Remarkable influence of alkaline earth ions on the enhancement of fluorescence from Eu3+ ion doped in sodium ortho-phosphate phosphors

Journal Pre-proof Remarkable influence of alkaline earth ions on the enhancement of fluorescence from Eu3+iondopedinsodiumortho-phosphate phosphors

A. Balakrishna, L. Reddy, O.M. Ntwaeaborwa, H.C. Swart PII:

S0022-2860(19)31484-X

DOI:

https://doi.org/10.1016/j.molstruc.2019.127375

Reference:

MOLSTR 127375

To appear in:

Journal of Molecular Structure

Received Date:

27 June 2019

Accepted Date:

06 November 2019

Please cite this article as: A. Balakrishna, L. Reddy, O.M. Ntwaeaborwa, H.C. Swart, Remarkable influence of alkaline earth ions on the enhancement of fluorescence from Eu3+ iondopedinsodiumortho-phosphate phosphors, Journal of Molecular Structure (2019), https://doi.org /10.1016/j.molstruc.2019.127375

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Remarkable influence of alkaline earth ions on the enhancement of fluorescence from Eu3+iondopedinsodiumortho-phosphate phosphors A. Balakrishna1*, L. Reddy1, O.M. Ntwaeaborwa3*and H.C. Swart2* Department of Physics, University of Johannesburg, Doornfontein Campus ZA2028, South Africa Department of Physics, University of the Free State, P.O. Box 339, Bloemfontein, ZA9300, South Africa 3School of Physics, University of the Witwatersrand, Private Bag 3, Wits, 2050, South Africa 1

2

Abstract: A series of different alkaline earth based sodium-ortho phosphate (NaMPO4, where M= Mg, Ca, Sr and Ba) phosphors doped with 1mol% of Europium (Eu3+) were prepared by solution combustion method. The phase evolution controlled morphology and fluorescence properties were systematically examined by various techniques such as X-Ray diffraction (XRD), field emission scanning electron microscope (FE-SEM) and photoluminescence excitation (PLE) and photoluminescence (PL) spectroscopy. Our materials crystallized into monoclinic and orthorhombic phases that were found to be consistent with those listed in the standard JCPDS cards. The XRD results confirmed that the samples contained a mixture of phases in the crystals. The PLE spectra of the NaMPO4 phosphors consisted of a broad band with a maximum at 278 nm, while the Eu3+ doped phosphors contained a similar broad band of lower intensity and characteristic Eu3+excitation bands. These phosphors could be efficiently excited by near ultraviolet at 393 nm and were found to exhibit excellent red emission at 593 nm corresponding to the 5D07F4 transition of the Eu3+ ion. The calculated Commission Internationale de l’Eclairge (CIE) chromaticity color coordinates were calculated. The color of the emitting light could be tuned from orange-red to red through incorporation of different alkaline metal ions. All the results suggested that the examined sodium-ortho phosphates could potentially be used as sources of red light in tricolor white light emitting diodes.

Keywords: Eu3+ ion, alkaline cations, photoluminescence (PL) excitation spectra, Red color emission Corresponding author E-mail addresses: [email protected],

