Erbium-Doped Upconversion Phosphor in the Li2MoO4–BaMoO4–Y2(MoO4)3 System

Article abstract of DOI:10.1134/S0020168520030085

Abstract—: We have investigated subsolidus phase equilibria in the Li₂MoO₄–BaMoO₄–Y₂(MoO₄)₃ system and constructed its phase compatibility diagram. A ternary molybdate with the composition Li₃Ba₂Y₃(MoO₄)₈ and a monoclinic scheelite-like structure (sp. gr. C2/c) has been identified on the BaMoO₄–LiY(MoO₄)₃ join. Doping Li₃Ba₂Y₃(MoO₄)₈ with Er³⁺ ions, we obtained a Li₃Ba₂Y_2.85Er_0.15(MoO₄)₈ upconversion phosphor possessing efficient anti-Stokes luminescence in the range 530–850 nm under IR excitation (λ_ex = 977 nm). The synthesized phosphor has been characterized by X-ray diffraction, differential thermal analysis, and IR spectroscopy.

Full text of DOI:10.1134/S0020168520030085

ISSN 0020-1685, Inorganic Materials, 2020, Vol. 56, No. 3, pp. 286–291. © Pleiades Publishing, Ltd., 2020.
Russian Text © The Author(s), 2020, published in Neorganicheskie Materialy, 2020, Vol. 56, No. 3, pp. 299–304.
Erbium-Doped Upconversion Phosphor
in the Li MoO –BaMoO –Y (MoO ) System
2
4
4
2
4 3
a,
a
N. M. Kozhevnikova * and S. Yu. Batueva
a
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 June 4, 2019; revised September 6, 2019; accepted October 1, 2019
Abstract—We have investigated subsolidus phase equilibria in the Li MoO –BaMoO –Y (MoO ) system and con-
2
4
4
2
4 3
structed its phase compatibility diagram. A ternary molybdate with the composition Li Ba Y (MoO ) and a mono-
3
2
3
4 8
clinic scheelite-like structure (sp. gr. C2/c) has been identified on the BaMoO –LiY(MoO ) join. Doping
4
4 3
3+
Li Ba Y (MoO ) with Er ions, we obtained a Li Ba Y2.85Er0.15(MoO ) upconversion phosphor possessing
3
2
3
4 8
3
2
4 8
efficient anti-Stokes luminescence in the range 530–850 nm under IR excitation (λ = 977 nm). The synthesized
ex
phosphor has been characterized by X-ray diffraction, differential thermal analysis, and IR spectroscopy.
Keywords: phase equilibria, ternary molybdate, erbium ion luminescence
DOI: 10.1134/S0020168520030085
INTRODUCTION
The objectives of this work were to synthesize an
upconversion phosphor based on the ternary molyb-
Possessing small thermal expansion coefficients
and high chemical and thermal stability, molybdates
and tungstates having a layered scheelite-like structure
and containing alkaline-earth and rare-earth elements
have attracted researchers' attention owing to their
potential practical applications in quantum electron-
ics, fiber optics, and laser devices [1–7].
3+
date Li Ba Y (MoO ) :Er with a layered scheelite-
3
2
3
4 8
like structure and investigate its luminescence and
physicochemical properties.
EXPERIMENTAL
The starting chemicals used to study phase rela-
In recent years, a search for multifunctional phos-
phors offering bright luminescence under UV exci-
tation and anti-Stokes luminescence under IR exci- were Li
tions in the Li MoO –BaMoO –Y (MoO ) system
2
4
4
2
4 3
MoO
, BaMoO , and Y
4
(MoO ) presynthe-
2
4
2
4 3
tation has been a topical issue. One effective way of sized by solid-state reactions in mixtures of Li CO ,
2
3
modifying luminescence properties of materials is by BaO, Y O , and MoO in the temperature range 400–
2
3
3
3+
activating them with Er ions, which are capable of 650°C with repeated intermediate grindings every 20–
absorbing IR radiation and converting it to visible 30 h. The firing time at each temperature was 100–120 h.
