Effects of slight structural distortion on the luminescence performance in (Ca1-xEux)WO4 luminescent materials

Article abstract of DOI:10.1002/bio.3941

(Ca_1-xEu_x)WO₄ (x = 0–21 mol%) phosphors were prepared using the classical solid-state reaction method. The influence of Eu³⁺ ion doping on lattice structure was observed using powder X-ray diffraction and Fourier transform infrared spectroscopy. Furthermore, under this influence, the luminescence properties of all samples were analyzed. The results clearly illustrated that the element europium was successfully incorporated into the CaWO₄ lattice with a scheelite structure in the form of a Eu³⁺ ion, which introduced a slight lattice distortion into the CaWO₄ matrix. These lattice distortions had no effect on phase purity, but had regular effects on the intrinsic luminescence of the matrix and the f–f excitation transitions of Eu³⁺ activators. When the Eu³⁺ concentration was increased to 21 mol%, a local luminescence centre of [WO₄]^2? groups was detected in the matrix and manifested as the decay curves of [WO₄]^2? groups and luminescence changed from single exponential to double exponential fitting. Furthermore, the excitation transitions of Eu³⁺ between different energy levels (such as ⁷F₀→⁵L₆, ⁷F₀→⁵D₂) also produced interesting changes. Based on analysis of photoluminescence spectra and the chromaticity coordinates in this study, it could be verified that the nonreversing energy transfer of [WO₄]^2?→Eu³⁺ was efficient and incomplete.

Full text of DOI:10.1002/bio.3941

Yang Yuguo (Orcid ID: 0000-0002-5910-3296)
Effects of slight structural distortion on the luminescence performance in
(Ca1-xEux)WO4 luminescent materials
Fengnian Wu1, 2, , Hua-Jian Yu1, 3, Yan-Yan Hu1, 3, Hua-Di Zhang1, 3, Rui Zhang1, 3, Jing Li1, 4,
Bing Liu1, 3, Xu-Ping Wang1, 3, Yuguo Yang1, 3, , Lei Wei1, 3
1. Advanced Materials Institute, Qilu University of Technology (Shandong Academy of
Sciences), Jinan 250014, China;
2. Shandong Provincial Key Lab. of Preparation and Measurement of Building Materials,
University of Jinan, Jinan 250022, China;
3. Qilu University of Technology (Shandong Academy of Sciences), Advanced Materials
Institute, Key Laboratory of Light Conversion Materials and Technology of Shandong
Academy of Sciences, Jinan 250014, China;
4. Qilu University of Technology (Shandong Academy of Sciences), Advanced Materials
 fnwu@sdas.org (Corresponding author: Fengnian Wu)
 yangyuguo@sdas.org (Corresponding author: Yuguo Yang)
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doi: 10.1002/bio.3941
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Institute, Shandong Provincial Key Laboratory of High Strength Lightweight Metallic
Materials, Jinan 250014, China.
Abstract
(Ca1-xEux)WO4 (x=0-21 mol%) phosphors were prepared by the classical solid-state
reaction method. The influence of Eu3+ ion doping on lattice structure was observed by
Powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR).
Furthermore, under this influence, the luminescence properties of all the samples were
analyzed. The results clearly illustrated that the europium element was successfully
incorporated into the CaWO4 lattice with scheelite structure in the form of Eu3+ ion, which
introduced a slight lattice distortion into the CaWO4 matrix. These lattice distortions had no
effect on the phase purity, but had regular effects on the intrinsic luminescence of the matrix
and the f-f excitation transitions of Eu3+ activators. When the concentration of Eu3+ increased
to 21 mol%, the local luminescence center of [WO4]2- groups was detected in the matrix and
manifested as the decay curves of [WO4]2- groups luminescence changed from single
exponential to double exponential fitting. Meanwhile, the excitation transitions of Eu3+
7
7
between different energy levels (such as F0→5L6, F0→5D2) also existe interesting changes.
Based on the analyses of the photoluminescence spectra and the chromaticity coordinate in
this work, it could be verified that the nonreversing energy transfer of [WO4]2-→Eu3+ was
efficient and incomplete.
Keywords: CaWO4:Eu3+; Photoluminescence; Structural distortion; Incomplete energy
transfer; Adjustable luminescence.
