ3
0.01%
(b)
Fig. 1. Temperature-composition phase diagrams of EuxSr1−xTiO3 bulk (a) and thin films (b). Plot (b) is calculated for the
matched substrate corresponding to zero misfit um = 0 (vertical boundary Φ1
/Φ3), um
*= −0.01% (left Φ3 region), um
* = +0.01%
(right Φ3 region). Designations represent the nonzero components of order parameter – oxygen tilt component Φi in a given
phase. The abbreviation “para” stands for the paraelectric non-ferrodistortive phase. (Adapted from Ref. [15].)
Among others, perovskites with accommodated Eu
ions are promising materials due to strong interaction
between the structural order parameter, polarization and
magnetization, which leads to their application in the
field of multiferroic nano-systems. Indeed, the matrix
based on the SrTiO3 structure has attracted great interest
due to its special structure features, excellent physical
and chemical stability. Instead of the fact that this
interest is unrelaxing more than half century, many
questions are still open and the new ones continually
arise. For example, nanosized materials based on
EuxSr1−xTiO3 solid solution can exhibit not only all the
interesting structural and polar mode interactions of
individual EuTiO3 and SrTiO3 films but also new
phenomena and properties [4-9]. There has been one
experimental study on the structural antiferrodistortive
and other physical properties of bulk solid solution
EuxSr1−xTiO3 [10]. Theoretically, possible multiferroic
properties of EuxSr1−xTiO3 nanotubes and nanowires [11]
have been predicted using the
Landau−Ginzburg−Devonshire theory. The vector nature
of the antiferrodistortive order parameter can strongly
influence the phase stability, twin domain structure,
polar and pyroelectric properties of quantum
paraelectrics [12] at interfaces [13], or entire thin films
[14] of SrTiO3 and EuxSr1−xTiO3 [15] (Fig. 1). Hence,
the study of the optical and electro-physical properties,
long-range structural, magnetic and polar ordering as
well as the phase diagrams of EuxSr1−xTiO3 is an
important problem for fundamental science and
promising for advanced application. To achieve that, it is
necessary to design the synthetic protocols that make it
possible to produce EuxSr1−xTiO3 structures with a wide
range of the doped europium concentration x inside the
perovskite matrix.
Among other rare earth ions, europium is a special
element as dopant, because it exhibits not only the
property of valence fluctuation (i.e., the valence state
may be divalent or trivalent), but the luminescent of Eu3+
doped materials are greatly influenced by the matrix as
well (the so-called spectroscopic probe). Indeed, the
short overview of the emission properties of europium-
doped materials clearly confirms this statement –
luminescence of the accommodated Eu3+ ions strongly
depends on both spatial distribution and nature of
coordinated ligands [16-21]. So, the difference in optical
properties of supposedly the same SrTiO3:Eu3+ product
obtained by different synthetic procedure clearly
indicates the differences of the local environment of
emitting ions [16, 22-26]. One of the possible reasons for
that is the fact that the most of these materials obtained
using the solid phase synthesis, which is peculiar to have
spatial heterogeneity in the final material with different
phases. Modern tendencies suggest the need for
development of liquid-phase techniques, which have the
advantage of uniform distribution of components and
dopant materials [27-29]. In accord with the mentioned
above, the aim of this work is development of
technological procedure for production of SrТiO3 with
accommodated Eu ions by liquid-phase sol-gel method
and detailed characterization of the product concerning
the real amount and local surrounding of the europium
ions within the perovskite matrix.
2. Structure and morphology of Eu3+ doped SrТiO3
SrTiO3:Eu3+ powders were prepared as described in [27]
using strontium chloride SrCl2 (99.99), europium
chloride, EuCl3 (99.9) and titanium tetra-isopropoxide,
Ti(O-i-C3H7)4 (99.999). All reagents were received from
KOJUNDO CHEMICAL LABORATORY CO., LTD
(Japan) and used without any other additional treatment.
The obtained dried gel was calcinated at high
temperature for crystallization of final material. The
annealing temperature was 1300 °С, annealing time: 5 or
7 hours. The polycrystalline powders were examined to
determine their structures as well as phase and elemental
compositions. The crystalline structures of the prepared
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
345
Table. Content of elements and separate phases in accord with the data of different analytical methods as a function of
the Eu content.
