Accepted Manuscript Structure and optoelectronic properties of palmierite structured Ba2Y0.67δ0.33V2O8: 3+ Eu red phosphors for n-UV and blue diode based warm white light systems S. Shisina, Subrata Das, S. Som, Shahzad Ahmad, V. Vinduja, P. Merin, K.G. Nishanth, Puja Kumari, Mukesh Kumar Pandey PII: S0925-8388(19)32047-X DOI: https://doi.org/10.1016/j.jallcom.2019.05.355 Reference: JALCOM 50887 To appear in: Journal of Alloys and Compounds Received Date: 13 March 2019 Revised Date: 15 May 2019 Accepted Date: 31 May 2019 Please cite this article as: S. Shisina, S. Das, S. Som, S. Ahmad, V. Vinduja, P. Merin, K.G. Nishanth, P. Kumari, M.K. Pandey, Structure and optoelectronic properties of palmierite structured 3+ Ba2Y0.67δ0.33V2O8: Eu red phosphors for n-UV and blue diode based warm white light systems, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.05.355. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. ACCEPTED MANUSCRIPT AC C EP TE D M AN US C RI PT Graphical abstract ACCEPTED MANUSCRIPT Structure and optoelectronic properties of palmierite structured Ba2Y0.67δ0.33V2O8: Eu3+ red phosphors for n-UV and blue diode based warm white light systems Puja Kumari,d Mukesh Kumar Pandeye RI PT Shisina S.,a Subrata Das,*a S. Som,b Shahzad Ahmad,*c Vinduja V.,a Merin P.,a K.G. Nishanth,a M AN US C *E-mail: subratadas@niist.res.in, shahzadncfm@gmail.com; Fax: +91-471-2491712; a. b. c. d. e. AC C EP TE D Tel: +91-471-2515360 Materials Science and Technology Division, CSIR – National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala-695019, India. Department of Chemical Engineering, National Taiwan University, Taipei-10617, Taiwan, ROC. Department of Chemistry, Zakir Husain Delhi College, University of Delhi, Delhi110002, India. Department of Physics, Darbhanga College of Engineering, Darbhanga-846005, India. Department of Physics, National Taiwan University, Taipei-10617, Taiwan, ROC. 1 ACCEPTED MANUSCRIPT Abstract Palmierite structured Ba2Y0.67δ0.33V2O8 in hexagonal symmetry have successfully been RI PT synthesized by employing a conventional solid state reaction. Considering the structural model of hexagonal palmierite in the R-3m (#166) space group, the observed powder X-ray diffraction pattern was fitted by the Rietveld refinement with lattice constants a =5.7797 (1) Å and c = 21.2894 (3) Å. Ba2Y0.67δ0.33V2O8 showed broad blue emission at 442 nm under the UV excitation M AN US C of 320 nm owing to [VO43−] group. A series of Eu3+ doped samples, Ba2Y0.67-xEuxδ0.33V2O8, showed bright orange-red luminescence (5D0→7F1, 2) under the UV and blue excitations. The optimum doping amount of Eu3+ ions was found to be x =0.2 and the energy transfer mechanism for the concentration quenching effect was determined to be dipole−quadrupole interaction. The CIE coordinates of the optimized Ba2Y0.47Eu0.20δ0.33V2O8 phosphor at λex = 320 and 394 nm are (0.67, 0.33) and (0.66, 0.34), respectively. Meanwhile, the optimized phosphor also showed high D red color purity (Ra) of 99.6 % and 99.4 % at λex = 320 and 394 nm, respectively, suggesting that TE it could be the preferred choice as a red component for white light emitting diodes. The internal quantum yield (η) and the absorption efficiency (α) of the optimized Ba2Y0.47Eu0.20δ0.33V2O8 EP phosphor were found to be 59 and 28% respectively, at an excitation wavelength of 394 nm. AC C Furthermore, the value of η and α for the optimized phosphor at the blue excitation of 464 nm were found to be 55% and 26%, respectively. The optimized Ba2Y0.47Eu0.20δ0.33V2O8 phosphor showed excellent thermal stability (75% up to 200◦C) with an activation energy of 0.4 eV. A white light emitting diode comprising the optimized and commercial yellow phosphor showed bright white emission with a Ra of 86, color temperature of 5478 K, and CIE coordinates of (0.34, 0.33). The investigated results indicated that the Ba2Y0.47Eu0.20δ0.33V2O8 phosphor is a 2 ACCEPTED MANUSCRIPT suitable red emitting phosphor for making white light emitting diodes under near-UV and blue excitations. RI PT Keywords: Vanadate; Energy transfer; Thermal stability; White-LED; Solid state lighting Introduction During the past few decades, luminescent materials that convert frequency into desired M AN US C wavelengths has been widely used in various fields, such as light-emitting diodes (LEDs), upconversion, solar cells and sensors.1-5 Nowadays, white-LEDs are considered to be the most important solid-state light sources owing to their excellent performances, such as high efficiency, high reliability, high durability, environmental friendliness, and low energy consumption.6 Commercial methods for producing white-LEDs involve the coating of appropriate amounts of a yellow-emitting phosphor, such as cerium-doped yttrium aluminum garnet (YAG:Ce3+) over a D blue-emitting GaN or InGaN chips.6 However, this combination causes several serious problems, such as thermal quenching, poor color rendering index (CRI), narrow visible range due to the TE lack of red component leading to the restriction of their use in practical applications.7 Many EP efforts have been carried out to improve CRI by incorporating red-emitting components using quantum dots phosphors (CdSe/Sr3SiO5:Ce3+, Li+), nitride phosphors (Sr2Si5N8:Eu2+) or fluoride AC C phosphors (Na2SiF6:Mn4+).8-10 In fact, these aforementioned phosphors possess either toxicity, high manufacturing expense, harsh syntheses conditions, poor chemical stability or uses of corrosive starting materials which restrict their practical utility. Consequently, it is noteworthy to search for newer red emitting phosphors that have excellent chemical stability, better color purity and CRI, high luminescence efficiency, and it should also possess high absorption in nearUV/blue wavelength region for its application in white-LEDs. 3 ACCEPTED MANUSCRIPT Orthovanadates with general formula A3(VO4)2 [A = Ba2+, Sr2+, Ca2+, Zn2+] has attracted the attention of many researchers as an efficient host matrix for their used in lighting, display devices, down-shifting photoluminescence, IR-laser and white-LED.11-16 In A3(VO4)2 structure, RI PT V occupy positions with C3v symmetry (distorted tetrahedra geometry) i.e one of the oxygen ligands is distinct