Structural and optical characterization of Eu and Dy doped CaWO4 nanoparticles for white light emission

Article abstract of DOI:10.1016/j.jallcom.2020.154804

The structural and light emission properties of calcium tungstate (CaWO₄) nanoparticles containing Eu³⁺ and Dy³⁺ are studied. CaWO₄ nanoparticles were prepared by chemical routes followed by drying at 80°C, doping with 1, 3, 5, 7 and 10 mol% of Eu₂O₃ and/or Dy₂O₃ and final annealing at 800^oC. The samples were characterized by field emission-scanning electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS), synchrotron X-ray diffraction (XRD), Raman and photoluminescence (PL) spectroscopy. FE-SEM studies show that CaWO₄ sample dried at 80°C contains spherical particles of diameter ～5 μm, and upon annealing at 800°C, dense and irregular shaped structures are formed. EDS mapping found a uniform distribution of Eu and Dy ions and only a small segregation in the CaWO₄ matrix is observed. XRD studies revealed the co-existence of the tetragonal CaWO₄ and cubic Dy₂O₃ and/or Eu₂O₃ phases in the doped samples and rule out the replacement of Ca²⁺ by rare-earth ions. The structural parameters: crystallite size, lattice strain, unit cell dimensions, atomic position coordinates, bond-lengths, bond-angles and cation-oxygen coordination numbers were determined. The short-range structure of CaWO₄ consists of interconnected CaO₈ and WO₄ units. All the W–O bonds in WO₄ units have the same length whereas two types of Ca–O bond-lengths exist in CaO₈ units. The undoped CaWO₄ sample annealed at 800°C, has the smallest Ca–O bond-lengths and unit cell dimensions due to the compressive macrostrain induced by heat-treatment. On incorporating Eu³⁺ and Dy³⁺, the W–O and Ca–O bond-lengths increase slightly. Raman spectra of all the samples are similar and show W–O vibrational modes. PL studies found that the undoped CaWO₄, Eu and Dy-doped CaWO₄ samples emit blue, red and yellow light respectively. The co-doped sample: 2 mol% Eu₂O₃–5 mol% Dy₂O₃–CaWO₄ was closest to the ideal white light emission properties.

Full text of DOI:10.1016/j.jallcom.2020.154804

Journal Pre-proof
Structural and optical characterization OF Eu and Dy doped CaWO nanoparticles for
4
white light emission
Puneet Kaur, Atul Khanna, M.N. Singh, A.K. Sinha
PII:
S0925-8388(20)31167-1
DOI:
https://doi.org/10.1016/j.jallcom.2020.154804
JALCOM 154804
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 29 November 2019
Revised Date: 9 March 2020
Accepted Date: 15 March 2020
Please cite this article as: P. Kaur, A. Khanna, M.N. Singh, A.K. Sinha, Structural and optical
characterization OF Eu and Dy doped CaWO nanoparticles for white light emission, Journal of Alloys
4
and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154804.
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition
of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of
record. This version will undergo additional copyediting, typesetting and review before it is published
in its final form, but we are providing this version to give early visibility of the article. Please note that,
during the production process, errors may be discovered which could affect the content, and all legal
disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.

CRedit authorship contribution statement
Puneet Kaur: Methodology, Data curation, Writing- review and editing. Atul Khanna:
Conceptualization, Methodology, Data curation, Writing- review and editing. M N Singh:
Methodiology, Data Curation, Writing- review and editing. A K Sinha: Data Curation, Writing-
review and editing.

STRUCTURAL AND OPTICAL CHARACTERIZATION OF Eu
AND Dy DOPED CaWO4 NANOPARTICLES FOR WHITE
LIGHT EMISSION
Puneet Kaur1, Atul Khanna1, a), M N Singh2, A K Sinha2, 3
1Sensors and Glass Physics Laboratory, Department of Physics, Guru Nanak Dev
University, Amritsar, Punjab, India
2Hard X-ray Applications Laboratory, SUS, Raja Ramanna Centre for Advanced
Technology, Indore, India
3Homi Bhabha National Institute, Anushakti Nagar, Mumbai, India
a) Corresponding author: atul.phy@gndu.ac.in
1

ABSTRACT
The structural and light emission properties of calcium tungstate (CaWO4) nanoparticles
containing Eu3+ and Dy3+ are studied. CaWO4 nanoparticles were prepared by chemical routes
followed by drying at 80oC, doping with 1, 3, 5, 7 and 10 mol% of Eu2O3 and/or Dy2O3 and final
annealing at 800oC. The samples were characterized by field emission-scanning electron
microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDX), synchrotron X-ray
diffraction (XRD), Raman and photoluminescence (PL) spectroscopy. FE-SEM studies show
that CaWO4 sample dried at 80oC contains spherical particles of diameter ~5 µm, and upon
annealing at 800oC, dense and irregular shaped structures are formed. EDX mapping found a
uniform distribution of Eu and Dy ions and only a small segregation in the CaWO4 matrix is
observed. XRD studies revealed the co-existence of the tetragonal CaWO4 and cubic Dy2O3
and/or Eu2O3 phases in the doped samples and rule out the replacement of Ca2+ by rare-earth
ions. The structural parameters: crystallite size, lattice strain, unit cell dimensions, atomic
position coordinates bond-lengths, bond-angles and cation-oxygen coordination numbers were
determined. The short-range structure of CaWO4 consists of interconnected CaO8 and WO4 units.
All the W—O bonds in WO4 units have the same length whereas two types of Ca—O bond-
lengths exist in CaO8 units. The undoped CaWO4 sample annealed at 800oC, has the smallest
Ca—O bond-lengths and unit cell dimensions due to the compressive macrostrain induced by
heat-treatment. On incorporating Eu3+ and Dy3+, the W—O and Ca—O bond-lengths increase
slightly. Raman spectra of all the samples are similar and show W—O vibrational modes. PL
studies found that the undoped CaWO4, Eu and Dy-doped CaWO4 samples emit blue, red and
yellow light respectively. The co-doped sample: 2 mol% Eu2O3–5 mol% Dy2O3–CaWO4 was
closest to the ideal white light emission properties.
Keywords: Tungstate compounds; rare-earth ions; X-ray diffraction; microstrain and
macrostrain; Rietveld refinement; Raman spectra; photoluminescence intensity; concentration
quenching phenomenon; chromaticity coordinates.
2