XRD Elemental analysis Magnetic resonance
Peak signal Ipp, rel. un.
Content
of Eu in
the initial
material,
mol.%
tann,
hour Eu, wt.% SrTiO3,
wt.%
TiO2,
wt.%
Ti,
wt.%
Sr,
wt.%
Eu,
wt.% FMR signal
with Нres =
800 Oe
EPR signal
with Нres =
3360 Oe
0 5 − 78.4 21.5 59.5 38.5 − 879 150
0 7 − 75.6 24.4 − − − − −
8 5 − 79.8 20.2 − − − 430 180
50 5 30.2 −
EuSrTi2O7
60.4 9.4 52.6 17.1 28.8 260 101
powders were investigated with X-ray diffraction (XRD)
on a DRON-3M X-ray diffraction apparatus with Cu Kα
radiation (λ = 1.542 Å) as the incident one. The
elemental analysis was performed using the X-ray
fluorescence spectrometer Х “Unique ІІ” (Philips).
Fig. 2 shows the X-ray diffraction patterns for the
SrTiO3:Eu3+ phosphor with equimolar Eu:Sr ratio in
initial solutions. The presented data shows that this
material is a polycrystalline powder consisting of several
phases: SrТiO3 is the dominating one with an
appreciable content of titanium oxide phase (rutile);
additional phase is EuSrTi2O7, which is present only at
high content of Eu in initial solutions, despite the fact
that the ratio of Eu and Sr in the initial solution was 1:1.
If the concentration of europium was less than 8%, Eu
compound in crystalline state has not been detected; at
the same time, the part of rutile increases up to
20…25%.
30 40 50 60 70 80
0
2
4
6
8
10
Ti
O
2 (
32
0)
Eu
Sr
Ti
2O
7 (
62
2)
Eu
Sr
Ti
2O
7 (4
40
)
Sr
Ti
O
3 (1
11
)
Eu
Sr
Ti
2O
7 (3
31
)
Eu
Sr
Ti
2O
7 (4
00
)
Eu
Sr
Ti
2O
7 (2
22
)
Eu
Sr
Ti
2O
7 (
31
1)
SrTiO3 60.4%
TiO2 9.4%
EuSrTi2O7 30.2%
Ti
O
2 (
21
1)
Ti
O
2 (1
10
)
Sr
Ti
O
3 (3
10
)
Sr
Ti
O
3 (
22
0)
Sr
Ti
O
3 (2
11
)
Sr
Ti
O
3 (2
00
)
Sr
Ti
O
3 (1
10
)
Sr
Ti
O
3 (1
00
)In
te
ns
ity
, a
.u
.
2Θ
Fig. 2. X-ray spectrum of the sample prepared with the ratio of
Sr and Eu in the initial solution as 1:1.
The overview of the results presented in the Table
allows to assume that, under the given process
conditions, formation of SrТiO3 is energetically more
favorable and accommodation of europium occurs only
in the case of its “excess” during the synthesis.
It is interesting to highlight that pure SrTiO3
contains a large amount of TiO2 (20…25%, Table) in the
crystalline modification of rutile. Increasing the
annealing time does not reduce the amount of TiO2 in the
sample whilst addition of Eu decreases. Moreover, in
line with elemental analysis Ti is the predominant
component of the powder. One of the possible
explanations is as follows. We used the strontium
chloride as the initial reagent. Owing to its relatively low
boiling temperature, Sr partly evaporated during the
annealing process resulting in Ti excess. Redundant Ti
precipitated as TiO2 [30].
As can be seen from the data, as the concentration of
europium in the initial solution increases, the proportion
of rutile in the final powder reduces. So, addition of Eu
diminishes formation of titanium dioxide. Keeping in
mind that the amount of europium in the phosphors under
consideration is less than the limit of detection, it is
reasonable to suggest that the initially formed structures
containing both europium and Ti destroyed during
annealing at high temperature. As a result, pure perovskite
matrix with high temperature stability dominates in this
powder. And only in the case of essential “excess” of Eu
in the original solutions, a reasonable amount of
europium-containing material can be detected
simultaneously with relatively low impact of TiO2.
