Synthesis and Luminescence Properties of a CsBaGd(MoO4)3:Er3+ Phosphor with a Scheelite-Like Structure

Article abstract of DOI:10.1134/S0020168518060079

A CsBaGd(MoO₄)₃:Er³⁺ phosphor with a scheelite-like structure (sp. gr. P2₁/n) has been synthesized and its luminescence properties have been studied. The synthesized material has been characterized by X-ray diffraction, differential thermal analysis, Raman spectroscopy, and IR spectroscopy.

Full text of DOI:10.1134/S0020168518060079

ISSN 0020-1685, Inorganic Materials, 2018, Vol. 54, No. 6, pp. 585–590. © Pleiades Publishing, Ltd., 2018.
Original Russian Text © N.M. Kozhevnikova, 2018, published in Neorganicheskie Materialy, 2018, Vol. 54, No. 6, pp. 616–621.
Synthesis and Luminescence Properties of a CsBaGd(MoO4)3:Er3+
Phosphor with a Scheelite-Like Structure
N. M. Kozhevnikova
Baikal Institute of Nature Management, Siberian Branch, Russian Academy of Sciences,
ul. Sakh’yanovoi 6, Ulan-Ude, 670047 Buryat Republic, Russia
e-mail: nicas@binm.ru
Received October 19, 2017
Abstract—A CsBaGd(MoO4)3:Er3+ phosphor with a scheelite-like structure (sp. gr. P21/n) has been synthe-
sized and its luminescence properties have been studied. The synthesized material has been characterized by
X-ray diffraction, differential thermal analysis, Raman spectroscopy, and IR spectroscopy.
Keywords: phase equilibrium, vibrational spectra, Er3+ luminescence
DOI: 10.1134/S0020168518060079
alkali metal cation Cs+ on phase relations in such sys-
tems, because ternary molybdates containing an alka-
line-earth and a rare-earth element are potentially
attractive as materials for phosphors, light-emitting
diodes, functional electronics, and low-threshold
lasers [1–6, 12].
The objectives of this work were to synthesize a
CsBaGd(MoO4)3:Er3+ phosphor with a scheelite-like
structure and investigate its spectroscopic and physi-
cochemical characteristics.
INTRODUCTION
Molybdates and tungstates having a scheelite-like
structure and containing alkaline-earth and rare-earth
elements offer small thermal expansion coefficients
and high chemical and thermal stability. Owing to the
possibility of varying their physicochemical and opti-
cal characteristics, they find wide application in opti-
cal devices and as laser materials, phosphors, and solid
electrolytes [1–6]. In recent years, a search for multi-
functional phosphors offering bright luminescence
under UV excitation and anti-Stokes luminescence
under IR excitation has been a topical issue. One
effective way of modifying luminescence properties of
materials is by activating them with Er3+ ions, which
are capable of absorbing IR radiation and converting it
to visible anti-Stokes luminescence [7–12].
The large extent of the primary crystallization field
of distinct compounds and solid solutions with a
scheelite-like structure is due to the possibility of iso-
and heterovalently replacing some of the alkaline-
earth cations by cations of various natures and sizes,
which occupy crystallographic sites in the structural
framework and structural voids. A wide isomorphism
of cations differing in nature leads to charge disbal-
ance in the scheelite structure as a consequence of
specific geometric features in the arrangement of
nearest neighbor polyhedra and to local and coopera-
tive distortions and makes it possible to control the
spectroscopic properties of phases with scheelite-like
structures [2–6, 12].
EXPERIMENTAL
The starting chemicals used to study phase rela-
tions in the Cs2MoO4–BaMoO4–Gd2(MoO4)3 system
were Cs2MoO4, BaMoO4, and Gd2(MoO4)3 presyn-
thesized by solid-state reactions in mixtures of the
Cs2CO3 and BaCO3 carbonates; molybdenum triox-
ide, MoO3 (analytical grade); and gadolinium and
erbium oxides (99.99+% purity). The Cs2CO3,
BaCO3, and MoO3 compounds were calcined for 10 h
at 400°C, and the rare-earth oxides were calcined in
the temperature range 400–700°C. CsBaGd(MoO4)3
was synthesized using a reaction mixture of the
cesium, barium, and gadolinium molybdates,
Cs2MoO4 + 2BaMoO4 + Gd2(MoO4)3, which was cal-
cined in the temperature range 500–850°C with
repeated intermediate grindings every 20–30 h. The
calcination time at each temperature was 80–120 h.
