Photoluminescence of single-phased white light emission materials based on simultaneous Tb3+, Eu3+and Dy3+doping in CaWO4matrix

Article abstract of DOI:10.1016/j.jallcom.2016.11.378

The triply-doped xTb³⁺/xEu³⁺/xDy³⁺CaWO₄(x: 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0?mol%) nanomaterials were prepared at room temperature by a coprecipitation method. The luminescence materials were characterized by X-ray powder diffraction (XPD), thermal analysis (TG), infrared absorption spectroscopy (FTIR) and UV excited photoluminescence. The prepared powder phosphors are single-phase scheelite structure with porous morphology and particle sizes around 11?nm. The materials display white color from cool to the warm white, under UV excitation, as a result of the intraconfigurational 4f transitions from trivalent rare earth ions. The non-radiative energy transfer processes from the O→W and O→Eu LMCT as well as the 4f⁸→4f⁷5d¹states to the intraconfigurational 4f excited levels of rare earth ions and simultaneous emissions from the⁴F_9/2(Dy³⁺)→⁵D₄(Tb³⁺)→⁵D₁,⁵D₀(Eu³⁺) emitter levels are reported. Furthermore, CIE parameters and the color correlated temperature (CCT) are discussed in order to characterize the color emission. Based on the results, these luminescence materials may be potential candidates for white light emitting diodes and solid-state lighting.

Full text of DOI:10.1016/j.jallcom.2016.11.378

Accepted Manuscript
Photoluminescence of single-phased white light emission materials based on
3+
3+
3+
simultaneous Tb , Eu and Dy doping in CaWO matrix
4
Helliomar P. Barbosa, Ivan G.N. Silva, Maria Claudia F.C. Felinto, Ercules E.S.
Teotonio, Oscar L. Malta, Hermi F. Brito
PII:
S0925-8388(16)33861-0
10.1016/j.jallcom.2016.11.378
JALCOM 39875
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 4 October 2016
Revised Date: 24 November 2016
Accepted Date: 26 November 2016
Please cite this article as: H.P. Barbosa, I.G.N. Silva, M.C.F.C. Felinto, E.E.S. Teotonio, O.L. Malta,
H.F. Brito, Photoluminescence of single-phased white light emission materials based on simultaneous
3+
3+
3+
Tb , Eu and Dy doping in CaWO matrix, Journal of Alloys and Compounds (2016), doi: 10.1016/
4
j.jallcom.2016.11.378.
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ACCEPTED MANUSCRIPT
White light emission of single-phased material
ET
ET
Dy3+
Tb3+
Eu3+
CaWO :RE
4
3+
ET: Energy transfer

ACCEPTED MANUSCRIPT
1
2
Photoluminescence of single-phased white light emission materials based on
3
+
3+
3+
simultaneous Tb , Eu and Dy doping in CaWO matrix
4
3
1
1
2
3
4
5
Helliomar P. Barbosa , Ivan G.N. Silva , Maria Claudia F.C. Felinto , Ercules E.S. Teotonio ,
4
1,*
Oscar L. Malta , and Hermi F. Brito
6
1
7
8
Department of Fundamental Chemistry, Institute of Chemistry, University of São Paulo,
05508-000, São Paulo, SP, Brazil.
2
9
0
Nuclear and Energy Research Institute (IPEN-CQMA), 05508-000 São Paulo, SP, Brazil.
3
1
Department of Chemistry, Federal University of Paraiba, 58051-970 João Pessoa, PB, Brazil.
4
1
1
2
Department of Fundamental Chemistry, Federal University of Pernambuco, 50670-901
1
Recife, PE, Brazil.
13
14
15
16
*Corresponding author:
e-mail: hefbrito@iq.usp.br
Phone: +55 11 3091-3708
Fax: +55 11 3815-5579
17
18
19
20
21
22
Keywords: rare earth; alkaline earth tungstate; charge transfer; white LEDs

ACCEPTED MANUSCRIPT
23
Abstract
3
+
3+
3+
24
25
26
27
28
29
30
31
The triply-doped xTb /xEu /xDy CaWO (x: 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mol%)
4
nanomaterials were prepared at room temperature by a coprecipitation method. The
luminescence materials were characterized by X-ray powder diffraction (XPD), thermal
analysis (TG), infrared absorption spectroscopy (FTIR) and UV excited photoluminescence.
The prepared powder phosphors are single-phase scheelite structure with porous morphology
and particle sizes around 11 nm. The materials display white colour from cool to the warm
white, under UV excitation, as a result of the intraconfigurational 4f transitions from trivalent
rare earth ions. The non-radiative energy transfer processes from the O→W and O→Eu
8
7
1
32
LMCT as well as the 4f →4f 5d states to the intraconfigurational 4f excited levels of rare
4
3+
5
3+
5
5
3+
33
34
35
36
37
earth ions and simultaneous emissions from the F (Dy )→ D (Tb )→ D , D (Eu )
9/2 4 1 0
emitter levels are reported. Furthermore, CIE parameters and the colour correlated
temperature (CCT) are discussed in order to characterize the colour emission. Based on the
results, these luminescence materials may be potential candidates for white light emitting
diodes and solid-state lighting.
3
3
8
9
1. Introduction
40
41
42
43
44
45
46
47
Nowadays, there is an increasing attention to the development of white emitting materials
applied to the light-emitting diodes (LEDs) instead of conventional incandescent and
fluorescent lamps. These luminescence materials exhibit new advantageous as power
efficiency, energy saving, environmental friendliness, no mercury and extended operation
time compared to the predominant incandescent and fluorescent lamps [1–5]. Usually, many
luminophores applied in white-LEDs are based on two or three different phosphors consisting
of the blue, green and red emission colours, exhibiting different thermal stability and
degradation leading to a higher costs [5]. For example, the white light emitting phosphors can

