1. INTRODUCTION:

Phosphate compounds with the apatite structures are known to be capable of forming solid solutions by substitutions and also effective hosts for luminescence and laser materials. Lanthanide ions doped halo-phosphate compound with M₅(PO₄)₃X formula (apatite structure), where M is alkaline earth metals (Ca, Ba, Sr) and X is halogens (Cl, Br, F) are extensively studied due to their applications as light emitting diode (LED), phosphor, laser, fluorescent lamp, etc.^1-5. Apatite structure has two types of cationic site (M₁ and M₂), M₁ site has C₃ point group symmetry due to the tricapped trigonal prism formed by six oxygen ions of the PO₄ surrounding the cationic site, and M₂ site is coordinated with six oxygen ions and two X ions, with C_s point group^6-8 has observed luminescence of Eu³⁺ and Eu²⁺ from Eu doped M₅(PO₄)₃X where M = Ca²⁺, Sr²⁺, Ba²⁺ and X = F^-, Cl^-, Br^-, OH^-.

From this report it is highlighted that Eu³⁺ and Eu²⁺ can co-exist in halo-apatite and hydroxyl-apatite due to occupancy of metal sites (M₁, M₂) by Eu³⁺/Eu²⁺ ions; but dominant Eu²⁺ emission arises from M₂ site. Later on it is also reported that Eu²⁺ doped halo-apatite can be used as a potential phosphor for UV-LED⁵ reported that Eu²⁺ and Mn²⁺ co-activated Sr₅(PO₄)₃Cl to be a potential candidate for UV- LED based white LED. Zhang and Gong⁴ fabricated white light emitting diode (LED) by coating Ca₅(PO₄)₃Cl:Eu²⁺ as well as Ca₂BO₃Cl:Eu²⁺ onto 395 nm emitting InGaN chips. Since, white LED are seen as being the next generation of solid state lighting for their high energy efficiency and eco-friendly, researchers have keen interest to develop new phosphors with good luminescent properties to increase the efficiency of white LEDs. Halo-apatite host shows some interesting features when doped with Eu, as both Eu²⁺ and Eu³⁺ can co-exist in this host and the luminescent color can be tuned in a wide range of emission wavelength (450 – 650 nm)⁹^,10. Up to our knowledge, in most of the reported work halo-apatite were prepared in a high temperature by a solid state reaction. Thus, in this work we synthesized nanoparticles of Ba₅(PO₄)₃Cl:Eu³⁺co-activated with Li⁺ at relatively low temperature stabilized by ethylene glycol (EG). In the process, we found two types of barium phosphate compound, BaHPO₄ at pH 6 and Ba₅(PO₄)₃Cl at pH 11.5. The possibility of forming different barium compounds and the photoluminescence properties of the prepared samples are studied.

2. EXPERIMENTAL DETAILS:

2.1. Sample preparation

The starting materials are europium oxide (Eu₂O₃, 99.99% Alfa Aesar), barium nitrate (Ba(NO₃)₂, CDH), lithium carbonate (Li₂CO₃, CDH), ammonium dihydrogen phosphate (NH₄H₂PO₄, Sigma Aldrich). In typical preparation of Ba₅(PO₄)₃Cl: 10 at.% Eu³⁺ co-activated with 10 at.% Li⁺, 0.5 g of Ba(NO₃)₂, 0.0170 g of Li₂CO₃ and 0.0283 g of Eu₂O₃ are dissolved together with 1 ml of conc. HCl in a 250 ml round bottom flask. Excess acid is removed by evaporating three times with double distilled water. To this reaction medium, 50 ml of ethylene glycol (EG) and 0.2651 g of NH₄H₂PO₄ are added. Then the pH of the solution is adjusted from 6 to 11.5 using prepared NaOH solution (5 ml), and then the solution is refluxed for 3 h at 150 ^oC. The precipitate formed is separated with centrifugation at 12,000 rpm, washed with water once and with acetone for three times and dry in ambient atmosphere.