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1. Introduction
Along with the rapid development of electronic technology, the research of luminescence
and display material had become a hotspot in recent decades.[1-4] The rare-earth doped
luminescent materials became popular objects because of their abundant and outstanding
luminescence properties. And that's why the luminescent materials related to rare earth were
widely used in many fields, such as lighting, display, laser materials, biomedicine,
photoelectric technology and so on.[5-11] As is well-known, calcium tungstate (CaWO4) is a
typical metal tungstate with stable scheelite structure and has self-activated blue
luminescence property.[12-14] More intriguing things perhaps were the changeable
luminescence properties and the energy transfer from the matrix to the doping ions when
doped with different rare earth or transition metal ions.[15-20] Therefore, undoped and doped
CaWO4 luminescent materials have attracted intense attention and application in the fields of
functional phosphors, bio-medicine, sensors and so forth.[21-24] Meanwhile, Trivalent
europium ion (Eu3+) had excellent red emission characteristics and became a very efficient
activator of red luminescent substances, which has been widely studied and applied in display
and luminescence engineering.[25-31] Over the past decade, CaWO4:Eu3+ phosphors has
received particular attention. This is because that CaWO4:Eu3+ phosphors can be efficiently
excited by ultraviolet rays, cathode rays, X-rays, etc., showing high-efficiency and
color-adjustable emission through both energy migration and effective energy transfer
between the host matrix and the luminescent center.[15, 27, 32-34] Previous studies have shown
that the CaWO4 based phosphors with different morphologies and sizes can be achieved by
using different experimental methods or adjusting experimental parameters, thus affecting
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their luminescence performance.[35-40] For instance, Su et al. synthesized doped CaWO4
core-shell microspheres with multicolor luminescence properties by using citric acid and PVP
assisted hydrothermal method.[41] By combining sol-gel process and electrospinning, Hou et
al. prepared one dimensional CaWO4:Tb3+ nano-material with adjustable luminescence.[42]
Yang et al. studied the influence mechanism of ultrasonic time and pH value on the
morphology-controlled synthesis of CaWO4 microcrystals in the sonochemical method.[43]
Recently, spindle-like CaWO4:Eu3+/Tb3+ nanoparticles and corresponding transparent
CaWO4:Ln/PMMA nanocomposites with strong trichromatic photoluminescence have been
acquired through reverse microemulsion by Zhang groups.[15] Zhang et al. reported that low
temperature is conducive to the controllable preparation of color adjustable CaWO4:Eu3+
nanoparticles with different morphology, structure and size.[44] In addition, some researchers
employed methods of co-doping different kinds of cations to realize charge compensation or
regulate the microenvironment of luminescence center, so as to achieve regulable
luminescence or performance improvement. These could include alkali metal cations (e.g.,
Li+, Na+, K+),[45, 46] alkali earth metal cations (e.g., Mg2+, Sr2+),[47] transition metal cations
(e.g., Mo6+, Y3+, Zn2+, Mn2+),[40, 48-51] main group metal cations (e.g., Bi3+),[33] lanthanide
cations (e.g., Tb3+, Sm3+, Gd3+, Nb5+, Ta5+),[20, 40, 51-54] and nonmetallic cations (e.g., Si4+).[50]
Looking at the related literatures, quite a few studies of CaWO4:Eu3+ phosphors have been
focused mainly on new synthetic methods, particle size and morphology control, perfecting
the red light-emitting efficiency though a cation co-doping technique, etc. Nevertheless, there
was little detail on the structural effects related to the substitution of Ca2+ by Eu3+ in CaWO4
lattice. In addition, no decent reports about the influence of lattice distortion on the
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luminescence property in CaWO4:Eu3+ phosphors with different Eu3+ concentration were
available. Especially, the influence of structural distortion on excitation effects of different
excitation wavelengths. However, this would be helpful to explore the mechanism of
luminous efficiency and precisely tune the luminescence of CaWO4:Eu3+ phosphors. Given
that, we subtly analyzed the microstructures of (Ca1-xEux)WO4（x=0-0.21 mol%) phosphors
which obtained by classical solid-state reaction and discussed the effect of structural
distortion on the photoluminescence properties. This work innovatively studied the
luminescence properties of (Ca1-xEux)WO4 materials with various Eu3+ concentrations from
the perspective of structural distortion. For example, the decay curves of intrinsic
luminescence origined from CaWO4 matrix changed from single exponential to double
exponential, the change of optimal excitation wavelength, etc. Furthermore, this paper more
precisely and clearly realized the continuous adjustable luminescence of (Ca1-xEux)WO4
fluorescent material from blue to red.