from the three others ligands in the [VO4]3− group. This [VO4]3− group makes the orthovanadates structure an excellent luminescent host material for rare-earth ions as they M AN US C can strongly absorb ultraviolet light and then efficiently transfer the energy to rare-earth ions. Bedyal and co-workers synthesized Eu3+ doped Sr3(VO4)2 phosphors for white-LEDs and flat panel displays.11 Yu and co-workers studied the Sm3+ doped Ba3(VO4)2 microparticles and found it as a promising bi-functional platform for optical thermometry and safety sign in the hightemperature environment.12 Recently, Zhang et al. reported near IR laser action of Mn5+ doped Sr3V2O8 and Ba3V2O8 samples at 1168 nm.13 These orthovanadates have also been extensively D investigated for their potential applications as supercapacitor, photocatalyst, photoresponse TE electrocatalyst, ferroelectric, dielectric and microwave devices.17-21 Recently, Fang and co-workers demonstrated that the color of the luminescence of the EP phosphors (λex = 365 nm) changed with the ionic radius of the M-cation, from yellow [Mg3(VO4)2] to light blue [Ba3(VO4)2] via blue-green [Sr3(VO4)2].22 The light blue emission in AC C Ba3(VO4)2 makes the host matrix most appropriate for white-LEDs when co-doped with Eu3+ ions. Furthermore, the optical properties of a host matrix can also be tuned by adopting different synthetic strategies, surface modification, controlling the size, shape, and phase purity of the crystals and multicolor emission optimization.23 The doping of Ba3(VO4)2 system by rare earth ions has been widely carried out, however the effect of the substitution of transition elements on the structural and optical properties are yet to be studied. Skakle et al. reported the substitution of 4 ACCEPTED MANUSCRIPT lanthanum ions in Ba2La2/3V2O8 and Sr2La2/3V2O8 (i.e. substitution at site A) in the Ba3V2O8 and Sr3V2O8 respectively.24 According to Skakle, substitution of La3+ ions for Ba2+ ions causes no change in structure type, but produces cation vacancies i.e. aliovalent substitution.24 RI PT Ba2Ln2/3V2O8 also belong to the palmierite class of compounds consisting of double vanadate units in the binary system Ba3V2O8–LnVO4 and its stoichiometry can also be written as A3LnV3O12.24 M AN US C In this research, a novel palmierite Ba2Y0.67δ0.33V2O8 sample in hexagonal symmetry has been synthesized for its use as a host for Eu3+ ions to generate suitable red emitting systems for white-LEDs. A detailed study on the structural and optical properties of the synthesized phosphor has been successfully carried out. A simple solid state synthesis has been employed as this methodology provided an efficient control over the phase, crystallinity, and homogeneity of the products. Additionally, to find the optimize doping concentration of Eu3+ ions, optical D properties of the various concentrations of Eu3+ ions doped in a Ba2Y0.67δ0.33V2O8 host has also TE been studied. The optimized sample showed superior thermal stability, high red color purity and internal quantum yield, which make the red phosphor a suitable component for white-LEDs Experimental EP based on the near-UV/blue LED chip. AC C The undoped and Eu3+ doped Ba2Y0.67δ0.33V2O8 samples were prepared by solid state reaction. Highly pure BaCO3 (99.8%), V2O5 (≥ 99.6%), Y2O3 (99.99%), and Eu2O3 (99.999%) obtained from Sigma-Aldrich were used. Stoichiometric amounts of BaCO3, Y2O3, V2O5 and Eu2O3 were thoroughly grounded via mortar pestle for 30 minutes for the syntheses of Ba2Y0.67xEuxδ0.33V2O8 (x = 0.00, 0.10, 0.20, 0.30, 0.40, 0.50 and 0.67) samples. Moreover, 2.5 wt% of H3BO3 flux was also added to achieve a homogeneous reaction during the heating process. The 5 ACCEPTED MANUSCRIPT mixture was first heated in an alumina crucible at 700˚C for 20 minutes, followed by firing at 1200˚C for 2.5 hours. Powder X-ray diffraction patterns (PXRD) of synthesized materials were recorded in a RI PT broad range of Bragg angle 2θ (10˚≤ 2θ ≤ 70˚) using a Philip’s x’pert pro diffractometer, Ni filtered Cu-Kα (λ = 1.54 Å) at 45 kV and 40 mA. Energy dispersive X-ray study (EDS) was done using a Silicon Drift Detector-X-MaxN combined with the Carl Zeiss EVO SEM. UV-Vis-NIR M AN US C reflectance properties were studied using UV-Vis-NIR spectrometer (Shimadzu UV 3600 with an integrating sphere attachment, ISR-2000) in which Barium Sulphate as a reference for UV– Vis range (300-700 nm) and Polytetrafluoroethylene (PTFE) for NIR range (700-2500 nm). Photoluminescence excitation, emission spectra were recorded using an Yvon Fluorolog 3 spectrofluorimeter with a 450W Xenon flash lamp as the exiting source. The Commission Internationale de I’Eclairage (CIE) coordinates were converted from the mixing the obtained red phosphors with the commercial yellow-emitting TE via D photoluminescence spectra using the color calculator software. The white-LEDs were fabricated Y3Al5O12:Ce3+ phosphor followed by dispersing in transparent silicon resin to prepare phosphor EP mixtures. The mixtures were then coated on 460 nm InGaN-based LED chips to fabricate LED devices. The photoluminescence characteristics of fabricated LEDs were measured using a CCE AC C spectrophotometer (BRC112E). Result and discussion The PXRD pattern of the product from the reaction of Ba2CO3, Y2O3 and V2O5 is shown in Fig. S1a. Reflections in the PXRD pattern were search matched for the known phases of BaY-V-O using ICDD software yielding no exact match with the orthorhombic forms of Ba2V3YO8 and Ba2YVO6 (JCPDS No. 42-0293 and 42-0310). In fact, the positions and intensities of the 6 ACCEPTED MANUSCRIPT observed reflections in the PXRD pattern resembled closely to hexagonal Ba3V2O8 (JCPDS No. 71-2060) except the two peaks observed at 25.02o and 33.62o which is correspond to the tetragonal YVO4 (JCPDS No. 70-1281). The observed reflections were moderately strong RI PT implying that the crystallinity of the product must be enhanced in order to have high luminescence intensity. The PXRD patterns of product prepared using 2.5 wt % H3BO3 flux is shown in Fig. S1b. As seen from the PXRD patterns, the addition of flux elevated the PXRD M AN US C intensity appreciably without any changes in the crystal structure. This phenomenon can be