ACCEPTED MANUSCRIPT
3
+
4+
48
49
50
51
52
53
be reached by the combination of the blue LED chip GaN with YAG:Ce , K SiF :Mn and
2 6
2
+
β-SiAlON:Eu materials, showing the yellow, red and green emission colours, respectively
[1]. However, the mixture of different phosphors can result in both low reproducibility and
low Colour Rendering Index (CRI). Another failing includes high Corrected Colour
Temperature (CCT > 7000 K) due to the lack of a red emission light component that limits its
multiple applications [6,7].
54
55
56
57
58
59
60
These disadvantages can be overcome by using different activator ions in the same host
matrix called the single-phase white light-emitting phosphor that is usually excited by UV
radiation. This kind of luminescence materials has a great advantage such as high CRI, high
luminescence efficiency and lower manufacturing cost that are excellent alternatives for
applications in LEDs and in white-light-emitting diodes (WLEDs). Generally, the single-
phase materials applied in WLEDs work as a result of non-radiative energy transfers (ET)
from a sensitizer to activators as an efficient way to obtain a warm white light.
3
+
2+
61
62
63
64
65
66
67
68
69
Currently, luminescence materials based on ET process such as Sr B O :Ce ,Eu ,
3 2 6
3
+
3+
2+
2+
2+
3+
2+
Sr Gd (Si O ) :Ce ,Tb /Mn , Ca Mg(PO ) F : Eu ,Mn and NaCaBO :Ce /Mn have
3
2
3
9 2
9
4 6
2
3
been used [8–11]. However, in preparation of these photonic materials, a reducing atmosphere
1
2
and long reaction time is necessary, leading to a high cost and energy spent. In the last years,
were reported single-doped tricolor emission phosphors in only one matrix containing rare
n+
3+
2+
earth ions (RE ) prepared by different synthetic routes like Ca SiO :RE (RE: Ce , Eu ,
2
4
3+
3+
3+
3+
3+
3+
Sm ) and Ca Mg Al Si O :RE (Ln: Ce , Dy , Eu , Sm ) by the conventional
2
0.75 0.5 1.75
7
3
+
3+
3+
solid-state method and CaO-Al O -B O -RE O (RE: Eu , Tb , Tm ) prepared by melt
2
3
2
3
2
3
quenching method [12–14].
3
+
70
71
72
The CaWO :RE nanophosphors have demonstrated interesting luminescence properties with
4
potential applications in several fields as waveguide lasers host materials, optical fibers,
organic light-emitting diodes (OLEDs), photovoltaic devices, luminescent materials [15–19].

ACCEPTED MANUSCRIPT
2
-
73
74
75
76
77
78
In addition, tungstate [WO ] based matrices are an important family of phosphors due to
4
their excellent thermal stability and interesting optical behavior [20–22]. The CaWO can be
4
doped and activated with NUV-blue light-absorbing ions for applications in WLEDs due the
2
-
3+
efficient energy transfer from tetragonal tungstate groups [WO ] to the activator RE ions
4
3
+
2+
2-
[23–29]. Besides, the RE doped in the MWO matrices (M : Ca, Sr, Ba) present O
4
3
+
2-
VI
3+
(2p)→Eu (4f), O (2p)→W (5d) Ligand–to–Metal Charge Transfer (LMCT) states and Tb
8
7
1
3+
79
80
81
82
83
84
85
86
87
88
4f →4f 5d interconfigurational transitions as well as the RE 4f-intraconfigurational
transitions, which may to improve the luminescence efficiency of these materials. The design
of the phosphors leading to the colour tuning arising from the narrow line emission bands
3+
3+
assigned to the 4f-4f transitions of RE ions. It is important to point out that the Dy ion may
4
5
5
acts as a sensitizer and can partially transfer energy from the F state to the D and D
0
9
/2
4
3
+
3+
3+
emitting levels of the Tb and Eu ions, respectively. In addition, the Tb ion acts as a good
3+
5
sensitizer to Eu activator transferring part of the energy from the D level to the emitting
4
5
5
3+
D , D level of Eu ion [30,31]. The full-color single-phased phosphors arising from the
1
0
3
+
3+
3+
combination of green (Tb ), yellow (Dy ) and red (Eu ) emission bands can potentially be
used for high luminous efficient white LEDs [32].
89
90
91
92
93
94
The tungstate materials prepared by ceramic method need high temperatures (> 1000 °C) and
long reaction time and reducing atmosphere, leading to a high energy consumption as well as
inhomogeneity of the material particles [33]. However, the coprecipitation method exhibits
many benefits such as short reaction time, straightforward operation at room temperature.
This procedure is environmental friendly producing materials with high crystallinity and
controlled particle size [22,34].
3
+
3+
3+
95
96
97
In this work, the simultaneous incorporation of the Tb /Eu /Dy triply-doped CaWO
4
matrix as a possible alternative to solid state synthesis is to use the “green procedure”
applying a coprecipitation method as well as their luminescence properties are described.