2. Experimental
2.1 Preparation
The phosphors with designed composition (Ca1-xEux)WO4 (x=0, 0.03, 0.07, 0.11, 0.15,
0.19, 0.21) were prepared by classical solid-state reaction and marked as A, B, C, D, E, F, G,
respectively. The starting materials, CaO (analytical reagent, Alfa Aesar), WO3 (99.99%,
Aladdin Industrial co.), Eu2O3 (99.99%, Aladdin Industrial co.) were used directly without
any further treatment. After precise weighing, the stoichiometric mixtures were mixed in the
mixing bottle for 30 minutes and then ground evenly in an agate mortar for another 30
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minutes. Whereafter, the homogeneous mixtures were transferred to a clean corundum
crucible and reacted in a resistance furnace for 2 hours at 800 °c in air. Finally, the products
were cooled to room temperature in the furnace and reground into fine powders for related
studies.
2.2 Characterization
Powder X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) were
used to investigate the crystal phase purity and structural characteristics of the as-prepared
samples. The XRD patterns were obtained by a Rigaku ZSX Primus Ⅱ diffractometer (Cu Kα,
λ= 0.154187 nm, 40kV/30 mA) continuous scanning in 2θ range from 5° to 80° at step of
0.01. A Thermo Scientific Nicolet iS5 FT-IR spectrometer with KBr beam splitter was used
to record infrared spectra from 4000 cm-1 to 400 cm-1. The photoluminescence excitation
(PLE) and emission (PL) spectra were collected by an Edinburgh Instruments FS5
spectrofluorometer with a Xe lamp excitation, and the bandwidth of excitation and emission
slits was kept at 0.45 nm. An FLSP-920 spectrometer (Edinburgh) was used to record
fluorescent decay curves. All the tests were conducted at room temperature.
3. Results and Discussion
3.1. Crystal structure
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Fig.1 XRD patterns of samples (A-G) and the standard diffraction data (Vertical bars) of
CaWO4 (JCPDS#41-1431)
The crystallization characteristics and phase purity of the obtained samples were analyzed
by XRD. Fig. 1 shown the XRD patterns of (Ca1-xEux)WO4 (x=0, 0.03, 0.07, 0.11, 0.15, 0.19,
6
0.21) phosphors and the standard data for scheelite phase CaWO4 (JCPDS #41-1431, C4h
I41/a(88) space group, a=b=5.24294 ,c=11.373, Z=4). It could be seen that both the pure
CaWO4 and the Eu3+ doped CaWO4 phosphors had narrow and strong diffraction peaks,
which matched well with the standard data of JCPDS card 41-1431. It meant that all the
samples had good crystallinity and normative CaWO4 scheelite phase with tetragonal
structure. The diffraction peaks were similar and no discernable diffractions of impurities
appeared, indicating that Eu3+ ions were successfully doped into calcium tungstate crystal
lattice and had no significant effect on the purity of crystalline phase. According to Bragg's
law, it could be found that the doping of Eu3+ ion has little influence on the cell parameter of
samples in this paper. Combining with the analysis of effective ion radius and charge
difference, it was easy to deduce that the Eu3+ ( r=1.066 Å for eight coordination) occupied
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the site of Ca2+ ( r=1.12 Å for eight coordination) in calcium tungstate lattice instead of the
W6+ site (r=0.42 Å for four coordination) .[20, 33, 46, 50, 52] In addition, it was obvious that the
intensity of the diffraction peaks in Fig. 1 tended to decrease with the increasing
concentration of Eu3+ ion, which meant that the doping of Eu3+ ion had the effect of making
the crystallinity of CaWO4 phosphors worse. This was agreed with some previous studies.[19,
42, 53, 55]
According to the crystal defect theory, this may be the manifestation of local lattice
distortion caused by the slight difference in radius and charge of Eu3+ ion replacing Ca2+ ion.