explained on the basis of the lower melting point of H3BO3 (171°C) compare to the final calcination temperature (1200°C) employed in the synthesis. During calcination, H3BO3 flux melted quickly, which significantly increased the mobility and homogeneity of solid reactants during its synthesis.25 The hexagonal Ba3V2O8 (space group R-3m # 166) was chosen to be the structural model D for the simulation of the diffraction peaks using the GSAS software with one of the Ba2+ ion TE replaced by 2/3Y3+ ion.26 The diffractogram was very well fitted after incorporating tetragonal YVO4 as a minor impurity in the Le-Bail procedure (Fig. S2). In Ba3V2O8 structure, Ba2+ EP occupies two Ba sites: 3a (0, 0, 0) and 6c (0, 0, z) and the distribution of Ba2+ and Y3+ ions in our product over the two sites cannot be determined using Le-Bail procedure. Consequently, two set AC C of Rietveld refinement were performed to find the site symmetry and occupancy of Ba2+ and Y3+ ions over the two available crystallographic sites (3a and 6c). In case-I: Ba and Y were distributed equally over the two sites in the ratio of 2/3: 2/9 and whereas in case-II: 3a site was fully occupied by Ba, and that the 6c site was occupied by 1/2 Ba and 1/3 Y. The occupancies were allowed to refine freely while other refinement parameters were kept fixed. The refinement converged successfully to give final site occupancies of: Ba = 0.5076, Y = 0.3257 at the 6c site 7 ACCEPTED MANUSCRIPT (Fig. 1). It was thus concluded that the 3a site was fully occupied by Ba, and that the 6c site was occupied by 1/2 Ba, 1/3 Y and vacancies. Our results obtained from Rietveld refinement are similar to those reported by Skakle and co-workers for Sr2La2/3V2O8.24 The occupancy of Y at RI PT the 6c site was further confirmed from optical studies which will be discussed later. The positions of the diffraction peaks generated considering the Ba3V2O8 structural model are in good agreement with the measured PXRD pattern. It is therefore concluded that the obtained M AN US C Ba2Y0.67δ0.33V2O8 possessed palmierite structure. The lattice parameters obtained from the Rietveld refinement of Ba2Y0.67δ0.33V2O8 (we abbreviated it as BYVO) were a = 5.7797 (1) Å and c = 21.2894 (3) Å which is slightly smaller than the standard parameters of Ba3V2O8. Since the ionic size decrease from Ba2+ (BaVI: 1.49 Å) to Y3+ (YVI: 1.04 Å), it is logical that there should be a slightly reduction in the lattice parameter of Ba2Y0.67δ0.33V2O8 and the reflections in PXRD spectrum should shift to the higher 2θ side.27 The crystal data and structure refinement D parameters of hexagonal Ba2Y0.67δ0.33V2O8 are summarized in Table 1. The refined unit cell and TE position parameters, after the final cycle of refinement, are provided in Table 2. The structure of Ba2Y0.67δ0.33V2O8 crystal is shown as an inset of Fig.1. Each cationic site in the structure has a EP different co-ordination environment: Ba at site 3a formed regular octahedra (coordinated to 6 oxygen atoms with equal bond distances), V at site 6c formed distorted tetrahedra (coordinated AC C to 4 oxygen atoms with two different bond distances) whereas Ba/Y at site 6c do not formed any polyhedra and is coordinated to 10 oxygen atoms with three different values of bond distances (Table S1). The Ba/Y polyhedron edge-shares with three BaO6 octahedra and corner-shares with four VO4 tetrahedra.11 The elemental mapping on various locations of the sample was carried out by SEM−EDS analysis (Fig. S3). From the analysis, the concentrations of barium, yttrium, vanadium, and oxygen are found to be 15.30, 6.93, 14.00 and 63.76%, respectively, which is 8 ACCEPTED MANUSCRIPT close to their corresponding atomic percentage (15.79, 5.21, 15.79 and 63.19%, respectively) in Ba2Y0.67δ0.33V2O8 crystal. Figure 2 shows the diffused reflectance spectra (DRS) of the undoped and Eu3+ doped RI PT BYVO samples. These spectra consist of a broad absorption band in the range of 200-400 nm with λmax = 300 nm. The band gap in orthovanadates consists of a charge transfer from nonbonding O2- 2p levels of the valence band to the V5+ 3d levels of the conduction band.28 Thus, M AN US C the strong broad absorption band in the DRS is a charge transfer band (CTB) which is attributed to the (O2- – V5+) bond in VO43− group. The introduction of Eu3+ ions in the host lattice causes a blue shift of the CTB towards the shorter wavelength (λmax = 292 nm). Using the Kubelka−Munk function, the band gap of the BYVO was estimated to be 3.92 eV (close to the literature value of Ba3V2O8 (3.85 eV)) which increased to 3.99 eV upon Eu3+ (x = 0.1) doping (Fig. 2). The observed band gap widening of Eu3+ doped sample might be due to the higher electropositive D character of Eu3+ ions as compared to as compared to Y3+ ions.29 BYVO: Eu0.1 sample also TE shows several narrow characteristic absorption lines at 466, 537 and 588 nm which are corresponds to the 7F0 → 5D2, 7F0 → 5D1 and 7F0 → 5D0 transitions of Eu3+ ions respectively EP (inset of Fig. 2).30 The photoluminescence (PL) excitation and emission spectra of BYVO sample are shown AC C in Fig. 3. The excitation spectrum recorded at λem = 440 nm consists of a broadband ranging from 240 to 350 nm with maxima at around 320 nm. While the emission spectrum recorded at λex = 320 nm consists of a broadband ranging from 370 to 570 nm with maxima at around 440 nm. According to molecular orbital theory, this broadband (370-570 nm) corresponds to electric or magnetic dipole allowed transitions of the [VO43−] group.31 The broad excitation band has been deconvoluted and fitted with two Gaussian curves peaked at 300 and 320 nm which are supposed 9 ACCEPTED MANUSCRIPT to originate from the 1A1 → 1T2 (EX1) and 1A1 → 1T1 (EX2) transitions, respectively (Fig. S3).32 Whereas the broad emission band has also been deconvoluted and fitted with two Gaussian curves peaked at 430 and 476 nm which are supposed to originate from the 3T2 → 1A1 (EM1) and T1 → 1A1 (EM2) transitions, respectively (Fig. S3).32 According to the spin selection rule, the RI PT 3 excitation process (1A1 → 1T1, 1T2) is allowed, while the intersystem crossing (1T1, 1T2 → 3T1, 3 T2) and luminescence process (3T1, 3T2 → 1A1) are forbidden in the ideal Td symmetry.15 