1. INTRODUCTION
Rare-earth (Re) doped tungstate and molybdate materials have been extensively studied
because of their technological applications in solid-state optoelectronic devices, phosphors,
optical fibers, laser hosts, light-emitting diodes, microwave dielectrics and scintillators [1, 2]. On
adding trivalent lanthanide ions in these materials, interesting luminescent properties are
observed [3-5]. One of the most promising applications of Re-doped materials is the generation
of white light [6-8]. The white light emission is usually observed by a set of different
luminophores with distinct emission colors, typically two (blue and orange/yellow) or three
(blue, green and red) [9]. As the tungstate matrix itself has blue-green color, the narrow line-
emission of lanthanide ions can help in achieving white light emission [10]. The
photoluminescence (PL) in Re ions is associated with the characteristic narrow-emission lines
arising from the intense and sharp 4f‰4f or 5d‰4f transitions [3, 11].
CaWO4 nanoparticles show blue light emission (~420 nm) under UV excitation, due to
2-
electronic transitions in the WO4 group [1, 12]. The luminescence arises from the electronic
1
1
2-
transition: T2‰ A1 within WO4 anions, creating charge transfer from oxygen to metal ions
[13, 14]. Zhang et al. analyzed the temperature-dependent photo-luminescent behavior of
CaWO4 microspheres and observed the decrease in blue emission with the increase in
temperature from 20 K to 270 K [15]. CaWO4 acts as a good host material for a variety of Re
ions such as Eu, Tb, Dy, Ho, Sm, Er, Yb, etc. [2, 4-6, 8, 16]. Xu et al. studied the optical
thermometry based on the temperature-dependent green up-conversion emissions of Er/Yb co-
doped CaWO4 phosphor in the temperature range of 294 K to 923 K [16]. CaWO4: Eu3+ has the
potential of white light emission due to the red emission from Eu3+ and blue-green emission from
CaWO4 host [17]. Ramakrishna et al. used Li+ as a charge compensating element in Eu-doped
CaWO4 and reported that 10 at% Li+ incorporation in 3 at% Eu-doped CaWO4 nanophosphors
enhances the crystallinity and PL yield [4].
The PL properties of Eu3+ activated alkaline earth tungstates crystals (MgWO4, CaWO4,
SrWO4) have been investigated using Na+ as the charge compensating element and it was found
that CaWO4 has the maximum emission while MgWO4 has the minimum [18]. Piskula et al.
studied the up-conversion and down-conversion luminescence characteristics in lanthanides-
3

doped CaWO4 phosphors and found that the up-conversion luminescence intensity increases in
the order: Pr3+/Yb3+ < Nd3+/Yb3+ < Tb3+/Yb3+ [3]. The white light emission under UV excitation
was studied by Barbosa et al. in Eu, Dy, Tb triply-doped CaWO4 phosphor [6]. Zhang et al. also
reported the white light emission from Eu-Tb co-doped CaWO4 phosphor by adjusting the
concentration of dopants [19]. CaWO4 nanoparticles can be synthesized by a variety of
techniques such as sol-gel [20], precipitation [21], hydrothermal [2], solid-state [3], molten salt
[22], microwave irradiation method [12], etc. The precipitation method is widely used for the
synthesis of uniformly distributed tungstate nanoparticles because of the narrow size distribution
of the particles so formed.
In several earlier structural studies on Re-doped CaWO4 materials, it has been reported
that all the X-ray diffraction (XRD) peaks belong only to the tetragonal phase of CaWO4 , and no
peaks of Re2O3 phases were observed [4, 6, 23]. According to these reports, Re ions replace Ca
sites due to the similar ionic radius of Ca2+ (=1.12 Å) and Re3+ (Eu3+=1.06 Å, Dy3+=1.02 Å) [6].
Gayatri et al. synthesized Eu3+ doped CaWO4 samples by chemical precipitation upto 30 at%
Eu3+ concentration and subjected it to heat treatment at 125oC for 3 h, these authors reported the
observation of the XRD peaks only due to tetragonal CaWO4 in their samples [24]. Du et al.
synthesized Ca0.95-xDy0.05EuxWO4 (x =0.01 to 0.05 mol%) by solid-state reaction method
followed by sintering at 1100oC for 6 h and reported the formation of only tetragonal CaWO4
without any separate cubic Eu2O3 phases in their samples [25]. The non-detection of crystalline
phases of Re2O3 in the XRD patterns of doped CaWO4 and the erroneous conclusions that all the
Re dopant ions replace Ca sites [6, 24, 25], are possibly due to the use of low-resolution
laboratory source XRD that are incapable of resolving closely spaced peaks of different phases
or due to the use of very small concentration of Re2O3 (for example, 1-mol % Eu2O3 doping in
CaWO4 matrix in the reference [26]) that cannot be detected by the XRD technique where the
intensity of X-ray source and instrument resolution is low as compared to synchrotron radiation
sources that offer higher photon flux and high monochromaticity of incident X-ray sources
(higher resolution due to use of double crystal monochromator in the beamline).
In the present investigation, two rare-earth ions (Eu and Dy) were incorporated in
CaWO4 powder samples by solid-state reaction method upto 10-mol % of Eu2O3 and 10-mol %
Dy2O3 and it is found by high-resolution synchrotron powder XRD studies that Eu2O3 and Dy2O3
4