Fig.2 Fourier transform Infrared transmittance spectra of samples (A-G)
The Fourier transform Infrared transmission mode analysis was adopted to investigate the
short-range bonding structure of the obtained samples and further explored its crystallization
characteristics. Fig. 2 presented the IR spectra of all the samples in the wavenumber range
from 4000 to 400 cm-1. As could be known from the preparation part above that the samples
in this paper were prepared through high temperature solid-state reaction with high purity
oxide as raw materials. Therefore, the infrared absorption maps of all the samples were
similar, and only a set of absorption bands below 1000 cm-1 appeared (included a strong peak
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around 810 cm-1 and a relatively weak peak near 445 cm-1). Other miscellaneous peaks (such
as characteristic peak of physically absorbed water) were so weak as to be indistinguishable
in the spectra. Basing on the space group theory, the CaWO4 crystal with scheelite structure
had S4 point symmetry and eight infrared active vibration modes (4Au+4Eu). Consistent with
many previous literature,[19, 20, 51, 54, 56] here, the stronger broad absorption band situated near
810 cm-1 could be defined as an overlapping peak of the anti-symmetric stretching vibrations
[ν3(Au) and ν3(Eu)] , and the weak absorption band detected at about 445 cm-1 was ascribed
to the anti-symmetric bending vibrations [δ4(Au + Eu)], which both belong to the W–O bond
in [WO4]2- tetrahedrons. This was a good indication that pure calcium tungstate crystals with
scheelite structure had crystallized well. In addition, Fig.2 also shown that the peaks around
810 cm-1 and 425 cm-1 all slightly shifted towards lower wave number with the increase of
Eu3+ doping concentration, meanwhile, the stronger peaks (810 cm-1) gradually became wider
and obvious shoulder peaks appeared. All these illustrated that the doping of Eu3+ slightly
distorted the crystal lattice of the samples, decreased the force constants, changed the
length/angle of the W-O bond, raised the structural disorder and indicated a trend to become
less crystallinity.[32, 54, 56-58] These results were consistent with XRD analysis.
3.2 Photoluminescence properties
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Fig.3 Photoluminescence excitation and emission spectra of sample A (pure CaWO4)
Fig. 3 presented the photoluminescence excitation and emission spectra of sample A (pure
CaWO4). There were two excitation curves (a) (λem=430 nm), (b) (λem=615 nm) and four
emission curves(c), (d), (e), (f) with different excitation wavelength of 242, 250, 393, 463 nm,
respectively. It was clearly seen that a broad excitation band (220-275 nm) with a maximum
at 242 nm was only observed by monitoring at 430 nm. Correspondingly, the broad emission
peaks appeared in the wavelength range of 300-650 nm (the maximum value at about 430 nm)
only when the short-wave ultraviolet irradiation occurred and no obvious responses to other
excitation wavelength of 393 or 463 nm (the main excitation wavelengths of Eu3+ as shown
in Fig.4). In other words, sample A (pure CaWO4) only had intrinsic broadband blue
luminescence which was response to the short-wave ultraviolet excitation. This was
consistent with earlier literatures.[20, 23, 50, 54, 59] According to the molecular orbital theory, the
intrinsic luminescence of pure CaWO4 was caused by the charge transfer transitions in the
[WO4]2- groups. When excited by short-wave ultraviolet radiation, the electrons transferred
from O2-(2p) to W6+(5d), and the transitions from the ground state (1A1) to the high vibration
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level of 1B (1T2) was realized within the [WO4]2- groups. Whereafter, the transitions from low
1
1
vibration level of B (1T2) back to the ground state of A1 generated the blue intrinsic
emission.[60] Therefore, it was evident that the CaWO4 matrix in this paper was a pure
self-activated blue luminescent material.
Fig.4 Photoluminescence excitation (a) (λem=615 nm) and emission (b) (λex=250 nm), (c)
(λex=393 nm), (d) (λex=463 nm) spectra of samples (A-G)
Fig. 4 shown the photoluminescent excitation (PLE) and emission (PL) spectra of all the
samples (Ca1-xEux)WO4 (x= 0, 0.03, 0.07, 0.11, 0.15, 0.19, 0.21) prepared in this study. As
could be observed from Fig. 4 (a), when monitored at 615 nm, no discernible excitation peak
was detected for sample A(pure CaWO4) , nevertheless, the other samples (B-G) with
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different Eu3+ ion concentrations showed similar excitation spectra which included a
broadband excitation peak (215-300 nm) and a set of linear excitation peaks within the
wavelength range of 300-550 nm. Firstly, the broadband excitation peak (215-300 nm) with a
maximum at about 250 nm was similar to the broad excitation band (λem=430 nm) of the