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Introduction: A great interest in phosphors has resulted in the rapid developments of a variety of display and other related illumination technologies. For general lighting applications, such as UV light-emitting diodes, various hosts of photoluminescent (PL) materials including silicates, aluminates, aluminosilicates, nitrides, borates, etc., play a vital role [1-4]. Among all these hosts studied, phosphates are considered excellent hosts for preparing phosphors due to advantages such as low synthesis temperature, ease of preparation and incorporation dopant ions, good chemical and thermal stability and high luminescent brightness. Different researchers have successfully synthesized and studied rare earths doped phosphate phosphors for various applications including solid state lighting and photodynamic therapy [5-7]. Orthophosphates with the general formula AMPO4 (where A= alkali and M = alkaline earth metal) have emerged as preferred host matrices for rare-earths dopant ions to prepare phosphors due to their excellent properties such as large band gaps and high absorption of the PO43- in the vacuum ultraviolet (VUV) region, ease of incorporation of dopants, moderate phonon energy, high thermal and chemical stability, exceptional optical damage threshold and low sintering temperatures[8–11]. Among various tri-valent rare-earths ions, Eu3+ is the preferred source of red light due to, among other things, its simple energy level structure that is used to probe the local environment of the Eu3+ ion based on the relative intensities of the transitions in the absorption and luminescence spectra. Additionally, a series of alkaline earth ions such as Mg+2, Ca+2, Sr+2 and Ba+2 have played a key role in enhancing luminescent efficiency of phosphate-based phosphors by modifying their chemical composition and charge compensation density [12, 13]. In the alkali-alkaline earth orthophosphate group with general formula AMPO4, NaMgPO4, NaCaPO4, NaSrPO4 and NaBaPO4are widely used as host lattice for the preparation of phosphors used in phosphor-converted-white light emitting diodes (WLEDs) [14].For example, Eu3+doped NaCaPO4[15], LiZnPO4 [16], LiBaPO4[17], NaBaPO4[18], KBaPO4[19], and KSrPO4[20] have recently been reported and they are emerging as a new class of luminescent materials with the potential application in w-LEDs. It is necessary to elucidate extensively on the PL emission properties of different alkaline metal based orthophosphates doped with rare-earths (REs). It is a well-known phenomenon that some RE ions are very sensitive to the crystal structure, phase transitions and chemical surroundings of the host lattice[21]. In particular,Eu3+ ions are used as luminescent activators in various inorganic phosphors that emit red light commonly in tricolor 2

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fluorescent device and can also be used as a very efficient structural probe that can reveal some important information about the host lattice[22]. The aim of the present work was to study the specific effects of alkaline metal ions on the PL properties of Eu3+ doped NaMPO4 phosphors. Their phase structure, particle morphology and luminescence properties were investigated for a potential application as sources of red light in tricolour w-LEDs. The PL properties of Eu3+ ions were investigated by measuring excitation and emission spectra. The influence of alkaline cations on the red emission of the 5D07F2 transition of the Eu3+is reported.Our results indicate that the phosphor with the host lattices incorporating Ca2+displayed intense red emission compared to those incorporating Mg2+, Sr2+ and Ba2+. Materials and methods: 2.1 Sample preparation: The samples were synthesized via solution combustion method. The starting materials were Mg(NO3)2.6H2O) calcium nitrate Ca(NO3)2.6H2O, strontium nitrate (Sr(NO3)2and barium nitrate

Ba(NO3)2,

ammonium

di-hydrogen

phosphate

(NH4H2(PO4),

sodium

nitrate

(NaNO3.9H2O) and urea (NH2-CO-NH2). These were ground using pestle and an agate mortar and dissolved in de-ionized water. The solutions were stirred vigorously for 45 min. until homogeneous solutions were obtained. The molar ratios of theEu3+were varied in the samples with respect to the Mg/Ca/Sr/Ba ions. For various compositions of the metal nitrates (oxidizers), the amount of urea (fuel) was calculated with the total oxidizing and reducing valences equal to unity, so that the heat liberated during combustion was maximized. The solutions were transferred to a furnace preheated to 600oC, followed by the formation of fluffy porous products. After cooling down to room temperature, the samples were ground again to get fine powders and were ready for characterization. 2.2 Characterization: The crystal structure and phase composition of the samples were examined by powder Xray diffractometer using a Bruker AXS D8 Advance X-ray diffractometer using a nickel-filtered CuKα target (λ = 0.154056 nm). The data were recorded in a wide range of Bragg angles 2θ (15◦ ≤ 2θ ≤ 70◦ ) with a counting rate of 1s for each step size of 0.00756043o. The particle morphology study was carried out using a JEOL JSM-7800F field emission scanning electron 3

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microscope (FE-SEM) coupled with an Oxford Aztec 350 X-Max80 energy-dispersive X-ray spectroscope (EDS) for chemical composition analysis. Fourier transform-Infrared spectra (FTIR) were recorded with a Nicolet 6700 Fourier transform infrared spectrometer with a KBr pellet technique in the spectral range from 4000 to 500 cm-1. UV–Vis diffusion reflectance spectra were recorded using a Perkin Elmer Lambda 950 UV–vis spectrometer in the range of 300–800 nm. The PL excitation and emission spectra, and lifetime were measured using the Edinburgh Instruments FLS980 Fluorescence Spectrometer using a monochromatized Xenon flash lamp as an excitation source. The chemical composition and electronic states were analyzed using the PHI 5000 Versa probe (the X-ray beam was a monochromatic AlKα lines with 100 µm diameter, 25 W, 15 kV). All measurements were performed at room temperature.

