Structural evolution induced by substitution of designated molybdate sites (MoO4?2) with different anionic groups (BO3?3, PO4?3 and SO4?2) in CaMoO4:Sm3+ phosphors-A study on color tunable luminescent properties

Article abstract of DOI:10.1016/j.jallcom.2017.08.117

A series of novel red emitting CaMoO₄:Sm³⁺ (1.0 mol %) phosphors substituted with different anionic groups (BO₃^?3, PO₄^?3 and SO₄^?2) were prepared using a high temperature solid state reaction method. The effects of anionic substitution on the crystalline structure and photoluminescence (PL) properties of the CaMoO₄:Sm³⁺, CaMoO₄-BO₃:Sm³⁺, CaMoO₄-PO₄:Sm³⁺and CaMoO₄-SO₄:Sm³⁺ phosphors were investigated. The structure, particle morphology, chemical composition, vibration modes, and PL properties of the phosphors were investigated by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectrometry (FT-IR) and PL spectroscopy, respectively. The XRD patterns indicated that the crystalline structures of all the samples were consistent with the standard scheelite structure of CaMoO₄. The structural parameters of the pure phase of CaMoO₄:Sm³⁺ phosphor were obtained from the Rietveld analysis. Red PL attributed to the ⁴G_5/2 → ⁶H_9/2 transition of Sm³⁺ was observed at 646 nm when the CaMoO₄:Sm³⁺ samples were excited by 404 nm using a monochromatized Xenon lamp. Furthermore, orange-red color tunable emission has been achieved by substitution of different anionic groups (BO₃^?3, PO₄^?3 and SO₄^?2) into the CaMoO₄:Sm³⁺ phosphors. Among all the studied phosphors, the CaMoO₄-SO₄:Sm³⁺ phosphor showed the strongest PL emission compared to all other phosphors suggesting that it is a promising potential candidate for red emission in the near UV excited white LED applications.

Full text of DOI:10.1016/j.jallcom.2017.08.117

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Accepted Manuscript
−2
Structural evolution induced by substitution of designated molybdate sites (MoO
)
4
−3
−3
−2
and SO ) in CaMoO :Sm
4
3+
with different anionic groups (BO
, PO
3
4
4
phosphors-A study on color tunable luminescent properties
A. Balakrishna, H.C. Swart, R. Ramaraghavulu, A.K. Bedyal, R.E. Kroon, O.M.
Ntwaeaborwa
PII:
S0925-8388(17)32846-3
10.1016/j.jallcom.2017.08.117
JALCOM 42874
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 14 June 2017
Revised Date: 9 August 2017
Accepted Date: 13 August 2017
Please cite this article as: A. Balakrishna, H.C. Swart, R. Ramaraghavulu, A.K. Bedyal, R.E. Kroon, O.M.
−2
Ntwaeaborwa, Structural evolution induced by substitution of designated molybdate sites (MoO
)
4
−3
−3
−2
and SO ) in CaMoO :Sm phosphors-A study
4
3+
with different anionic groups (BO
, PO
3
4
4
on color tunable luminescent properties, Journal of Alloys and Compounds (2017), doi: 10.1016/
j.jallcom.2017.08.117.
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ACCEPTED MANUSCRIPT
Structural evolution induced by substitution of designated molybdate
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-3
-3
-2
sites (MoO4 ) with different anionic groups (BO3 , PO4 and SO4 ) in
CaMoO4:Sm3+ phosphors-A study on color tunable luminescent properties
A.Balakrishna1*, H.C. Swart1* R. Ramaraghavulu3, A.K. Bedyal1, R.E. Kroon1, O.M. Ntwaeaborwa2*
1Department of Physics, University of the Free State, Bloemfontein, ZA-9300
2School of Physics, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa
3Department of Materials Science and Engineering, Korea University, Anam-ro, Seoul, 02841, South Korea
Abstract
A series of novel red emitting CaMoO4:Sm3+ (1.0 mol %) phosphors substituted with different
-3
-3
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anionic groups (BO3 , PO4 and SO4 ) were prepared using a high temperature solid state
reaction method. The effects of anionic substitution on the crystalline structure and
photoluminescence (PL) properties of the CaMoO4:Sm3+, CaMoO4-BO3:Sm3+, CaMoO4-
PO4:Sm3+and CaMoO4-SO4:Sm3+ phosphors were investigated. The structure, particle
morphology, chemical composition, vibration modes, and PL properties of the phosphors were
investigated by X-ray diffraction (XRD), Field emission scanning electron microscopy (FE-
SEM), energy dispersive spectroscopy (EDS), Fourier transform infrared spectrometry (FT-IR)
and PL spectroscopy, respectively. The XRD patterns indicated that the crystalline structures of
all the samples were consistent with the standard scheelite structure of CaMoO4. The structural
parameters of the pure phase of CaMoO4:Sm3+ phosphor were obtained from the Rietveld
4
analysis. Red PL attributed to the G5/2→6H9/2 transition of Sm3+ was observed at 646 nm when
the CaMoO4:Sm3+ samples were excited by 404 nm using a monchromatized Xenon lamp.
Furthermore, orange-red color tunable emission has been achieved by substitution of different
anionic groups (BO3 , PO4 and SO4 ) into the CaMoO4:Sm3+ phosphors. Among all the
studied phosphors, the CaMoO4-SO4:Sm3+ phosphor showed the strongest PL emission
compared to all other phosphors suggesting that it is a promising potential candidate for red
emission in the near UV excited white LED applications.
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-3
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Keywords: CaMoO4 powders, anionic group, White-LED lightning, molybdate
Corresponding authors E-mail addresses: balakrishna.veera@gmail.com, swarthc@ufs.ac.za
and ntwaeab@gmail.com

