Article
pubs.acs.org/JPCC
Discovery of New Solid Solution Phosphors via Cation SubstitutionDependent Phase Transition in M3(PO4)2:Eu2+ (M = Ca/Sr/Ba) QuasiBinary Sets
Haipeng Ji,† Zhaohui Huang,*,† Zhiguo Xia,*,‡ Maxim S. Molokeev,§,∥ Victor V. Atuchin,⊥,#,∇
Minghao Fang,† and Yangai Liu†
†
School of Materials Science and Technology, Beijing Key Laboratory of Materials Utilization of Nonmetallic Minerals and Solid
Wastes, National Laboratory of Mineral Materials, China University of Geosciences (Beijing), Beijing 100083, China
‡
School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China
§
Laboratory of Crystal Physics, Kirensky Institute of Physics, SB RAS, Krasnoyarsk 660036, Russia
∥
Department of Physics, Far Eastern State Transport University, Khabarovsk 680021, Russia
⊥
Laboratory of Optical Materials and Structures, Institute of Semiconductor Physics, SB RAS, Novosibirsk 630090, Russia
#
Functional Electronics Laboratory, Tomsk State University, Tomsk 634050, Russia
∇
Laboratory of Semiconductor and Dielectric Materials, Novosibirsk State University, Novosibirsk 630090, Russia
S Supporting Information
*
ABSTRACT: The cation substitution-dependent phase transition was used
as a strategy to discover new solid solution phosphors and to efficiently
tune the luminescence property of divalent europium (Eu2+) in the
M3(PO4)2:Eu2+ (M = Ca/Sr/Ba) quasi-binary sets. Several new phosphors
including the greenish-white SrCa2(PO4)2:Eu2+, the yellow Sr2Ca(PO4)2:Eu2+, and the cyan Ba2Ca(PO4)2:Eu2+ were reported, and the
drastic red shift of the emission toward the phase transition point was
discussed. Different behavior of luminescence evolution in response to
structural variation was verified among the three M3(PO4)2:Eu2+ joins.
Sr3(PO4)2 and Ba3(PO4)2 form a continuous isostructural solid solution set
in which Eu2+ exhibits a similar symmetric narrow-band blue emission
centered at 416 nm, whereas Sr2+ substituting Ca2+ in Ca3(PO4)2 induces a
composition-dependent phase transition and the peaking emission gets red
shifted to 527 nm approaching the phase transition point. In the
Ca3−xBax(PO4)2:Eu2+ set, the validity of crystallochemical design of phosphor between the phase transition boundary was
further verified. This cation substitution strategy may assist in developing new phosphors with controllably tuned optical
properties based on the phase transition.
Sr2CeO4,11 Y0.845Al0.070La0.060Eu0.025VO4,12 Ba(Si,Al)5(O,N)8:Eu2+,13 and Ca1.5Ba0.5Si5N6O3:Eu2+ 14 have
been discovered. Recently, the usage of combinatorial
chemistry in phosphor discovery was reviewed.15 Another
emerging strategy to tune the spectroscopic properties and find
new phosphors is based on the solid solution design via cationic
substitution or anionic polyhedron replacement.16−19 Since the
emission of Eu2+ is very sensitive to the local environment, the
structural property of the Eu2+ site is expected to be tuned via
chemical composition variation of the solid solution host, which
can serve as a color-tuning technique. Our contributions on
apatite,20 melilite,21,22 and clinopyroxene23,24 type phosphors
suggest that solid solution design is promising to form new
1. INTRODUCTION
Solid state lighting which concentrates on white light-emitting
diodes (w-LEDs) is becoming increasingly crucial to the world’s
energy saving.1 An important approach to achieve color-stable
w-LEDs is to combine a blue, ultraviolet (UV), or near-UV
LED chip with phosphors, known as phosphor-converted (pc)
w-LEDs. Rare-earth ions such as Eu2+, Eu3+, Ce3+, and Mn4+ are
the most frequently used activators for wavelength converting
due to the high efficiency and abundant emissions based on 4f
→ 4f or 5d → 4f transitions.2−4 In the past decade, extensive
efforts have been made to develop new phosphors for
application in w-LEDs based on fluoride, phosphate, silicate,
aluminate, oxynitride, nitride, etc.5−9
One efficient strategy to explore new phosphor materials is
the heuristic optimization involving combinatorial chemistry,
where a large number of samples are prepared in parallel
synthesis.10 Using this method, new phosphors such as
© 2015 American Chemical Society
Received: September 25, 2014
Revised: January 2, 2015
Published: January 6, 2015
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tube furnace under reducing 5% H2−95% N2 flow. In the
Ca3−xBax(PO4)2:Eu2+ set, the optimized temperature of 1350
°C was used to synthesize Ba2Ca(PO4)2:Eu2+ because 1250 °C
was not high enough to form phase-pure sample. After slow
cooling to room temperature, the products were ground into
fine powders for subsequent characterization.