M AN US C However, in our sample, the structure of VO43- tetrahedron is distorted to some extent (with two different values of V-O bond lengths (Table S1)) from that of the ideal tetrahedron, these forbidden processes become partially allowed.15 Figure 4 shows the PL excited spectra of Eu3+ doped BYVO samples recorded at 615 nm. The spectra consist of several emission peaks at 594, 615, 650 and 701 nm which corresponds to Eu3+ ion transitions, 5D0 → 7FJ (J = 1-4) respectively. The 1A1 → 1T2, 1 transition of a [VO43−] D group is expected to be much stronger than the partially allowed 7F0 → 5H6 transition of Eu3+ TE ions. Thus, the electrons in [VO43−] group are excited from the ground state (1A1) to higher excited states (1T2 and 1T1) upon excitation of λex = 320 nm and then it transfers to a higher EP excited state of Eu3+ ion (5H4-7) using charge transfer process. Subsequently, the electrons from this higher excited state (5H4-7) relaxed to lower excited states (5D0) by non-radiative process and AC C then finally transfer to 7FJ (J = 1-4) level states by emitting light.33 The high emission intensity of Eu3+ ions and the absence of CTB in the Eu3+ doped samples indicate that there is the strong attraction of Eu3+ ions towards O2- ions, which increases the probability of mixing of 4f-orbitals of Eu3+ ions and 2p orbitals of O2- ions.34 Inset of Fig. 4 shows fewer emission peaks obtained from the transitions of electrons from higher excited state 5DJ (J = 0–3) level to 7FJ (J = 0–3) level. The appearance of these peaks suggested that some of the electrons from excited states 10 ACCEPTED MANUSCRIPT (1T2 and 1T1) may relax to lower excited states (3T2 and 3T1) by the non-radiative process, followed by energy transfer to excited states of Eu3+ ion (5D1-3) by resonance process.35 The transfer of energy in the Eu3+ doped BYVO system can be depicted schematically as Fig. 5. RI PT Also, the presence of the 5D0 → 7F0 transition at 578 nm indicates that the Eu3+ ions are embedded in a low symmetry environment without an inversion center.36 Figure 6a shows the PL excitation spectra of Eu3+ doped BYVO samples recorded at 615 M AN US C nm. In all spectrums, a CTB is present in the region from 235 to 355 nm. The broad CTB band has been deconvoluted and fitted with two Gaussian curves peaked at 300 and 320 nm which are supposed to originate due to transitions take place in between 1A1 and 1TJ (J = 1, 2) level (Fig. S4). The absence of O2- (2p) – Eu3+ (4f) CTB which usually lie in the regions 250-300 nm may be due to the higher molar absorptivity of O2- (2p) – Eu3+ (4f) charge transfer transition than O2(2p) – V5+ (3d).36 The sharp peaks in the spectra at 361, 380 and 394 nm are due to the intra f–f D transitions 7F0 → 5D4, 7F0 → 6G2–6 / 5L7, 7F0 → 5L6, respectively of Eu3+ ions.30 Figure 6b shows TE the PL emission spectra of Eu3+ doped BYVO samples recorded at 394 nm. All the emission spectrums obtained in this investigation show mainly two peaks; a strong red emission peak at EP 615 nm and orange emission peak at 594 nm. Moreover, the hypersensitive 5D0 → 7F2 (615 nm) electric dipole transition has a higher intensity than the non-hypersensitive 5D0 → 7F1 (594 nm) AC C magnetic dipole transition, confirming that Eu3+ ions occupy a low symmetry site and without an inversion center.30 The presence of multiplets in the emission spectra of samples is attributed to the partial lifting of degeneracy due to 2J+1 Stark splitting. According to Binnemans, a field possessing low symmetry cause the splitting of the spectral lines.30 This effect is more pronounced in even J terms as they are more hypersensitive to the coordination environment (5D0 → 7F2, 4 transitions).30 This further confirmed that the Y3+ ions which are substituted by Er3+ ions 11 ACCEPTED MANUSCRIPT occupied non-centrosymmetric sites 6c (ten Ba/Y/Er-O bond with three different values of bond lengths). Whereas the site 3a is a regular octahedral (all the six Ba-O bond are at an equivalent distance) and thus it a centrosymmetric site. RI PT The variation of Eu3+ concentration can be determined from the asymmetric ratio (red to orange ratio) which is the intensity ratio of (5D0 → 7F2) to (5D0 → 7F1) transition. High value of asymmetry ratio (Table 3) indicates that the optimal value of color chromaticity will be closer.37 M AN US C Of the all, the 5D0 → 7F2 transition is stronger and is responsible for the bright orange-red luminescence and the intensities show a systematic enhancement with the increase of Eu3+doping concentration up to x = 0.20. Subsequently, when the concentration of Eu3+ ions was further increased, the PL intensity decreased due to the concentration quenching effect, which was triggered by the non-radiative energy transfer among the nearest Eu3+ ions (Fig. 7). The redorange color intensity of the optimum phosphor BYVO: Eu0.2 is even visible to the naked eye D (Inset of Fig. 6). The relative R/O ratios as a function of the Eu3+ content in the present TE phosphors are not much varied, indicating that the overall Eu3+ local environments are almost the same for different doping concentrations. Therefore, it can be guessed that the Eu3+ ions have EP conveniently occupied the sites 6c in BYVO. The concentration quenching phenomena was further explored by calculating the distance AC C between two nearest Eu3+ ions, known as the critical distance (Rc). The value of Rc for Eu3+ in BYVO: Eu0.2 system was calculated using Blasse’s equation:38 / R ≈2 (1) where V is the volume of the unit cell, N is the number of formula units per unit cell, and XC is the optimized concentration of the activator ion. For the optimized system, V = 616.18 Å, N = 3 12 ACCEPTED MANUSCRIPT and the XC = 0.2. The value of RC was calculated to be 12.52 Å, which is greater than 5 Å.38 Therefore, the dominant mechanism of concentration quenching occurred via a multi-polar interaction in the BYVO: Eu3+ system. Further, the nature of multi-polar interaction was = RI PT analyzed using the Dexter’s relation:39 (2) ( ) / M AN US C where k and β are constants and θ assumes values of 6, 8, and 10 for dipole-dipole (d-d), dipole−quadrupole (d-q), and quadrupole−quadrupole (q-q) interactions, respectively. The slope of the linear