After firing, the samples were slowly furnace-cooled.
Nonequilibrium samples were annealed further. Equi-
librium was thought to be reached if the phase compo-
Phase relations in the M2MoO4–AMoO4–
Ln2(MoO4)3 (M = Li–Rb; A = divalent element; Ln =
La–Lu, Y) systems were studied previously [4]. It is of sition of the samples remained unchanged after two
interest to assess the effect of the nature of the large sequential anneals. The synthesis products were iden-
585

586
KOZHEVNIKOVA
Cs2MoO4–Gd2(MoO4)3, Cs2MoO4–BaMoO4, and
Cs2MoO4
BaMoO4–Gd2(MoO4)3 contain compounds with the
composition. The double molybdates
1
:
1
CsGd(MoO4)2 and Cs2Ba(MoO4)2 have a narrow
homogeneity range (no more than 2 mol % wide). The
homogeneity range of BaGd2(MoO4)4 is ~10 mol %.
The double molybdate CsGd(MoO4)2, isostructural
with CsPr(MoO4)2 and crystallizing in orthorhombic
symmetry, melts incongruently at a temperature of
1100°C. In the Cs2MoO4–BaMoO4 system, we obtained
a compound with the composition Cs2Ba(MoO4)2 and
palmierite structure (K2Pb(MoO4)2 structure type).
Its melting point is 910°C. BaGd2(MoO4)4 crystallizes
in monoclinic symmetry and melts congruently at
1020°C [1].
Cs2Ba(MoO4)2
Cs2Gd(MoO4)2
T
BaMoO4
Gd2(MoO4)3
BaGd2(MoO4)4
T = CsBaGd(MoO4)3
Analysis of the phase composition of the samples
corresponding to the intersections of the joins between
the intermediate and double molybdates indicates
that, in the temperature range 650–700°C, weak
reflections appear from a new phase: the ternary
molybdate CsBaGd(MoO4)3. Raising the temperature
to 750–800°C increases the reaction rate. At 850°C,
CsBaGd(MoO4)3 was obtained in a single-phase state
(calcination time of 120 h). X-ray diffraction and DTA
data for annealed and quenched samples indicate that
the 850°C phase relations in the Cs2MoO4–
BaMoO4–Gd2(MoO4)3 system involve six joins:
Fig. 1. Phase relations in the Cs MoO –BaMoO –
2
4
4
Gd (MoO ) system (scheelite-like structure).
2
4 3
tified by X-ray diffraction on a Bruker D8 Advance
diffractometer (CuKα radiation). X-ray diffraction
data were analyzed using Rentgen software.
IR and Raman spectra of polycrystalline
CsBaGd(MoO4)3 samples were measured on a Bruker
Fourier transform IR (FTIR) spectrometer and Spe-
cord M-80 spectrometer (backscattering geometry,
resolution of 3–5 cm–1). Excitation was provided by a
near-IR laser (1.06 μm). The samples were prepared
as Vaseline mulls on polyethylene plates or by pressing
with KBr. Molybdenum isotope substituted samples
were prepared using 92MoO3 and 100MoO3 oxides con-
taining at least 95% major isotope.
Cs2Ba(MoO4)2–CsGd(MoO4)2,
CsBaGd(MoO4)3, BaMoO4–CsBaGd(MoO4)3,
BaGd2(MoO4)4–CsBaGd(MoO4)3, CsGd(MoO4)2–
CsBaGd(MoO4)3, and CsGd(MoO4)2–
Cs2Ba(MoO4)2–
BaGd2(MoO4)4 (Fig. 1). Further raising the tempera-
ture, to above 900°C, is unreasonable because
Cs2Ba(MoO4)2 begins to decompose peritectically at
910°C [1].
Differential thermal analysis (DTA) was carried out
using an MOM OD-103 thermoanalytical system (Hun-
gary) (heating rate, 10°C/min; sample weight, 0.3–0.4 g).