ACCEPTED MANUSCRIPT
2
-
3+
8
7
1
3+
9
8
9
Moreover, the [WO ] and Eu LMCT states and 4f →4f 5d (Tb ) transitions are also
4
9
discussed. Finally, the non-energy transfer process from the tungstate host lattice to the rare
4
3+
5
3+
5
1
00
01
earth ions as well as between their 4f-4f transitions of the F (Dy ), D (Tb ) and D
9/2 4 1
3
+
1
(Eu ) emitting levels are also analyzed and elucidated.
1
1
02
03
2. Experimental Section
3
+
104
105
106
107
108
109
110
111
112
The RE dopant ions were obtained from their respective oxides RE O (Cstarm, 99.99%) for
2 3
3
+
3+
3+
Eu and Dy and Tb O (Cstarm, 99.99%) for Tb by digestion of their aqueous
4
7
suspensions with the addition of concentrated hydrochloric acid until pH reaches 6. The
RECl ⋅6H O resulted white crystalline solids were dried and filtered and afterwards stored
3
2
3+
3+
3+
under reduced pressure in vacuum desiccator. The CaWO :xTb ,xEu ,xDy (x: 0.5 – 5.0
4
mol%) materials were prepared with conventional coprecipitation reaction using aqueous
3
+
solution of Na WO (99.99 % Vetec), CaCl (99.99 % Alfa) and RECl . The RE doping
2
4
2
3
concentrations were identical for each rare earth ion in following concentrations: 0.5, 1.0, 2.0,
2
+
3.0, 4.0 and 5.0 mol% of the Ca amount.
3
+
1
13
14
The CaWO :RE (5.0 mol%) material was prepared from a mixture of CaCl ⋅2H O (0.085
4 2 2
-1
3+
-1
1
mol⋅L ) and RECl ⋅6H O aqueous solutions (RE : Tb, Eu and Dy; 0.05 mol⋅L ). After, the
3
2
-
1
115
116
117
118
119
120
resulting solution was dropwise added into the Na WO ⋅2H O (0.1 mol⋅L ) aqueous solution
2 4 2
under magnetic stirring, leading to white precipitate immediately. The crystalline powders
were washed with distilled water for thrice, filtered and after dried under reduced pressure in
vacuum desiccator. The as-prepared materials are air stable and non-hygroscopic. The
3
+
synthesis of the materials at different doping RE concentrations 0.5, 1.0, 2.0, 3.0 and 4.0 are
similar to 5.0 mol% one.

ACCEPTED MANUSCRIPT
121
122
123
124
The infrared absorption spectra were performed with KBr pellets with the equipment Bomem
-1
MB100 FTIR apparatus in the spectral region from 400 to 4000 cm . The X-ray powder
diffraction (XPD) patterns were registered by a Miniflex Rigaku (Cu K 1.5406 Å) from 5 to
α1
60º (2θ) with the step of 0.02° and step integration time of 1 s.
125
126
127
128
129
130
131
The scanning electron microscopy (SEM) images and the energy dispersive spectroscopy
(EDS) elemental mapping were obtained with a JEOL JSM-7401F Field Emission Scanning
Electron Microscope with the acceleration voltages of 5 and 15 kV, respectively. For the SEM
images, the solid samples were deposited on carbon graphite sample holder. In the EDS
mapping, the samples were prepared by dispersing the materials in chloroform. The
suspension was submitted to ultrasound for 15 min and then placed on a carbon graphite grid
sample holder.
132
133
134
135
136
137
138
139
140
Thermogravimetric (TG) curves were obtained with the 2950 TGA HR V5.4A equipment
from 40 to 950 ºC in a dynamic atmosphere of synthetic air with constant heating rate of 10
-1
ºC min . The photoluminescence study was conducted based on the excitation and emission
spectra recorded at room temperature (300 K) by use of the front-face data collection mode
(22.5º) with a 450 W Xenon lamp as the irradiation source coupled to the equipment SPEX-
Fluorolog 2 with double monochromators. The luminescence decay curves were recorded
with a 150W pulsed xenon lamp attached to the SPEX 1934D phosphorimeter accessory. The
CIE (Commission Internationale de l’Eclairage) color coordinates were calculated from the
emission spectra by use of SpectraLux v2.0 software [35].
1
1
41
42
3. Results and discussion
3
+
3+
3+
143
144
145
The thermogravimetric curves of triply-doped CaWO :xTb ,xEu ,xDy (x: 0.5 – 5.0 mol%)
4
materials exhibit low weight loss event in the temperature interval from 40 to 800 ºC,
indicating good thermal stability of the materials (Figure 1). The first weight loss (1.5%)