exist as separate, secondary cubic phases in the tetragonal CaWO4 matrix and that Re ions do not
replace Ca2+ in the crystal structure of CaWO4. The effects of Re ions on the distortion of the
crystal structure of CaWO4 and on the Ca—O, W—O bond lengths and bond-angles have been
studied by FullProf Rietveld refinement of the XRD data. The investigation provides new
insights on the phase formation and structure-property relationships in Re-doped CaWO4 for
white light emission applications using highly monochromatic intense synchrotron X-rays, Field
Emission Scanning Electron Microscopy, Energy dispersive X-ray analysis, Raman and
photoluminescence spectroscopy.
2. EXPERIMENTAL METHODS
2.1. SYNTHESIS
0.4 M aqueous solution of calcium nitrate was prepared from calcium carbonate (Fluka,
˃99.5%) in 100 ml solution of dilute HNO3 acid in distilled water. The aqueous solution of
sodium tungstate dihydrate (Sigma Aldrich ˃99%) was used as the source of W ions. The
dropwise mixing of these two solutions in 1:1 molarity resulted in the formation of white
precipitates of calcium tungstate by the following chemical reaction:
Ca(NOꢀ)ꢁ + NaꢁWOꢂ. 2HꢁO → CaWOꢂ ↓ + 2 NaNOꢀ + 2 HꢁO
(1)
The precipitates were subjected to ultra-sonication and filtration followed by washing
with distilled water and ethanol twice. The precipitates were dried by keeping in an oven at 80oC
for 30 min. The stoichiometric proportions of Eu2O3 and Dy2O3 were used to prepare the
following nominal compositions of the samples:
(i)
xRe2O3–(100-x)CaWO4 (Re = Eu and Dy; x = 0, 1, 3, 5, 7 and 10 mol%)
xEu2O3–yDy2O3–(100-x-y)CaWO4 (x = 2, 5, 10 mol% and y = 5, 10 mol%)
(ii)
The resulting powder mixture was ground in pestle and mortar for 1.5 h for thorough
mixing. The ground powder was mixed with 3% polyvinyl alcohol solution (PVA, Sigma
Aldrich, 99%) and pressed in the form of cylindrical pellets using a hydraulic press and sintered
at 800oC for 6 h in air. The pellets of the undoped CaWO4 sample were also prepared and
5

sintered at 800oC for 6 h. The sample codes, composition and the preparation conditions of
CaWO4 samples are given in Table 1.
Table 1. Sample code, composition and preparation conditions of calcium tungstate
samples.
Composition (mol %)
Eu2O3 Dy2O3 CaWO4
Sintering
temperature (oC)
Sintering
Sr. No.
Sample code
time (h)
-
-
-
-
100
100
99
97
95
93
90
99
97
95
93
90
93
90
80
80
0.5
6
1
2
CW-80
CW-800
800
800
800
800
800
800
800
800
800
800
800
800
800
800
1
3
5
7
10
-
-
6
3
1Eu-CW
-
6
4
3Eu-CW
-
6
5
5Eu-CW
-
6
6
7Eu-CW
-
6
7
10Eu-CW
1Dy-CW
1
3
5
7
10
5
5
10
6
8
9
3Dy-CW
-
6
-
6
10
11
12
13
14
15
5Dy-CW
-
6
7Dy-CW
-
6
10Dy-CW
2Eu5Dy-CW
5Eu5Dy-CW
10Eu10Dy-CW
2
5
10
6
6
6
2.2. CHARACTERIZATION STUDIES
2.2.1. Structural Properties
The surface morphology, phase, composition, crystal structure and short-range order
properties of CaWO4 samples were studied by field emission-scanning electron microscopy (FE-
SEM), energy-dispersive X-ray spectroscopy (EDX), synchrotron X-ray diffraction and Raman
spectroscopy. FE-SEM studies were carried out on Zeiss Supra 55 operated at 5 kV and the
6

images were taken from secondary electron detector. The average crystallite size was determined
using ImageJ software [27]. The elemental composition and phase segregation were determined
by EDX mapping technique. The powder samples were coated with a thin gold layer to reduce
surface charging effects during SEM and EDX analysis.
The XRD studies were carried out on finely ground powder samples at the BL-12
beamline [28] of Indus-2 synchrotron facility at Raja Ramanna Center for Advanced
Technology, Indore, India. The measurements were performed in the transmission mode using
the image plate (MAR-345 Dtb) area detector. Highly monochromatic X-rays of wavelength
0.78681 Å and the sample-to-detector distance were accurately calibrated by performing XRD
on LaB6 NIST standard on the same set-up. However, one sample (undoped CaWO4 annealed
at 800oC) was characterized at a wavelength of 0.71271 Å.
Fit2D image processing software was used to convert the two-dimensional powder
diffraction data of image plate detector into one-dimensional intensity versus 2θ patterns.
Rietveld refinement was performed on XRD data using FullProf software [29]. The crystal
structure of the unit cells was generated using VESTA (Visualization for Electronic and
Structure Analysis) program [30].
Raman spectra of the samples were recorded on In-Via Reflex Renishaw micro-Raman
spectrometer. The samples were excited by 488 nm argon-ion laser and all the measurements
were performed at room temperature using an edge-filter and diffraction grating consisting of
2400 lines mm-1 and a Peltier cooled charged-coupled detector.
2.2.2. Photoluminescence (PL) studies:
The light emission properties were studied by PL spectroscopy and the patterns were
recorded on Horiba Scientific Fluorolog spectrofluorometer. The spectra were measured at
excitation wavelengths: 350, 388 and 395 nm (generated by xenon flash lamp) in the undoped
CaWO4, Dy and Eu-doped CaWO4 samples respectively.
7