anti-Stokes luminescence [7–19]. In scheelite-like After firing, the samples were slowly furnace-cooled.
molybdates and tungstates, one can stabilize a combi- Nonequilibrium samples were annealed further. Equi-
nation of various cations, atypical coordination poly- librium was thought to be reached if the phase compo-
hedra, and metastable polymorphs of three-dimen- sition of the samples remained unchanged after two
sional structures in the form of layers [3–7]. Which sequential anneals.
structural variety will be realized depends on the
The synthesis products were identified by X-ray
nature of the cations present in the composition of the
diffraction on a Bruker D8 Advance diffractometer
compound, their combination, their distribution over
(
CuK radiation). X-ray diffraction data were ana-
α
structural sites, and site occupancies. A wide isomor-
phism of cations differing in nature leads to charge dis-
balance in the structure as a consequence of specific
geometric features in the arrangement of nearest
lyzed using Rentgen software.
Vibrational spectra of polycrystalline Li Ba R -
3
2
3
(MoO ) samples were measured on a Bruker Fourier
4 8
neighbor polyhedra and to local and cooperative dis- transform IR (FTIR) spectrometer and Specord M-80
tortions and makes it possible to control the spectro- spectrometer (backscattering geometry, resolution of
–1
scopic properties of compounds with layered struc- 3–5 cm ). Excitation was provided by a near-IR laser
tures [7–21].
(1.06 μm). The samples were prepared as Vaseline
86
2

ERBIUM-DOPED UPCONVERSION PHOSPHOR
287
Li MoO
dination, the Y atoms reside in the center position of
the eight-vertex polyhedra, and the Ba atoms are in
tenfold coordination, sandwiched between the layers
formed by the Mo and Y polyhedra [3, 15, 16].
2
4
L
Li Y (MoO )
4 8
7
3
The Li MoO –BaMoO –Y (MoO ) system was
2
4
4
2
4 3
LiY(MoO4)2
studied in two steps. First, we determined the phase
composition at the intersection points of the joins
between the intermediate and binary molybdates existing
in the constituent binaries Li MoO –Y (MoO ) and
T
2
4
2
4 3
BaMoO –Y (MoO ) . The Li MoO –BaMoO sys-
4
2
4 3
2
4
4
tem was shown to have eutectic phase relations. In the
second step, we studied the pseudobinary joins identi-
fied, which allowed us to construct the phase compat-
ibility diagram of the system (Fig. 1).
BaMoO4 BaY (MoO )
Y (MoO )
2
4 4
2
4 3
Fig. 1. Phase relations in the Li MoO –BaMoO –
2
4
4
Phase relations along the BaMoO –LiY(MoO )
4
4 2
Y (MoO ) system at 700°C; T = Li Ba Y (MoO ) .
2
4 3
3
2
3
4 8
join were studied in greatest detail (at 1 to 2 mol %
intervals), and the formation of the ternary molybdate
Li Ba Y (MoO ) in the range 650–700°C was
mulls on a polyethylene substrate or by pressing with
3
2
3
4 8
6
demonstrated. To reach equilibrium, the reaction
mixtures were annealed for 120–140 h with intermedi-
ate grindings for homogenization.
KBr. Lithium isotope ( Li) substituted samples were
prepared using Li CO containing 90.5% major iso-
2
3
tope as a starting reagent.
The 700°C phase equilibria in the Li MoO –
2
4
Differential thermal analysis (DTA) was carried
BaMoO –Y (MoO ) system are characterized by the
out using an MOM QD-103 thermoanalytical system
4
2
4 3
following pseudobinary joins: Li MoO –Li -
(
heating rate, 10°C/min; sample weight, 0.3–0.4 g).