ACCEPTED MANUSCRIPT
146
147
148
149
150
151
event is observed between 40 and ~130 ºC due to adsorbed/absorbed water, suggesting that
the crystalline material has no phase impurities in its composition. The water loss corresponds
3+
3+
3+
to 0.25 mol H O of the CaWO :Tb ,Eu ,Dy (5.0 mol%) material. It is expected the
2
4
existence of hemihydrate form WO ꢀ0.5H O in the system, since the material contains high
4
2
3
+
RE doping concentration [36]. Due to the proximity, the rare earth ions can share the
molecules of water resulting in a hemihydrate compound.
152
153
154
Please, insert Figure 1 here
3
+
3+
3+
155
156
157
158
159
160
161
162
The infrared absorption spectra (FTIR) of the CaWO :Tb ,Eu ,Dy (x mol%; x: 0.5 – 5.0)
4
-1
materials show medium absorption band at around 3400 cm assigned to the O–H stretching
vibrations owing to the adsorbed water by the KBr pellets and the compounds (Figure S1).
-1
The intense absorption band located at 800 cm is attributed to the W–O symmetric stretching
-1
mode and the absorption peak at 400 cm are assigned to the δ (Ca–O) bending modes
[22,37]. Besides, the undoped and doped materials present similar FTIR features despite the
3
+
3+
3+
high doping concentrations of the rare earth ions (Tb , Eu and Dy from 0.5 to 5.0 mol%)
in the phosphors.
3
+
3+
3+
163
164
165
166
167
168
169
170
The X-ray powder diffraction (XPD) patterns of the CaWO :Tb ,Eu ,Dy materials suggest
4
the tetragonal scheelite phase with the I4 /a space group (PDF entries: CaWO 41-1431
1
4
strongest reflections at 2θ values of 18.58 [101] and 28.74 [112] (Figure 2a) [38]. There are
no additional diffraction peaks arising from the impurities or the RE (WO ) phase,
2
4 3
3+
3+
3+
confirming the successfully preparing of the CaWO :Tb ,Eu ,Dy materials, even at higher
4
doping concentrations. The XPD data indicate the efficient incorporation of rare earth ions
into the CaWO matrix owing to the similarity of 1.06, 1.04, 1.02 and 1.12 Å ionic radii of the
4
3
+
3+
3+
2+
Eu , Tb , Dy and Ca metal ions, respectively [39]. The ionic radii values obeys the

ACCEPTED MANUSCRIPT
171
172
173
174
175
Vegard's law that establishes a limit of around 15% to obtain complete solid solubility
3
+
2+
3+
between the RE ions that occupy the Ca site in host lattice [39]. The RE ions replace the
2
+
Ca with 8 oxygen atoms in the distorted dodecahedral sites [CaO ] (Figure 2b) and the
8
tungsten ions [WO ] are coordinated to four oxygens with the O–W–O angles slightly
4
distorted from the tetrahedral ones of CaWO scheelite (Figure 2c) [40].
4
3
+
176
177
178
179
180
181
182
It is noteworthy that the incorporation of RE ions into a CaWO host lattice substitute a
4
2
+
3+
2+
divalent metal ion (Ca ), charge balancing is required. The RE ion replace one Ca ion, but
3
+
the ionic radii mismatch would make accommodation of RE more difficult. Furthermore, in
+
3+
solution with excess of Na (from Na WO ), this ion goes into the CaWO :RE lattice since
2
4
4
+
2+
the size of Na (1.18 Å, CN: 8) is slightly larger than that of Ca (1.12 Å, CN: 8). In these
2
+
+
materials, two Ca sites are substituted by trivalent rare earth ion and one Na ion ensuring
overall charge equilibrium compensating the charge mismatch [22].
3
+
3+
3+
183
184
185
186
The average crystallite sizes of the CaWO :xTb ,xEu ,xDy materials (x: 0.5, 1.0, 2.0, 3.0,
4
4.0, 5.0 mol%) are 12.9, 12.8, 12.1, 10.8, 10.7, 11.5 nm, respectively (Figure 3). The average
crystallite sizes were estimated from the diffraction data by the Scherrer formula D =
hkl
0.9λ/βcosθ [41]. Where D is the average grain size, λ the X-ray wavelength, θ half of the
hkl
187
188
189
190
191
192
193
194
195
Bragg angle, and β is the full width at half maximum (FWHM) of the selected reflection. In
this work, the [112] reflection (2θ: 28°) was used in the calculations. In order to correct the
ꢁ
ꢁ
ꢁ
broadening due to the diffractometer setup from the β was used ꢀ = ꢀ −ꢃꢀ where the β
s
ꢂ
ꢄ
s
and β are the FWHM of the material and reference (NaCl: 0.149; 31.7º; hkl: [2 0 0]),
r
respectively. The reduction in the crystallite size from 0.5 to 4.0 mol% (Figure 3) is mainly
3
+
due to the distortion in the host CaWO lattice by the dopants (i.e., RE ), that can retard the
4
crystal growth rate of CaWO reaching a minimum when the doping concentration of each
4
3
+
RE ion is 4.0 mol% [42]. The XPD results confirm that the coprecipitation method produced
materials with high crystallinity.