Luminescence spectra of CsBaGd(MoO4)3:Er3+ under
UV excitation (λex = 365 nm) were obtained on an SM
2203 spectrofluorometer (Solar, Belarus). The exci-
tation was provided by a DKsSh 150-1M high-pres-
sure xenon arc lamp. In the IR spectral region, lumi-
nescence was excited by an InGaAs laser diode (λex =
977 nm), and luminescence spectra were measured on
an Ocean Optics QE 65000 spectrometer.
According to X-ray diffraction results for CsBaGd-
(MoO4)3, the ternary molybdate CsBaGd(MoO4)3
crystallizes in a monoclinically distorted scheelite-like
structure (sp. gr. P21/n, Z = 9) and is isostructural with
the double molybdate α-KSm(MoO4)2 and the previ-
ously synthesized potassium and rubidium analogs
MBaGd(MoO4)3 (M = K or Rb). The X-ray diffrac-
tion pattern of CsBaGd(MoO4)3 could be indexed
with unit-cell parameters а = 11.076(4) Å, b =
11.873(3) Å, с = 15.911(2) Å, and β = 86.71°. The
indexing scheme of the X-ray diffraction pattern of
CsBaGd(MoO4)3 is presented in Table 1. The struc-
ture of CsBaGd(MoO4)3 can be thought of as the
result of heterovalent substitution for the alkaline-
earth element in the scheelite structure according to
the following scheme: 2Ba = Cs+ + Gd3+.
RESULTS AND DISCUSSION
The phase relations in the Cs2MoO4–BaMoO4–
Gd2(MoO4)3 system were studied in the range 500–
850°C by the method of intersecting cuts. The system
was investigated in two steps. First, we determined the
phase composition at the intersections of joins
between intermediate and double molybdates in the
constituent binary systems. In the temperature range
of interest here, the constituent binary systems
The ternary molybdate CsBaGd(MoO4)3 melts
incongruently at 945°C, without undergoing poly-
morphic transformations (Fig. 2).
Along the Cs2Ba(MoO4)2–CsGd(MoO4)2 join,
CsGd(MoO4)2 has a slight homogeneity range. The
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SYNTHESIS AND LUMINESCENCE PROPERTIES
587
Table 1. Indexing scheme of the X-ray diffraction pattern of CsBaGd(MoO4)3
2
2
2
2
) × 104
(1/
) × 104
(1/
) × 104
(1/
dcalc
) × 104
I, %
d, Å
hkl
I, %
d, Å
hkl
(1/
dobs
200
dcalc
199
dobs
2200
1
7.072
5
2.131
512
2199
11
1
2
2
1
5.026
4.902
3.881
396
416
664
121; 210
103
397; 398
416
30
2
1
2.078
2.051
2.008
2314
2377
2480
326
245
2313
2377
2478
664
22
23
6
1
100
90
3.326
3.177
903
990
204
231
902
989
1
20
1.982
1.973
2559
2570
531
2560
2571
35
1
5
2.935
1159
1158
10
1.954
2619
352
2621
30
3
25
40
2.904
2.763
1186
1307
224
323
1185
1309
15
15
1.884
1.853
2817
2911
218; 028
2816; 2818
2909; 2911
40 ; 53
6
2
25
1
2.665
2.616
1407
1460
402
1406
1
2
1.801
1.786
3082
3135
3082
3134
43
525
5
1460; 1461
240; 21
016
5
2
1
2.585
2.542
1497
1548
1497
1549
1
1
1.777
1.752
3166
3258
318
3166
3258
143
61
2
1
5
2.479
2.452
1627
1663
035
1627
15
50
1.735
1.684
3324
3526
3322; 3327
3525
15 ; 427
6
333; 206
1663; 1665
41
7
5
10
2
2.419
2.346
2.221
1709
1817
2027
026
144
1709
1819
2027
15
10
10
1.660
1.622
1.584
3629
3803
3983
072
633
3628
3804
3984; 3984
23
174; 64
5
1
1
2.182
2098
250
2097
800°C solubility of Cs2Ba(MoO4)2 in CsGd(MoO4)2 does is due to the reduction of the symmetry of the MoO4
not exceed 3 mol %. In the joins containing the ternary
molybdate CsBaGd(MoO4)3, solubility is 6–8 mol %.
Barium molybdate dissolves up to 11 mol %
CsBaGd(MoO4)3. On the side of the double molybdate
BaGd2(MoO4)4, the homogeneity range is 12–13 mol %.