undoped CaWO4 sample A [Fig. 3 (a)], but became wider and tended to shift towards
short-wave with the increase of Eu3+ doping concentration. Referring to previous studies,
those broadband peaks could be ascribed to the superposition of both O2-→W6+ (in [WO4]2-
groups) and O2-→Eu3+ charge transfer bands, moreover, as Eu3+ ion concentration increases,
the charge transfer band of O2-→Eu3+ enhances, making them wider and asymmetrical blue
shift.[33, 44, 54, 55, 61] The other linear excitation peaks were originated from the 4f6 electron
configuration of Eu3+ ion, mainly including the bands located at around 317, 360, 380, 393,
7
5
414, 463, 534 nm, corresponding to the electronic transitions of F0→5H3, 5D4, 5L7, L6, 5D3,
5
5D2, D1, respectively.[19, 20, 33, 52, 55] Meanwhile , in addition to the intensity, the shape and
position of the linear peaks were not affected by the doping content of Eu3+. According to the
present analyses, it could be clearly seen that Eu element was doped into the CaWO4 lattice
as Eu3+ ions and all the doped samples (B-G) shown three strong excitation bands located at
250 nm (short-wave ultraviolet), 393 nm (near-ultraviolet), 463 nm (blue light). In addition,
the presence of strong intrinsic excitation bands of [WO4]2- groups in Fig. 4 (a) indicated that
there were efficient energy transfer between the host and the luminescent center ions.[13, 20, 40,
54, 59, 62]
The emission spectra of all the as-prepared samples excited at different single wavelength
of 250 nm (short-wave ultraviolet), 393 nm (near-ultraviolet) and 463 nm (blue light) were
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presented in Fig. 4 (b), (c) and (d), respectively. Corresponding to Fig. 4 (a), the emission
spectra in Fig. 4 (b) contained the wide emission bands at around 430 nm arisen from
[WO4]2- groups in the matrix and the characteristic emission peaks of Eu3+ ion at 591, 615,
5
7
7
7
654 and 701 nm, assigned to D0→7F1, F2, F3 and F4 transitions, respectively.[13, 19, 23, 54, 55]
With the increase of Eu3+ doping concentration, the broad emission band (around 430 nm)
was gradually consumed to unidentifiable, as shown in Fig. 4 (b), inversely, the characteristic
emission bands of Eu3+ ion increased continuously and reached the maximum value. On the
one hand, this also showed the presence of energy transfer from the host to Eu3+ ion in the
samples.[13, 15, 42, 44, 55] On the other hand, this energy transfer was incomplete at low Eu3+
doping concentration.[13, 40, 50, 59, 62] Namely, with the increase of Eu3+ doping, more energy
would transfer from [WO4]2- groups to Eu3+ ion and yielded more characteristic luminescence
of Eu3+ ion. Fig. 4 (c) and (d) illustrated the emission spectra of all the samples excited at 393
and 463 nm, respectively. The similar linear emission peaks at 591, 615, 654, 701 nm were
5
7
7
7
originated from D0→7F1, F2, F3, F4 transitions of Eu3+ ion, respectively, which were
conforming with that in Fig. 4 (b). As we all known, the 5D0→7F2 electric dipole transition of
Eu3+ ion was hypersensitive to the local lattice symmetry environment and would become the
dominant luminescence peak only when Eu3+ ion occupied a lattice site without inversion
5
symmetry and the parity-forbidden rule was broken.[51, 62, 63] In this paper, the D0→7F2
transition was dominant obviously, indicating that Eu3+ ions successfully occupied the Ca2+
sites (S4 point symmetry, without inversion symmetry center), which was in good agreement
with the structure analysis of XRD and FTIR above. From another viewpoint, the intensity of
Eu3+ ion characteristic emission was highly dependent on the doping concentration of Eu3+
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ions, so it was necessary to find the optimal doping concentration. A comprehensive analysis
of both the excitation spectra which monitored at 615 nm and the emission spectra pumped
by various wavelengths at 250, 393, 463 nm in Fig. 4 was conducted to investigated the
influence of Eu3+ doping concentration on the 615 nm emission intensity. As shown in Fig. 5
and Fig. 6 (a) and (b), a consistent law could be obtained that the 615 nm emission intensity
of Eu3+ ion increases with the increment of Eu3+ doping concentration under the excitation of
different wavelengths and all reached the maximum at sample F (19 mol% Eu3+).
Simultaneously, other weak emission peaks of Eu3+ ion (5D0→7F1, 7F3, 7F4) also had the same
phenomena. This could conclusively demonstrate that the optimal doping concentration of
Eu3+ ion in this study was 19 mol% and the concentration quenching effect of Eu3+ would be
caused by a non-radiation process of cross-relaxation if the concentration continued to
increase, which was agree with other literatures.[54, 55, 63]
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Fig.5 (a), (b), (c) and (d) are the corresponding magnified patterns for Fig. 4 (a), (b), (c), (d)
respectively.