3. Magnetic properties
Magnetic properties of polycrystalline phosphors were
investigated using the EPR method with Radiopan SE/X
2544 X-band radio-spectrometer. Results of the
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
346
magnetic properties study of the samples with different
Eu-content in the initial materials are shown in Fig. 4
and Table in the form of magnetic resonance spectra.
The measurements were performed at room temperature
with the frequency 9.386 GHz. The test powder was
placed into a silica ampoule. The spectra were
normalized to the weight of the samples, which allowed
to compare the signals from different samples
accurately.
The spectra of all samples are identical and consist
of three lines, А, М and В. However, the line A in the
range of magnetic field 1500 Oe – signal from Fe3+-ions,
located in the glass ampoules, into which the
investigated powders were placed. Thus, the samples
show two proper signals, М and В. The resonant
magnetic field (Нrеs) and width (ΔН) for each of those
signals remain constant from sample to sample. Only
their peak intensity (Ipp) varies.
The signal M of the sample placed in a weak
magnetic field Нres = 800 Oe is very wide: ΔН =
1600…1700 Oe. Because of the Нres low values and a
large width, M signal is not fully registered. These
signals often have a ferromagnetic origin [31].
Obviously, this signal is provided by an uncontrolled
magnetic impurity located in the initial reagents that
were used in the strontium titanate synthesis. Under
doping, the peak intensity of М signal decreases, and the
line becomes very weak in the sample doped with 50%
EuCl3. Possibly, the signal M corresponds to the α-Fe
phase. Thus, in [32] the authors investigated the FMR
signal of α-Fe phase formed after annealing the
amorphous alloy Fe90Zr7B3. This signal has
characteristics similar to the characteristics of the signal
М. It should be noted that the signal M has a line shape
of the resonant absorption derivative and is not a
reflection of the initial magnetization processes, which
are sometimes observed in the FMR spectra of magnetic
compounds (called DARMA peaks [33]).
0 10 20 30 40 50
8
10
12
14
16
18
20
22
24
Ti
O
2
co
nc
en
tra
tio
n,
m
ol
. %
Eu concentration, mol. %
t
an
= 5 hours
tan = 7 hours
Fig. 3. Content of the oxide phase TiO2 in the obtained
powders SrTiO3:Eu depending on the Eu concentration.
Fig. 4. EPR spectra of the samples comprising in the starting
materials 8% EuCl3 (spectrum 1), 50% EuCl3 (spectrum 2) and
control sample containing no EuCl3 (spectrum 3).
The narrow signal В at Нres = 3360 Oe can
correspond to both extrinsic and intrinsic defects. The
former include, for example Fe3+-ions, randomly
included in the material during the synthesis and
forming the paramagnetic centers, in the structure of
which the oxygen vacancy V(O) is present. This signal
from paramagnetic centers Fe3+-V(O) was observed in
KTaO3 [34, 35] and SrTiO3 [36]. Defects of internal
origin in the investigated powders are, for example,
centers of О− and О2−. EPR signals from these centers
were observed in SrTiO3 [37], and in TiO2 [38]. As it is
obvious from Fig. 4, the signal B has the lowest
intensity in a sample doped with 50% EuCl3.
According to the data of X-ray and EPR, listed in
Table, SrTiO3 phase amount correlates with the
intensity of the signal from sample to sample. From
this comparison, it can be assumed that the signal B
corresponds to the paramagnetic centers formed
precisely in this phase. The exact determination of the
signal’s origin requires a separate investigation and is
beyond the scope of this work.
Unlike EuxSr1−xTiO3 solid solutions described in
[2], the doped samples obtained in this study not exhibit
long-range magnetic order resulting from the exchange
interaction between the Eu2+-ions. This is quite natural,
since according to the X-ray data the sample doped with
8% EuCl3 does not contain europium, and in the sample
doped with 50% EuCl3, EuSrTi2O7 phase is formed. It
should be noted that according to [5], as a result of the
exchange interaction between Eu2+ ions, the low-
temperature phase in EuxSr1−xTiO3 solid solutions is
antiferromagnetic, but the long-range magnetic
ordering is realized only in the compositions with
х > 0.25. This is critical concentration, below which
the crossover occurs in the properties of the
EuxSr1−xTiO3 solutions. Despite the fact that the content
of europium in the sample doped with 50% EuCl3 is
28.8% (see Table), all Eu is in the phase of EuSrTi2O7.