2.2. Characterization

Structural studies of the samples were examined using X-ray Diffractomer (Rigaku-Miniflex-II). The crystallite size (D) was calculated from Scherrer’s relation D = 0.9l/BCosq where l is the wavelength of x-ray, B half-width at peak intensity and q Bragg’s angle. CM-200 transmission electron microscopy (TEM) was used for recording images of particles. The samples were ground and mixed together with EG and dispersed under ultrasonication for 30 min. A drop of dispersed sample was put over carbon coated copper grid and evaporated to dryness under an IR lamp. Fourier transform infra-red (FT-IR) spectra of the samples were recorded using Shimadzu (IR Prestige 21) spectrometer using ZnSe sample holder. All the luminescence spectra of the prepared samples were recorded using Shimadzu (RF-5301 PC) equipped with source of Xenon discharge lamp.

3. RESULTS AND DISCUSSION:

3.1. Crystal structure study

The X-ray diffraction (XRD) patterns of the as-prepared compounds (10 at.% Li and 10 at.% Eu co-doped compounds) at different pH together with Joint Committee on Powder Diffraction Standards (JCPDS) card No. 72-1370 (for BaHPO₄) and 16-0686 (for Ba₅(PO₄)₃Cl) are shown in the Figure 1. The prepared samples show two different XRD patterns indicating that different species of barium phosphate are formed in this process with change in the pH of the medium. The XRD pattern at pH 6 or 8 corresponds to the BaHPO₄ and that at pH 11.5 corresponds to Ba₅(PO₄)₃Cl. Song Y et al.¹¹ reported that precipitation of phosphate solution depend on the pH of the precipitant solution since pH value influenced the supersaturation of the precipitant solution at certain phosphate concentration. At higher pH value the concentration of the basic species, such as BaPO₄^- and PO₄^3- increases and at lower pH value acidic species, HPO₄^2- and H₂PO₄^- increases. Below pH 10 formation of BaHPO₄ is dominant¹². Thus, the prepared samples are BaHPO₄ in the lower pH value and Ba₅(PO₄)₃Cl in the higher pH value.

Figure 1-XRD patterns of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu (10 at.%) at different pH.

Table 1 shows the calculated lattice parameter, unit cell volume and mean crystallite size of the as-prepared samples along with the corresponding value of pure BaHPO₄ and Ba₅(PO₄)₃Cl. The crystallite size of the as-prepared samples calculated using Scherrer equation decreases from 57 to 32 nm with increasing the pH value from 6 to 11.5. The unit cell volume of the as-prepared sample is less than the corresponding JCPDS value. It is suggested that the smaller Eu³⁺ ions (0.106 nm, C.N. = 8), and Li⁺ (0.092 nm, C.N. = 8) are occupying the sites of the larger Ba²⁺ ions (0.145 nm, C.N. = 8) in the host matrix¹³.

3.2. Microstructure study

TEM micrographs of the as-prepared samples (10 at.% Li and 10 at.% Eu co-doped compounds) at pH 6 and 11.5 are shown in Figure 2. The sample prepared in pH 6 shows large particles having diameter around 500 nm (Figure 2a). Each larger particle is composed of a cluster of smaller particles/crystallites due to agglomeration. Its selected area electron diffraction (SAED) pattern is shown in Figure 2b. While the samples prepared in the higher pH 11.5, the particles are well dispersed with length 55 to 179 nm and diameter in the range of 45 to 79 nm (Figure 2c). Its high-resolution transmission electron diffraction (HRTEM) image is shown in Figure 2d and corresponding plane/d-spacing is calculated. Also, the SAED pattern is shown in Figure 2e. The (hkl) planes are assigned.

Figure 2-TEM image of as-prepared 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu (10 at.%) prepared at (a) pH 6 (b) its SEAD pattern, and TEM (c) and HRTEM (d) images of sample prepared at pH 11.5, and (e) its SEAD pattern.