3. Results and discussion: 3.1. Structural and morphological studies: Fig. 1 shows the XRD patterns of Eu3+ doped NaMPO4(M= Mg, Ca, Sr and Ba) phosphor samples. Some of the patterns are consistent with the standard JCPDS data files as indicated in the diagrams. These phases are α–monoclinic phase with space group pn21a for NaMgPO4 and NaSrPO4 and orthorhombic phase with space group P-3m1 for NaCaPO4 and NaBaPO4 [23-26]. The impurity phases were minimal for NaCaPO4 and NaBaPO4 indicating a successful crystallization of the major phases. For NaMgPO4 and NaSrPO4 samples, there is a noticeable mismatch with the standard data files in terms of peak position and peak intensity as shown in Fig 1 (a) and (c). The characteristic diffraction patterns of NaMgPO4 and NaSrPO4 phases were shifted to lower angles, and there are additional peaks which we attribute to secondary phases associated with Na3PO4 and EuPO4 [27-28]. As the heating temperature of the combustion furnace (600oC) was the same for all the as-synthesized NaMPO4-Eu3+(M= Mg, Ca, Sr and Ba) phosphors, the characteristic diffraction patterns of NaMgPO4 and NaSrPO4 phases were shifted to the lower angles, while these diffraction peaks of the impurity phases became stronger and dominate the XRD patterns of the as-synthesized samples [29]. This also indicates that a higher calcination temperature is needed to obtain pure crystalline phases. The incorporation of Eu3+ did not cause any significant changes to the major structures, probably due to similarities between the ionic radii (r) of Eu3+(r = 0.94 Å) and the different cationic ions. Eu3+ is more likely to

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substitute Ca2+ ion (r = 1.12 Å) due to close similarities in their ionic radii than Mg+2 (r = 0.86 Å), Sr2+(r = 1.32 Å) and Ba2+ (r = 1.49 Å) ions.

a)

b)

c)

d)

Fig.1. X-ray diffraction patterns of(a) NaMgPO4-Eu3+ (b)NaCaPO4-Eu3+ (c)NaSrPO4-Eu3+ and (d) NaBaPO4-Eu3+phosphor samples along with their respective standard JCPDS data files. 3.2. Surface morphological analysis with EDS: The SEM micrographs of the NaMPO4phosphors are presented in Fig. 2 (a)-(d). All the micrographs, except that of Fig 2 (b), show that the particles were made of rod-like particles that were randomly aligned. Although all the samples were prepared using the same method, the NaCaPO4-Eu3+ (Fig 2(b)) irregular and relatively bigger particles, suggesting that their growth mechanism was different from that of rod-like morphology of Fig (a), (c) and (d).The Energy Dispersive Spectroscopy (EDS) spectra, Fig 3(a)-(d) and the elemental maps (Fig 3(e)-(h)) were 5

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recorded and they confirm the presence of all the elements (Na, O, P, Mg, Ca, Sr, Ba and Eu) present in our samples and their homogeneous and uniform on the surface.

Fig. 2. SEM micrographs of the (a)NaMgPO4-Eu3+(b)NaCaPO4-Eu3+(c)NaSrPO4-Eu3+ and (d)NaBaPO4-Eu3+phosphor samples.

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Fig. 3EDS spectra and elemental mapping of the (a)&(e)NaMgPO4-Eu3+(b)&(f)NaCaPO4Eu3+(c)&(g)NaSrPO4-Eu3+ and (d)&(h)NaBaPO4-Eu3+phosphor samples. 3.3. Surface characterization by X-ray photoelectron spectroscopy (XPS): The XPS survey spectra of the NaCaPO4-Eu3+ phosphors before and after Ar ion sputtering are presented in Fig. 4. The spectra confirm the presence of all the chemical elements, namely Na 1s, P 2p, O 1s, C 1s, Mg 1s, Ca 2s, Sr 3p, Ba 4d and Eu 4d in our materials.