2.2. Characterization. The X-ray diffraction (XRD)
patterns were recorded by an X-ray powder diffractometer
(D/max-IIIA, Rigaku, Japan) using Cu Kα radiation (1.5406 Å)
under operating electric voltage and current of 40 kV and 100
mA, respectively. The step scanning rate (2θ range 5−110°)
used for Rietveld refinements was 2 s per step with a step size
of 0.02°. The XRD patterns were analyzed using the program
TOPAS 4.2.33 The photoluminescence emission (PL) and
excitation (PLE) spectra were measured at room temperature
on a fluorescence spectrophotometer (F-4600, Hitachi, Japan)
with a photomultiplier tube operating at 400 V using a 150 W
Xe lamp as the excitation source. The temperature-dependent
luminescence was measured on the same device which was
combined with a computer-controlled electric furnace. The 31P
magic angle spinning nuclear magnetic resonance (MAS NMR)
measurements were performed on a Bruker AVANCE III 400
MHz NMR spectrometer at 9.4 T. The device is equipped with
a 4 mm double-resonance MAS probe at a spinning speed of 10
kHz. The recycle delay is 5.00 s, and the pulse length is 90°;
H3PO4 solution is used as a reference. High-resolution
transmission electron microscopy (HRTEM) images were
obtained using the JEM-2011 microscope (JEOL, Japan), which
is equipped with energy-dispersive X-ray spectroscopy (EDS,
INCA, Oxford Instruments, U.K.). Quantum efficiency was
calculated using the data measured on the FLS920 fluorescence
spectrophotometer (Edinburgh Instruments Ltd., U.K.) using
the integrating sphere at room temperature.
phosphors with predetermined structural parameters. Solid
solution provides feasibility for preparing smart artificial
materials using isomorphism and/or pseudoisomorphism
relationships with the alteration of local crystal field strength
around the activators, which have been used to shift the
emission spectra, optimize the luminescence property, and
improve the rigidity of the host lattice of commercial phosphors
such as M 2 Si 2 O 2 N 2 :Eu 2 + (M = Ca, Sr, Ba) and
SrxBa2−xSiO4:Eu2+.18,19
A conventional solid solution design strategy in phosphors is
to prepare isostructural compounds; here, we propose another
attempt: to prepare solid solutions within a phase transition
boundary. In the solids, the crystal lattice becomes less rigid
and bears more distortion around the vicinity of the phase
transition; in addition to the generation of point defects, the
abnormal critical behavior of physical properties, including
structural, thermal, and optical parameters, is commonly
observed,25,26 related effects of which are found in many
solution systems.27−30 Interestingly, similar effects on the
luminescence property can also be achieved, with the
involvement of the internal pressure on the lattice induced by
cation substitution. Such effect is supposed in M3(PO4)2 (M =
Ca/Sr/Ba), and the aim of the present study is to study the
structural and photoluminescence evolution of such quasibinary sets. The M3(PO4)2 phosphates with Ca, Sr, and Ba are
selected as they are big enough to be substituted by Eu2+. The
Mg2+ phosphate seems to be less suitable for the Eu2+
incorporation and is ignored.
