Elucidating the structure and optimising the photoluminescence properties of Sr2Al3O6F: Eu3+ oxyfluorides for cool white-LEDs

Journal Pre-proof Elucidating the structure and optimising the photoluminescence properties of 3+ Sr2Al3O6F: Eu oxyfluorides for cool white-LEDs P. Ranjith, S. Sreevalsa, Jyoti Tyagi, K. Jayanthi, G. Jagannath, Pritha Patra, Shahzad Ahmad, K. Annapurna, Amarnath R. Allu, Subrata Das PII:

S0925-8388(20)30378-9

DOI:

https://doi.org/10.1016/j.jallcom.2020.154015

Reference:

JALCOM 154015

To appear in:

Journal of Alloys and Compounds

Received Date: 17 November 2019 Revised Date:

21 January 2020

Accepted Date: 22 January 2020

Please cite this article as: P. Ranjith, S. Sreevalsa, J. Tyagi, K. Jayanthi, G. Jagannath, P. Patra, S. Ahmad, K. Annapurna, A.R. Allu, S. Das, Elucidating the structure and optimising the 3+ photoluminescence properties of Sr2Al3O6F: Eu oxyfluorides for cool white-LEDs, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.154015. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Credit author statement

Ranjith P. and Sreevalsa S.: Carried the materials synthesis, characterization, PL recording, device making, plotting, etc. Jyoti Tyagi: Performed the DFT calculations. K.

Jayanthi: Carried

Jagannath: Completed

the

JO

the

crystallographic parameters

analysis. G.

calculations. Pritha

Patra: Recorded the PL spectra. Shahzad Ahmad: Performed the XRD refinement and structural analysis. Annapurna K: Contributed to concept discussion and in paper writing. Amarnath R. Allu: Helped in PL analysis, concept discussion and in paper writing. Subrata Das: Mainly supervised the whole research from content to final analysis.

Elucidating the structure and optimising the photoluminescence properties of Sr2Al3O6F: Eu3+ oxyfluorides for cool white-LEDs Ranjith P.1, Sreevalsa S.1, Jyoti Tyagi2, K. Jayanthi3, G. Jagannath4, Pritha Patra5, Shahzad Ahmad2, *, Annapurna K.5, Amarnath R. Allu5, *, Subrata Das1, *

*E-mail: [email protected]; [email protected]; [email protected]; Fax: +91-471-2491712; Tel: +91-471-2515360

1

2

3

4 5

Materials Science and Technology Division, CSIR – National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram, Kerala695019, (India). Department of Chemistry, Zakir Husain Delhi College, University of Delhi, Delhi110002, India Peter A. Rock Thermochemistry Laboratory and Neat ORU, University of California, Davis Davis CA 95616 Department of Physics, Bangalore University, Bengaluru, Karnataka 560056, India Glass Division, CSIR-Central Glass and Ceramic Research Institute, 700032 Kolkata, India

1

Abstract Herein, Sr2Al3O6F in hexagonal symmetry was synthesized via a solid-state methodology. The X-ray diffraction pattern of Sr2Al3O6F was refined by the Rietveld refinement with lattice parameters a = 17.8232(1) Å and c = 7.2168(0) Å. The stability of the crystal structure is further confirmed from the results of bond valence sums and the global instability index. The theoretical calculations of the electronic and optical behaviors of the Sr2Al3O6F were analyzed by density functional theory and the obtained results of the lattice parameters and direct bandgap were found close to the experimental data. The chemical states and elemental composition of Sr2Al3O6F were also authenticated by X-ray photoelectron spectroscopy (XPS). To evaluate the suitability of the Sr2Al3O6F structure as high efficient red phosphor, a series of Eu3+ doped Sr2xEuxAl3O6F

(x = 0.0 to 0.10) were synthesized, which showed intense red-orange emission

(5D0→7F1,2) at UV and blue excitations. The photoluminescence intensity corresponding to 5

D0→7F2 transition decreased significantly for x = 0.10 due to the luminescence quenching.