2
4
3
Ba Y (MoO ) ,
Li Y (MoO ) –Li Ba Y (MoO ) ,
7 3 4 8 3 2 3 4 8
2
3
4 8
To obtain different activator ion concentrations,
BaMoO –Li Ba Y (MoO ) ,
Li Ba Y (MoO ) –
yttrium oxide was replaced by an equimolar amount of
erbium oxide in Li Ba Y (MoO ) . In this way, we
4
3
2
3
4 8
3 2 3 4 8
LiY(MoO ) , Li Ba Y (MoO ) –BaY (MoO ) , and
4 2
3
2
3
4 8
2
4 4
3
2
3
4 8
BaY (MoO ) –LiY(MoO ) . According to X-ray dif-
obtained phosphor samples containing 1–6 mol % Er O .
2
4 4
4 2
2
3
fraction data, the ternary molybdate Li Ba Y (MoO )
3+
3
2
3
4 8
Luminescence spectra of Li Ba Y (MoO ) :Er
3
2
3
4 8
dissolves 18 mol % BaMoO . The solubility of the ter-
4
under UV excitation (λ = 365 nm) were obtained on
ex
nary molybdate in BaY (MoO ) does not exceed
2
4 4
an SM 2203 spectrofluorometer (Solar, Belarus).
Excitation was provided by a DKsSh 150-1M high-
pressure xenon arc lamp. IR luminescence was excited
5
mol %. LiY(MoO ) solubility in Li Ba Y (MoO )
4 2 3 2 3 4 8
is 12 mol %, and LiY(MoO ) dissolves no more than
4
2
10 mol % Li Ba Y (MoO ) .
3 2 3 4 8
by an InGaAs laser diode (λ = 977 nm). In this case,
ex
luminescence spectra were measured on an Ocean
The X-ray diffraction pattern of Li Ba Y (MoO )
3 2 3 4 8
Optics QE 65000 spectrometer. Measurement uncer- (Fig. 2) could be indexed in monoclinic symmetry
tainty was ±2 nm.
using the unit-cell parameters of a Li Ba Gd (MoO )
3 2 3 4 8
single crystal. The structure of Li Ba Gd (MoO )
4 8
3
2
3
was solved by the heavy atom method using a three-
dimensional Patterson function. The lighter atoms O
RESULTS AND DISCUSSION
The phase relations in the Li MoO –BaMoO – and Li were located using ρ(хуz) and Δρ(хуz) electron
2
4
4
Y (MoO ) system were studied in the range 400– density map calculations [2]. Anisotropic structure
2
4 3
9
00°C. The constituent binary system Li MoO – refinement, involving gadolinium site occupancy
2 4
refinement, converged to R = 0.044. We evaluated the
local valence force balance, taking into account the
site occupancies. The data thus obtained are well
consistent with the stoichiometric composition of
the Li Ba Gd (MoO ) compound. The valence bal-
Y (MoO ) contains 1 : 1 and 7 : 3 molybdates with a
2
4 3
scheelite-like structure [1, 3]. The double molybdate
LiY(MoO ) has a disordered tetragonal scheelite
4
2
structure (sp. gr. I4 /a). Li Y (MoO ) (sp. gr. P1) has
1
7
3
4 8
3
2
3
4 8
a layered structure made up of layers of M (M = 0.75Y +
ance quality D is 2.10% [2, 3]. Li Ba Y (MoO )
4 8
0
3
2
3
0
.25Li) edge-sharing eight-vertex polyhedra, which
melts incongruently at 960°C, without undergoing any
share corners with neighboring MoO tetrahedra.
4
polymorphic transformations.
