Photoluminescence of Eu(III) Doped Double Perovskite Phosphor for Red LED Application


 The luminescence property of Ba2CeNbO6 has been studying by varying Eu(III) concentration. Ba2CeNbO6 doped with trivalent ions of Eu has a monoclinic phase with P21/n space group. FTIR of Ba2CeNbO6 gives information that phosphor has a nano–size perovskite structure with an edge-connected Nb-O octahedral. Photoluminescence properties of Eu(III) doped Ba2CeNbO6 have been examined with excitation wavelength 275nm and 466 nm. Luminescence emission spectra exhibited three prominent peaks around 580 nm, 596 nm, and 610nm wavelength for Eu(III). Using xenon lamp as a source and having excitation wavelength 275 and 466nm, observing that the maximum light emission yielded in the region presenting a color-correlated temperature in the range of 1500 – 2900 K.


INTRODUCTION
The A2BB'O6 type double perovskite structure has a vital interest to materials scientists and solid-state chemists. This attention is often due to their potential application as substrates for high-Tc superconductors, especially for applications in the microwave region [1 -5]. Double perovskite-type oxides are suitable for such purposes due to their outstanding dielectric properties, including a low dielectric loss, low chemical reactivity, and their compatibility with some of the most regularly used superconductors, particularly as a result of owning an appropriate thermal expansion co-efficient. They are concern as they give a material where the magnetic properties of lanthanides studied in a relatively significant octahedral environment [6 -10] and have been recently reviewed for their potential use as luminescent materials [11].
Perovskite is a new derivative of a close-packed structure in which the oxygen arranged in a cubic close-packed organization except one oxygen ion is missing in every second layer, leaving a large volume void in the center. The strength of perovskite to accommodate large cations of rare earth elements makes it novel among the close-packed oxides. If the 12-coordinated cation is smaller compared to the oxide ion, the tilting of ~ 2 ~ octahedra changes the perovskite structure [12]. The modified structure produced after tilting could be tetragonal, orthorhombic, monoclinic, or sometimes without any center of symmetry. Non-centric perovskites display piezoelectricity or ferroelectricity, and synthetic perovskites used as electronic sensors. The structure of the newly discovered hightemperature superconductor shows resemblance with perovskite structure [13].
Around 90% of the natural metallic elements of the periodic table are known to be stable in a perovskite-type oxide structure. Perovskites have sub-metallic to metallic luster, colorless streak, cube-like structure along with imperfect cleavage, and brittle tenacity.
Perovskites can be prepared in various forms like nanocrystalline, bulk, thin films, and rods depending on their applications [14].
It's well known that the complex oxides having perovskite-type structure usually show functional properties [15,16]. Among them, the researcher paid attention to the Ba2LnNbO6 (Ln = lanthanide elements) compounds. Ba2LnNbO6 type perovskite compound was first synthesized by Brixner [15] and found to have a cubic perovskite-type structure for all except for Ln = La, which is tetragonally distorted structure, and some of them have ferroelectric properties. Later, Filip'ev et al. [17] reported that the structure of Ln = La had a rhombohedral distorted structure. Anderson et al. [18], however, point out that the crystal system of A2BB'O6 double perovskites is cubic, orthorhombic, or monoclinic because the B/B'-cation arrangement is limited to be a random type, a rock salt type, or a layered type. When the B/B'-cation arrangement is a rock salt type, the crystal system should be either a cubic one ( 3 ̅ ) [19,20]or a monoclinic one (P21/n) [21,22], for random type arrangement, the crystal system should be either a cubic one ( 3 ̅ ) [20,23] or a orthorhombic one (Pbnm) [21,24] and for layered type, the crystal system should be monoclinic one (P21/n) [25].
Practically all kinds of optical centres were studied in the perovskite structure for various doped concentrations. The studies of optical properties of perovskite systems have contributed considerably to the development of many optical materials like LASER materials, solar energy conversion and many more [26,27].
There is no reported data were found on photoluminescence properties of rare earth doped Ba2CeNbO6 during the literature survey. Our group report the photoluminescence studies of Ba2CeNbO6 doped with Eu (III).