Nevertheless, further enhancement in photoluminescence of Sr1.9Al3O6F: Eu0.1 sample was realized with the substitution of 0.1 mol Ba2+ ion for 0.1 mol Sr2+ ion. The various radiative properties of the emission bands were also analyzed through the Judd-Ofelt theory. The optimized Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor showed high red color purity (> 95%), and moderate thermal stability of around 72% at 150 oC, suggesting that it could be an ideal red component for white-LEDs. A white-LED comprising the commercial yellow phosphor and the optimized sample showed bright white light having the CRI of 80.5%, CCT of 5510 K, and CIE of (0.33, 0.36) indicating that Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor is an appropriate red component for cool white-LEDs.

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Keywords: Hexagonal Sr2Al3O6F; Rietveld refinement; density functional theory; Eu3+-doping; red phosphors; white-LEDs 1. Introduction Phosphor based White-LEDs have become a very interesting topics of research due to their high-end utilization in solid-state lighting. These types of lighting exhibit excellent features including higher brightness, less energy consumption, high durability, low thermal radiation, long lifetime, etc.1 Commercial white-LED appliances are produced by combining a yellow color emitting YAG: Ce3+ with a blue color emitting InGaN chip.2 Unfortunately, this system faces some severe issues, such as thermal quenching, reproducibility of color, low color rendering index (CRI) due to the absence of red color component leads to their restricted practical applications.3 To improve CRI, many attempts have been made by integrating red color emitting materials such as nitride phosphors (Sr2Si5N8: Eu2+),4 sulfide phosphors (SrGa2S4 / Ca1-xSrxS: Eu2+),5 and quantum dots (Sr3SiO5:Ce3+, Li+ / CdSe).6 However, these materials possess either tough synthesis conditions, toxicity, poor stability or uses of corrosive starting materials, and the production cost is also high. These facts are largely restricting their applied utility.7 Therefore, exploring newer red-emitting phosphors with better stability, prominent luminescence efficiency as well as good color purity is required for improved white-LED devices. Recently, fluoride-containing oxides activated with rare earths are immerged as the potential phosphors for white-LEDs.8-13 The introduction of fluoride ions into an oxide lattice generates a distortion of the coordination polyhedrons i.e. generation of non-centrosymmetric sites, which causes unusual spectroscopic properties.14 Moreover, the fluoride host matrices are known to provide a wide bandgap, less phonon energies and low inter-configurational transitions whereas the chemically stable oxides host matrices are known to provide high absorption in UV

3

to near-UV zone.9 Additionally, the ionic radii of O2- and F- ions are also analogous and thus steady crystals are usually obtained in oxyfluorides at various O/F contents. Because of such unique nature, the oxyfluorides are widely investigated in various energy sectors including battery, catalyst, magnetic and UV shielding properties, etc.15-20 Sr3AlO4F is one of the most widely investigated oxyfluorides phosphor due to its good chemical and thermal stability, economical starting materials, facile synthesis conditions, and efficient luminescence properties.21-25 Shang and co-workers published the white-colored light emission from Sr3AlO4F: Tm3+/Tb3+/Eu3+ at 360 nm UV excitation.12 Chen et. al. reported the exciting luminescence efficiency of Sr3AlO4F: Ce3+/Na+ under the 395 nm UV light.26 Jang and co-workers showed Eu3+/Dy3+ co-doped Sr3AlO4F phosphor as a prominent host for a white-light emission under the excitation of 384 nm.9 Recently, oxyfluoride with the general composition Ca2Al3O6F

show

interesting

luminescence

properties

when

doped

with

rare-earth

activators.14,27,28 Although this system is associated with the CaO–Al2O3–CaF2 ternary system,29 its preliminary structural characterization and luminescence properties are firstly reported by Xia and co-workers. 14,27,28 Their results specify that Ca2Al3O6F: Ce3+/Tb3+ and Ca2Al3O6F: Eu2+ are appropriate green-emitting phosphors for the near-UV based white-LEDs. Nevertheless, the luminescence behavior of a phosphor can be altered by the chemical substitutions of alkaline earth ions with another.8,10,30 Thus an attempt has been made to synthesize Sr2Al3O6F sample with hexagonal symmetry are synthesized by replacing Ca with Sr in Ca2Al3O6F. It is to be noted at this point that up to the author’s knowledge Sr2Al3O6F phosphor material has not been reported so far in the open literature. In the present investigation, a facile solid-state methodology has been adopted to produce highly crystalline and homogeneous Sr2Al3O6F material. We used Sr2Al3O6F system as the host matrix, which was activated with