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
347
Thus, the EPR signal from the Eu2+ ions in our samples
was not observed. The result is not so unexpected,
keeping in mind that for production of Eu2+ containing
phosphors usually used is annealing Eu3+ phosphors at
high temperature (e.g., 1100 °C) in a reductive
atmosphere (a mixture of H2 and N2 etc.) for several
hours [39]. Application of the Eu2+ compound during the
synthesis is not reasonable, since in solutions Eu2+ ions
are quickly oxidized up to Eu3+ [40].
4. Photoluminescent properties
of SrTiO3:Eu3+ phosphors
The optical properties of crystalline phosphors
SrTiO3:Eu3+ were investigated for samples annealed at
the temperature 1300 °С for 5 hours. The luminescence
spectra were recorded using photo-multianalyzer
Hamamatsu PMA-12 (Japan) at room temperature. The
excitation source was He-Cd laser with the wavelength
λ = 325 nm. The concentration of europium in initial
solutions during fabrication varied within the range 0.1
to 10.0 mol.%. Fig. 5 demonstrates typical photo-
luminescence of SrTiO3: Eu3+ samples (0.2 mol.%).
Pure strontium titanate displays a broad spectrum
of luminescence in the visible region without any
specific bands. This emission spectrum is similar for
many perovskite crystals, associated with the presence of
imperfections or defects and is typical for materials with
localized exciton excitations. For the temperature range
more than c.a. 1100 °C, this band usually disappears.
Thus, mixed ionic-covalent bonding properties of the
SrTiO3 perovskite matrix with a unique electronic
structure can be efficiently used to investigate the optical
behavior of accommodated ions with their proper
emission spectrum. The latter opens the ways to finely
tune and adjust properties of SrTiO3 material due to
cation substitution.
Fig. 5. Crystalline phosphor SrТiO3:Eu photoluminescence
spectrum (concentration of Eu is equal to 0.2 mol.%).
Therefore, the presence of Eu3+ in the SrTiO3 host
results in photoluminescence properties specific for
europium ions. Fig. 5 presents the emission spectrum of
the Eu3+-doped SrTiO3 powder, excited at 325 nm. The
line positions are in good agreement with the energy
levels for Eu3+ transitions arising from their 4f electrons
and described in Ref. [16]. The emission spectrum
shows the typical emissions of Eu3+ ions occupying the
range 500…800 nm; the luminescence peaks can be
assigned to the 5D0→7FJ (J = 0, 1, 2, 3, 4) transitions.
The 5D0→7F1 transition near 593 nm dominates the
spectrum and is more intense than the other ones.
Furthermore, the 5D0→7F2 and 5D0→7F4 transitions are
easy detectable and located within 610…630 and
680…710 nm ranges, correspondingly. Similar spectra
of the Eu3+-doped SrTiO3 powder were observed
F. Fujishiro for the material obtained by the sol–gel
based Pechini technique [41], C.R. García et al. –
prepared by pressure-assisted combustion synthesis with
powder post-annealing at 1200 °C [22] and L. Dong
et al. in SrxBa1−xTiO3:Eu3+ phosphors synthesized using
the high temperature solid-phase method with
calcination at 1100 °C, excited with 466 nm [23].
However, at the same time photoluminescence of
europium-doped strontium titanate prepared at lower
temperature (microwave hydrothermal method at 140 °C
[42], in a molten NaCl flux at 950 °C and dehydrated at
120 °C, excitation 488 nm [26], by the sol-gel process
with annealing at 750 °C, excitation 460 nm, 77 K) [24])
demonstrate the dominant band around 620 nm that
corresponds to the 5D0→7F2 transition with a low
intensity of the 5D0→7F1 band.