plot of ln(I/X) vs lnX, as shown in the inset of Fig. 7, was found to be −2.56. The θ value calculated using eqn. (2) was found to be 7.68, which is approximately equal to 8. Thus, d– q interaction is responsible for the occurrence of concentration quenching effects.39 To studies the Eu3+ concentration dependent site symmetry and luminescence behaviour D of BYVO: Eu3+ phosphors, Judd–Ofelt (J-O) parameters t (2 and 4) were calculated from the TE emission spectra.40 As the emission due to 5D0 → 7F6 transition cannot be observed, the value of Ω6 cannot be estimated in the present case. Ω2 determines the covalency, polarization and the EP asymmetric nature of the activator and the ligand (short range effects), whereas the other two AC C parameters depend on long-range effects.41 The integrated emission intensities of the radiative emission of the 5D0 → 7FJ (J = 2, 4) transitions are associated with the radiative emission rates as: where the A0–2, 4 A , =A !," # $υ $υ # !," (3) and A0–1 is the radiative emission rates for 5D0 → 7F2, 4 and 5D0 → 7F1 transitions; I0–2, 4 and I0–1 are integral intensities for 5D0 → 7F2, 4 and 5D0 → 7F1 transitions and hν0–2, 4 and hν0–1 are their energies, respectively. The magnetic dipole radiative emission rate A0–1 13 ACCEPTED MANUSCRIPT has a value of 50 s-1. The electric dipole radiative emission rates A0–2, 4 may be written as a function of the J–O intensity parameters as below: % = & " (υ !," ) $( '! ) χ ∑%+ , Ω% , 5 D0 -U (%) - 7 FJ / 2 (4) RI PT A where, χ is the Lorentz local field correction factor, and its related with the index of refraction 1(1! 2)! “n” of the host using the following equation χ = . The non-zero square reduced matrix 〈 5D0 | U ( 2) |7 F2 〉 2 =0.0032 and 〈5D0 | U ( 4) |7 F4 〉 2 =0.0023. Using M AN US C elements are taken from the references as 2 equations (3) and (4) the values of Ω2 and Ω4 were calculated and tabulated in Table 3. The calculated J–O parameters have been used to predict some important radiative properties such asradiative transition probabilities (Arad), total radiative transition probability (AT), radiative lifetime τrad(ψJ) and the branching ratios β(ψJ) for the excited states of Eu3+ ions using the following equations; D A456 (ψJ, ψ9 J9 ) = A% (6) %′ TE A: (ψJ) = ∑%′ A% (5) %9 τ456 (ψJ) = EP β(ψJ) = (7) <= (ψ%) < ((ψ%,ψ′%? ) <= (ψ%) (8) of 2 AC C The results of these aforementioned radiative properties are also tabulated in Table 3. The value is very high, indicating the asymmetry of the local environment at the Eu3+ ion site in this host; this is also evidenced from their higher asymmetric ratio (Table 3). Also, the value of and asymmetric ratio is highest for the BYVO: Eu0.2 phosphor. The value of 4 2 is not directly related to the symmetry of the Eu3+ ion but to the electron density on the surrounding ligands. The trend for the t parameters in our host is 2> 4 which is in agreement with those reported previously by Rao and co-workers.41 This implies that the efficiency of the 5D0 → 7F2 transition 14 ACCEPTED MANUSCRIPT becomes weak at the cost of the 5D0 → 7F1 transition. This is further supported by the other radiative properties which include radiative transition probabilities (A0-4), total radiative transition probability (AT) and the branching ratios β(ψJ) which is maximum for the BYVO: RI PT Eu0.2 phosphor. The highest branching ratio is calculated for the 5D0 → 7F2 transition, as this is the dominant emission line, responsible for the red color emission. Moreover, the radiative lifetime for the BYVO: Eu0.2 phosphor was also found to be very low (0.0022 s-1). The M AN US C assessment of the J–O intensity parameters indicates that Eu3+ doped BYVO phosphors can be an effective luminescence materials. The CIE (X, Y) coordinate diagram showing the chromaticity points of Eu3+ doped BYVO phosphors for the emission spectra recorded at λex = 394 nm is shown in Fig. 8. It is evident from the CIE that the color coordinates for all the doped samples are confined within the red-orange area, and there is not much variation in the emission color with the variation in Eu3+ D concentrations (Table 4). A red emission around (0.66, 0.34) has been detected for the optimum TE BYVO: Eu0.2 phosphor which is quite close to value of the ideal red chromaticity value given by National Television Standard Committee (0.67, 0.33). The color purity was further investigated EP as follows:42 I(J JK )! (L LK )! I(JM JK )! (LM LK )! × 100% (9) AC C ColorPurity = where (x, y), (xi, yi), and (xd, yd) present the CIE chromaticity coordinates of the sample, white illumination, and dominated wavelength, respectively. The results of color purity of Eu3+ doped BYVO phosphors are listed in Table 4. The value of color purity for the optimum BYVO: Eu0.2 phosphor is calculated to be 99.6 and 99.4 % at the excitation wavelengths 320 and 394 nm, respectively which were higher than that of Ca2SiO4: Eu3+ (91.2%), CaWO4: Eu3+(91.3%), SrMoO4: Eu3+ (85.8%), and many others red emitting phosphors.43-45 15 ACCEPTED MANUSCRIPT Meanwhile, the emission properties of the optimized BYVO: Eu0.2 phosphor was compared with the commercially available Y2O3: Eu3+ red phosphor under the UV and blue radiations with excitations wavelengths of 394 and 464 nm, respectively (Fig. 9). These two RI PT excitation wavelengths are selected based on the excitation spectrum recorded for the BYVO: Eu0.2 phosphor at λem = 615 nm (Fig. 9). As seen from the figure, the emission intensities as well as the emitting regions of the commercial red color emitting Y2O3:Eu3+ phosphors are lower than M AN US C that of the optimized BYVO: Eu0.2 phosphor. The FWHM value of the emission band ranging from 600 to 620 nm of the BYVO: Eu0.2 phosphor is found to be 10.3 nm whereas the commercial Y2O3:Eu3+ phosphor had a FWHM value of 7.4 nm, indicating that BYVO: Eu0.2 phosphor have much broader emission as compared to commercial Y2O3:Eu3+ phosphor. It is well known that Y2O3:Eu3+ phosphor is one of the best luminescent phosphors that has been widely used in display technology to generate red-color light. It shows excellent luminous D efficiency and significant red color purity.46 Therefore, better photoluminescence intensity and TE red color purity of the