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Eu3+ ions to produce red emission suitable for white-LEDs. Thorough structural studies including Rietveld refinement, bond valence sums, and Raman spectroscopy, were carried out. The quantifications of elemental composition were employed using X-ray photoelectron spectroscopy (XPS). The electronic properties like band structure of the obtained sample were studied via density functional theory (DFT). The optical behavior of a series of Eu3+ doped Sr2Al3O6F host have also been studied to elaborate the optimized concentration of Eu3+ ions. Additionally, an attempt has been made to further enhance the optical properties of the Sr1.9Al3O6F: Eu0.1 sample by partially substituting 0.1 mol Ba2+ ion for 0.1 mol Sr2+ ion. Both Eu3+ doped as well as Ba2+ co-doped samples showed high red color purity, CRI, and CCT, which makes the red phosphor a suitable component for near-UV/blue LED chip-based whiteLEDs.

2. Experimental 2.1. Materials and methodology A series of Sr2-xAl3EuxO6F (x = 0.00, 0.01, 0.03, 0.05, 0.07 and 0.10) phosphors were synthesized via conventional solid-state reaction route. Pure SrCO3 (99.8%), Al2O3 (≥ 99.6%), SrF2 (99.99%), and Eu2O3 (99.999%) purchased from Sigma-Aldrich were used. Stoichiometric contents of starting materials excluding slightly excessive SrF2 (5 wt%) were meticulously grounded. The resultant grounded mixture was then transferred to an alumina cup crucible and fired in the air at 1250˚C for 4 hrs. The Sr1.8Al3O6F: Ba0.1/Eu0.1 sample was also prepared by employing a similar method wherein BaCO3 (99.8%) was used as a Ba2+ source. The X-ray diffraction (XRD) characteristics of synthesized materials were measured using Ni filtered Cu-Kα (λ = 1.54 Å) at 45 kV and 40 mA via Philip’s x’pert pro diffractometer.

5

The electronic structure of Sr2Al3O6F was estimated using the DFT modelling, and the calculation details are given in supporting information. The Raman spectrum of the undoped Sr2Al3O6F was collected from a Confocal Raman Microscope (alpha 300 R WITEC Germany). The UV-Vis diffused reflectance spectra (DRS) were recorded using Shimadzu UV 3600. The photoluminescence

(PL)

characteristics

were

measured

using

a

YvonFluorolog

3

spectrofluorimeter with a 450W Xenon flash lamp as the exiting source. The XPS scans were carried out using Omicron ESCA+, Oxford Instruments Germany. Al Kα radiation (1.487 keV) at a resolution of 0.1 eV was utilized to obtain the core-level spectra of various specimens. Furthermore, the core level spectrum of C 1s at 285.0 eV was recorded as a reference and the peak positions were calibrated according to it. The peak positions and the areas of chemically dissimilar types were determined using the Gaussian-Lorentzian distribution fitting process. The color coordinates of Commission Internationale de I’Eclairage (CIE) were estimated via a color calculator software. The white-LED modules were fabricated by mixing the optimal ratio of red phosphor with the Y3Al5O12: Ce3+ yellow phosphor. This mixture was then dispersed in transparent silicon resin for making the phosphor blend which was pasted on a 460 nm blue InGaN LED chip, which was operated at 300 mA current. The PL properties of the fabricated LEDs were measured using a CCE spectrophotometer (OCEAN-FX-XRI-EX).