M3(PO4)2 (M = Ca, Sr, and Ba) are found to crystallize in
space groups of R3c, R-3m, and R-3m, respectively, under an
identical sintering procedure. Thus, different cations apply
different internal pressure on the M3(PO4)2 lattice, and this
pressure, if appropriately tuned, will result in compositiondependent phase transition. Among the related binary joins, it
can be reasonably supposed that the isostructural Sr3(PO4)2
and Ba3(PO4)2 form a continuous solid solution set with
linearly changed structural parameters, whereas Ca3(PO4)2 and
Sr3(PO4)2 are not isostructural, and the solution range in quasibinary set Sr3(PO4)2−Ca3(PO4)2 should be limited, where at
least one phase transition is supposed to occur. Thus, it is
suitable to compare the relationship between effects in the
structural and photoluminescence properties in the
Ba3−xSrx(PO4)2:Eu2+ and Sr3−xCax(PO4)2:Eu2+ joins. Furthermore, the Ca3−xBax(PO4)2:Eu2+ join is additively studied to
verify the photoluminescence response to the structure
transition.
3. RESULTS AND DISCUSSION
The structural evolution and photoluminescence response are
first investigated in the Ba3−xSrx(PO4)2 set. The XRD patterns,
variation of cell parameters, typical PLE and PL spectra, as well
as digital images (under 365 nm light excitation) of the
Ba3−xSrx(PO4)2:Eu2+ (x = 0, 1, 2, 3) phosphors are shown in
Figure 1. The four XRD patterns were analyzed by Rietveld
refinement with crystallographic information files (CIFs)
provided in the Supporting Information. Main parameters of
processing and refinement results are presented in Table 1. The
patterns recorded from Ba3(PO4)2:Eu2+ and Sr3(PO4)2:Eu2+
can be well fitted by a rhombohedral cell in space group R-3m.
As shown in Figure 1, progressive substitution of Ba by Sr
induces shifting of the diffraction peaks to larger 2θ angles.
Fitting analysis of the cell parameters a, c, and V reveal a linear
decreasing trend. The crystal structure of the solutions remains
the same despite the composition variation. Earlier, Popović et
al. 34 reported the isostructural nature of undoped
Ba3−xSrx(PO4)2. As demonstrated, the unit cell parameters’
decrease is in favor of the occupation of 3a and 6c sites by Sr2+
(ri = 1.36 Å with coordination number (CN) of 10 and 1.44 Å
with CN of 12), the effective ion radius of which is smaller than
that of Ba2+ (ri = 1.52 Å with CN of 10 and 1.61 Å with CN of
12). Moreover, the refinements in ref 26 give cell volumes of
572.3, 556.3, 523.1, and 498.0 Å3 for Ba3(PO4)2, Ba2Sr(PO4)2,
BaSr2(PO4)2, and Sr3(PO4)2, respectively. Our refinements on
the Eu2+-doped samples yield cell volumes of 570.39(7),
549.08(6), 523.02(2), and 497.54(3) Å3 (Table 1), respectively.
Thus, the V of pure Ba3−xSrx(PO4)2 are slightly bigger than
2. EXPERIMENTAL SECTION
2.1. Materials and Synthesis. Although sol−gel, hydrothermal, spray pyrolysis, and combustion methods have been
reported to synthesize phosphors,31,32 the simplest method is
solid-state reaction which can be easily scaled up in large
quantity and is chosen in the present study. Three sets of Eu2+doped phosphors Sr3−xCax (PO 4) 2 , Ba 3−xSrx(PO 4) 2 , and
Ca3−xBax(PO4)2 (x = 0, 1, 2, and 3) were prepared starting
from SrCO3 (A.R.), CaCO3 (A.R.), BaCO3 (A.R.), NH4H2PO4
(A.R.), and Eu2O3 (99.99%) (Beijing Chemical Co., China).
Briefly, the starting materials were weighed according to
nominal x values and thoroughly ground in an agate mortar by
hand. Then, the mixtures were placed in corundum crucibles
and preheated at 300 °C for 2 h and again at 800 °C for 3 h in a
muff furnace in the air to release NH3, H2O, and CO2. Finally,
the samples were reground and sintered at 1250 °C for 8 h in a
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Sr3(PO4)2−Ca3(PO4)2 set are found to exhibit interestingly
unusual emission features. Crystal structures of Sr3(PO4)235 and
Ca2.83Sr0.17(PO4)236 were used as starting models to refine the
structure of Sr3(PO4)2:Eu2+ and those of Sr2Ca(PO4)2:Eu2+,
SrCa2(PO4)2:Eu2+, and Ca3(PO4)2:Eu2+, respectively. All
patterns were well indexed, and all peaks in each pattern
were verified to be related to one phase (except in
Ca3(PO4)2:Eu2+ where one tiny peak at 31.86° was unfitted).