CsGd(MoO4)2 dissolves 3–4 mol % CsBaGd(MoO4)3.
The IR and Raman spectra of MBaLn(MoO4)3
(M = Cs, Rb; Ln = Gd, Sm) and CsBa-
Gd(MoO4)3:Er3+ are presented in Fig. 3 and the vibra-
tional frequencies in the spectra are listed in Table 2
together with the vibrational frequencies of BaMoO4
and assignment to internal modes of the MoO4 groups
and lattice modes (librational modes of the MoO4
groups (R) and translational modes of the different
sublattices in the crystals (T)).
These data demonstrate that the vibrational spectra
of MBaLn(MoO4)3 are similar to the spectrum of
BaMoO4, which allows for adequate interpretation of
the observed bands. The observed distinctions can be
understood in terms of changes in interatomic dis-
tances and, as a consequence, in Mo–O bond energies
in response to the heterovalent isomorphous substitu-
tions on the cation sites. The number of frequencies
observed in this region is three times that in the spec-
trum of BaMoO4. The increase in the number of bands
groups, complete degeneracy lifting of the triply degener-
ate modes, and resonance coupling between modes of
translationally inequivalent MoO4 groups [4, 13].
The measured spectra have an increased number of
IR-active librational modes of the MoO4 groups.
Their frequencies are insensitive to molybdenum iso-
T
900
700
500
300
DTG
DTA
945
100
0
5
Time
TG
Δm, mg
Fig. 2. Thermal analysis results for the ternary molybdate
CsBaGd(MoO ) .
4 3
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588
KOZHEVNIKOVA
substitutions in the scheelite structure is caused by the
distortion of the molybdenum–oxygen tetrahedra.
1
Doping of luminescent materials with erbium ions
is an effective way of raising the luminescence bright-
ness of IR and visible phosphors with a scheelite-like
structure. Activation of phosphors with Er3+ ions,
which are capable of absorbing IR light and converting
it to visible anti-Stokes luminescence, makes it possi-
ble to raise the efficiency of lasers. The spectral com-
position of steady-state CsBaGd(MoO4)3:Er3+ lumi-
nescence at λex = 365 nm is determined by bands cor-
2
responding to optical transitions of the Er3+ ion. As the
Er3+ content is raised to 5 mol %, several groups of
erbium-related narrow emission bands emerge in the
green (535–610 nm) and red (660–705 nm) regions of
the visible spectrum (Fig. 4). The emission bands in the
3
4
range 590–610 nm, corresponding to the S3/2 → 4I15/2
transition, have the simplest and best-resolved structure.
To the left and right of these green bands, we observe
weaker emission bands, in the ranges 535–538 and 556–
560 nm, due to the 2H11/2 → 4I15/2 and 2H9/2 → 4I13/2 tran-
sitions, respectively. The bands in the red spectral region
are due to the 4F9/2 → 4I15/2 and 2H9/2 → 4I11/2 transitions,
whose spectrum extends from 660 to 705 nm (Fig. 4) and
has a more complex structure, because the 4F9/2 excited
state is split into five multiplets.
CsBaGd(MoO4)3:Er3+ possesses efficient anti-
Stokes luminescence in the visible range under IR
excitation. The origin of the bands observed in its anti-
Stokes luminescence spectra under excitation at λex =
977 nm can be understood as follows: According to a
known energy diagram [8], after a sequential, two-step
excitation of the Er3+ ion to the 4F7/2 level, nonradia-
200
400
600
800
1000
Wavenumber, cm–1
Fig. 3. IR spectra of (1) α-KSm(MoO ) ,
(2) CsBaGd(MoO ) , and (3) CsBaGd(MoO ) :Er
4 2
3+
.
4 3
4 3
1200
1000
800
600
400
200
0
4S3/2 → 4I15/2
tive relaxation processes lead to the filling of the 4Н11/2
,
4S3/2, 4F9/2, and 4I9/2 excited-state levels, giving rise to
anti-Stokes luminescence in the range 530–850 nm.
Figure 5 shows the anti-Stokes luminescence spec-
trum between 450 and 850 nm. There are four strong
bands, located around 530, 550, 660, and 800 nm. Two
bands, those at 530 and 550 nm, correspond to the
350
450
550
650
750
850
Wavelength, nm
3+
Fig. 4. Luminescence spectrum of CsBaGd(MoO ) :Er
(λ = 365 nm).