3.3. Characteristic vibration/phonon study

FT-IR spectra of the as-prepared samples (10 at.% Li and 5, 10 at.% Eu co-doped compounds) along with magnified spectra in the range of 2390 to 3500 cm^-1 are shown in Figure 3. Broad band around 3434 cm^-1 in the magnified spectra is attributed to the stretching vibration of O-H. Stretching vibration of C-H is observed at 2946, 2884 cm^-1 and wagging vibration band at 1247 cm^-1of ethylene glycol which is used as capping agent in the reaction process¹⁴. These bands are disappeared when the sample is heated at 800 ^oC. Bands at 2358 cm^-1 and around 1597 cm^-1 are attributed to CO₂ and CO₃ respectively due to the absorption from the atmosphere during the preparation process^{14, 15}. In the region between 534 to 1100 cm^-1, three significant bands of PO₄^3- can be assigned as υ₁- symmetric stretching mode of O-P-O at 978 cm^-1, υ₃ asymmetric stretching of P-O at 1064 cm^-1 and υ₄ asymmetric bending mode of O-P-O at 560 cm^-1. Slight change in peak position can be noticed in this region with change in pH and for the heated sample. It is obvious that the sample prepared in the lower pH is BaHPO₄ and the samples prepared at higher pH (above pH-10) is Ba₅(PO₄)₃Cl as indicated in the XRD study. Thus, weak bands observed for the sample prepared in pH 6 at 884 cm^-1 and at 581 cm^-1 can be attributed to the deformation vibration bands of P=O and P=O-H respectively¹⁶.

Figure 3- FTIR spectra of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu (5,15 at.%) prepared at pH 6, 11.5 and 800 ^oC heated sample.

3.4. Photoluminescence study

The photoluminescence characteristics of Li⁺ (10 at.%) co-activated Ba₅(PO₄)₃Cl:Eu³⁺ (10 at.%) nanophosphors are investigated by monitoring the excitation and emission spectra at room temperature. Figure 4 shows the excitation spectra of Li⁺ (10 at.%) co-activated Ba₅(PO₄)₃Cl: Eu³⁺ (10 at.%) by monitoring the emission wavelength at 592 nm and 615 nm. The excitation spectra consist of a broad intense absorption band between 220 and 280 nm with centre at 233 nm and various sharp weak peaks from 280 to 400 nm. The peak centred at 233 nm is attributed to the charge transfer from p-orbital of O to f-orbital of Eu (Eu-O CT). This arises due to the transition of 2p electrons of O^2- to the empty 4f orbitals of Eu³⁺ ions¹⁷. The different excitation bands obtained with centre at 323, 360, 378, and 393 nm correspond to ⁷F_0,1→⁵H_3,6, ⁷F_0,1→⁵D₀, ⁷E_0,1→⁵G₁, ⁵L₇, and ⁷F₀→⁵L₆ transitions of Eu³⁺ respectively^18-20. These peaks are weak as compared to Eu-O charge transfer band.

Figure 4- Excitation spectra of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu³⁺ (10 at.%) at two different emission wavelengths

Figure 5- Emission spectra of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu³⁺ (10 at.%) at different excitation wavelengths.