The refinement was stable and ended with low R factors. Main
parameters of processing and refinement results are shown in
Table 2. The refined CIFs of the four samples are provided in
Table 2. Main Parameters of Processing and Refinement of
the Sr3−xCax(PO4)2:Eu2+ Samples
x
formula
space group
a, Å
c, Å
V, Å3
Z
2θ interval,
deg
Rwp, %
Rp, %
Rexp, %
χ2
RB, %
Table 1. Main Parameters of Processing and Refinement of
the Ba3−xSrx(PO4)2:Eu2+ Samples
Ba3(PO4)
Ba2Sr(PO4)2
BaSr2(PO4)2
Sr3(PO4)3
R-3m
5.6019(3)
20.988(1)
570.39(7)
3
5−110
13.39
9.41
6.14
2.18
3.32
R-3m
5.5401(2)
20.657(1)
549.08(6)
3
5−110
17.14
11.92
5.99
2.86
5.77
R-3m
5.4647(1)
20.2235(4)
523.02(2)
3
5−110
13.35
9.15
5.70
2.34
2.38
R-3m
5.3882(1)
19.7888(5)
497.54(3)
3
5−110
22.15
13.56
7.64
2.90
2.77
1
Sr2Ca(PO4)2
R3c
10.6686(2)
38.9832(8)
3842.5(2)
21
5−110
2
SrCa2(PO4)2
R3c
10.5620(3)
38.0712(1)
3678.0(2)
21
5−110
3
Ca3(PO4)2
R3c
10.4414(2)
37.4208(8)
3533.1(2)
21
5−110
5.41
3.99
9.19
1.04
1.99
11.28
7.61
5.26
2.15
5.07
9.94
6.23
5.97
1.67
4.45
9.87
6.89
7.01
1.41
4.40
supplemental files. Their crystal structures are depicted in
Figure 2. Sr3(PO4)2 (space group R-3m, No. 166) can be
considered as parent phase for the other three phases in this set
due to higher symmetry; superstructure peaks appear in the
XRD patterns of Sr2Ca(PO4)2, SrCa2(PO4)2, and Ca3(PO4)2 in
comparison with that of Sr3(PO4)2, indicating the three phases
have another space group (R3c, No. 161) in reference to the
parent phase and should have bigger cell volumes. Moreover, of
the two intermediate phases Sr2Ca(PO4)2 and SrCa2(PO4)2,
the occupation of Sr2+ in the Ca5 site is refined to be 0.0(1);
analysis of the Ca5−O bond lengths reveals that the Ca5 site is
too small to be occupied by Sr2+, and we thus decide to occupy
this site by Ca ion only.
The Sr 2 Ca(PO 4 ) 2 :Eu 2 + , SrCa 2 (PO 4 ) 2 :Eu 2 + , and
Ca3(PO4)2:Eu2+ have identical space group and similar cell
parameters, indicating that they are isostructural.
SrCa2(PO4)2:Eu2+ has cell parameters (a = 10.5620(3) Å, c =
38.0712 Å) which are very close to values given by Belik37 (card
no. 52-467) for SrCa2(PO4)2: a = 10.5642 Å, c = 38.065 Å.
Pure Ca3(PO4)2 and Sr3(PO4)2 were also reported earlier (no.
70-2065 for Ca3(PO4)2 and no. 80-1614 for Sr3(PO4)2), and
cell parameters of the above Eu2+-doped compounds are very
close to those in the reports. However, there is no available
structural information for Sr2Ca(PO4)2, which is verified to be a
new phase. The linear dependence of the formula unit volume
Vfu on x shown in Figure 3 indicates that the refined Sr/Ca
ratios in all compounds are very close to the nominal ones. The
two end members, Sr3(PO4)2 and Ca3(PO4)2, are fitted to be in
the R-3m and R3c space group, respectively. To exhibit a
complete solid solution range, it is essential that the end
members are isostructural; otherwise, partial or limited solid
solution ranges should occur. Thus, not all compositions falling
in this join could form a single phase due to limited solubility.