The 5D0→7F1 transition is the most intense one in
the spectra of solids with the centrosymmetric crystal
structure [16, 43]. It well correlates with experimental
results, if we assume that Eu3+ enters the
centrosymmetric Sr2+ site within the cubic perovskite
structure of SrTiO3 [41]. The intensity of this transition
is often considered to be constant, since the intensity of a
magnetic dipole transition is largely independent of the
environment of the Eu3+ ion [16]. Moreover, the
5D0→7F1 transition directly reflects the crystal-field
splitting of the 7F1 level. Fig. 5 demonstrates that the 7F1
level is not split, so, europium ions are mainly in highly
symmetric environment. However, negligible crystal-
field splittings may be a consequence of a high
coordination number possible in perovskite matrix
(probably, ion is twelve-coordinated): a large number of
coordinating atoms distributed fairly evenly around the
central metal ion tends to produce approximately
spherical field, with small effective asymmetry [16].
The intensity of 5D0→7F2 transition is essentially
smaller in respect to the 5D0→7F1 band, but still
relatively high. Keeping in mind that the intensity of this
so-called “hypersensitive transition” is influenced by the
local symmetry of the Eu3+ ion and the nature of the
ligands, it’s reasonable to conclude that some distortion
can be attributed to local ion surrounding. The reasoning
550 600 650 700 750
0
30
60
90
7F0 7F3
7F4
7F2
7F1 SrTiO3: Eu3+
annealing: T=1300 oC, t=5 h
[Eu]
solution
=0,2%
P
ho
to
lu
m
in
es
ce
nc
e
In
te
ns
ity
(a
rb
. u
n.
)
Wavelength, nm
5D0
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
348
is that the 5D0→7F2 is strictly forbidden for the Eu3+ ion
at the site with a center of symmetry, so that the stronger
the distortion of the site from a highly symmetric
coordination polyhedron, the more intense the 5D0→7F2
transition will become [16]. Taking into account that the
intensity of the 5D0→7F4 transition is also easy
detectable, it is very likely that high symmetry of
coordination polyhedron is partly distorted near the Eu3+
ion. It is well known that the substitution of divalent ions
by trivalent ions produces distortion of the site
symmetry; so, if the Eu3+ surely enters into the Sr2+ sites,
this in turn promotes the 5D0→7F2 transition. Contrary to
that, the higher intensity of the 5D0→7F2 transition as
compared with the 5D0→7F1 one observed in the
luminescence spectrum of SrTiO3:Eu3+ materials in Refs.
[26, 24, 42] may be attributed to low symmetry of Eu3+,
if suggesting strong distortion around the Eu3+ sites. This
different behavior can be explained, if the europium
enters not only into Sr2+ (as in the latter case) but into
Ti4+ sites at the same time, too, as reported by Jiang
et al. [44]. This mechanism can lead to a low degree of
distortions and, probably, can be realized only at high
temperatures (more than 1000 °C). This suggestion well
correlates with the difference in emission spectra
discussed above.
540 560 580 600 620 640 660
0
50
100
150
5
4
6
3
2
1
1 - 0.2 mol. %
2 - 2 mol. %
3 - 4 mol. %
4 - 6 mol. %
5 - 8 mol. %
6 - 10 mol. %
In
te
ns
ity
, a
. u
.
Wavelength, nm
7F1
5D0
7F2
5D0
а)
0 2 4 6 8 10
0
20
40
60
80
100
Concentration, mol. %
In
te
ns
ity
, a
.u
.
b)
Fig. 6. PL spectrum of SrTiO3:Eu3+ at various concentrations
of Eu (а) and dependence of the PL intensity at λ = 593 nm on
the concentration of europium (b).