Figure 5 shows the emission spectra of Li⁺ (10 at.%) co-activated Ba₅(PO₄)₃Cl:Eu³⁺(10 at. % )at four different excitation wavelengths and for the host under 233 nm and 393 nm excitation. Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu³⁺ shows five main emission peaks, with centre at 483, 592, 615, 652 and 698 nm. The broad emission peak centred on 483 nm is observed for all the emission spectra including for those of host. Such peak from the host may arise due to the impurities/defect in the host since Ba²⁺ and PO₄^3- ions doesn’t give emission and similar defect emission in the case of Ca₅(PO₄)₃F host has been reported²⁰. Upon excitation from 233 to 393 nm, the emission intensity for the host deceases, whereas it increases for the Eu³⁺ doped sample. Also, the intensity at 483 nm in case of Li/Eu doped sample is much higher than that for host suggesting that there is contribution from Eu²⁺ emission in the former. The broad band of Eu²⁺ is due to allowed 4f⁶5d¹→4f⁷transition. The peak intensity of Eu²⁺ at 483 nm is much higher than that of Eu³⁺ at 592, 615, 652 and 698 nm upon excitation at 393 nm, whereas it is opposite in case of 233 nm excitation. It is suggested that excitation band at 393 nm is corresponding to the 4f⁷ →4f⁶5d¹ transition of Eu²⁺ which is overlapping with the excitation transition ⁷F₀ →⁵L₆ of Eu^3+21,
22. Upon excitation at 233 nm, there is an energy transfer from Eu-O to Eu³⁺. Further, there is a high chances to form Eu²⁺ in the present host considering the charge imbalance between Eu³⁺ and Ba²⁺, some Eu³⁺ ions will reduce to Eu²⁺ (radius = 0.125 nm) and perfectly substitute the Ba²⁺ sites (radius = 0.145 nm) ions. Thus, the broad intense emission peak at 483 nm for the Eu³⁺ doped sample may be due to the presence of Eu²⁺ in the host^23,
24. The emission peak observed at 592 nm, 615 nm, 652 nm and 698 nm is corresponds to ⁵D₀ →⁷F₁, ⁷F₂, ⁷F₃ and ⁷F₄ transition of Eu³⁺ respectively. The emission peak with centre at 615 nm (⁵D₀→⁷F₂) is dominant over 592 nm (⁵D₀→⁷F₁). It is well known that the transition ⁵D₀→⁷F₁ (magnetic dipole transition) is independent of the local symmetry while the transition ⁵D₀→⁷F₂ (electric dipole transition) strongly dependent on the symmetry site occupied by the dopant ions in the host lattice^{25, 26}.The magnetic dipole transition is permitted whereas the electric dipole transition, which is sensitive to local symmetry, is allowed exceptionally on the condition that the europium ion occupies a site without an inversion centre. Since in the halo apatite structure, the two sites of metal ions (M₁ and M₂) have no inversion symmetry, the dominant intensity of electric dipole over magnetic dipole transition is justified. A small peak with centre at 580 nm is attributed to the ⁵D₀→⁷F₀ transition, such transition has also been reported earlier and indicated that the forbidden transition ⁵D₀→⁷F₀ is relaxed by distortion of Eu³⁺environment and the intensity of ⁵D₀→⁷F₀transition arises due to the mixing of j-j at ground state of F_j=0,1,2,3,4due to thermal population²⁷. Figure 6 shows the emission spectra of Ba₅(PO₄)₃Cl: Eu³⁺(7 at.%) co-doped with Li⁺ at different concentration (0, 5, 10, 15 at.%) after excitation at 233 nm. The emission intensity without Li⁺ ions is almost negligible comparing to that of the samples doped with Li⁺. It is suggested that Li⁺ co-doping improves luminescence intensity significantly^28,29.

Figure 6-Emission spectra of Li+ co-activated Ba₅(PO₄)₃Cl:Eu³⁺ (7 at.%) at different conc. (0, 5, 10, 15 at.%) of Li⁺ monitoring excitation at 233 nm.

Figure 7- Integrated area under the emission curve (at 615 nm) verses concentration of Eu³⁺ in 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu³⁺. Also integrated area under the emission curve of samples prepared at different pH (doted line) under 233 nm excitation wavelength.

Figure 7 shows the integrated area under the curve (⁵D₀→⁷F₂)verses the concentration of the Eu³⁺ dopant ions (at fixed 10 at.% Li) in Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu³⁺ and also integrated area under the curve (⁵D₀→⁷F₂) of samples with 10 at.% Eu³⁺ and 10 at.% Li at different pH. The integrated area is calculated using Gaussian equation²⁷ for the emission peak at 615 nm when excited at 233 nm. It is clearly shown that with increasing concentration of the dopant ions up to 10 at. % the emission intensity increases and decreases again at higher concentration. This is the typical character of lanthanide ions due to the cross-relaxation among the dopant ions at higher concentration³⁰. And the emission intensity of the sample prepared in different pH value increases with the increase of pH value from 6 to 11.5 (Figure 7 doted arrow).