In the BaMoO –Y (MoO ) system, we synthe-
4
2
4 3
The Li Ba Y (MoO ) molybdate has a monoclin-
3
2
3
4 8
sized the BaY (MoO ) molybdate with a broad homo-
2
4 4
ically distorted scheelite structure (sp. gr. C2/c, Z = 2),
geneity range and a triclinic structure (sp. gr. P1) [3] which contains honeycomb layers of eight-vertex Y poly-
containing honeycomb layers with slight variations. hedra. Each layer shares oxygen corners with Mo tetra-
The Mo atoms in this structure are in tetrahedral coor- hedra on both sides. The compound is isostructural with
INORGANIC MATERIALS Vol. 56 No. 3 2020

2
88
KOZHEVNIKOVA, BATUEVA
the double molybdate BaGd (MoO ) [1–3]. The
I/I0
2
4 4
lithium atoms in the structure of Li Ba Y (MoO )
4 8
3
2
3
occupy different crystallographic sites. One-third of
the lithium atoms are located at random on the rare-
earth site (coordination number CN = 8). The other
two-thirds of the lithium atoms occupy a special posi-
tion on the twofold rotation axis and have an octahe-
dral oxygen coordination. Note that this position is
occupied in an ordered or partially ordered manner,
without resulting in a new superstructure, even though
a new structure appears. The crystal chemical formula
of the compound can be written in the following form:
1
2
3
Li (Ba
Y
) (Y Ba Li ) (MoO ) [2]. The
2
0.85 0.15 2 0.675 0.075 0.25 4 4 8
unit-cell parameters of Li Ba Y (MoO ) are а =
3
2
3
4 8
5
.189(1) Å, b = 12.694(2) Å, c = 19.109(2) Å, β =
10
20
30
θ, deg
40
50
3
9
1.57(2)°, and V/Z = 629.1 Å .
2
The measured vibrational frequencies in the long-
wavelength part of the IR spectrum of Li Ba Y (MoO )
Fig. 2. X-ray diffraction patterns of (1) Li Ba Gd (MoO ) ,
3
2
3
4 8
3
2
3
4 8
3+
(
2) Li Ba Y (MoO ) , and (3) Li Ba Y (MoO ) :Er .
3 2 3 4 8 3 2 3 4 8
are listed in Table 1. Group-theoretical analysis results
for vibrations of a Li Ba Y (MoO ) crystal indicate
3
2
3
4 8
that, of the 144 modes corresponding to the total
vibrational representation, only 24 normal vibrational
modes correspond to motion of the atoms in the three
cation sublattices. They can be described by the fol-
lowing irreducible representations:
ΓМ1 = 1A + 2B + 1A + 2B
u
g
g
u
for the barium and yttrium cations on the twofold
rotation axes (M1 site); and
ΓМ2 = 3A + 3B + 3A + 3B
u
ΓLi = 1A + 2B +1A + 2B
u
g
g
u
g
g
u
for the lithium cations residing on the twofold rotation for the lithium, barium, and yttrium cations in general
axes (in octahedra); position (M2 site in the eight-vertex polyhedra).
Table 1. Vibrational frequencies in the long-wavelength part of IR absorption spectra
Frequency, cm–1
Assignment
δ(МоО4)
6
7
3+
Li Ba Y (MoO )
Li NaBa Y (MoO )
Li Ba Y (MoO ) :Er
3 2 3 4 8
3
2
3
4 8
2
2
3
4 8
456
454
416
384
350
319
297
286
265
258
227
–
455
417
384
344
319
290
–
4
18
84
52
25
07
δ(МоО4)
δ(МоО4)
TLi + TNa
TLi
3
3
3
3
TLi + TNa
TLi
290
–
265
258
231
216
204
–
TMo + TBa
TLi
2
63
32
–
2
TLi
TNa
–
–
TNa
1
86
64
56
186
–
156
92
TMo + TBa
R
R
1
1
164
156
90
9
0
TBa
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ERBIUM-DOPED UPCONVERSION PHOSPHOR
289
The frequencies of the external vibrational modes
1
of the MoO groups and the vibrational modes of the
4
heavy cation sublattices are roughly the same as in
BaGd (MoO ) .
2
4 4
The librational modes of the MoO groups are
4
responsible for the bands at frequencies of 164 and
–1
2
156 cm , and the vibrational modes of the barium
–1
atoms are responsible for the band at 90 cm . The
translational modes of the molybdenum are inter-
mixed with vibrations of atoms in the yttrium sublat-
–1
tice and have frequencies of 265 and 186 cm .