3. RESULTS AND DISCUSSION
3.1. FIELD EMISSION-SCANNING ELECTRON MICROSCOPY (FE-SEM)
Fig. 1(a) shows the FE-SEM micrographs of the undoped sample: CW-80. The overall
morphology of the sample indicates the presence of solid microspheres with good mono-
dispersity, a few of them showed cracks. The inset in Fig. 1(a) shows that the microspheres have
~5 µm diameter formed from the agglomeration of nanoparticles with an average size of 50 nm.
Thus, the precipitation method yields a fairly homogenous size-distribution of the microspheres.
The sample annealed at 800oC (CW-800) shows a dense agglomeration of the particles
(Fig. 1(b)) having different shapes and sizes that are produced by the high-temperature heat
treatment [3]. The irregular distribution of small and large particles shows that the smaller
particles coalesce to form larger clusters.
Figure 1. FE-SEM images of the samples (a) CW-80 and (b) CW-800 (inset shows the
higher magnification image).
Energy dispersive X-ray (EDX) analysis (Fig. 2(a)-(g)) was used to study the distribution
of rare-earth ions in the sample: 10Eu10Dy-CW. The elemental composition was measured at
several positions on the sample (Fig. 2(a)) and these measurements found a fairly uniform
distribution of Eu and Dy ions in CaWO4 sample and only a small amount of their segregation in
8

the CaWO4 matrix (Fig. 2(c)-(g)). The average atomic % of all the elements i.e. Ca, W, O, Eu
and Dy were close to the nominal batch compositions.
The EDX findings on the samples: 10Eu-CW, 10Dy-CW, 2Eu5Dy-CW and 5Eu5Dy-CW
are similar and given in Table S1. Further, the concentrations of Eu2O3, Dy2O3 and CaWO4, as
found from the Rietveld refinement on the samples, are in good agreement with the nominal
batch compositions (Table 2).
Figure 2(a)FE-SEM image of the sample 10Eu10Dy-CW (b) EDX spectra (c-g) Elemental
mapping scans. EDX results at different positions in the sample are given in the table.
9

3.2.
SYNCHROTRON X-RAY DIFFRACTION (XRD)
Fig. 3(a-c) shows the XRD patterns of the samples: CW-80, CW-800, 10Eu-CW, 10Dy-
CW and 10Eu10Dy-CW in terms of d-spacing. The undoped samples: CW-80 and CW-800 show
well-resolved peaks of the tetragonal phase of CaWO4 (space group: I4ꢃ/a), that can be indexed
to JCPDS (Joint Committee Powder Diffraction Database) #772234. The presence of strong and
sharp diffraction peaks confirms the formation of the structurally ordered crystalline phases [26].
In the XRD pattern of the sample 10Eu-CW, the small peaks at d-values: 4.42, 2.70, 1.90
and 1.62 Å confirm the existence of secondary cubic Eu2O3 (JCPDS #120393) along with the
majority tetragonal phase. The sample 10Dy-CW has small XRD peaks at d = 4.43, 2.66, 1.88
and 1.60 Å due to the cubic Dy2O3 (JCPDS #90197) and all the other unlabeled peaks in Fig.
3(a)-(c) are due to CaWO4. The XRD pattern of 10Eu10Dy-CW sample shows peaks at d = 4.43,
2.71, 2.56, 1.91, 1.62 Å due to cubic Eu2O3, and at d = 4.36, 2.67, 2.52, 1.89, 1.61 and 1.58 Å
due to cubic Dy2O3, along with the peaks of CaWO4. Besides this, the peaks at d-values of 1.54,
1.51 and 1.48 Å in the sample 10Eu10Dy-CW match with both Eu2O3 and Dy2O3. It is evident
from the XRD data in Fig. 3(a-c) that the peaks in the undoped CW-800 sample are shifted
towards lower d-spacings as compared to other samples. This is due to the lattice compression
induced by annealing of defects at 800oC (Fig. S2).
The average crystallite sizes in the samples: CW-80, CW-800, 10Eu-CW, 10Dy-CW and
10Eu10Dy-CW were calculated by Debye-Scherrer and Williamson-Hall (W-H) methods [31]
and the values for all samples are given in Table S2. The average lattice microstrain values were
determined from W-H analysis of the width of the XRD peaks, while the shift in peak positions
was used to determine the macrostrain. The sample CW-800 shows compressive macrostrain,
with an average value of -0.99 ± 0.16%, while the other samples are under tensile macrostrain
(Table S2).
10

Figure 3. Synchrotron XRD patterns of the samples CW-80, CW-800, 10Eu-CW, 10Dy-CW
and 10Eu10Dy-CW in d-range: (a) 2.7 to 5.0 Å (b) 2.0 to 2.9 Å and (c) 1.5 to 2.1 Å. Peaks
marked as ‘*’ and ‘#’ are due to cubic Eu2O3 and cubic Dy2O3 respectively, and all the
other unmarked peaks are due to tetragonal CaWO4.
11