SYNTHESIS
Eu(III) doped Ba2CeNbO6 phosphor had been synthesized by the combustion method.
Starting chemicals used for this objective are Ba(NO3)2, Ce(NO3)3.6H2O, Nb2O5, Eu2O3, and urea as flux. These chemicals were intimately mixed in the stoichiometric ratio corresponding to the nominal composition of Eu(III) doped Ba2CeNbO6 and transferred into alumina crucible. Then, this mixture was fired at 800⁰C in a muffle furnace for 3 hours and then allowed it naturally cool to room temperature. The synthesized material ground using mortar pastel was obtained in powder form owning white color.

CHARACTERIZATION
XRD measurements of undoped and Eu(III) doped Ba2CeNbO6, were done on D8 Bruker advance X-Ray Diffractometer with the Cu Kα radiation with 8.05keV energy, and 1.5406 Å wavelength (λ) at room temperature by step scanning in an angle range of 20° ≤ 2θ ≤ 70° with increments of 0.02⁰. FTIR spectra of phosphors were, recorded using Jasco FTIR-4100, spectrophotometer (Japan) by mixing phosphor with KBr in mortar -pestle in a ratio of 1:10. The PL of the samples examined using a Shimadzu spectrofluorophotometer (RF-5301 PC) at room temperature with a xenon lamp as an excitation source. indicating that prepared materials have a highly crystalline nature. There is a good agreement between the observed and calculated inter-planar spacing suggests that the compound has a monoclinic phase with a P21/n (#14) space group. The relative coordinates and occupancy of each site for Ba2CeNbO6 are listed in Table 1. The values of bond distances of cations relative to the oxygen anion and occupancy were obtained from the Rietveld refinement are shown in Table 2. These diffraction lines are consistent and confirm the formation of a double perovskite structure for all prepared samples. The average crystalline size were estimated by the Scherrer's equation using the full width at half maximum (FWHM) of all ~ 4 ~ intense peak. The average crystalline size was calculated using the Debye-Scherrer formula [28] which is given as equation (1),
Where, D is the average crystalline size, k is the constant equal to 0.94, λ is the wavelength of the X-rays equal to 0.1542 nm, θ is the Bragg angle and β is FWHM. All the reflection peaks of the X-ray profile indexed and lattice parameters are determined with the help of a standard computer program Powder-X. In double perovskite oxide, the study of distortion from the ideal cubic perovskite structure is clear as the Ba2CeNbO6 complex perovskite has general formula A2BB'O6. The tolerance factor Tf [29] of the Ba2CeNbO6 is calculated by using equation (2), Where, RBa, RCe, RNb and RO are the ionic radii of Ba, Ce, Nb and O respectively [30]. By the geometry of crystal, the ideal cubic structure should have Tf = 1, whereas it will be monoclinic structure for values of Tf < 1 and it following the SPuDs prediction [31]. The value of tolerance factor for Ba2CeNbO6 was found to be approximately 0.9505, which suggests that sample under study has the monoclinic structure.  Table 2 Inter-atomic distance and occupancy calculated through Rietveld refinement of experimental data.   (3) 6.0986 (9) 6.0943 (5) 6.1013 (7) 6.0972 (8) c(Å) 8.0586 (3) 8.0795 (2) 8.0626 (3) 8.0746 (6) 8.0816 (8)    To discover the atomic bonds in a compound FTIR analysis has been carried out. In wavenumber. The FTIR spectrum of Ba2CeNbO6 shows some well-defined bands in the above figure. It is in good accordance with group theory predictions what orderly found for pervoskite type structures [32]. In pervoskite-type material, significant vibrational couplings may expect between the different coordination polyhedral compounds. All the peaks in the spectra are typical of the material. One small hump is at 1750 cm -1 , which is due to the presence of adsorbed moisture in KBr [33]. The lower energy band found at around 450-650 cm -1 is related to the deformational mode of CeO6 octahedra [34]. The medium energy peak appearing near 850 cm -1 is due to the asymmetric NbO6 stretching vibration [35] due to the higher charge of the cation. The strong intensity peak at approximately 1430 cm -1 can eventually be related to the symmetric stretching vibration of NbO6 octahedra [36]. A small intensity peak at 1100 cm -1 likely corresponds to the presence of overtones of the fundamental vibrations in Ba2CeNbO6 [37]. The peak at 552cm -1 is due to the Ba-O.   recognized by Ryan and Jørgensen [43] but, it was studied by Dorenbos [44,45] in more detail. Dorenbos gives an extensive compilation of the energies of the charge-transfer transitions of Eu(III) compounds in the solid state [45]. The general trend of the energies of the charge-transfer transitions is: fluorides > oxides > nitrides > chlorides > bromides > iodides > sulfides > selenides > phosphides > arsenides > tellurides > antimonides [38]. ~ 12 ~ Charge-transfer states of Eu 3+ at low energies possess severe consequences for the spectroscopic characteristics of Eu 3+ ion, due to the non-negligible mixing of the 4 f6 electronic states and the charge-transfer states. This mixing has been utilized to explain the high intensity of the 5 D0→ 7 F0 transition in some europium(III) compounds [46]. Chargetransfer bands also useful for the sensitization of europium(III) luminescence as they can work as aerial to consume energy and transfer this excitation energy to the Eu 3+ ion.  The emission peak at 596 nm is due to the magnetic dipole transition of 5 D0 → 7 F1, which has higher intensity compare to peak at 613nm of 5 D0 → 7 F2 electric dipole transition during excitation at 275nm. This asymmetricity confirms that the Eu 3+ ions are located at the noninversion symmetric sites in host material [47 -49]. When this phosphor was excited at 466nm emission peaks are observed at around 580-600 nm, this is mainly due to the magnetic ~ 13 ~ dipole transition of 5 D0 → 7 F0 and 5 D0 → 7 F1 and emission peak at 613nm and 633nm are due to electric dipole transition of the 5 D0 → 7 F2.