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1. Introduction:
Today, light-emitting diodes (LEDs) are widely used in solid state lighting thereby
generating research interest in the development of advanced materials that can serve as source of
light in LED applications. Since the realization of GaN or near-ultraviolet (NUV) based light-
emitting diode (LED), solid-state lighting emission based on phosphor converted (pc) white
LEDs have generated interest because of their great advantages over the conventional
incandescent and fluorescent lamps [1–6]. The general approach of producing white light is by
combining a blue LED chip with a yellow color emitting phosphor (YAG:Ce3+). However, this
combination has a low color rendering index factor (CRI) because of the poor red component in
the YAG:Ce3+ phosphor. The combination of a NUV LED with red, green and blue color
emitting phosphors is another alternative that may provide a high CRI factor [7]. Therefore, the
development of new red-emitting phosphors that can be excited by a near ultraviolet (NUV)
(350–420 nm) LED is necessary. Rare earth ions (RE3+) doped molybdate phosphors have been
evaluated extensively and they have made significant advances in white LEDs [8–11]. Among
the rare-earth ions, the Sm3+(4f5) ion is one of the most promising sources of red light. The red
4
emission associated with the G5/2→6H9/2 transition of Sm3+ exhibits a relatively high quantum
efficiency. In addition, the Sm3+ ions exhibit different quenching emission channels defined from
the features of the excited state relaxation dynamics of the luminescence in the host materials
[12,13]. Compared to other available rare earth dopants, Sm3+ ions are very popular because of
its reddish orange emission in the visible region. Sm3+ ions belongs to the 4f5 configuration and
in any crystal field perturbation it is doubly generated. The excitation spectra of Sm3+ ions doped
materials covers the UV, blue and bluish-green spectral range Therefore it is possible to get the
reddish orange emission by excitation in this region. The emission profile of Sm3+ ions is narrow
and it shows longer life times, similar to Eu3+ ions. Sm3+ ions doped materials are, however, a
better alternative. The reddish-orange fluorescence from the Sm3+ ions is aligned to the emission
peak of the NUV-emitting InGaN chips. Today, the development of Sm3+ doped phosphors for
NUV LEDs such as NaGd(WO4)2:Sm3+[14], Ba2CaWO6:Sm3+[15], Ba3La(PO4)3:Sm3+ [16] and
La2WMo2O9:Sm3+[17] has been reported. Molybdates are being considered as attractive host
materials due to their chemical and thermal stability and excellent luminescent properties, among
other things. Recently, Sm3+-doped molybdates, such as CaLa(MoO4)2:Sm3+ [18], CdMoO4:Sm3+
[19] and LaMoBO6:Sm3+ [20] have been investigated extensively as potential red-emitting

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phosphors for NUV GaN chip-based W-LEDs. In addition, alkaline-earth metal molybdates have
important applications in various fields that involve optical fibers, scintillator materials,
catalysis, lithium ion batteries, sensors, detectors, light emitting devices and microwave systems
devices [21–25]. For luminescent application, CaMoO4 is an excellent material due to ease of
energy transfer (ET) from the MoO4 absorption band to the excited states of the luminescent
centres (lanthanide-ions) [26,27].
Calcium molybdate CaMoO4 is a typical scheelite compound and has a tetragonal
symmetry with space group I41/a [28–30]. In general, it is an excellent host lattice as it can be
excited via the O/Mo ligand-to-metal charge transfer (LMCT). CaMoO4 has been doped with
different rare-earths ions such as Tb3+, Sm3+ and Eu3+ resulting in phosphors that emit green,
orange and red light respectively [31,32]. Up to now, there are only a few reports on the Sm3+
doped CaMoO4 phosphor [33–35] and their luminescence properties are not good-enough for
LEDs application. Therefore, some efforts are necessary to improve the luminescence intensity
of Sm3+ doped molybdate based phosphors. Xi and co-workers synthesized Na5Eu(MoO4)4-
x(PO4)x (x≤0.10) phosphors by solid-state reaction and observed the PL emission intensity from
Na5Eu(MoO4)3.96(PO4)0.04 that was 5.0 times higher than that of commercial Y2O2S:Eu3+
phosphor [36]. In a study conducted by Zhang et al, an improvement in the red PL emission
2-
2-
intensity of NaEu(MoO4)2 and anionic (SO4 and SiO3 )-doped NaEu(MoO4)2 was observed
2-
[37]. Furthermore, the crystal sites of MoO4 ions in improved red photoluminescence was
2-
3-
observed from NaLa(MoO4)2:Eu3+ due to incorporation of SO4 or BO3 anions [38]. We
2−
therefore, substituted different anionic groups into the MoO4 sites to improve the emission
intensity of the CaMoO4:Sm3+.
In this study, CaMoO4:Sm3+, CaMoO4-BO3:Sm3+, CaMoO4-PO4:Sm3+ and CaMoO4-
SO4:Sm3+phosphors were prepared by the conventional solid state reaction method. The effects
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-2
of (BO3 , PO4-3 and SO4 ) substitution on the PL properties of CaMoO4:Sm3+ phosphors and the
possible mechanism for the PL intensity enhancement are discussed in detail.
2. Experimental
2.1 Sample preparation