Europium free SrTiO3 does not show
luminescence. With increasing the concentration of
Eu3+, the intensity of the bands increases without
changing the overall spectrum shape (Fig. 6). Indeed,
this figure shows that with increasing the Eu
concentration within the range 0.2 to 10 mol.% all
peaks in the wavelength range λ = 580…710 nm
remain the same. A similar dependence was obtained in
[23], the optimum doping concentration 5%
corresponding to the strongest emission intensity was
determined. Authors suggest that it is the result of
concentration quenching owing to the nonradiative
energy transfer between luminescent centers increased
when the doping concentration rises up. Keeping in
mind that the observed concentration of Eu in the
materials under investigations was so low, that one
cannot be detected by elemental analysis, it’s
reasonable to assume that concentration quenching is a
rare event in these structures. Indeed, the ratio of
intensities specific to 5D0→7F2 and 5D0→7F1 transitions
remains constant for all the measured concentrations of
the doped europium. Any broadening or change in the
spectrum shape was not observed as well. So, it is
reasonable to assume that structural effects related to
the maximal level of possible doping specific for
perovskite matrix is dominant instead of Eu-Eu
electronic interactions. In particular, in [41] it was
suggested that solid-solubility limit of europium would
be 1…2 mol.% due to large ionic radius difference
between Eu3+ and Sr2+. In this study, we define that for
samples annealed at 1300 °C this limit is lower than
1 mol.%.
In order to check typical for perovskite matrix
based materials procedure for decreasing the intrinsic
distortion in phosphors containing rare-earth ions by the
addition metal ions [45], different amounts of aluminium
(1.0 to 10.0 mol.%) were added during the synthetic
procedure with a constant effective concentration of Eu
equal to 8 mol.%. The results indicate that a monotonic
linear increase of the SrTiO3:Eu3+, Al luminescence
brightness has been observed with growth of the
aluminum concentration. For example, at the Al
concentration 10 mol.%, the photoluminescence
intensity increases by 2.2 times.
4. Concluding remarks
The SrTiO3:Eu3+ red phosphors with different
concentrations of accommodated Eu3+ ions were
prepared using the sol-gel method. The structural
analysis showed that the resulting material is a
polycrystalline powder consisting of several phases with
predominance of SrТiO3 and additional appreciable
content of titanium oxide phase in the rutile form. The
amount of europium in the phosphors synthesized with
less than 8 mol.% of Eu is extremely low; so, the ratio of
concentrations Eu/Sr in the final product is essentially
low as compared to the original solutions. It was found
Semiconductor Physics, Quantum Electronics & Optoelectronics, 2016. V. 19, N 4. P. 343-351.
doi: https://doi.org/10.15407/spqeo19.04.343
© 2016, V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine
349
that phosphors do not possess magnetic properties, as
they contain Eu in a trivalent state. The latter fact
confirms the results of luminescence study; the intensity
of photoluminescence increases with the Eu
concentration up to the maximum level at c.a. 8 mol.%.
With increasing the Al concentration up to 10 mol.%,
the photoluminescence intensity increases by 2.2 times
as well.
Summarizing the results, we can formulate the
following conclusions for the samples, the relative
concentration of europium in their original solution does
not exceed 8 mol.%:
1) the concentration of Eu in phosphors under
consideration is lower than the detection limit of
the equipment used;
2) the magnetic properties do not appear;
3) luminescent study demonstrates typical
luminescence of Eu3+ ions in highly symmetrical
environment with relatively low distortion of
coordination polyhedron.
The collection of the data allows assuming that
doped Eu present in a SrТiO3 matrix in very small
quantities not correlated with the initial reagents ratio.
This suggests that either the europium ions are in the
amorphous regions of the material with a large number
of structural or crystallite surface defects and do not
form a crystalline phase of stable europium compounds
with detectable emission or the amount of formed
europium compounds is extremely small. The existence
of different europium centers in these matrices
mentioned for example in Ref. [25]. A possible reason
for a small amount of Eu3+ in SrTiO3 may be the result
of chosen technological conditions including the high
temperature ceramic annealing step. However, similar
emission is observed in similar phosphors obtained
under the high temperature annealing with the
temperature exceeding 1200 °С [22, 23, 25] and, at least
in [41] mentioned similar low solid-solubility limit of
Eu3+ in the perovskite matrix. Possible explanation may
be related to the low probability process assuming
simultaneous exchange substitution of Sr2+ and Ti4+ sites
by Eu3+ ions with compensation both electrostatic and
spatial distortion within the crystalline structure.
However, the ultimate conclusion of this issue requires
an additional research.
Acknowledgments
Assistance of Dr. Sc. Miroslav V. Karpets
with X-ray measurements is highly appreciated. Authors
acknowledge multiple discussions with
Dr. Anna N. Morozovska.
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