optimized phosphor than that of the commercially available Y2O3:Eu3+ phosphor (Fig. 9) indicated the suitability of the present system for display applications. EP Moreover, the emission properties of the optimized BYVO: Eu0.2 phosphor was also compared with the similar vanadate host Sr3(VO4)2: Eu3+ red phosphor (Fig. S5). The synthetic procedure AC C of Sr2.8(VO4)2: Eu0.2 phosphor is given in supporting information. Likewise, Y2O3: Eu3+ phosphors, the emission intensities as well as the emitting regions of the Sr2.8(VO4)2: Eu0.2 red phosphor are also lower than that of the optimized BYVO: Eu0.2 phosphor. Due to the intense 5D0 → 7F4 emission, BYVO: Eu0.2 phosphor offer natural white light when combined with the Y3Al5O12:Ce3+ (YAG:Ce3+) yellow phosphor because the excitation peaks (394 and 464 nm) of the BYVO: Eu0.2 phosphor overlapped well with the excitation band 16 ACCEPTED MANUSCRIPT of the YAG:Ce3+ phosphors. This type of phosphor-converted white-LED could contribute to high CRI display systems and the warm white light is also useful to create a decent atmosphere in the indoors. RI PT Generally, junction temperature of LEDs may rise up to 200◦C during its operation whereas the thermal quenching as well as the shift in the emission color start significantly even at above 120◦C. Consequently, the temperature-dependent PL of the optimum BYVO: Eu0.2 M AN US C phosphor was measured as a function of temperature in the range of 25-200°C (at an interval of 25°C) under the excitation wavelength of 394 nm (Fig. 10). At 200°C, the integrated emission intensity of the BYVO: Eu0.2 phosphor remained around 75% of those recorded at room temperature. This indicates that the thermal stability is good enough for the present phosphor for lighting applications especially in the white-LEDs. The electron-phonon interaction increases with increases intemperature. Due to this interaction, more electrons from excited state can be D thermally relaxed to the ground state, where upon release the energy by generating lattice TE vibration.47 However in the present host, the Ba/Y polyhedron edge-shares with three BaO6 octahedra and corner-shares with four VO4 tetrahedra giving a stable structure and that could be EP the main reason of the excellent thermal stability. Another parameter, activation energy was also AC C used to evaluate the thermal stability of the host lattice using Arrhenius equation:47-50 I: = S ( 'JTU VW Y X= (10) where Io is the emission intensity at room temperature, IT is the emission intensity at temperature T, A is the constant, and k is the Boltzmann constant. From the plot of ((Io/IT)-1) vs 1/kT (inset of Fig. 10), the Ea was calculated to be 0.40 eV for the present optimized BYVO: Eu0.2 phosphor which is higher than the previously developed Eu3+ based red-emitting phosphors such as 17 ACCEPTED MANUSCRIPT Na2Gd(PO4)(MoO4) (0.22 eV), Gd2ZnTiO6 (0.23 eV) and La2CaZnO5 (0.24 eV).48-50 High activation energy of BYVO: Eu0.2 sample indicating that the phosphor has a good thermal stability, and is suitable for white-LEDs applications.47 Eu0.2 phosphor were calculated using integrating sphere method:51 [ \ . ](\)6\ [ \^_(\) `(\)a6\ (11) α= [^_(\) `(\)a6\ [ \ ._(\) 6\ (12) M AN US C η= RI PT Moreover, the internal quantum yield (η) and the absorption efficiency (α) of the BYVO: where E(λ), R(λ), and P(λ) are the number of photons in the spectrum of excitation, reflectance and emission of the phosphors, respectively. The value of η and α for the BYVO: Eu0.2 phosphor at λex = 394 nm was found to be 59% and 28%, respectively. It is also a noticeable fact that the η value of the commercial Y2O3:Eu3+ at λex = 394 nm is 43%, which is lower than that of the D BYVO: Eu0.2 phosphor. Furthermore, the value of η and α for the BYVO: Eu0.2 phosphor at λex = TE 464 nm were found to be 55% and 26%, respectively. The excellent internal efficiency and the absorption efficiency indicated the suitability of the present materials for practical applications. EP For further testing the application of BYVO: Eu0.2 phosphor in white-LEDs, a white-LED device was fabricated using a combination of commercial YAG phosphors coated on a blue LED AC C chip along with the LED fabricated from the combination of the our optimum BYVO: Eu0.2 phosphor and commercial YAG phosphor coated on a blue LED chip and their EL results are shown in Fig. 11. The resultant CIE coordinate approached towards even more bright white light [(0.38, 0.36) → (0.34, 0.33)] upon coating with our red emitting BYVO: Eu0.2 phosphor (Fig. 12). Their digital images showing bright warm white light under 280 mA current are shown in the inset of Fig. 12. Similarly, the correlated color temperature (CCT) value was also improved from 3245 K to 5478 K and a Ra value was increased from 83 to 86 with the incorporation of the 18 ACCEPTED MANUSCRIPT present phosphor. The results obtained from this combination of phosphors suggested its potential applications for warm white-LEDs. RI PT Conclusion Herein, undoped and Eu3+-doped Ba2Y0.67δ0.33V2O8 microcrystals in hexagonal symmetry have been successfully synthesized by employing a conventional solid state reaction. The PXRD pattern was fitted by the Rietveld refinement by considering the structural model of hexagonal M AN US C palmierite in the R-3m (#166) space group. Results of photoluminescence excitation and emission spectra suggested that the optimum doping concentration of Eu3+ ions is x = 0.20. The results of CIE coordinates and the red color purity of the BYVO: Eu0.2 phosphor at λex = 320, 394 and 464 nm are found to be very close to the commercial red emitting phosphor. Additionally, BYVO: Eu0.2 phosphor showed an excellent thermal stability, which was around 75% at 200⁰C. Furthermore, the optimized phosphor exhibited high internal quantum yield and absorption D efficiency of around 59%, 28% and 55%, 26%, respectively, at excitation wavelengths of 394 nm TE and 464 nm. Consequently, a white-LED comprising the optimized phosphor was also fabricated which showed bright white emission with a color rendering index (Ra) of 86, a color temperature EP of 5478 K, and CIE color coordinates of (0.34, 0.33). Therefore, our results indicated that the AC C BYVO: Eu0.2 phosphor may be a suitable candidate as a red phosphor for white-LEDs under effective excitation at near-ultraviolet and blue light. Acknowledgements The present research is financially supported by the Science and Engineering Research Board (SERB), DST, Government of India, through Early Career Research Award (ECR/2017/002210). We thank Dr. Prabhakar Rao P. and Ms. Sreena T., CSIR-NIIST, Thiruvananthapuram for the PL and X-ray diffraction measurements. 