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Fig.4XPS survey scan spectra of NaCaPO4-Eu3+before and after Ar ion sputtering. Fig. 5(a)-(d) shows the high resolution XPS spectrum of Na1s, P 2p and O 1s in NaCaPO4Eu3+before and after sputtering. Fig. 5 (a) depicts the Na 1s peak with a maximum at 1071.7 eV, which indicates oxidation state of +1 [30-32]. Fig. 5 (b) depicts the P 2p peak with maximum binding energy at 133.4 eV, suggesting that phosphorus was in the form of pentavalent oxidation state (P5+) [33].The de-convoluted O 1s profile of the NaCaPO4is presented in Fig. 5 (c) and (d) and it shows three peaks with the maximum binding energy at 531.2 eV and minor peaks at 532.0 eV and 530.4 eV associated respectively with O-Ca-O bonding, O-P-O bonding of the PO4-3 network and Ca-O bonding [34-37].

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b)

Counts/s

a)

O-Ca-O

c)

O-Ca-O

d) O-P-O

Ca-O

O-P-O

Ca-O

Binding Energy (eV) Fig.5. The high resolution XPS scan spectra of (a) Na1s (b) P 2p and Gaussian fit of (c)O 1s before and d)O 1s after sputtering for the NaCaPO4-Eu3+ phosphors. The high resolution XPS peak of Ca 2p core levels with binding energies of 347.2 and 350.7 eV, are attributed to the spin-orbit splitting of the Ca(2p) components, namely Ca(2p3/2) and Ca(2p1/2) as shown in Fig. 6 (a). These confirm that Ca2+ ions occupied two different lattice sites [38, 39]. The C 1s is located at the binding energies (BE) ranging from 283.1 eV to 286.8 eV. The C 1s curve shows a strong peak at 284.9 eV, which is assigned to the aliphatic chain (C–C), which is consistent with the residual carbon and the carbon concomitant in the vacuum chamber. Similarly, the high-resolution XPS scan spectra for the Eu 4d and Eu 3d regions are shown in Fig. 6 (d), respectively. The presence of two peaks at 1133.5 and 1161.7 eV is indicative of the Eu3+ 3d5/2 and 3d3/2 core levels, respectively. Both peaks are due to multiple spin-orbit interactions and are consistent with the binding energy values reported elsewhere [40-42]. Additional peaks are also present at the low binding energy side of Eu3+ ions shown in Fig. 6 (c). 9

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A broad peak at around 1167.1 eV corresponds to Eu 4d3/2 which is attributed to the spin-orbit splitting and the well-known peak around 1135.4 eV associated with Eu2+ (4d5/2) was not observed. The high signal-to-noise ratio in Fig 6 (d) is due to the low concentration of Eu in the samples.

b)

Counts/s

a)

c)

d)

Binding Energy (eV) Fig.6 (a)-(d). The high resolution XPS spectra of (a) Ca 2p (b) C 1s (c) Eu 4d and (d) Eu 3d. 3.4. Fourier transform infra-red spectroscopic studies: The FT-IR spectra were recorded in the range of 400-4000 cm-1are shown in Fig. 7. The peaks at 3472, 1388, 1038, 839, 728, 563 and 488 cm-1 are attributed to the (PO4)−3 vibrational bands [43, 44]. A series of bands between 950 and 1150 cm-1 are associated with PO4-3 ions and correspond to asymmetric stretching vibrations. The IR absorption band of (PO4)3− are located at 10

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650–540 cm−1. Three significant bands were observed at frequencies 578 and 1038 cm-1 [45, 46]. The narrow band at 563 cm-1 is due to the O–P–O asymmetric bending vibrations, while the prominent band at 1038 cm-1 is due to the asymmetric stretching vibrations of [PO4]3tetrahedron[47]. A medium IR band at 839 cm–1 indicates the wagging modes of vibration of the coordinated water and the metal-oxygen bond in the complex.