ACCEPTED MANUSCRIPT
196
197
198
Please, insert Figure 2 here
2
+
199
200
201
202
The fast precipitation process of CaWO doped phosphors due to the attraction of Ca ,
4
2
-
3+
[WO ] and RE ions and subsequent bond between them can result in formation of defects,
4
3
8
deformations, or strains in the crystal structure. The microstrain can be calculated using the
Equation 1:[42]
203
204
205
β
ꢃꢆꢇꢈ
θ
ꢅ =
(1)
ꢉ
206
207
208
209
210
211
212
213
214
215
The microstrain values of the doped materials 0.148, 0.148, 0.159, 0.175, 0.177 and 0.166
-3
-2
-4
(10 lines ⋅m ) increase with decrease of crystallite size 12.9, 12.8, 12.1, 10.8, 10.7, 11.5 nm,
respectively (Figure 3). This effect is a consequence of the small distortion of CaO8
3
+
dodecahedra up to 4.0 mol% RE doping concentration. On the other hand, the phosphor
3+
with 5.0 mol% RE doping concentration the crystallite size increases slightly and the
3
+
microstrain decreases simultaneously. Although the material with RE 5.0 mol% has shown a
-3
-2
-4
smaller microstrain value (0.166⋅10 lines ⋅m ; 11.5 nm) this can indicate the slightly
increase of crystallinity of the material with this doping concentration due to reach for upper
limits of the Vegard’s law. In addition, the microstrain curve profile (Figure 3) are
corroborated to those reported for Gao et al.[42]
216
217
218
219
The scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX)
3
+
3+
3+
images present the morphological features and composition of the CaWO :Tb ,Eu ,Dy
4
3
+
(5.0 mol%) phosphors (Figure 4). The material with lower RE doping concentration (0.5
mol%) exhibits a spherical micrometric particle with diameter around 4 ꢁm. In addition, the

ACCEPTED MANUSCRIPT
3
+
2
2
20
21
increase of RE doping concentration (5.0 mol%) has shown effect on the morphological
features, disturbing the spherical shape.
222
223
224
Please, insert Figure 3 here
225
226
227
228
229
The EDX mapping provides the chemical composition of the sample containing O, Ca, Eu, Tb,
Dy and W and no other impurity can be detected. This clearly depicts the incorporation of rare
earth ions and their distribution inside the spherical structure like “tennis ball form”.
Combined with above XPD patterns, confirm a homogeneous incorporation of rare earths ions
in the CaWO host lattice.
4
230
231
232
233
Please, insert Figure 4 here
4. Photoluminescence properties
3
+
3+
3+
234
235
236
237
238
239
240
241
The excitation spectra of the CaWO :Tb ,Eu ,Dy (x mol%; x: 0.5 – 5.0) materials
4
5
7
3+
recorded by monitoring the emission intensity of the D → F transition (at 545 nm) of Tb
4
5
in the spectral range from 225 to 510 nm at room temperature showing similar spectral
features (Figure 5). It is observed absorption bands in the range of 220–320 nm, which are
assigned to the O–W (~270 nm) and O→Eu (~310 nm) Ligand-to-Metal Charge Transfer
2
−
2-
(LMCT) states from the excited 2p orbitals of O belong to the [WO ] groups to the empty
4
VI
3+
orbitals of the W and Eu ions, respectively. These bands are overlapped with that one the
8
7
1
3+
4f →4f 5d interconfigurational transition of the Tb ion around 280 nm [43]. It is important
8
7
1
2
42
43
to mention that the LMCT and 4f →4f 5d transitions have broad absorption bands with high
2
intensities because they involve parity-allowed electronic transitions to the outer 5d orbitals.