19 ACCEPTED MANUSCRIPT References 1. X. Ren, X. Zhang, N. Liu, L. Wen, L. Ding, Z. Ma, J. Su, L. Li, J. Han and Y. Gao, White RI PT light-emitting diode from Sb-doped p-ZnO nanowire arrays/n-GaN film, Adv. Funct. Mater. 25 (2015) 2182−2128. 2. X. Zhang, L. Li, J. Su, Y. Wang, Y. Shi, X. Ren, N. Liu, A. Zhang, J. Zhou and Y. Gao, M AN US C Bandgap engineering of GaxZn1–xO nanowire arrays for wavelength-tunable light-emitting diodes, Laser Photonics Rev. 8 (2014) 429–435. 3. S. Ahmad, R. Nagarajan, P. Raj and G. V. Prakash, Novel fluorite structured superparamagnetic RbGdF4 nanocrystals as versatile upconversion host, Inorg. Chem. 53 (2014) 10257−10265. 4. N. J. 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Swart, Red-light-emitting inorganic La2CaZnO5 frameworks with high photoluminescence quantum efficiency: Theoretical approach, Mater. Des. 93 (2016) 203–215. standard samples, J. Illum. Eng. Inst. Jpn., 83 (1999) 87–93. Fig. 1 M AN US C Figure Captions RI PT 51. K. Ohkubo and T. Shigeta, Absolute fluorescent quantum efficiency of NBS phosphor Rietveld refinement of the PXRD pattern of the Ba2Y0.67δ0.33V2O8 sample. Observed, calculated (profile matching), and difference profiles are given respectively as black, red, and blue lines and the Bragg positions as pink vertical lines. Structure of hexagonal Ba2Y0.67δ0.33V2O8 using the DIAMOND 3 program is shown as an inset. DRS of the Eu3+ doped and undoped BYVO samples. Fig. 3 PL excitation (λem = 440 nm) and emission (λex = 320 nm) spectrum of BYVO TE D Fig. 2 sample. PL emission spectra of Eu3+ doped BYVO phosphors at λex = 320 nm. Fig. 5 Schematic model of excitation, energy transfer (ET), non-radiative (NR), EP Fig. 4 AC C resonance (RN) and emission processes in Eu3+ doped BYVO phosphors under 320 and 394 nm excitation energy. Fig. 6 (a) PL excitation spectra of Eu3+ doped BYVO phosphors at λem = 615 nm. (b) PL emission spectra of Eu3+ doped BYVO phosphors at λex = 394 nm. The digital photograph of the red-orange glow emission of the BaYVO: Eu0.2 phosphor is shown in the inset. 26 ACCEPTED MANUSCRIPT Fig. 7 Relative emission intensity of 5D0→7F2 transition as a function of Eu3+ ions concentrations. Inset shows the relationship plot between ln(I/X) vs lnX for emission of 5D0→7F2 transition under the excitation of 394 nm. CIE (X, Y) coordinate diagram of Eu3+ doped BYVO phosphors for the emission spectra recorded at λex = 394 nm. Fig. 9 RI PT Fig. 8 PL emission spectra of the BYVO: Eu0.2 and commercial Y2O3:Eu3+ phosphors M AN US C under excitations wavelengths of 394 and 464 nm. PL excitation spectrum of the BYVO: Eu0.2 phosphor at λem = 615 nm. Fig. 10 Temperature-dependent PL emission intensities of the BYVO: Eu0.2 phosphor. Inset shows the plot of ln(I0/IT - 1) versus 1/kT of the BYVO: Eu0.2phosphor. Fig. 11 The EL spectra of a (a) commercial YAG phosphors coated on a blue LED chip and (b) BYVO: Eu0.2 and commercial YAG phosphors coated on a blue LED TE CIE (X, Y) coordinate diagram of commercial (a) YAG phosphors coated on a blue LED chip (0.38, 0.36) and (b) BYVO: Eu0.2 and commercial YAG phosphors EP coated on a blue LED chip (0.34, 0.33). Their digital photographs of the bright warm white light under 280 mA is shown in the insets. AC C Fig. 12 D chip. 27 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. 1 Fig. 2 28 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. 3 Fig. 4 29 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. 5 30 AC C EP TE D M AN US C RI PT ACCEPTED MANUSCRIPT Fig. 6 31 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. 7 Fig. 8 32 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. 9 Fig. 10 33 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. 11 Fig. 12 34 ACCEPTED MANUSCRIPT Table 1: Crystallographic data of hexagonal Ba2Y0.67δ0.33V2O8 (Ba2Y0.67□0.33V2O8)3 Crystal system Hexagonal Space Group R-3m(# 166) a [Å] 5.7797 (1) c [Å] 21.2894 (3) V [Å3] 616.18 (2) Z 3 M AN US C RI PT Formula ρcalc [g/cm3] 4.558 1.52 χ2 Rp(%) 0.1369 Rwp (%) 0.1876 GOF (S) 1.23 0.0334225°/20.32 sec Step size/ Step time Number of data 2394 (2θ = 10-90º) Number of D 298 K TE Temperature 32 AC C EP Table 2: Refined Atomic parameters after the Final cycle of Refinement. Atom Site X Y z Ba1 6c 0 0 0.20652 0.5000 0.025 Y1 6c 0 0 0.20652 0.3333 0.025 Ba2 3a 0 0 0 1.0 0.025 O1 18h 0.50126 0.49874 0.22889 1.0 0.025 O2 6c 0 0 0.33211 1.0 0.025 V1 6c 0 0 0.40855 1.0 0.025 35 S.O.F Uiso ACCEPTED MANUSCRIPT Table 3:J-O intensity parameters ( 2 and 4),radiative transition probabilities (Arad), branching ratios (β), total radiative transition probability (AT), radiative lifetime (τrad) and asymmetry ratio of the Ba2Y0.67-xδ0.33V2O8: xEu3+ (x = 0.10 to 0.67) phosphors. A0-1 (s-1) (s-1) 5 - 50 5 191.11 5 51.72 5 - 5 375.58 5 34.32 5 - 5 337.42 5 5 4 (pm2) (pm2) 1.66 0.93 D0- 7F1 D0- 7F2 3.27 0.62 65.3 - 17.6 50 10.8 - 81.7 - 7.5 50 11.6 - 78.1 44.55 - 10.3 - 50 10.9 363.87 - 79.1 46.27 - 10.0 5 - 50 16.8 5 220.18 - 73.9 5 27.41 - 9.3 5 - 50 12.5 5 288.45 - 71.9 5 62.39 - 15.5 D0- 7F1 D0- 7F2 D0- 7F4 0.30 2.94 0.80 D0- 7F1 D0- 7F2 D0- 7F4 3.17 0.84 D0- 7F1 5 TE 0.40 D0- 7F2 D0- 7F4 0.49 AC C 0.67 1.92 EP 5 0.50 2.51 1.12 17.1 - D0- 7F4 0.20 Branching ratio (%) D0- 7F1 D0- 7F2 D0- 7F4 D0- 7F1 D0- 7F2 D0- 7F4 AT 36 τrad Asymmetric ratio RI PT 0.10 2 A0-2,4 Transiti ons (s-1) (s) 292.83 0.0034 3.68555 459.90 0.0022 7.24303 431.97 0.0023 6.50706 460.14 0.0022 7.0172 297.59 0.0034 4.24605 400.83 0.0025 5.56265 M AN US C x J-O intensity parameters D Eu3+ Conc ACCEPTED MANUSCRIPT Table 4: The asymmetry ratio, CIE chromaticity coordinates, and color purity of Ba2Y0.67δ0.33V2O8:Eu3+ phosphors with different Eu3+ concentrations (λex = 320 and 394 nm) CIE coordinates (x, y) Red colour purity (Ra) RI PT Eu3+ 394 nm 320 nm 0.00 (0.16, 0.13) (0.16, 0.13) -- 0.10 (0.66, 