The peak at 1388 cm-1 is

attributed to the asymmetric stretching of C–O bonds[48]. For the FT-IR spectra of all studied samples, a broad band centered in 3472 cm-1 can be attributed to the adsorbed moisture while the 1983 cm-1 is assigned to the H–O–H bending vibration mode of the H2O molecule.

Fig. 7.FT-IR spectra of combustion-driven phosphors (NaMPO4, where M= Mg, Ca, Sr and Ba) doped with Eu3+ ions. 3.5. UV-VIS-NIR diffuse reflection spectroscopy-optical band gap studies: Fig. 8 presents the reflection spectra of alkaline based NaMgPO4, NaCaPO4, NaSrPO4 and NaBaPO4 phosphors doped with Eu3+ ions. Typical absorption bands attributed to Eu3+ ion transitions from ground state 7F0 and 7F1 to different excited state levels were observed and were assigned according to the literature cited [49, 50]. Generally, phosphors with high absorption efficiency in the 350-400 nm spectra region of UV-excitation and suitable transmittance efficiency in further visible bands have potential applications in w-LEDs. 11

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Fig.8.The UV–visible DRS spectra of different alkaline sodium-phosphate-phosphors (NaMPO4, where M= Mg, Ca, Sr and Ba) doped with Eu3+ ions. The absorption peaks around 350 nm are due to the 7F05L6 transitions of Eu3+. From the DRS spectra, it is noticed that the full width at half-maximums (FWHMs) of all Eu3+ ions absorption bands become wider, compared with those reported in the literature[51, 52]. The wider FWHMs of absorption bands of Eu3+ in NaMPO4:Eu3+ are helpful in allowing the excitation wavelength shift of UV light emitting sources.

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a)

b)

c)

Photon Energy h (eV) Fig.9. Energy band gap calculations for different alkaline sodium-phosphate-phosphors (NaMPO4, where M= Mg, Ca, Sr and Ba) doped with Eu3+ ion. The line represents the best linear fit to determine the energy band gap. The optical band gap calculations were explained by the Kubelka-Munk theory [53] and were discussed extensively in our previous work [28]. From the plot of [F(R)hν]2 versus hν, the value of Eg was obtained by extrapolating the linear fitted regions to [F(R)hν]2=0 as shown in Fig. 9. According to the K–M function, the relationships between (αhν)2 and photon energy demonstrated that the band gap energies for all studied Eu3+ doped NaMgPO4, NaCaPO4, NaSrPO4 and NaBaPO4 phosphors were found to be 3.26, 3.21, 3.97 and 3.95 eV, respectively. Among all the prepared phosphors, the largest and least band gap energy was observed from NaSrPO4-Eu3+ and NaCaPO4-Eu3+ phosphors, respectively, as shown in Fig. 9. So, the different energy band gap observed in all phosphors may be due to the different crystalline morphology of 13

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the material. Furthermore, it can be seen from Fig. 9 that the band gap energies have increased from 3.213.97 eV in the case ofNaMgPO4:Eu3+ and NaBaPO4:Eu3+. The variation of energy band gap values for this studied phosphor series may be due to dissimilar crystalline size of the material based on different alkaline metal ions [54-56]. 3.6. Photoluminescence spectroscopic studies of Eu3+ ion in NaMPO4 phosphors: Fig. 10 (a) & (b) present the PLE and PL spectra of theNaMPO4-Eu3+ phosphors. The PLE spectrum obtained when monitoring the emission at 613 nm shows a broad band extending from 240 to 530 nm and several sharp peaks in the range of 240–320 nm. The broad band was attributed to the charge transfer band (CTB) of O2−→Eu3+ transition, while the sharp peaks were attributed to the 4f–4f forbidden transitions of the Eu3+, namely, 316 nm (7F05H6), 361 nm (7F05D4), 394 nm (7F05L7), 422 nm (7F05L6), 464 nm (7F05D3) and 524(7F1 5D0). This is a good indication that this phosphor can be efficiently pumped by a near-UV chip. Except for the peak intensities, all the spectra are the same indicating that all the excitation peaks were due to the f→f transitions of Eu3+. The most intense excitation peak at 394 nm was noticed from the NaCaPO4-Eu3+ and the least intense was recorded from the NaBaPO4-Eu3+ in the visible region. This is consistent with the fact that the ionic radii of Ca2+ and Eu3+ are similar therefore more Eu3+ could effectively substitute Ca2+ in the host lattice compared to other cations. The PL emission spectra of the different alkaline earth metal ion based NaMPO4-Eu3+ phosphors measured when exciting at 394 nm are shown in Fig. 11. Intense characteristic emission peaks of Eu3+ at 572, 591, 613, 651 and 700 nm assigned to the 5D0 7F0, 5D0 7F1, 5D0 7F2, 5D0 7F3 and 5D0 7F4 transitions were observed under 394 nm excitation. The most intense emission at 613 nm is assigned to the 5D0 7F2 transition. Except for the peak intensities, all the spectra are the same indicating that all the excitation peaks were due to the f→f transitions of Eu3+.