ACCEPTED MANUSCRIPT
244
245
246
Excitation spectra also exhibit groups of narrow absorption bands assigned to the 4f
7
3+
6
3+
intraconfigurational transitions (Figure 5) from the ground states F (Tb ) and H (Dy )
6
15/2
-1
7
5
3+
to excited states in order of increasing energy as following (in cm ): 20,508 F → D (Tb ),
6
4
6
4
3+
6
4
3+
6
4
3+
247
248
249
250
21,159 H → F (Dy ), 22,099 H → I (Dy ), 23,339 H → G (Dy ), 26,399
15/2
9/2
15/2
15/2
15/2
11/2
7
5
5
3+
7
5
3+
7
5
5
3+
F → D , G (Tb ), 27085 F → L (Tb ), 27,824 F → L , G (Tb ), 28,312
6
3
6
6
10
6
9
5
7
5
3+
7
5
3+
6
4
3+
7
5
3+
F → D ,(Tb ), 29,308 F → L (Tb ), 30599 H → K (Dy ), 31,368 F → H (Tb ),
6
2
6
7,8
15/2
15/2
6
7
7
5
3+
7
5
5
3+
32852 F → H (Tb ) and 35,088 F → I , F (Tb ) [43,44].
6
6
6
8
5
251
252
253
254
255
256
257
Usually, the 4f – 4f transitions of the rare earth ions have low absorption and emission
intensities due to their parity-forbidden by Laporte’s rules (∆l = ±1), but this rule is slightly
relaxed due to the mixing of opposite parity electronic configurations, produced by the odd
components of a non-centrosymmetric ligand field [45]. It is noteworthy that the relative
absorption intensities between from 4f intraconfigurational transitions and those ones arising
from other kind of transitions decrease significantly with the increasing of the rare earth ions
concentration indicating the cross relaxation process among rare earth ions.
258
259
260
Please, insert Figure 5 here
261
262
263
264
265
266
267
Interestingly, Figure 5 shows several narrow absorption bands from terbium and dysprosium
5
7
ions when the excitation is monitored at 545 nm corresponding to the emission D → F
4
5
3
+
transition of Tb ion. These spectroscopic data indicate that the non-radiative energy transfer
3
+
3+
occur from Dy to Tb ion. On the other hand, the narrow absorption bands arising from the
7
2S+1
3+
F0→ L transitions of Eu ion are absent in the excitation spectra presented in Figure 5,
J
3
+
3+
indicating than non-radiative energy transfer from Eu to Tb ion is not an operative process.
5
3+
5
As well known, the D excited level of the Eu presents lower energy than the D acceptor
0
4

ACCEPTED MANUSCRIPT
3
+
2-
268
269
270
271
excited level of the Tb ion [45,46]. Therefore, the efficient absorption by the [WO ]
4
tungstate group, non-radiative energy transfer and emission by the metal ion, thus overcoming
3
+
the typically very small absorption coefficients of the RE ions. This optical feature makes
the rare earth ions an excellent activator for diverse applications in photonic area.
3
+
3+
3+
272
273
274
275
276
277
278
The emission spectra of the CaWO :Tb ,Eu ,Dy (x mol%; x: 0.5 – 5.0) materials were
4
recorded at room temperature in the range of 420–725 nm under excitation centered on
7
5
5
3+
narrow absorption bands at 378 nm assigned to the F → D , G transitions of Tb ion
6
3
6
(Figure 6). These emission spectra show only narrow emission bands arising from the doping
3
+
3+
3+
Tb , Dy and Eu ions. These emission bands are assigned to the 4f transitions from the
4
3+
5
3+
5
3+
F (Dy ), D (Tb ) and D (Eu ) emitting states to their energy levels corresponding to
9
/2
4
0
-1
5
7
3+
5
7
3+
5
7
3+
(in cm ): 14,238 D → F (Eu ), 15,267 D → F (Eu ), 16,260 D → F (Eu ), 16,886
0
4
0
3
0
2
5
7
3+
4
6
3+
5
7
3+
5
7
3+
2
79
80
D → F (Eu ), 17,391 F → H (Dy ), 18,362 D → F (Tb ), 20,450 D → F (Tb ),
0 1 9/2 13/2 4 5 4 6
4
6
3+
2
20,920 F → H (Dy ).
9
/2
15/2
281
282
283
Please, insert Figure 6 here
2
84
85
The presence of the emission bands assigned to the doping dysprosium, terbium and europium
7
5
5
3+
2
ions, with excitation in 378 nm F → D , G (Tb ), suggests clear evidence of non-radiative
6
3
6
4
3+
5
3+
5
3+
286
287
288
289
290
291
energy transfer from F (Dy )→ D (Tb )→ D (Eu ) emitter states (Figure 7). This
9/2 4 0
energy transfer process is due to the energy level structure of these rare earth ions present
-
1
3+
their emitter levels very close, only by a few hundred cm (Figure 7). Since the Eu can be
3
+
excited through Tb excitation, it is possible consider that there exists an overlapping
5
7
5
7
7
5
7
5
between terbium D → F , D → F emissions and europium F → D , F → D
0
4
5
4
4
1
1
0,1
absorptions, respectively. In addition, the more prominent emissions band at around 16260