Our refinements indicate that samples with x = 1 and 2 are
Figure 1. XRD patterns, variation of cell parameters, typical
photoluminescence emission and excitation spectra, and digital images
under λex = 365 nm of Ba3−xSrx(PO4)2:Eu2+ (x = 0, 1, 2, 3) phosphors.
space group
a, Å
c, Å
V, Å3
Z
2θ interval, deg
Rwp, %
Rp, %
Rexp, %
χ2
RB, %
0
Sr3(PO4)2
R-3m
5.38799(3)
19.7860(1)
497.443(6)
3
5−110
those of the Eu2+-doped ones with the same x, because Ba2+ has
a larger radius than that of Eu2+ (for example, r(Eu2+)CN=6 is
1.17 Å and r(Ba2+)CN=6 is 1.35 Å). Besides, the PL spectra of all
four compositions are observed to be practically identical:
Sr3(PO4)2:Eu2+ and Ba3(PO4)2:Eu2+ both exhibit symmetric
narrow-band purplish-blue emission centered at ∼416 nm. The
intermediate Ba2Sr(PO4)2:Eu2+ and BaSr2(PO4)2:Eu2+ display
similar PL patterns peaking at ∼419 nm with no significant
emission wavelength shifting on x variation.
In the above Ba3−xSrx(PO4)2:Eu2+ join we see the structural
parameters well obeying Vegard’s law and blue emission from
Eu2+ in all compositions. In comparison, the phosphors in the
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phase is greater than that in R-3m phase. Also, the refinement
suggests a difference in the anion polyhedra between them: the
compositions in R3c contain three kinds of [PO4] groups, while
only one kind of [PO4] group exists in the composition in R3m (Figure 2).
To see the difference, the 31P solid-state magic angle
spinning nuclear magnetic resonance (MAS NMR) study was
performed; the spectra recorded are shown in Figure 4. A single
Figure 4. 31P solid-state nuclear magnetic resonance (NMR) spectra
of Sr3−xCax(PO4)2:Eu2+.
Figure 2. Refined crystal structures of Sr3−xCax(PO4)2:Eu2+: (a) x = 0,
(b) x = 1 and 2, and (c) x = 3. Of the four hosts, Sr3(PO4)2 is in space
group R-3m and Sr2Ca(PO4)2, SrCa2(PO4)2, and Ca3(PO4)2 are in
R3c. The Sr2+ in Sr2Ca(PO4)2 and SrCa2(PO4)2 can occupy four of the
five Ca sites in Ca3(PO4)2 based on the refined site occupation.
sharp peak observed in Sr3(PO4)2 shows the presence of only
one P position in the R-3m structure. Peak splitting occurs on
the Ca concentration increase, which indicates transformation
of the initial Sr3(PO4)2 structure. It is known that the chemical
shift of 31P is dominantly influenced by the paramagnetic
shielding σp resulted from the restriction of the chemical
bonding on the outer core−shell electronic flow. Thus,
Sr2Ca(PO4)2 should be a new phase (solid solution) with
structure different from that of Sr3(PO4)2. Moreover, the
chemical shift of P remains almost the same as Ca content
increases, suggesting similar P position in Sr2Ca(PO4)2,
SrCa2(PO4)2, and Ca3(PO4)2. The 31P solid-state NMR results
thus confirm the formation of two types of crystal structures
with x increase.
As for the new phosphor Sr2Ca(PO4)2:Eu2+, the coexistence
and element distribution of Sr, Ca, P, O, and Eu were
characterized by HRTEM equipped with EDS. The TEM image
of the selected crystal is shown in Figure 5a, and all elements
(including C and Cu detected from the holder) are observed
from one complete microcrystal (Figure 5d). The fine
structures examined by HRTEM were analyzed by Digital
Micrograph software. The fast Fourier transform (FFT) images
(inset in Figure 5b and 5c) suggest the highly crystalline nature.