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Fig.6 The changing trends of (a) the intensity of different excitation peaks in the excitation
spectra(λem=615 nm), (b) the emission peak intensity of 615 nm in the corresponding
emission spectra (λex=250 nm, λex=393 nm, λex=463 nm)and (c) the ratio(R) of 615 nm
emission peak intensity pumped by 463 nm and 393 nm in the excitation or emission spectra
with the increment of Eu3+ ion concentration
Furthermore, a meaningful phenomenon was revealed that the excitation effects of
different excitation wavelength varied with the Eu3+ doping concentration. Fig. 6 (a)
summarized the variations of different excitation peak intensities at 250, 393, 463 nm with
different Eu3+ concentrations from the excitation spectra in Fig. 4 (a). Meanwhile, Fig. 6 (b)
summarized the variations of the emission intensities at 615nm with different Eu3+
concentrations from the emission spectra excited by 250, 393, 463 nm in Fig. 5 (b-d). It was
clear that both Fig. 6 (a) and (b) had a uniform development tendency. For sample B with 3
mol% Eu3+ doping, the emission intensity of 5D0→7F2 (615 nm) transition excited by 250 nm
was stronger than those excited by 393 or 463 nm, meanwhile, the corresponding value of
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463 nm was stronger than that of 393 nm. Whereas, for sample C (7 mol% Eu3+), it changed
to 463 nm>250 nm>393 nm and with further increment of Eu3+ doping (D-G,11-21mol%),
the order continued to change into 393 nm>463 nm>250 nm. This indicated that the energy
transfer from the host to Eu3+ ion played a predominant role for the Eu3+ luminescence when
the Eu3+ doping concentration was smaller (<7 mol%). Here we deduced, when the Eu3+ ions
dispersed in the matrix sparsely, it was easier to acquire excitation energy from the host
though charge transfer and this energy transfer was high-efficiency. After that, with the
further increase of Eu3+ ion doping (>7 mol%), the distribution of Eu3+ ions in the matrix
became denser, the excitation energy obtained from the host was relatively reduced and the
self-absorption excitation of Eu3+ ion became the main energy source for the 615 nm
emission. In addition, it was noteworthy that the ratio of the Eu3+ emission intensity at 615
nm excited by 463 nm and 393 nm also changed with different doping concentrations. As
demonstrated in Fig. 6 (c), for samples B (3 mol% Eu3+) and C (7 mol% Eu3+), the excitation
at 463 nm (7F0→5D2) was easier to pump high-intensitive emission of 615 nm. In contrast,
the strongest excitation wavelength was 393 nm (7F0→5L6) in samples D-G (11-21 mol%
Eu3+). According to the relevant theories and reports,[46, 51, 55, 62] we speculated that this was
originated from the changes of Eu3+ self-excited transition ability with different Eu3+ doping
concentration. As Eu3+ ion doping concentration increases, more Eu3+ ions would substitute
the Ca2+ ion in the CaWO4 lattice, due to the small differences of ionic radius and valence
state between the two cations , more vacancies and distortions were introduced into crystal
structure, the crystallinity of CaWO4 matrix slight declined, further resulted in the changes of
the chemical bonds and the ionic electron clouds in the crystal lattice, which affected the
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ability of electron transition between different energy levels. The discovery of this
phenomenon was conducive to realize the precise luminescence of (Ca1-xEux)WO4 phosphors
through jointly regulating the Eu3+ doping concentration and the excitation wavelength. This
could make them further perfect in scientific research and practical application.
Fig.7 The decay curves of (a) intrinsic luminescence of host matrix (λex=250 nm, λem=430
nm) and (b) characteristic luminescence of Eu3+ (λex=250 nm, λem=615 nm) for the
(Ca1-xEux)WO4 samples (x=0, 0.03, 0.07, 0.11, 0.15, 0.19, 0.21).