ACCEPTED MANUSCRIPT
-
1
5
7
292
293
294
295
296
cm assigned to the rigorously forbidden electric dipole D → F hypersensitive transition as
0 2
5
7
-1
compared with the intensity of the D → F transition at 16,886 cm which is allowed by
0
1
3
+
magnetic dipole mechanism. This spectroscopic feature suggests that the Eu ion is in a
5
7
chemical environment without center of symmetry. Indeed, D → F transition is allowed by
0
2
forced electric dipole and dynamic coupling mechanisms.
3
+
297
298
299
300
Besides, when the concentration of RE ions increase from 0.5 to 5.0 mol% (Figure 6) it is
4
6
4
6
observed an increasing in relative emission intensities of the F → H and F → H
15/2
9
/2
13/2
9/2
3
+
transitions of the Dy ion due to a lower non-radiative energy transfer with an increasing
concentration [47–49].
7
5
3+
3
01
Interestingly, when materials are excited in the F → L transition of the Eu ion at 393 nm
0 6
5
7
302
303
304
305
306
the emission spectra show an intensity increasing of the D → F hypersensitive transition
0 2
with concentration from 0.5 to 5.0 mol% compared to the 4f transitions of terbium and
-
3+
3+
3+
dysprosium ion. This result suggests that the energy transfer process from Dy →Tb →Eu
become more efficient with the increase of doping concentration leading to higher red color
emission.
3
3
07
08
Please, insert Figure 7 here
309
310
311
The lifetime values τ (ms) were obtained from the luminescence decay curves of
3
+
3+
3+
CaWO :Eu ,Tb ,Dy materials recorded under different excitation wavelengths. The τ
4
7
5
5
values of the materials (Table 1) recorded under excitation at the F → D , G (~378 nm)
6
3
6
5
7
3+
312
313
314
315
transitions and emissions at the D → F (545 nm) transition of the Tb ion decrease from
4 5
0.865 to 0.684 ms according to the rare earth ion doping concentration increasing from 0.5 to
4.0 mol% (Figure 8). This result confirms that the cross-relaxation process is operative,
except to the lifetime value of 0.731 ms for the highest concentration (5.0 mol%), suggesting

ACCEPTED MANUSCRIPT
3
+
3
16
17
that energy migration between the RE ions becomes dominant, with respect to the cross-
3
relaxation process, in these luminescent materials [50,51].
318
319
320
Please, insert Figure 8 here
321
322
323
324
325
326
The coordinates in the CIE (Commission Internationale l'Éclairage) chromaticity diagram of
3+
3+
3+
the of CaWO :Tb ,Eu ,Dy suggests that the doping materials exhibit whitish emissions
4
colors when doping concentrations 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mol%, under excitation at
7
5
3+
378 nm corresponding to the F → G (Tb ) transition (Figure 9) [35]. The color correlated
6
6
temperature (CCT) values (summarized in Table 1) were calculated by the following Equation
2:[7,52]
327
328
329
ꢎ
ꢁ
ꢊꢊꢋ = −ꢉꢉꢌꢍ + ꢎꢏꢁꢏꢍ − ꢐꢑꢁꢎ. ꢎꢍ + ꢏꢏꢁꢒ. ꢎꢎ
(2)
330
331
332
333
334
where n = (x – x ) / (y – y ) is the reciprocal slope and x : 0.3320; y : 0.1858 is the
e
e
e
e
chromaticity epicenter. The CCT values of 4,308; 5,411; 5,933; 4,802; 4,195 and 5,479 K are
3
+
assigned to the 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mol% doping concentration of RE , respectively
3
+
(Table 1). The white light emission was mainly reached for 5.0 mol% RE with x: 0.333; y:
0.352 CIE coordinates.
3
3
3
35
36
37

ACCEPTED MANUSCRIPT
3
+
338
339
340
Table 1. Lifetimes values (τ, ms) obtained at different excitation wavelength for the Eu ,
3
+
3+
Tb and Dy ions and CIE chromaticity parameters and color correlated temperature (CCT)
3
+
3+
3+
values of the CaWO :xTb ,xEu ,xDy (x: 0.5 – 5.0 mol%) materials.
4
3
+
3+
3+
CaWO :xTb ,xEu ,xDy
Lifetimes (ms)
CIE
CCT
(K)
4
a)
3+
b)
3+
c)
3+
(
x mol%)
Tb
Eu
Dy
(x ; y)
0
1
2
3
4
5
.5
.0
.0
.0
.0
.0
0.865
0.834
0.707
0.687
0.684
0.731
0.715
0.686
0.584
0.583
0.578
0.628
0.333
0.312
0.289
0.262
0.263
0.265
0.377; 0.414
0.335; 0.372
0.321; 0.373
0.355; 0.392
0.380; 0.406
0.333; 0.352
4308
5411
5933
4802
4195
5479
a)
b)
c)
3+
341
342
343
344
Tb : λ = 378 nm; λem. = 545 nm
exc.
3+
Eu : λ = 393 nm; λem. = 614 nm
exc.
3+
Dy : λ = 385 nm; λem. = 576 nm
exc.
3
+
3+
3+
3
45
46
The presence of the Tb , Eu and Dy ions and tungstate host lattice provide different
2
-
3+
2-
VI
3
excitation wavelengths due to O (2p)→Eu (4f), O (2p)→W (5d) Ligand–to–Metal Charge
3
+
8
7
1
347
348
349
Transfer (LMCT) states and Tb 4f →4f 5d interconfigurational as well as 4f-4f transitions,
leading to white light emission tunable levels of chromaticity. Moreover, the doped materials
were excited at 260, 378 and 393 nm, yielding to tuning of emission colors between cool and