3
In addition, in the long-wavelength part of the
spectra of the compounds under study we detected two
–1
extra bands, at 297–307 and 258–263 cm , which
exhibited an isotope effect. The vibrational modes of
the alkali metal sublattice were assigned using their
shift in response to lithium isotope substitutions or
sodium substitution for lithium. The largest shifts were
observed for two groups of absorption bands: 325–
2
00
400
600
800
1000
–
1
Wavenumber, cm
–1
3
19, 307–297, and 290–286 cm ; 263–258, and
–
1
2
32–227 cm .
Fig. 3. IR spectra of (1) Li Ba Gd (MoO ) , (2) Li Ba -
3
2
3
4 8
3
2
3+
The former group of bands was assigned to vibra-
Y (MoO ) , and (3) Li Ba Y (MoO ) :Er
.
3
4 8
3
2
3
4 8
tions of the lithium cations on the octahedral site, and
the latter, to the lithium cations in the eight-vertex
polyhedra. It is these groups that include the two addi-
tional spectral bands missing in the spectrum of
These modes are active in IR absorption or Raman
spectra (with the rule of mutual exclusion met) and
should have frequencies under 500 cm . Moreover,
this part of the spectrum should contain 44 bands aris-
ing from bending, librational, and translational modes
–1
BaGd (MoO ) , in good agreement with previously
2 4 4
reported data on IR spectra of LiLn(MoO ) [2, 3].
4 2
As a result of isomorphous substitution of Na for
of the MoO groups [3, 22].
one Li atom per cell, two new absorption bands
4
–1
emerged in the IR spectrum: at 216 and 204 cm .
These frequencies correspond to the vibrational modes
of the sodium cations in the eight-vertex polyhedra.
–1
In their high-frequency part (650–950 cm ), the
measured spectra of Li Ba Y (MoO ) are very similar
3
2
3
4 8
–1
to the spectrum of Li Ba Gd (MoO ) (Fig. 3). The
3
2
3
4 8
However, the bands at 258–263 and 227–232 cm per-
absorption bands in this spectral region, due to the sisted in the same positions. At the same time, the
–1
valence modes of the MoO4 groups, are poorly
intensity of the band at 344–352 cm decreased,
resolved. This is a consequence of the described cation whereas the integrated intensity of the forming group
–1
distribution over the crystallographic sites of the crys-
tal and the associated statistics of distortions of the tet-
rahedral anion groups [2, 18].
of bands in the range 320–350 cm increased, con-
firming that the lithium and sodium cations were dis-
tributed over two distinct crystallographic sites.
Doping of the luminescent material Li Ba Y (MoO )
3
2
3
4 8
In the bending region of the MoO groups and the
4
with erbium ions is an efficient way of raising the lumi-
nescence brightness of IR and visible phosphors with
a scheelite-like structure. Activation of phosphors
region of vibrational modes of the cation sublattices
–1
(
below 500 cm ), up to 14 absorption bands were
detected in the spectrum of Li Ba Y (MoO ) . This is
3
2
3
4 8
3+
with Er ions, which are capable of absorbing IR light
much less than the expected number of vibrational
modes. Such discrepancies between group-theoretical
analysis results and the experimentally observed num-
ber of vibrational modes seem to result from a slight
splitting over the factor group of the internal (bending)
and converting it to visible anti-Stokes luminescence,
makes it possible to raise the efficiency of lasers. The spec-
3
+
tral composition of steady-state Li Ba Y (MoO ) :Er
3
2
3
4 8
luminescence at λ = 365 nm is determined by bands
ex
3+
corresponding to optical transitions of the Er ion
and external vibrational modes of the MoO groups.
4
3+
(
Fig. 4a). As the Er content is raised to 5 mol %, sev-
At the same time, in this region the spectra of the
compound under consideration are very similar to
those of the double molybdate BaGd (MoO ) [2].
eral groups of erbium-related narrow emission bands
emerge in the green (535–610 nm) and red (660–705 nm)
2
4 4
regions of the visible spectrum. The emission bands in
This suggests that the vibrational modes of the cation
4
the range 590–610 nm, corresponding to the S
→
sublattices are similar to each other and to the bending
3/2
4
modes of the MoO tetrahedra.