3.2.1. Rietveld refinement analyses:
The FullProf Rietveld refinement was performed on the samples: CW-80, CW-800,
10Eu-CW, 10Dy-CW and 10Eu10Dy-CW and the refined patterns are shown in Fig. S3(a)-(e).
The sharp XRD peaks match well with the tetragonal phase of CaWO4, but due to the presence
of distinct small peaks of cubic Re2O3, the second phase had to be incorporated for better
refinement. Thus, the refinement results of Re-doped CaWO4 samples confirm that the doped
samples contain two phases: tetragonal CaWO4 having space group I4ꢃ/a and cubic Re2O3
2+
ꢄ
having space group: Ia3 and that Ey and Dy ions do not substitute at Ca sites, contrary to what
is claimed in the earlier studies [6, 23, 24, 31].
Table 2 gives the values of fit parameters, mole-fraction of the phases along with the
refined unit-cell parameters for each phase. The Rietveld refinement was carried out atleast 3
times on each sample and the uncertainty values are the difference between the highest and the
lowest refined values. The small values of errors and residuals show that the refinements were
performed with success. The concentration of each phase from Rietveld refinement was found to
be close to the nominal composition. The atomic position coordinates are given in Table S3(a)-
(e).
Table 2 gives the unit cell dimensions of tetragonal CaWO4 and it is found the lattice
parameters decrease from a=5.237 Å and c=11.361 Å in the sample CW-80 to a=5.180 Å and
c=11.239 Å in the sample CW-800. This is because CaWO4 sample heated to 80oC contains
defects, such as oxygen vacancies [32-34]. Upon annealing, these defects are removed, and the
lattice contracts and reduces the unit cell dimensions [35]. However, with the incorporation of
secondary phases of Eu2O3 and Dy2O3 into the matrix, the unit cell dimensions of CaWO4 phase
again increase significantly.
Literature survey reveals that with the addition of Re ions (Eu and Dy) into CaWO4
matrix, all the XRD peaks are indexed to tetragonal CaWO4, and substitution of Ca sites by Re
ions has been reported [6, 23, 24]. The substitution of larger Ca2+ (1.12 Å) by smaller Re3+ ions
(Eu3+ = 1.06 Å, Dy3+ = 1.02 Å) was attributed to the lattice shrinkage and decrease in the unit cell
dimensions and cation—O bond-lengths in the doped samples [23, 36-38].
12

However, in the present study, the addition of Eu2O3 and Dy2O3 in CaWO4 by high-
temperature solid-state reaction method produces composite samples containing cubic phase(s)
of Re2O3 and the tetragonal CaWO4, as revealed by the detection of XRD peaks of Re2O3 in the
samples. The addition of Re2O3 induces tensile strain in the CaWO4 lattice (Fig. S2) and expands
the unit cell of CaWO4 (Table 2). The Ca—O bond-lengths (Table 3(b), discussed later) also
increase in the doped samples. The tetragonal unit cell dimensions of CaWO4 are found to
increase from a=5.180 Å and c=11.239 Å in CW-800 sample to a=5.254 Å and c=11.401 Å in
10Eu10Dy-CW sample. The present investigation shows that Re ions are not substituting Ca
sites but exist as an independent cubic phase of Re2O3.
Table 2. Unit cell dimensions, phase quantification and fit parameters from the Rietveld
refinements of undoped and 10% Re2O3 (Re=Eu and/or Dy) doped CaWO4 samples.
Unit cell dimensions (Å)
Phase 2
Fit parameters
Sample
Phase 1
CaWO4
Phase 3
Dy2O3
χ2 Rwp Rexp
Eu2O3
a=5.237 ± 0.001
c=11.361 ± 0.001
100%
-
-
CW-80
Mol %
1.5
3.7
2.4
1.4
2.8
5.6
7.3
6.9
5.0
8.2
4.6
3.6
4.2
4.3
4.6
-
-
a=5.180±0.001
c=11.239±0.001
100%
-
-
CW-800
Mol %
-
-
a=5.247±0.001
c=11.387±0.001
a=10.788±0.003
-
10Eu-CW
Mol %
90.9±0.8
9.1±0.8
-
a=5.242±0.001
c=11.370±0.001
91.2±2.5
-
a=10.641±0.001
8.8±2.5
10Dy-CW
Mol %
-
a=5.254±0.001
c=11.401±0.001
a=10.868±0.001
6.5±2.0
a=10.691±0.001
11.1±1.1
10Eu10Dy-CW
Mol %
82.4±2.4
13

3.2.2. CaWO4 unit cell:
The unit cell of CaWO4 is modeled by the program: VESTA [30] using the values of lattice
parameters (Table 2) and atomic position coordinates (Table S3(a)-(e)) obtained from the
Rietveld refinement of the XRD data.
Fig. 4 shows that the short-range order in the undoped sample: CW-80 consisting of CaO8
units having distorted snub disphenoid deltahedral configuration (12 faces, 8 vertices, 18 edges),
along with the tetrahedral WO4 units (4 faces, 4 vertices, 6 edges) in such a way that each Ca2+ is
in coordination with eight oxygens and W ions are in four-coordination with oxygens. Also, each
CaO8 shares its corners with eight adjacent WO4 units.
Figure 4. Unit cell of sample CW-80 as modeled by VESTA.
14

Tables 3(a) and 3(b) give the values of W—O and Ca—O bond-lengths, O—W—O,
O—Ca—O bond-angles and NW—O, NCa—O coordination number for W and Ca ions respectively.
In each sample, all the W—O bonds in each WO4 unit have exactly the same bond-lengths
whereas CaO8 has two types of Ca—O linkages, four of each kind. The distortion from regular
tetrahedron is reflected in two kinds of internal O—W—O angles (Table 3(a)). CaO8 units are
found to have eight types of O—Ca—O bond-angles (Table 3(b)).
Table 3(a). W—O bond-lengths and O—W—O bond-angles in WO4 units.
CW-80
CW-800
10Eu-CW
10Dy-CW
10Eu10Dy-CW
W—O bond-
length (Å)
O—W—O
bond angles
(o)
1.779 ± 0.001 1.774 ± 0.012 1.817 ± 0.002 1.794 ± 0.008
1.801 ± 0.022
107.1 ± 0.6
107.6 ± 0.2
107.5 ± 0.1
107.1 ± 0.2
107.4 ± 0.3
114.3 ± 1.4
4 .00
113.2 ± 0.5
4 .00
113.5 ± 0.1
4 .00
114.3 ± 0.4
4.00
113.7 ± 0.8
4.00
NW—O
Table 3(b). Ca—O bond-lengths and O—Ca—O bond-angles in CaO8 units.
CW-80
CW-800
10Eu-CW
2.443 ± 0.001
2.453 ± 0.002
10Dy-CW
2.432 ± 0.003
2.466 ± 0.008
10Eu10Dy-CW
2.421 ± 0.021
2.485 ± 0.006
Ca—O
bond-lengths
(Å)
2.445 ± 0.021 2.394 ± 0.007
2.468 ± 0.019 2.451 ± 0.004
68.7 ± 0.4
73.3 ± 0.2
76.3 ± 0.3
79.8 ± 0.3
98.8 ± 0.1
126.1 ± 0.2
133.9 ± 0.6
68.5 ± 0.1
73.6 ± 0.1
75.9 ± 0.3
80.1 ± 0.5
99.1 ± 0.2
125.8 ± 0.3
133.0 ± 0.6
68.6 ± 0.1
73.4 ± 0.1
75.5 ± 0.1
81.5 ± 0.1
99.5 ± 0.1
125.0 ± 0.1
132.1 ± 0.1
68.9 ± 0.1
73.6 ± 0.1
75.7 ± 0.3
79.5 ± 0.3
99.0 ± 0.1
126.2 ± 0.2
133.5 ± 0.3
68.7 ± 0.4
73.9 ± 0.4
75.6 ± 0.6
79.3 ± 0.8
99.0 ± 0.4
126.4 ± 0.4
133.3 ± 1.2
O—Ca—O
bond-angles
(o)
15