ACCEPTED MANUSCRIPT
350
351
352
warm white light (Figures S2, S3 and S4). Since warm white light is preferred for reading,
while cold white light is preferred for public lighting the tunability of this single-phase
emitting phosphors exhibit promising applications for solid-state lighting [53].
353
354
355
356
Please, insert Figure 9 here
5. Conclusion
3
+
3+
3+
357
358
359
360
361
362
363
The triply-doped CaWO :xTb ,xEu ,xDy (x mol%; x: 0.5 – 5.0) materials were
4
successfully prepared by the coprecipitation method at room temperature. The synthesis was
carried out under aqueous condition without the use of any catalyst, organic solvent or
surfactant. Furthermore, the crystallite sizes of the doped materials are at around 11 nm. The
3
+
3+
phosphors with different RE concentrations (RE : Tb, Eu and Dy) between 0.5 – 5.0 mol%
for each activator ion exhibited their characteristic excitation and emission transitions, leading
to white light emission under UV excitation.
3
64
65
The non-radiative energy transfer occurs via higher energy levels from the O→W and O→Eu
8
7
1
3
LMCT states as well as the 4f →4f 5d transitions to intraconfigurational 4f transitions of rare
4
3+
5
3+
5
5
3+
366
367
368
369
370
371
372
373
earth ions and mainly arises from the F (Dy )→ D (Tb )→ D , D (Eu ) emitter levels
9/2 4 1 0
which are very close (or in resonance) in energies. In addition, the energy transfer process
3
+
3+
between emitter levels from the Dy to Eu ions is also operative. With the increasing of
3
+
RE ions concentration (in the range 0.5 – 5.0 mol%) the color changes gradually and has
contribution from near whitish blue to warm white. Meanwhile, the CCT values of the
prepared phosphors were also varied and the emission color was tuned from cool white (5,479
K) to warm white (4,308 K). These phosphors could be suitable as triply-doped white light
emitters with only single-phased for solid state lighting applications.

ACCEPTED MANUSCRIPT
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375
376
377
378
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in the online version.
Acknowledgements
379
380
381
382
The authors thank the following Brazilian agencies for financial support: Conselho Nacional
de Desenvolvimento Cientifico e Tecnológico (CNPq), the Coordenação de Aperfeiçoamento
de Pessoal de Nível Superior (CAPES) and the Fundação de Amparo à Pesquisa do Estado de
São Paulo (FAPESP), as well as the photographer Cezar Guizzo.
3
3
3
3
3
3
3
3
3
3
3
3
3
83
84
85
86
87
88
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95

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Table 1. Lifetimes values (τ, ms) obtained at different excitation wavelength for the
3
+
3+
3+
Eu , Tb and Dy ions and CIE chromaticity parameters and color correlated
3
+
3+
3+
temperature (CCT) values of the CaWO :xTb ,xEu ,xDy (x: 0.5 – 5.0 mol%)
4
materials.
3
+
3+
3+
CaWO :xTb ,xEu ,xDy
Lifetimes (ms)
CIE
CCT
(K)
4
a)
3+
b)
3+
c)
3+
(
x mol%)
Tb
Eu
Dy
(x ; y)
0
1
2
3
4
5
.5
.0
.0
.0
.0
.0
0.865
0.834
0.707
0.687
0.684
0.731
0.715
0.686
0.584
0.583
0.578
0.628
0.333
0.312
0.289
0.262
0.263
0.265
0.377; 0.414
0.335; 0.372
0.321; 0.373
0.355; 0.392
0.380; 0.406
0.333; 0.352
4308
5411
5933
4802
4195
5479
a)
b)
c)
3+
Tb : λ = 378 nm; λem. = 545 nm
exc.
3+
Eu : λ = 393 nm; λem. = 614 nm
exc.
3+
Dy : λ = 385 nm; λem. = 576 nm
exc.

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3
+
3+
3+
Figure 1. Thermogravimetric curves of CaWO :xTb ,xEu ,xDy (x: 0.5, 1.0 and 5.0
4
mol%) materials recorded in dynamic synthetic air atmosphere.

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Figure 2. a) The X-ray powder diffraction (XPD) patterns of the
3
+
3+
3+
CaWO :xTb ,xEu ,xDy (x: 0.5, 1.0, 2.0, 3.0, 4.0 and 5.0 mol%) materials. Vertical
4
33
bars at the bottom of the figure indicate the standard PDF [ICDD entry #41-1431] data
for the tetragonal CaWO phase. b) CaO polyhedron distorted cube and c) WO
4
8
4
polyhedron exhibit in CaWO nanoparticles.
4