The continuous lattice fringe measurements with d spacing
values of 2.838 and 3.437 nm, which can be assigned to the
(030) and (042) planes, respectively, in the Sr2Ca(PO4)2
lattice, are in line with the refined corresponding crystal
structure parameters.
The PLE and PL spectra and color coordinates of
Sr3−xCax(PO4)2:Eu2+ (x = 0, 1, 2 and 3) phosphors are
presented in Figure 6. The inset shows the digital images under
λex = 365 nm with visible colors of blue, yellow, and greenishwhite. When x = 0, 1, 2, and 3, the corresponding band
Figure 3. Linear dependence of formula unit volume Vfu on x in
Sr3−xCax(PO4)2:Eu2+ (Vfu = V for R-3m phase and Vfu = V/8 for R3c
phase).
pure. In an earlier study on the polar and centrosymmetric
phases in Ca3−xSrx(PO4)2 (0 ≤ x ≤ 16/7), Belik et al.38 found
that, at 1000 °C, the solid solubility limit in this set is x ≤ 2.31,
confirming the rigidity of the present refinements. The two
crystal structures distinguish from each other in several aspects.
The R3c structure has no center of inversion, but it exists in R3m. Also, the cell volume of R3c phase is much bigger than that
of R-3m, which means that the number of different positions of
Sr/Ca ions in the independent part of the unit cell in the R3c
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Figure 5. (a) TEM image, (b and c) HRTEM images of the new
compound Sr2Ca(PO4)2:Eu2+, and (d) EDS pattern detected in a.
Inserts in b and c are the fast Fourier transforms (FFT) of the relevant
HRTEM image sections.
emissions are peaking at 413, 527, 500, and 416 nm,
respectively. The narrow symmetric emission bands of αSr3(PO4)2:Eu2+ and β-Ca3(PO4)2:Eu2+ are consistent with the
literature results.39 An unexpected broad-band emission curve
peaking at 527 nm with the fwhm (full-width at the halfmaximum) of 130 nm is observed on the new Sr2Ca(PO4)2:
Eu2+ phosphor; both the structural and the luminescence
characters are first reported. SrCa2(PO4)2:Eu2+ exhibits a wide
band emission peaking at 500 nm with a fwhm of 109 nm and
appears to be moderately bright greenish-white when visually
observed under λex = 365 nm. Eu2+ in these crystals is expected
to experience different crystal field strength and covalence
when entering the various crystallographic cation sites in the
lattices, which induces tunable emission wavelength and fwhm
values.40 A recent concentration-dependent PL study on
SrCa2(PO4)2:xEu2+, yMn2+ confirms that there are two sites
occupied by Eu2+ which emit at 430 and 500 nm, and the
relative intensity of the two peaks changes with the doping
content variation due to energy transfer between these sites.41
Our result for SrCa2(PO4)2:Eu2+ (Figure 6a) is consistent with
the situation where the emission at 500 nm is much higher than
that at 430 nm. Another earlier report42 on
Ca2Sr1−x(PO4)2:xEu2+ gave an asymmetric emission with the
maximum at 418 nm, together with a shoulder at 435 nm,
which is not in accordance with the ref 41. We find that
although both refs 41 and 42 cite the same Joint Committee on
Powder Diffraction Standards (JCPDS) card no. 52-0467, only
the standard PDF card in ref 41 is real.
We also evaluated the internal quantum efficiency (QE) of
concentration-optimized Sr 2 Ca(PO 4 ) 2 :0.015Eu 2+ and
SrCa2(PO4)2:0.015Eu2+ phosphors, which were determined to
be 44.03% and 41.74%, respectively, under the 380 nm
excitation. The temperature-dependent photoluminescence
emission spectra indicate that, when the temperature increases
to 100 and 150 °C, the emission intensity of Sr2Ca(PO4)2:0.015Eu2+ decreases to 64.9% and 41.2% in reference
to that at 30 °C and the emission intensity of
SrCa2(PO4)2:0.015Eu2+ decreases to 70.6% and 52.1% of that
at 30 °C.