3. Result and discussion The XRD pattern of Sr2Al3O6F is shown in Fig. 1. The observed reflections are strong indicating good crystallinity. The peak intensities and their positions are narrowly matching with the hexagonal Ca2Al3O6F (JCPDS. 17-0107). Nevertheless, the diffraction peaks of Sr2Al3O6F

6

are shifted slightly towards the lower angle side in comparison with Ca2Al3O6F. Such a shift might be happening due to the difference in the ionic size of Ca2+ (CaIX: 1.18 Å) to Sr2+ (SrIX: 1.31 Å).31 Therefore, the model of hexagonal Ca2Al3O6F (space group R-3 #148) has been utilized for the Rietveld refinement in the present work. The peak positions of the calculated XRD pattern produced from the Rietveld refinement are very narrowly matching with the experimental pattern (Fig. 2). The estimated lattice parameters from the Rietveld refinement of Sr2Al3O6F are a = 17.8232 (1) Å and c = 7.2168 (0) Å, which are higher than the standard parameters of Ca2Al3O6F. Also, these experimental values are in an excellent match with the computational values estimated from the DFT (a = 17.83 Å and c = 7.22 Å). The average difference between the experimental and calculated lattice parameters are 0.048% and 0.043% for the lattice parameter a and c respectively. A minor impurity of monoclinic SrAl2O4 (JCPDS No. 34-0379) based on the two very minute unmatched peaks observed at 28.36o and 29.21o (inset of Fig. 1) was also identified and incorporated in the Rietveld refinement. The relative amount of SrAl2O4 was found to be 0.04 mol%. Table 1 listed the crystallographic data and parameters of hexagonal Sr2Al3O6F. Whereas Table 2 exhibited values of the refined Wyckoff positions along with the isotropic thermal parameters. The two dimensional unit cell structure of Sr2Al3O6F crystal is shown in Fig. 2 inset. The structure of Sr2Al3O6F consists of two kinds of distorted [AlO4]5- tetrahedra such that the Al1 tetrahedrons linked through corners to form sixmembered rings while the Al2 tetrahedrons linked to these six-membered rings through its two corners to form a helix along the c-axis. The Sr atoms occupy two sites: 18f (Sr1) and 6c (Sr2). The Sr2 atom which lies at the center of the six-membered ring is surrounded by the six F atoms. The six-membered rings of the Al1 tetrahedrons are further surrounded by the six Sr1 atoms. Fig.

7

S1 of the supporting information shows the selected coordination models of Sr1 [SrO7F2] −14 and Sr2 [SrO3F6]−10 in hexagonal Sr2Al3O6F. The crystal structure of Sr2Al3O6F was further analyzed using the bond valence sum (BVS) and global instability index (GII) to measure the degree of strain present in a crystal structure and their detailed calculations are given in supporting information. The BVS calculated for the Sr1 is close to its formal charge (1.978), however, the BVS for Sr2 is slightly higher (2.213) (Table S1). Since the Sr1 is surrounded by 2 F & 7 O ions and the Sr2 ion is surrounded by 6 F & 3 O ions, it can be expected that the more positive charge due to higher electronegativity of F ions (Fig. S2). The GII of the Sr2Al3O6F was found to be 0.14 indicating a presence of a small amount of strain in its structure. Similar results are also reported in Sr3AlO4F compounds where the values of GII lie in the range of 0.23-0.40.32 According to Lufaso and Woodward,33 in compounds of oxides and halides with di- and trivalent cations, the relaxation of the strains can result from the atom being displaced from the center of the cavity, or by the ligands moving to ensure that the bonds have different lengths. Herein in this sample, the majority of the strain comes from the over-bonded Al(1) and the under-bonded Al(2) of a distorted [AlO4]5- tetrahedra. The BVS and the GII values of Sr2Al3O6F were further improved by doping with Ba2+ ions for Sr2+ ions. The stabilizing effect of the larger Ba2+ ions in Sr3AlO4F has been demonstrated by the Sullivan and co-workers.34 The ionic size of Sr2+ having ideal BVS is SrIX: 1.31 Å which decrease with increase in its BVS.31 Since the BVSs for Sr2Al3O6F show a minor under-bonding for the 18f sites and a slight over-bonding for 6c sites indicating that if the host is co-doped with Ba2+ and Eu3+ then the larger Ba2+ ions and smaller RE3+ ions would tend to occupy the 18f and 6c sites respectively. This is because the partial incorporation of the larger