Williomson -Hall Plot
It's important to note that in both the emission spectra emission peak at around 613nm is occurred in the red region. Though an admixture of odd-parity electronic configuration to the pure 4f (like a non-centrosymmetric crystal field component) [50,51] will allow the ED transitions partially and their possibility of occurrence is much higher than the probability of parity-allowed magnetic dipole (MD) ( 5 D0→ 7 F1) transition. The site symmetry of Eu 3+ ion in the host lattice can be predicted by asymmetry ratio, which can be defined as equation (4) Table 4 shows the asymmetry ratio for both emission spectra. In the present research, the asymmetric ratio is found to be less than unity (1), indicates that Eu (III) is substituted at a non-centrosymmetric site in the host lattice, and it's in good agreement with the earlier report [49, 52 -56]. It saw that the substitution of Eu (III) at the place of Ba (II) site in Ba2CeNbO6 is followed by Ba(II) ion vacancy due to a distinct ionic radius. These gaps in lattice reduce the local site symmetry at the Eu(III) site and act as a luminescence quenching center.  Table 4 Asymmetric ratio of Eu(III) doped Ba2CeNbO6 at 275nm and 466nm emission spectra.
The results presented in Fig. 6 (a & b) reveal that the spectral profile of the phosphors show noticeable changes with increasing europium concentration and the mixed overall emission light chromaticity.

CIE (Commission International del'éclairage)
The chromaticity color coordinates of all phosphors were determined, and the results are presented in the Commission International del'éclairage (CIE) 1931 diagram, as shown in Fig. 7 (a) and (b). With increasing europium (III) concentration, the color tone of the emanating light shifts from the orange region to the red region [55, 57 -59]. It's good to point out that the continuity of the emission tone corresponding to coordinate points shown in  Table 5.