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a)

b)

Fig.10. (a) Room temperature PLE and PL spectra ofNaBaPO4:Eu3+(b)PL spectra of thefour different NaMPO4-Eu3+. The PLE spectra were measured at emi = 613 nm. The PL spectra were measured at exc = 394 nm. Moreover, the variation in intensity of emission peaks with different alkaline metal ions follows the order: Ca2+>Mg2+>Sr2+>Ba2+. The observed emission peaks indicate that the Eu3+ ion occupied the different alkaline ion sites with lower symmetry. Finally radiative emission took place from the 5D0 state to lower lying states. Fig. 11 schematically shows the energy level diagram and the proposed relaxation mechanism for the luminescence emission in NaMPO4:Eu3+ phosphor materials under 394 nm excitation.The PL emission is due to electron transitions 15

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between energy levels that involve excitation of electrons from the ground states to the excited level 5L6 of Eu3+ after absorption of primary excitation photons and subsequent non-radiative relaxation to the lower 5D0 energy level of Eu3+ ion. A fast non-radiative (NR) multiphonon relaxation from excited states to the 5D0 level is followed by radiative emission corresponding to 5D

7 0 FJ

(J = 0, 1, 2, 3, 4) transitions of Eu3+. Emissions from 5D1, 5D2 and 5D3 levels were not

observed probably due to higher phonon energy of prepared phosphor material system.

Fig.11. The schematic energy level diagram of the Eu3+ ions showing the possible emission transitions and non-radiative decays mechanism in Eu3+doped four different alkaline sodium-phosphate-phosphors (NaMPO4 where M= Mg, Ca, Sr and Ba). 3.7. Fluorescence lifetime measurements: The PL decay curves and lifetimes of Eu3+ for NaMPO4-Eu3+ where M= Ca, Sr and Ba phosphors were recorded when monitoring the 613 nm (5D0 7F2) emission upon the excitation at 394 nm and are shown in Fig. 12. All the decay curves except that for NaMgPO4:Eu3+were fitted successfully with a second-order exponential decay equation:[57]

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()

I(t) = I0 + 𝐴𝑓exp –

t

𝑓

( )

+ 𝐴𝑠exp ―

t

(1)

𝑠

Where I is the luminescence intensity, τf and τs are the fast and slow decay times respectively, and Af and As are their respective weight. The average lifetime was calculated using the equation [58].

𝑎𝑣𝑔 =

(𝐴𝑓2𝑓 + 𝐴𝑠2𝑠 ) 𝐴𝑓𝑓 + 𝐴𝑠𝑠

(2)

From the decay curve analysis, the emission from the 5D0 level is not completely radiative, as there should also be non-radiative contributions. The energy transfer between Eu3+ ions or from multiphonon relaxation or from both contributed to the non-radiative transitions. The long decay time of the 5D0 level reflects the partially allowed Eu3+ 4f4f transitions. The values of average lifetime (τavg) as calculated from the fitted curves were 2994, 2860, 2633 and 1786 s for NaMPO4-Eu3+ where M= Mg, Ca, Sr and Ba phosphors respectively. The lifetime values of Eu3+ are in the range of microseconds due to the forbidden nature of the f–f transition [59].The life time value of the 5D0 level measured for the NaMgPO4-Eu3+ and NaCaPO4-Eu3+ phosphors are higher than those of the rest of the phosphors, suggesting that the magnesium and calcium phosphate host is suitable for the Eu3+ doping with low non-radiative energy. This is also consistent with the earlier observation where NaCaPO4 gave relatively higher PL intensity, confirming easy substitution of Ca+2 by Eu3+due to similar ionic radii [60, 61]. Considering higher substitution of the Eu3+ ions, in the Sr2+ and Ba2+ sites in NaMPO4-Eu3+, the probability of energy transfer to luminescent killer sites increased. As a consequence, the lifetimes will be shorten by favouring the non-radiative energy transfer processes when compared to NaMgPO4Eu3+and NaCaPO4-Eu3+phosphors, which is consistent with previous results discussed in other systems [62-67].