Figure 8- Emission spectra of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu^,3+ (7 at.%) at different heating temperatures.

Figure 9-Commission International de l’Eclairage (CIE) chromaticity co-ordinates for as-prepared, 600 ^oC and 800 ^oC annealed samples of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl: Eu³⁺ (10 at.%) at different excitation wavelength (A) 393 nm, (B) 317 nm, (C) 233 nm, (D) 255 nm, (E) 233 heated at 600 ^oC (Eu³⁺, 7 at.%) and (F) 233 nm heated at 800 ^oC (Eu³⁺, 7 at.%).

Figure 8 shows the emission spectra of as prepared sample 10 at.% Li⁺ co-doped Ba₅(PO₄)₃Cl: Eu³⁺(7 at.%) and annealed samples at 600 ^oC and 800 ^oC. The Eu³⁺ emission intensity of heated sample significantly enhanced up to 5-6 times than that of the as-prepared sample. Increase in intensity of Eu³⁺ emission with annealing may be attributed to the enhancement in crystallinity of the samples and decrease in the non-radiative transfer due to the removal of surface impurities. While the emission intensity at 483 nm decreases significantly may be due to the oxidation of Eu²⁺ ions to Eu³⁺ partially when the sample is heated at high temperature. The emission peak at 580 nm corresponding to the ⁵D₀→⁷F₀ transition become more prominent with increasing heating temperature form 600 to 800 ^oC indicating the j-j mixing of the Eu³⁺ ion.

Figure 9 shows the Commission international de l’Eclairage (CIE) chromaticity co-ordinate diagram for the as-prepared, 600 ^oC and 800 ^oC annealed samples of 10 at.% Li⁺ co-activated Ba₅(PO₄)₃Cl: Eu³⁺ (10 at.%). Different emission colors including blue, white and yellowish green are obtained. As-prepared sample shows blue emission (marked as A and B) under 393 nm and 317 nm excitation, white emission under 233 nm, 255 nm excitation (marked as C and D). For the sample (Eu³⁺, 7 at.%) heated at 600 ^oC under 233 nm excitation show white (marked as E) and yellowish green emission for the sample (Eu³⁺, 7 at.%) heated at 800 ^oC under excitation (marked as F).

4. CONCLUSIONS:

We prepared Li⁺ co-activated Ba₅(PO₄)₃Cl:Eu³⁺ by solution method using ethylene glycol as reaction medium at pH 11.5. The emission spectra under 233 nm excitation gives maximum emission peak at 615 nm and under 393 nm excitation gives maximum emission peak at 483 nm. The prepared samples give blue, white and yellowish green emission by tuning the excitation wavelength. The prepared samples can be used as potential candidate for blue, white emitting phosphor and near UV-based white light-emitting diodes (LED).

5. ACKNOWLEDGEMENT:

The authors thank R.S. Ningthoujam, BARC, Mumbai, India for his suggestion and the authors also thank Sophisticated Analytical Instrument Facility (SAIF), NEHU, Shillong, India for the TEM measurement. One of the authors (N. Yaiphaba) thanks University Grant Commission (UGC), New Delhi for financial assistant.

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Li:Ba₅(PO₄)₃Cl:Eu	Lattice parameter (A^o)			Unit Cell Volume (A^o)³	Size (nm)
Li:Ba₅(PO₄)₃Cl:Eu	a	b	c	Unit Cell Volume (A^o)³	Size (nm)
JCPDS-72 1370 Orthorhombic(BaHPO₄)	14.12	17.15	4.59	1111.51
pH-6	14.03	17.04	4.57	1093.59	57.92
pH-8	13.99	17.26	4.49	1085.43	47.55
JCPDS-16 0686 Hexagonal(Ba₅(PO₄)₃Cl)	10.26	-	7.64	696.50
pH-11.5	10.16	-	7.64	684.66	32.74