Inset: (1) Dependence of the relevant lifetime on Eu3+ concentration; (2) The partial
magnification image of the corresponding decay curve
Here, we studied the decay characteristics of the host intrinsic luminescence (430 nm) and
the Eu3+ ion characteristic luminescence (615 nm) when excited at 250 nm in room
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temperature, corresponding to Fig. 7 (a) and (b) respectively. As presented in figure 7 (a), the
decay for the intrinsic blue emission of [WO4]2- groups in matrix (λex=250 nm, λem=430 nm)
shown two kinds of fitting curves because of difference Eu3+ doping concentration. Firstly,
the decay curves of samples A-F (0-19 mol% Eu3+) were well fitted by the single exponential
function:
I = I0 exp(−t/τ ) +A0
(1)
where, I and I0 were the emission intensity at t and t = 0, respectively, t was the time and τ
was the lifetime, A0 was a constant. Here, the lifetimes of [WO4]2- luminescence in samples
A-F were drawn as 9.82, 8.80, 8.65, 8.39, 7.40, 6.41 μs, respectively. Secondly, while the
Eu3+ ion doping concentration was 21 mol%, the 430 nm luminescence decay curve of
sample G couldn't fit well with mono-exponential function and became a nice double
exponential function fitting (R2=0.99995):
I= A1 exp(t/τ1) + A2 exp(t/τ2) +A0
(2)
where, A1 and A2 were two fitting parameters, τ1 and τ2 were the fast- and slow-decay
components of the lifetimes, respectively. Meanwhile, the integrated lifetime could be
calculated with the following function:
2
2
2
2
τ= (A1 τ1 + A2 τ2 )/(A1τ1 + A1τ1)
(3)
So, the [WO4]2- luminescence lifetime of sample G could be obtained as 2.63 μs. In this
section, the intrinsic blue luminous lifetimes of the host were well in agreement with other
reports.[20, 59] On the basis of literatures,[34, 64, 65] it could be inferred that the transformation
for decay curves of the host luminescence could be attributed to the local lattice distortion in
the matrix which had mentioned above. While the Eu3+ doping concentration was small (0-19
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mol%), the matrix had little local lattice distortion and a relatively uniform luminescence
environment, showing the characteristics of single center luminescence. However, when the
Eu3+ concentration increased to 21 mol%, the partial lattice distortion was enough to cause
the appearance of local luminescence centers, so the sample G exhibited a double center
luminescence. At the same time, as illustrated in inset (1) of Fig. 7 (a), the lifetime of CaWO4
matrix luminescence decreased with the increase of Eu3+ ion concentration, which could
further prove the energy transfer between matrix and Eu3+ ions. Meanwhile, the energy
transfer efficiency between host and Eu3+ ions can be estimated using the intrinsic
luminescence lifetime of the samples and the equation (4).
ηet=1-(τ/τ0)
(4)
Where, ηet was the energy transfer efficiency, τ and τ0 were the intrinsic luminescence
lifetime in the presence and absence of Eu3+ ions.[34] The estimated values of energy transfer
efficiency were 10.39%, 11.91%, 14.56%, 24.64%, 34.72%, and 73.22% for as prepared
samples B, C, D, E, F, and G, respectively.
However, the decay curves of the Eu3+ characteristic luminescence (λex=250 nm, λem=615
nm) for the doped samples (B-G) prepared in this paper conformed to single exponential
function consistently, as exhibited in Fig. 7 (b), which indicated that the lattice distortion had
little effect on the Eu3+ luminescence at 615 nm (5D0→7F2 ) in this study. The Eu3+ ion
luminescence originated from the electron transitions in internal 4f orbit. The 4f electrons
were shielded by the external 5s5p electron shell and had a stable energy level structure, so
the f-f transitions of Eu3+ would not susceptible to external lattice environment. Therefore,
the position and shape of Eu3+ emission peaks stayed the same under different Eu3+ doping
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concentration and excitation wavelength, meanwhile, the Eu3+ luminescence at 615 nm could
maintain single exponential decay. From the fitting results, the fluorescence lifetimes of Eu3+
in samples B-G were determined to be 574.72, 566.44, 544.07, 533.86, 532.60, 528.31 μs,
respectively, and this were consistent with previous reports.[20, 23, 49, 64] In comparison, the
lifetimes of Eu3+ were much longer than those of [WO4]2- in CaWO4 matrix. This was
conducive to the energy transfer between [WO4]2- groups and Eu3+.[20, 59] In our case, as the
doping concentration increases, the lifetimes of Eu3+ at 615 nm gradually decreases, as shown
in Fig. 7 (b)(1), which could be attributed to the concentration quenching effect.[63]
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Fig.8 The distribution diagram of chromaticity coordinates（CIE）for the samples (A-G)
excited at 250, 393 and 463 nm. The images on the left column are digital photographs of the
samples upon 254 nm UV lamp box excitation
In order to better demonstrate the excellent luminescence tunability of the samples with
different Eu3+ doping concentrations under various wavelength excitation of 250, 393, 463
nm in this study, the chromaticity coordinates were calculated by employing the CIE color
matching functions (1931) and the data of corresponding emission spectra, as shown in Fig. 8.