The structural model able to explain the luminescence
evolution of Eu2+ in the Sr3(PO4)2−Ca3(PO4)2 join is
illustrated in Figure 7. Ca3(PO4)2:Eu2+ exhibits a blue emission;
Figure 6. (a) Photoluminescence emission and excitation (λex = 365
nm) spectra of the Sr3−xCax(PO4)2:Eu2+ (x = 0, 1, 2, and 3)
phosphors. With compositional variable x being 0, 1, 2, and 3, the
corresponding emission is peaking at 413, 527, 500, and 416 nm,
respectively. (b) Color coordinates of Sr3−xCax(PO4)2:Eu2+ under λex
= 365 nm in the CIE chromaticity diagram (insets show digital
images).
substitution of Ca2+ by bigger Sr2+ changes the coordination
spheres of Eu2+ and induces increasing pressure on the EuOn
polyhedra, in accordance with which the average bond length
d(Eu−O) could reasonably decrease (Figure 7b−d). This leads to
the red shifting of peaking emission and the color changing
from blue to yellow through greenish-white. Somewhere in the
composition range x = 0−1 the phosphate crystal experiences
the R3c ↔ R-3m phase transition; Sr3(PO4)2:Eu2+ then has
another structure (Figure 7a), and the emission drastically
changes to blue. Moreover, our recent detailed investigation on
the photoluminescence evolution of this set with a smaller x
interval of 0.10 in the 1.00 ≤ x ≤ 3.00 region reveals that the
strongest red shift in the emission spectra occurs at x = 1.00,43
which is closest to the phase transition point. Although direct
characterization of the EuOn polyhedra distortions or the Eu−
O bond length is desired to verify the model, we find it is very
difficult to accurately evaluate them because of the low Eu
doping concentration and the limitation of the X-ray method
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Figure 7. Structural model explaining the photoluminescence evolution observed in Sr3−xCax(PO4)2:Eu2+ phosphors: coordination of Eu2+ in the
phase with (a) R-3m structure and (b, c, d) R3c structure. (f) The substitution of Ca by bigger Sr ions directly affects the third coordination sphere of
Eu2+ and then distorts the EuOn polyhedra due to size mismatch.
which identifies an averaged structure of compounds. The first
three nearest coordination spheres around the Eu2+ are
constructed of O atoms, PO4 tetrahedrons, and surrounding
Ca/Sr ions.43 For the four Ca sites which can be occupied by
Eu, the same number (nine) of neighboring Ca/Sr ions are
observed (Figure 7f). The refinements reveal that the PO4
tetrahedrons in these phases remain similar, and the Ca/Sr
substitution distorts the inner EuOn polyhedron by affecting the
third coordination sphere. Thus, the peaking wavelength of the
band emitted by the EuOn−Ca9−ySry units (y = 1, 2, ..., 9)
should be different from the 416 nm peak position related to
the EuOn−Ca9 units (Figure 7f) existing in Ca3(PO4)2:Eu2+.
The random distribution of Sr2+ in Sr2Ca(PO4)2:Eu2+ or
SrCa2(PO4)2:Eu2+ leads to different combinations of EuOn−
Ca9−ySry emitting units, which give various peaking emission
wavelengths, and thus, the observed fwhms of luminescence
spectra differ from each other. The dependency of the
normalized emission peak intensity at 416 nm of the phosphors
and the occurrence probability of EuOn−Ca9 units in the solid
solutions on y well fits with each other,43 which supports the
structural model.
On the basis of the evolution of the structural and
luminescence properties in the Ba3−xSrx(PO4)2:Eu2+ and
Sr3−xCax(PO4)2:Eu2+ sets, it is deduced that the behavior of
the Ca3−xBax(PO4)2:Eu2+ set would be similar to that of the
Sr3−xCax(PO4)2:Eu2+ one because in both these sets the end
members are not isostructural. Previously, color-tunable
luminescence was observed by Li et al.,44 who studied the
effect of a small level substitution of Ca2+ by Ba2+ in monoclinic
Ca 3 (PO 4 ) 2 :Eu 2+ (space group P2 1 /a). The Ba-doped
Ca 3 (PO 4 ) 2 phase was obtained over the range of
(Ca0.95‑yBay)3(PO4)2:0.05Eu2+ (0 ≤ y ≤ 0.132), where Ba
doping stabilizes the high-temperature α form to room
temperature. Also, crystal structure a naly sis o f
(Ca1−xBax)3(PO4)2 (x = 0.05−0.15) through synchrotron
diffraction has been performed,45 showing that the cell
parameters a, b, c, and β increase with more Ba doping.