0.33) (0.66, 0.34) 97.8 0.20 (0.67, 0.33) (0.66, 0.34) 99.6 0.30 (0.67, 0.32) (0.65, 0.34) 97.3 95.6 0.40 (0.68, 0.31) (0.64, 0.33) 95.5 89.7 0.50 (0.68, 0.30) (0.64, 0.33) 95.4 88.3 0.67 (0.69, 0.28) (0.61, 0.34) 95.3 80.5 AC C EP TE D M AN US C 320 nm 37 394 nm -- 99.3 99.4 ACCEPTED MANUSCRIPT Supplementary material Structure and optoelectronic properties of palmierite structured Ba2Y0.67δ0.33V2O8: Eu3+ RI PT red phosphors for n-UV and blue diode based warm white light systems Shisina S.,a Subrata Das,*a S. Som,b Shahzad Ahmad,*c Vinduja V.,a Merin P.,a K.G. Nishanth,a M AN US C Puja Kumari,d Mukesh Kumar Pandeye *E-mail: subratadas@niist.res.in, shahzadncfm@gmail.com; Fax: +91-471-2491712; b. c. d. e. Materials Science and Technology Division, CSIR – National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala-695019, India. Department of Chemical Engineering, National Taiwan University, Taipei-10617, Taiwan, ROC. Department of Chemistry, Zakir Husain Delhi College, University of Delhi, Delhi110002, India. Department of Physics, Darbhanga College of Engineering, Darbhanga-846005, India. Department of Physics, National Taiwan University, Taipei-10617, Taiwan, ROC. AC C a. EP TE D Tel: +91-471-2515360 38 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. S1 PXRD patterns of the product from the reaction of Ba2CO3, Y2O3 and V2O5 Fig. S2 Le-Bail refinement (red line) of the observed PXRD pattern (black line) of the product and residuum. 39 M AN US C RI PT ACCEPTED MANUSCRIPT AC C EP TE D Fig. S3 SEM image and EDS spectrum of undoped BYVO sample. Fig. S4 Schematic model of absorption and emission processes of [VO43-] tetrahedron in BYVO crystal. 40 D0 - F2 ACCEPTED MANUSCRIPT BYVO:Eu0.2 D0 - F 2 Sr2.8(VO4)2:Eu0.2 7 7 BYVO:Eu0.2 625 650 675 575 600 RI PT 625 7 7 5 600 M AN US C 7 D0 - F3 5 575 D0 - F 3 λex = 464 nm D0 - F1 λex = 394 nm 5 5 7 Intensity (a.u.) D0 - F 1 5 5 Sr2.8(VO4)2:Eu0.2 650 675 Wavelength (nm) Fig. S5 PL emission spectra of the BYVO: Eu0.2 and Sr2.8(VO4)2: Eu0.2 phosphors under the TE D excitation wavelengths of 394 and 464 nm. AC C EP Table S1: Selected bond lengths and angles for Ba2Y0.67δ0.33V2O8 Bond Length V1 O2 × 1 O1 × 3 1.6276 1.7928 Ba1/Y1 O2 × 1 2.6742 O1 × 3 2.7326 O1 × 6 2.9293 O1 × 6 2.7727 Ba2 Bond O2 O1 41 V1 V1 O1 O1 110.31 108.62 ACCEPTED MANUSCRIPT Synthesis of Sr2.8(VO4)2: Eu0.2 In this work, Sr2.8(VO4)2: Eu0.2 sample was prepared by solid state reaction. Highly pure SrCO3 RI PT (99.8%), V2O5 (≥ 99.6%) and Eu2O3 (99.999%) obtained from Sigma-Aldrich were used. Stoichiometric amounts of SrCO3, V2O5 and Eu2O3 were thoroughly grounded via mortar pestle for 30 minutes. The mixture was first heated in an alumina crucible at 500˚C for 6 hours, followed by firing at 900˚C for 5 hours. Repeated grindings was performed between two M AN US C sintering processes to improve the homogeneity.1 Reference 1. R. Cao, D. Peng, H. Xu, Z. Luo, H. Ao, S. Guo and J. Fu, Synthesis and luminescence properties of Sr3(VO4)2:Eu3+ phosphor and emission enhancement by co-doping Li+ ion, Optik AC C EP TE D 127 (2016) 7896–7901. 42 ACCEPTED MANUSCRIPT (Ba2Y0.67□0.33V2O8)3 Crystal system Hexagonal Space Group R-3m(# 166) a [Å] 5.7797 (1) c [Å] 21.2894 (3) V [Å3] 616.18 (2) Z 3 ρcalc [g/cm3] 4.558 1.52 χ2 Rp(%) 0.1369 Rwp (%) 0.1876 GOF (S) 1.23 Step size/ Step time Number of data Number of EP 0.0334225°/20.32 sec 2394 (2θ = 10-90º) D 32 TE Temperature AC C M AN US C Formula RI PT Table 1: Crystallographic data of hexagonal Ba2Y0.67□0.33V2O8 298 K ACCEPTED MANUSCRIPT Table 2: Refined Atomic parameters after the Final cycle of Refinement. Site X Y z S.O.F Uiso Ba1 6c 0 0 0.20652 0.5000 0.025 Y1 6c 0 0 0.20652 0.3333 0.025 Ba2 3a 0 0 0 1.0 0.025 O1 18h 0.50126 0.49874 0.22889 1.0 0.025 O2 6c 0 0 0.33211 1.0 V1 6c 0 0 0.40855 1.0 AC C EP TE D M AN US C RI PT Atom 0.025 0.025 ACCEPTED MANUSCRIPT Table 3: J-O intensity parameters ( 2 and 4), radiative transition probabilities (Arad), branching ratios (β), total radiative transition probability (AT), radiative lifetime (τrad) and asymmetry ratio of the Ba2Y0.67-x□0.33V2O8: xEu3+ (x = 0.10 to 0.67) phosphors. 0.20 0.30 (pm2) 1.66 0.93 2.94 0.67 0.62 0.80 - 50 17.1 5 D0 - 7F2 191.11 - 65.3 5 D0 - 7F4 51.72 - 17.6 5 D0 - 7F1 - 50 10.8 5 D0 - 7F2 375.58 - 81.7 5 D0 - 7F4 34.32 - 7.5 5 D0 - 7F1 - 50 11.6 - 78.1 - 10.3 0.49 337.42 5 D0 - 7F4 44.55 5 D0 - 7F1 - 50 10.9 5 D0 - 7F2 363.87 - 79.1 5 D0 - 7F4 46.27 - 10.0 5 D0 - 7F1 - 50 16.8 5 D0 - 7F2 220.18 - 73.9 5 D0 - 7F4 27.41 - 9.3 2.51 1.12 5 D0 - 7F1 - 50 12.5 5 D0 - 7F2 288.45 - 71.9 5 D0 - 7F4 62.39 - 15.5 TE 0.84 D0- 7F2 EP 1.92 (s-1) D0 - 7F1 AC C 0.50 3.17 (s-1) Branching ratio (%) 5 5 0.40 A0-1 4 (pm2) 3.27 A0-2,4 AT τrad (s-1) (s) Asymmetric ratio RI PT 0.10 2 Transiti ons 292.83 0.0034 3.68555 459.90 0.0022 7.24303 431.97 0.0023 6.50706 460.14 0.0022 7.0172 297.59 0.0034 4.24605 400.83 0.0025 5.56265 M AN US C x J-O intensity parameters D Eu3+ Conc ACCEPTED MANUSCRIPT Table 4: The asymmetry ratio, CIE chromaticity coordinates, and colour purity of Ba2Y0.67□0.33V2O8: Eu3+ phosphors with different Eu3+ concentrations (λex = 320 and 394 nm) Eu3+ CIE coordinates (x, y) Red colour purity (Ra) 394 nm 320 nm 394 nm 0.00 (0.16, 0.13) (0.16, 0.13) -- -- 0.10 (0.66, 0.33) (0.66, 0.34) 97.8 0.20 (0.67, 0.33) (0.66, 0.34) 99.6 0.30 (0.67, 0.32) (0.65, 0.34) 97.3 0.40 (0.68, 0.31) (0.64, 0.33) 95.5 0.50 (0.68, 0.30) (0.64, 0.33) 95.4 88.3 0.67 (0.69, 0.28) (0.61, 0.34) 95.3 80.5 M AN US C D TE EP AC C RI PT 320 nm 99.3 99.4 95.6 89.7 ACCEPTED MANUSCRIPT Table S1: Selected bond lengths and angles for Ba2Y0.67□0.33V2O8 Bond Length O2 × 1 O1 × 3 1.6276 1.7928 Ba1/Y1 O2 × 1 2.6742 O1 × 3 2.7326 O1 × 6 2.9293 O1 × 6 2.7727 V1 V1 O1 O1 110.31 108.62 AC C EP TE D M AN US C Ba2 O2 O1 RI PT V1 Bond ACCEPTED MANUSCRIPT Hexagonal structured Ba2Y0.67□0.33V2O8 has been synthesized by solid state method • Ba2Y0.67□0.33V2O8 emits blue light under the UV excitation due to [VO43−] group. • Ba2Y0.67-xEux□0.33V2O8, emits bright red light under the UV/blue excitations • The optimized phosphor shows higher red purity and quantum yield than Y2O3:Eu3+ • Red and yellow phosphor combination shows bright white light with Ra;86 and RI PT • AC C EP TE D M AN US C CCT;5478K