4 3
3+ 2
4
Er
H
11/2
→ 4I15/2 and S3/2 → 4I15/2 transitions. The
ex
660-nm emission is due to the 4F9/2 → 4I15/2 transition,
and the 800-nm emission, to the I9/2 → 4I15/2 transi-
4
tope substitutions, are essentially constant across the
series of the compounds studied, and differ little from
those in the spectra of BaMoO4. The number of
observed bands of translational modes agrees with that
predicted by theory. In a number of cases, the corre-
sponding frequencies exhibit an isotope effect and
undergo systematic changes in response to cations
substitutions. The present data demonstrate that the
translational modes of MBaGd(MoO4)3 involve all of the
cations in equal measures. The observed reduction of the
tion. The weak blue band at 490 nm arises from the
4F7/2 → 4I15/2 transition. A characteristic feature of the
Er3+ ion is a narrow luminescence bandwidth. The
anti-Stokes luminescence excitation process typically
involves a sequential absorption of two IR photons of
the same energy, which leads to the following
4
4
sequence of electronic transitions: I15/2 → I11/2 and
4I11/2 → 4F7/2. Transitions from high energy levels may
result in visible luminescence. The shape of the spec-
tra can be understood in terms of the effect of the host
crystal lattice on the Stark structure of the ground and
lattice symmetry in response to the heterovalent cation excited-state levels of the activator ion.
INORGANIC MATERIALS Vol. 54 No. 6 2018

SYNTHESIS AND LUMINESCENCE PROPERTIES
589
Table 2. Vibrational frequencies (cm–1) in the Raman and IR spectra of CsBaGd(MoO4)3, RbBaSm(MoO4)3,
CsBaGd(MoO4)3:Er3+, and BaMoO4
CsBaGd(MoO4)3:Er3+
CsBaGd(MoO4)3
RbBaSm(MoO4)3
BaMoO4
Raman
Assignment
Raman
IR
Raman
IR
Raman
IR
IR
92Mo 100Mo 92Mo 100Mo
946
929
886
869
944
927
885
869
935
919
896
868
935
915
895
934
891
926
845
924
840
918
ν1
855
840
823
804
782
756
836
817
796
768
748
837
816
804
774
749
736
843
842
813
803
771
748
737
705
838
792
830
801
760
ν3
738
706
705
703
698
682
456
418
386
354
686
474
430
427
375
329
426
374
325
371
ν2
384
384
385
366
383
365
372
360
346
325
ν4
ν2
ν4
332
323
331
320
327
300
324
292
322
286
229
321
285
228
320
309
285
309
284
262
305
284
262
286
224
R
T
202
210
189
187
187
189
203
177
155
170
144
170
145
139
129
126
103
77
168
149
137
131
157
138
132
125
157
138
–
146
140
143
136
154
R
R
R
175
155
128
105
125
127
78
101
101
107
79
T
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KOZHEVNIKOVA
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12000
10000
8000
6000
4000
2000
0
4S3/2
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ex
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A
CsBaGd(MoO4)3:Er3+ phosphor with
a
scheelite-like structure (sp. gr. P21/n) has been pre-
pared by solid-state reaction. Analysis of its lumines-
cence characteristics indicates that, as the Er3+ con-
tent is raised to 5 mol %, several groups of erbium-
related narrow emission bands emerge in the green
(535–610 nm) and red (660–705 nm) regions of the
visible spectrum.
CsBaGd(MoO4)3:Er3+ possesses efficient anti-
Stokes luminescence in the visible range under IR
excitation. The origin of the bands observed in its anti-
Stokes luminescence spectra under excitation at λex =
977 nm can be understood in terms of the filling of the
4Н11/2, 4S3/2, 4F9/2, and 4I9/2 excited-state levels, giving
rise to anti-Stokes luminescence in the range 530–850 nm.
The CsBaGd(MoO4)3:Er3+ phosphor may find appli-
cation in lasers, converters of IR radiation to visible
light, color displays, biomedical diagnostics, and opti-
cal communication systems.
Translated by O. Tsarev
INORGANIC MATERIALS
Vol. 54
No. 6
2018