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Fig.12The luminescence decay curves of Eu3+ ion in different alkaline sodium-phosphatephosphors (NaMPO4 where M= Mg, Ca, Sr and Ba) by monitoring 613 nm from the transitions of 5D0 7F2 under the excitation of 394 nm. 3.8. CIE chromaticity color co-ordinate diagram: Commission Internationale de l'Eclairage (CIE) coordinates is frequently used to analyze and express colours of phosphors. The CIE coordinates (x, y) can be obtained from tristimulas values X, Y and Z (which were obtained by color matching functions and spectral power distribution) using formulae reported in previously [68]. Based on the corresponding PL spectra, the CIE software was used to calculate the chromaticity coordinates of the NaMPO4:Eu3+ (where M= Mg, Ca, Sr and Ba) phosphor compositions and are shown in Fig. 13. The calculated CIE chromaticity coordinates were (0.631, 0.372), (0.634, 0.376), (0.602, 0.392) and (0.604, 0.394), respectively, for Mg, Ca, Sr and Ba incorporated phosphors. These results show that the chromaticity coordinates of all samples are lying in the red region. It is also clearly observed that samples of different alkaline metal based sodium phosphate phosphors all emit red light. 18

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Fig.13. CIE chromaticity coordinates for the different alkaline sodium-phosphatephosphors (NaMPO4, where M= Mg, Ca, Sr and Ba) doped with Eu3+ ion. Conclusions: A series of NaMPO4-Eu3+powder phosphors, where M= Mg, Ca, Sr and Ba were successfully prepared using combustion method. Their crystal structure and optical properties were studied. The optical band gaps were calculated by DRS spectra and the Kubelka-Munk function. The band gap energy values were different for NaMPO4-Eu3+ phosphors and this was attributed to different crystalline morphology of the material. The SEM images exhibited that the powders consisted of randomly aligned nanorods. The absorption peaks obtained in the FT-IR spectra confirmed the presence of water of crystallization, symmetric as well as asymmetric stretching and bending vibration of PO4 units and metal-oxygen bonds. The NaCaPO4-Eu3+ display strong red emission at 611 nm ascribed to the 5D07F4 transition of the Eu3+ ion. The PL properties of the resultant phosphors were strongly dependent on the calcined temperature and

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were also influenced by the different alkaline metal ions. The PL results suggest that the NaCaPO4-Eu3+ phosphors can potentially in various LED applications. Acknowledgements: This research was financially supported by PDRFs, Global Excellence and Stature Funding (GES) programme, University of Johannesburg and Faculty of Science, University of Johannesburg, South Africa and the South African Research Chairs Initiative of the Department of Science and Technology and the National Research Foundation of South Africa (Grant no. 84415).OMN would like to acknowledge financial support from the Competitive Programme for Rated Researchers (CPRR) (Grant no. CPR20110724000021870) of the National Research Foundation (NRF)-South Africa.

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Declaration of interests (X)The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

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Different NaMPO4-Eu3+, where M= Mg, Ca, Sr and Ba phosphors were prepared via solution combustion method. The influence of different alkaline metal ions on the PL luminescent properties of NaMPO4-Eu3+ was discussed. Kubelka–Munk function analysis was used to determine the optical band gap energy values. Change in PL emission intensity was observed in NaMPO4-Eu3+ (where M=Mg,Ca, Sr and Ba) phosphors. The studied phosphors can be used potentially in WLEDs as a red component.