However, a higher Ba/Ca ratio would induce new phases
formation (such as Ba3Ca3(PO4)4 and Ba2Ca(PO4)2) with
other crystal structures. The successful synthesis of pure
Ba3Ca3(PO4)4:Eu3+ was demonstrated,46 but a big misfit
between the measured XRD pattern and the JCPDS card no.
24-0091 evidently exists. Our attempt to prepare Ba3Ca3(PO4)4
also fails, and the final product consists of Ba2Ca(PO4)2
(JCPDS no. 23-814) as the major phase. Although the XRD
pattern and PL spectrum of Ba 2 Ca(PO 4 ) 2 :Eu 2+ were
recorded,47,48 the crystal structure information on cell
parameters and space group were not reported. Ba2Ca(PO4)2:Eu2+ was thus synthesized, and its XRD pattern was
subjected to Rietveld refinement. The profile fitting of
Ba2Ca(PO4)2:Eu2+ by the rhombohedral unit cell with
parameters a = 5.4705(1) Å and c = 34.8547(8) Å is shown
in Figure 8a. Space group determination using systematic reflex
extinction conditions shows that a proper space group should
be one from the set R3m, R3, R-3, or R-3m. However, the
crystal structure solving in all these groups gave insufficient
results, and the detailed crystal structure remains unclear. The
Figure 8. (a) Profile fitting of Ba2Ca(PO4)2:Eu2+. Bragg reflections are
indicated with tick marks. (b) Photoluminescence emission and
excitation spectra and digital images of Ba 2 Ca(PO 4 ) 2 :Eu 2+ ,
Ca3(PO4)2:Eu2+, and Ba3(PO4)2:Eu2+ under λex = 365 nm excitation.
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The Journal of Physical Chemistry C
PL spectrum under λex = 365 nm (Figure 8b) shows an
asymmetric broad-band emission extending from 410 to 600
nm with a maximum at 456 nm, which differs from those of the
end members Ca3(PO4)2:Eu2+ and Ba3(PO4)2:Eu2+, where
symmetric emission curves peaking at ∼416 nm were observed.
Thus, tunable luminescence of Eu2+ is validated in the
Ca3−xBax(PO4)2 set, further proving the hypothesis that the
composition variation through the phase transition boundary is
beneficial to obtain new photoluminescence features.
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4. CONCLUSION
In summary, different behaviors of photoluminescence
response to the structural variation were verified among the
three M3(PO4)2:Eu2+ (M = Ca, Sr, and Ba) quasi-binary sets.
Several new solid solution phosphors including the greenishwhite SrCa2(PO4)2:Eu2+, the yellow Sr2Ca(PO4)2:Eu2+, and the
cyan Ba2Ca(PO4)2:Eu2+ were discovered. The color-tunable
photoluminescence emission in the Sr3−xCax(PO4)2:Eu2+ series
can be ascribed to the phase transition from R3c to R-3m as
well as the variation of the crystal field environment. The cation
substitution-dependent phase transition can be an efficient
approach to assist in the discovery of new phosphors and to
tune the photoluminescence spectra of phosphors.
■
ASSOCIATED CONTENT
S Supporting Information
*
Crystallographic information files of Sr3−xBax(PO4)2:Eu2+ (x =
1, 2) and Sr3−xCax(PO4)2:Eu2+ (x = 1, 2). This material is
available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*Phone: +86 10 82322186. E-mail: [email protected].
*Phone: +86 10 82377955. E-mail: [email protected].
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the National Natural Science
Foundations of China (Grant Nos. 51032007, 51002146, and
51272242), the Research Fund for the Doctoral Program of
Higher Education of China (Grant No. 20130022110006), the
Natural Science Foundations of Beijing (2132050), the
Program for New Century Excellent Talents in University of
Ministry of Education of China (NCET-12-0950), the Beijing
Nova Program (Z131103000413047), and Beijing Youth
Excellent Talent Program (YETP0635). V.V.A. acknowledges
the Ministry of Education and Science of the Russian
Federation for financial support.
■
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