152.5 ± 0.5
7.99 ± 0.02
153.1 ± 0.5
7.96 ± 0.01
154.4 ± 0.1
7.99 ± 0.01
152.7 ± 0.3
7.99 ± 0.01
152.7 ± 0.9
7.95 ± 0.01
NCa—O
The sample CW-800 has the smallest values of W—O (1.774 Å) and Ca—O (2.394 Å
and 2.451 Å) bond-lengths, due to the lattice compression induced by annealing at 800oC.
Further, it is noteworthy that amongst the samples annealed at 800oC (CW-800, 10Eu-CW,
10Dy-CW, and 10Eu10Dy-CW), Ca—O bond-lengths are found to be greater in the doped
samples. The increase in Ca—O bond-lengths from 2.394 Å and 2.451 Å (in CW-800 sample) to
2.421 Å and 2.485 Å (in 10Eu10Dy-CW sample) confirms that Ca2+ ions are not replaced by
Re3+ ions as the latter would have decreased the unit cell dimensions. On the contrary, literature
reports the replacement of Ca by Re ions and decrease in Ca—O bond-lengths [37, 38].
3.3.
RAMAN SPECTROSCOPY
The vibrational modes in the Raman spectra of scheelite CaWO4 are classified into two
2-
groups: internal and external. The external modes correspond to the vibrations of WO4
tetrahedron against the Ca2+, while the internal modes correspond to the internal vibrations of
WO42- tetrahedron in which Ca ions remains stationary [39]. All the vibrations (A1 (ν1), E (ν2), F2
(ν3), F2 (ν4)) of isolated WO4 clusters are in tetrahedral symmetry (Td) and are Raman active [1,
40]. But when WO4 clusters are located inside the scheelite structure, the doubly degenerate
vibrational mode, E (ν2), and triply degenerate vibrational modes: F2 (ν3) and F2 (ν4) split into
non-degenerate bands (A and B) due to the crystal field effect, reducing the point symmetry to S4
and give rise to 26 vibrations [1, 39, 40], i.e.
Γꢅꢆꢇꢆꢈꢉꢊꢈꢋꢌꢆꢌꢍꢎ = 3Bꢏ + 5Eꢏ + 5Aꢏ + 3Aꢐ + 5Bꢐ + 5Eꢐ
(2)
The subscripts ‘g’ and ‘u’ indicates the even and odd parity under inversion, respectively.
Thirteen possible even vibrations (subscripted by ‘g’) are Raman active and the odd vibrations:
4Au and 4Eu are IR active [1]. The remaining Au and Eu modes are acoustic vibrations whereas
3Bu vibrations are silent modes [1, 12].
16

Γꢅꢆꢇꢆꢈ = 3Aꢐ + 5Bꢐ + 5Eꢐ
(3)
Out of the thirteen expected Raman active modes, nine peaks were detected in the
sample, CW-80 at 83.5, 115.1, 196.1, 211.4, 331.9, 398.5, 796.6, 837.6 and 911.3 cm-1 (Fig. 5).
Among them, there are 3 Ag vibrations (211.4, 331.9, 911.3 cm-1), 4 Bg vibrations (83.5, 331.9,
398.5, 837.6 cm-1) a-nd 3 Eg vibrations (115.1, 196.1, 796.6 cm-1). The peak at 331.9 cm-1 is
assigned to the symmetric bending vibrations of O—W—O linkages and the peaks at 796.6 and
837.6 cm-1 are assigned to the antisymmetric stretching vibrations of O—W—O linkages. The
peak at 398.5 cm-1 is attributed to the antisymmetric bending of W—O bond while the sharp
peak at 911.3 cm-1 is attributed to the symmetric stretching vibrations of W—O bond. Peaks at
lower wave-number (83.5, 115.1, 196.1, 211.4 cm-1) belong to the external modes of CaWO4
[12, 36, 41-43] .
In the sample CW-800, a small shift (1 to 2 cm-1) in Raman peaks towards higher
frequencies is observed (Fig. 5). The increase of processing temperature to 800oC results in the
structural organization at short-range order and enhances the intensity of peaks at 197.8 and
274.8 cm-1 due to the free rotation of WO4 groups (Eg mode) [36].
17