Meanwhile, the acquired chromaticity coordinate values were arranged and presented in
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Table 1. It was clearly depicted that under the excitation of 250 nm wavelength, the
luminescence color of the corresponding samples could be changed from blue to orange by
adjusting the Eu3+ doping concentration from 0 to 21 mol%, and the coordinate value
scarcely kept changing after 19 mol%. It demonstrated that the luminescence ratio of [WO4]2-
groups and Eu3+ ions in the samples could be controlled by regulating the Eu3+ concentration
in CaWO4, realizing tunable multicolor luminescence. At the same time, compared with the
chromaticity coordinates excited at 393 nm and 463nm, the luminescence excited at 250 nm
could not be in accordance with the red luminescence of pure Eu3+ ions. this indicated that
the emission of [WO4]2- groups could not be completely disappeared, that is, the
energy transfers between the host and Eu3+ ions was incomplete in this study. It could also be
seen that the luminescence of the doped samples (B-G) excited at 393 nm and 463 nm were
all good red light and their chromaticity coordinates all slightly changed towards the standard
red light (0.670, 0.330) with the increment of Eu3+ doping concentration. This may be due to
the slight distortion of the lattice structure mentioned above, which made the hypersensitive
5D0→7F2 (615 nm) transition stronger,[51, 55, 61] thus slightly improved the purity of red
luminescence in the samples.
Table 1 CIE chromaticity coordinates for Ca(1-x)WO4: xEu3+ samples.
CIE chromaticity coordinates (x,y)
Samples
Ca(1-x)WO4:xEu3+
Excitation wavelength (nm)
250
393
-
463
-
A(x=0.00)
(0.1715, 0.1536)
B(x=0.03)
C(x=0.07)
D(x=0.11)
(0.2717, 0.1880)
(0.3136, 0.2018)
(0.3767, 0.2251)
(0.6357, 0.3257)
(0.651, 0.3289)
(0.6578, 0.3302)
(0.6336, 0.3336)
(0.6473, 0.3332)
(0.6536, 0.3329)
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E(x=0.15)
F(x=0.19)
(0.4647, 0.2615)
(0.5161, 0.2809)
(0.6605, 0.3311)
(0.6613, 0.3313)
(0.6576, 0.3331)
(0.6584, 0.3333)
G(x=0.21)
(0.5156, 0.2809)
(0.6613, 0.3316)
(0.6583, 0.3332)
4. Conclusions
In summary, the Ca(1-x)WO4: xEu3+ pure phase samples with scheelite structure were
prepared successfully in this work. The analyses of structure and luminescence properties
shown that europium elements were presented as Eu3+ ions and occupied the Ca2+ sites in the
CaWO4 lattice. When the concentration of Eu3+ reached to 21 mol%, the structure distortion
would be high enough to become a local luminescence center in the matrix, which led to a
double exponential fitting on the decay curve of [WO4]2- groups luminescence. However, this
distortion had no discernible influence on the decay of Eu3+ ions in this work. In addition, it
was also worth noting that the variation of Eu3+ concentration would regularly affect the
excitation effects of different excitation wavelengths. In particular, the excitation wavelength
derived from Eu3+ f-f transition. When the Eu3+ concentration was small (<7% mol%), the
463 nm (7F0→5D2) excitation transition played a dominant role for the 615 nm emission.
With the increment of Eu3+ doping (>7 mol%), the lattice distortions were getting larger, the
393 nm(7F0→5L6) excitation transition would becoming easier to occur and turned into the
strongest excitation wavelength. This study speculated that this was attributed to the different
sensitivity of the two excitation transitions to the changes of lattice environment around
luminescence center. The integrated analysis of the photoluminescence performance clearly
shown that there was an efficient one-way energy transfer from [WO4]2- to Eu3+ ions and this
energy transfer was incomplete. All the regular conclusions above were helpful to regulate
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the luminescence properties of CaWO4: Eu3+ phosphors more accurately and could also be
referred to other studies of similar luminescent materials.
Acknowledgements
This work was supported by National Natural Science Foundation of China (51772172,
51972179 and 51902168); Major Scientific and Technological Innovation Project in
Shandong (2018CXGC0412); Natural Science Foundation of Shandong Province
(ZR2018BEM023); Science and Technology Program for Young Innovation Team in
Colleges and Universities of Shandong Province, China (2019KJA003).
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