I15/2 transition, have the simplest and best-resolved
4
INORGANIC MATERIALS Vol. 56 No. 3 2020

2
90
KOZHEVNIKOVA, BATUEVA
structure. To the left and right of these green bands, we
observe weaker emission bands, in the ranges 535–538
(а)
1200
1000
800
4S3/2 →4I15/2
2
4
and 556–560 nm, due to the H
→ I
and
1
1/2
15/2
2
4
H9/2 → I
transitions, respectively. The bands in
13/2
4
4
the red spectral region are due to the F → I and
9/2
15/2
2
4
H9/2 → I
transitions, whose spectrum extends
11/2
from 660 to 705 nm and has a more complex structure,
600
4
because the F9 excited state is split into five multiplets.
/2
Li Ba Y Er (MoO ) possesses efficient anti-
3
2
2.85
0.15
4 8
400
Stokes luminescence in the visible range under IR
excitation. The origin of the bands observed in its anti-
200
Stokes luminescence spectra under excitation at λ
=
ex
9
77 nm can be understood as follows: According to a
0
known energy level diagram [13], after a sequential,
350
450
550
650
750
850
two-step excitation of the Er3+ ion to the 4F level,
Wavelength, nm
7/2
nonradiative relaxation processes lead to the filling of
(
b)
4
4
4
4
the Н , S , F , and I excited-state levels, giv-
12000
0000
11/2
3/2
9/2
9/2
4S
3
/2
ing rise to anti-Stokes luminescence in the range 530–
8
50 nm. Figure 4b shows the anti-Stokes lumines-
1
cence spectrum between 450 and 850 nm. There are
four strong bands, located around 531, 552, 660, and
8000
6000
4000
2000
0
8
00 nm. The 531- and 552-nm bands correspond to
3+ 2
4
4
4
the Er H11/2 → I
and S → I
transitions.
15/2
3/2
15/2
4
4
The 660-nm emission is due to the F9 → I tran-
/2
15/2
4
4
sition, and the 800-nm emission, to the I → I
2H
9/2
15/2
11/2
transition. The weak blue band at 490 nm arises from
4
4
the F7 → I transition. A characteristic feature of
/2
15/2
4I9/2
3
+
the Er 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
sequence of electronic transitions: I
I11/2 → F . Transitions from high energy levels may
4F
4F7/2
9/2
5
00
600
700
800
900
Wavelength, nm
4
4
→ I
and
15/2
11/2
4
4
7/2
Fig. 4. (a) Luminescence spectra of Li Ba Y2.85-
Er0.15(MoO ) at λ = (a) 365 and (b) 977 nm.
3
2
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
excited-state levels of the activator ion.
4 8
ex
FUNDING
This work was supported by the Russian Federation
Ministry of Science and Higher Education (state research
target for the Baikal Institute of Nature Management, Sibe-
rian Branch, Russian Academy of Sciences, project no.
CONCLUSIONS
3
+
A Li Ba Y (MoO ) :Er phosphor with a layered
3
2
3
4 8
scheelite-like structure (sp. gr. C2/c) has been pre-
0339-2016-007).
pared by solid-state reaction. The luminescence data
3
+
obtained in this study demonstrates that, as the Er
content is raised to 5 mol %, several groups of erbium-
related narrow emission bands emerge in the green
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3
+
Li Ba Y (MoO ) :Er
possesses efficient anti-
3
2
3
4 8
2
. Klevtsova, R.F., Vasil’ev, A.D., Glinskaya, L.A., et al.,
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3
2
3
4 8
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=
ex
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4
4
4
Н1 , S , F , I excited-state levels, giving rise
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3/2
9/2
9/2
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INORGANIC MATERIALS
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ERBIUM-DOPED UPCONVERSION PHOSPHOR
291
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Translated by O. Tsarev
INORGANIC MATERIALS Vol. 56 No. 3 2020