Figure 5. Raman spectra of undoped and Eu-doped CaWO4 samples.
In the case of 1, 3, 5, 7 and 10 mol% Eu2O3-doped and Dy2O3-doped CaWO4 samples,
peaks are located at the same positions, within the limits of the spectral resolution of the
instrument, but are of enhanced intensity as compared to CW-800 (Fig. 5 and Fig. 6). All the
Raman peaks in Eu and Dy-doped CaWO4 samples are only due to W—O vibrational modes and
no Raman peak of Eu2O3 at 338 cm-1 or that of Dy2O3 at 372 cm-1 (Fig. S4) were detected,
possibly due to overlapping of these peaks by sharp and strong W—O vibrations of CaWO4. The
similar Raman patterns of all the doped samples show that the concentration and the nature of
dopants do not modify the vibrational modes of W—O bonds, however, the addition of Eu and
Dy ions enhanced the Raman signal of CaWO4 (Fig. 5 and Fig. 6).
Figure 6. Raman patterns of undoped and Dy-doped CaWO4 samples.
Fig. 7 shows the shift in Raman peak centered at 911 cm-1 in the samples: CW-80, CW-
800, 10Eu-CW, 10Dy-CW and 10Eu10Dy-CW. The sample CW-800 shows the shifting of
Raman peaks towards higher frequency as compared to other samples. The blue-shift in Raman
18

peaks implies shorter W—O bond-lengths in the sample CW-800, a result which is in agreement
with the XRD findings that reveal the compression of the crystal lattice (Fig. S2). The shift in
positions of Raman modes in Eu-doped CaWO4 samples was also reported by Gonçalves et al
.
[26].
Figure 7. Shift in Raman peak at 911.1 cm-1 in CW-80, CW-800, 10Eu-CW, 10Dy-CW,
10Eu10Dy-CW samples.
3.4. PHOTOLUMINESCENCE (PL) SPECTROSCOPY
3.4.1. CaWO4:
The PL spectra of the undoped CaWO4 samples: CW-80 and CW-800 were measured at
an excitation wavelength of 350 nm. Fig. 8 shows that the PL spectra of both the samples consist
of a broad emission band centered at 437 nm (blue region), due to the radiative transitions within
2-
WO4 complexes, creating charge transfer from O to W ions [13, 44]. In the PL spectrum of
CW-80, a peak at 570 nm (greenish-yellow region) is also observed whose intensity is found to
become negligible after heat treatment at 800oC. This peak is an effect of two different
19

mechanisms: one is the disorder caused by the WO42- complexes due to the oxygen vacant WO3
units and the other one is the intrinsic disordered WO42- tetrahedron in short-range order [33, 44,
45]. Thus, the absence of a peak at 570 nm in the sample CW-800 indicates the removal of
oxygen vacancies and the transformation of WO3 into WO4 units with annealing.
Figure 8. PL spectra of CW-80 and CW-800 samples at an excitation wavelength of 350
nm.
3.4.2. Eu2O3 doped CaWO4:
(a) Excitation spectrum: The excitation spectrum of 3Eu-CW sample was measured at 615 nm
(Fig. 9(a)). The spectrum shows sharp peaks corresponding to the transitions from the ground
state 7F0 to the excited states: 362 nm (5D4), 380 nm (5G2), 394 nm (5L6), 415 nm (5D3) and 465
nm (5D2) of Eu3+ [11]. The intense peaks at 394 and 465 nm show that the samples can be
efficiently excited by near UV (394 nm) or blue light (465 nm).
(b) Emission spectra: The PL emission spectra of Eu-doped CaWO4 samples were measured at
an excitation wavelength of 395 nm (Fig. 9(b)). The spectra show prominent emission peaks at
592, 596, 612, 615, 653 and 700 nm corresponding to the transitions from the excited level, 5D0,
7
to the ground level, FJ, of 4f6 configuration within Eu3+. The emission spectra are mainly
20

dominated by the hypersensitive red emission at 612 nm and 615 nm corresponding to the parity-
5
forbidden electric dipole transition D0
‰
7F2 of Eu3+, which is strongly influenced by the
chemical environment of Eu3+ in the host lattice [11]. The electric dipole transitions are
prominent if Eu ions occupy sites without an inversion center i.e. low symmetry sites [4, 17, 23].
‰
The transition: 5D0 7F1, at 592 nm, originates from the parity-allowed magnetic dipole transition
and is insensitive to the crystal field environment [11]. This transition becomes prominent for the
cases when Eu ions are located at high symmetry sites. The feeble broad peaks at 653 and 700
‰ ‰
nm corresponds to 5D0 7F3 and 5D0 7F4, respectively [11].
The higher intensity of electric dipolar transitions (612 and 615 nm) than magnetic
dipolar transitions (592 nm) shows that Eu ions in CaWO4 lattice are located at low symmetry
sites. The ratio of intensities of electric and magnetic dipolar transition can be used to compute
the relative fluorescence intensity ratio (R) which is inversely related to local ion symmetry and
is mathematically defined as [46]:
ꢕ
ꢔ
ꢘ
ꢑ = ꢒ (ꢓ → ꢖ )
(4)
ꢗ
ꢕ
ꢘ
ꢒ (ꢓ → ꢖ )
ꢔ
ꢙ
R-value is a measure of degree of distortion from the inversion symmetry of the local
environment of Eu3+ in the lattice. Its value is ~1 for symmetric environments and is higher for
asymmetric environments [46]. In the case of 1, 3, 5, 7 and 10 mol% Eu2O3-doped CaWO4
samples, R-values are 9.1, 9.2, 8.9, 8.9 and 8.1 respectively. Larger R-values show the distortion
of lattice from the inversion symmetry. It is clear from Fig. 9(b) that Re concentration has no
effect on the profile of PL spectra except on the photoluminescence intensity. Further, the inset
of Fig. 9(b) shows maximum luminescence in 3 mol% Eu2O3-doped CaWO4 sample due to the
concentration quenching mechanism (discussed below) [47].
21

Figure 9(a). Excitation spectra of sample 3Eu-CW at an emission wavelength of 615 nm (b)
Emission spectra of Eu2O3-doped CaWO4 samples excited at 395 nm. Inset shows the
magnified view of the emission peaks at 615 nm.
22

3.4.3. Dy2O3-doped CaWO4:
(a) Excitation spectra: The excitation spectrum of 3Dy-CW sample was measured at emission
wavelength of 575 nm (Fig. 10(a)) and shows the transitions from the ground state, 6H15/2, to the
different excited states: 326 nm (6P3/2), 353 nm (6P7/2), 367 nm (6P5/2), 388 nm (4I13/2), 428 nm
(4G11/2), 452 nm (4I15/2), 476 nm (4F9/2) of Dy3+ respectively [23].
(b) Emission spectra: Fig. 10(b) shows the emission spectra of Dy-doped samples recorded at
an excitation wavelength of 388 nm. The sharp characteristic emissions peaks are at 479, 487,
1
569, 575 and 663 nm. The spectra show a small peak at 420 nm, due to the transition: T2
‰
1A1
2-
within WO4 anions. The peaks in the range: 460-500 nm correspond to the magnetic dipole
4
transitions F9/2
‰
6H15/2 [5, 23]. The presence of a major peak at 575 nm (yellow region)
4
corresponds to the electric dipole transition: F9/2
‰
6H13/2 [5, 23]. Since the electric dipole
transition (575 nm) is more prominent than the magnetic dipole transition (~480 nm), it shows
that the Dy 3+ are located at low symmetry sites. A broad band at 663 nm corresponds to the
‰
transition 4F9/2 6H11/2 [5]. The R-values for 1, 3, 5, 7 and 10 mol% Dy2O3-doped samples, were
calculated from the ratio of peaks centered at 575 and 480 nm and these are 3.3, 3.2, 3.1, 3.4 and
3.3 respectively, which shows the distortion of the environment of Dy ions. Finally, the sample
containing 1 mol% Dy2O3 exhibits maximum PL intensity (inset of Fig. 10(b)).
23

Figure 10(a) Excitation spectra of sample 3Dy-CW at emission wavelength of 575 nm (b)
Emission spectra of Dy2O3-doped CaWO4 samples at excitation of 388 nm. Inset shows the
magnified view of peak at ~575 nm.
24

3.4.4. Concentration Effects on PL Intensity:
Fig. 11 shows the dependence of PL intensities of the transitions: 5D0 7F2 (615 nm) and
‰
‰
4F9/2 6H13/2 (575 nm) with the concentration of Eu and Dy ions, respectively. The emission in
Eu-doped samples first increases with the increase in the concentration of Eu2O3 from 1 mol% to
3 mol%, but when Eu2O3 content increases beyond 3 mol%, the light emission intensity
decreases due to the concentration quenching mechanism [47]. The dipole-dipole interaction
increases with the increase in Eu/Dy concentration, and enhances the cross-relaxation among the
dopant ions, making the dopants to transfer their energy to the neighboring atoms and contribute
to non-radiative transitions [36]. In Dy-doped samples, PL emission intensity is observed to
decrease continuously with the increase in the Dy2O3 concentration from 1 to 3, 5, 7 and 10
mol%, with maximum emission at 1 mol% of Dy2O3 in CaWO4.
Figure 11. Variation of luminescence intensity of PL peaks at 615 nm (Eu) and 575 nm
(Dy) with the variation in the concentration of Re ions.
25

Gayatri et al. synthesized Eu3+: CaWO4 nanoparticles by precipitation method and
reported the maximum luminescence in 20 mol% Eu3+-doped CaWO4 sample [24]. Du et al
.
studied the PL emission spectra of x-mol% Dy3+: CaWO4 (0.01≤x≤0.11) samples prepared by
solid-state reaction between CaCO3, WO3 and Dy2O3 and found intense emission at 572 nm due
to the electric dipolar transitions occurring within Dy3+ with maximum luminescence at x = 0.05
mol% [31].
3.4.5. Eu-Dy co-doped CaWO4:
The emission spectra of Eu-Dy co-doped samples have been recorded at two excitation
wavelengths: 353 and 388 nm (Fig. 12(a)-(b)). The spectra show light emissions in different
color regimes: blue emission due to the host CaWO4, greenish-yellow emission due to Dy3+
(4F9/2
‰ ‰
6H13/2) and red emission due to Eu3+ (5D0 7F2).
26

Figure 12. PL spectra of co-doped samples 2Eu5Dy-CW, 5Eu5Dy-CW and 10Eu10Dy-CW
at excitation wavelengths of (a) 353 and (b) 388 nm.
27

3.4.6. CIE chromaticity co-ordinates
The Commission Internationale d I’Eclairge (CIE) chromaticity diagrams 1961 of
undoped, Eu/ Dy doped samples are shown in Fig. 13(a) at excitation wavelengths of 350, 395
and 388 nm respectively. The chromaticity color coordinates (x, y) for the samples are tabulated
in Table S4 which vary with the dopant concentration. Eu-doped samples show the reddish-
orange emission at lower concentrations of Eu2O3 and the color shifts towards purplish-pink at
10 mol% doping. Dy-doped samples show emission in the yellowish region approaching nearly
white color with an increase in Dy2O3-concentration.
The CIE chromaticity plots for the co-doped samples are presented in Fig. 13(b) which
shows that under an excitation wavelength of 353 nm, the increase in the concentration of
dopants shifts the CIE color coordinates from (0.36, 0.37) in 2Eu5Dy-CW to (0.34, 0.34) in
10Eu10Dy-CWsample resulting in white emission.
On the other hand, with 388 nm excitation, a decrease in the concentration of dopants shifts
the CIE coordinates from (0.33, 0.30) in 10Eu10Dy-CW to (0.34, 0.33) in 2Eu5Dy-CW making
pure white emission (Fig. 13(b)). The reason for the slight shift in color coordinates is that the
appropriate amounts of red, green and blue light mix to create white light emission. The color
coordinates for the co-doped samples are given in Table S5.
28