8

isovalent Ba2+ ions (BaIX: 1.47 Å) onto the Sr1 occupying 18f site has a stabilizing effect on the structure. On the other hand, by similar consideration, the smaller trivalent Eu3+ (EuIX: 1.06 Å) ions will stabilize the structure by substitute for over-bonding Sr2 site.31 The core-level binding energies (BE) of an ion situated in a particular oxidation state depend on its chemical environment, which is decided by several factors such as coordination number, electronegativity, inter-atomic distances and the configuration of its ligands.35 A survey XPS scan is presented in Fig. 3, which confirms the presence of O, F, Al, and Sr atoms. The core-level spectrum of Sr 3d was initially deconvoluted into two peaks centered at 133.83 and 135.73 eV, with a separation of ~1.9 eV, are corresponding to Sr 3d5/2 and Sr 3d3/2, respectively with an intensity ratio of 1:0.41. Although the value of their energy separations can be accepted as it is close to their ideal separations gap (~1.79 eV) however, their intensity ratio is largely deviated from their ideal ratio (1:0.69).36 Since the Sr2+ ions occupied two crystallographic positions with slight differences in their valence oxidation states as calculated from BVS, it was assumed that the Sr 3d core level spectrum can be deconvoluted into two spin–orbit-split doublets. Consequently, the broadband of Sr 3d was deconvoluted into four peaks which are centered at 133.23, 133.98, 135.02 and 135.85 eV (Fig. 4 (a)). The first two peaks at 133.23 eV and 133.98 eV corresponds to Sr-3d5/2 while the latter two peaks at 135.02 eV and 135.85 eV corresponds to Sr-3d3/2.36 Since ions of a higher BVS exhibits a higher BE due to the increased columbic interaction between the photo-emitted electron and the nucleus of the ions. And the BVS of Sr2 (2.213) is higher as compared to Sr1 (1.978), therefore the bands at 133.23 and 135.02 eV are corresponds to Sr1 ions while the bands at 133.98 and 135.85 eV are corresponds to Sr2 ions. Moreover, the energy separations gap and intensity ratios of 3d5/2 and 3d3/2 both in Sr1 and Sr2 are also close to their ideal values. Similar deconvoluted peaks of Sr2+ in the XPS

9

were also reported earlier by Nsimama and co-workers for SrAl2O4.37 The core-level spectrum of Al 3p is centered at 73.7 eV which is a characteristic of Al atoms in an oxygen environment (Fig. 4 (b)).36 Unlike Sr 3d, the splitting of Al 2p into Al 2p3/2 and Al 2p1/2 components cannot be resolved in this spectrum due to their low spin-orbit splitting. The core-level spectrum of O 1s and F 1s are centered at 531.67 and 685.39 eV, respectively, which are close to their ideal values (Fig. 4 (c) & (d)).38 The elemental percentage composition can be estimated by the following formula:39 C =

∑

× 100 %

(1)

where Ix is the intensity of the element x and Sx is standing for the equivalent atomic sensitivity factor. The estimated values of concentration of Sr, Al, O and F were 16.58, 29.70, 43.64 and 10.08 %, respectively which is close to elemental composition. The Raman spectrum of a hexagonal Sr2Al3O6F is shown as Fig. 5. Simplified factor group analyses of the hexagonal Sr2Al3O6F using the Symmetry Adapted Modes (SAM) yielded the following number of phonon modes:40 Γvib = 25(Ag) + 25(1Eg) + 25(2Eg) + 25(Au) + 25(1Eu) + 25(2Eu)

(2)

Γacoustic = Au + 1Eu + 2Eu

(3)

Γoptic = 25Ag + 251Eg + 252Eg + 24Au + 241Eu + 242Eu

(4)

Γinfrared = 25Au + 251Eu + 252Eu

(5)

ΓRaman = 25Ag + 251Eg + 252Eg

(6)

The structure of hexagonal Sr2Al3O6F consists of a large number of crystallographically distinguishable Sr, Al, O and F atoms in the same Wyckoff-position leading to the lower number of observed peaks in the Raman spectrum than the number of peaks theoretically predicted from their Raman-active modes. This phenomenon is common for a crystal with a large number of 10

atoms in the same Wyckoff-position.41 These greater numbers of modes make the interpretation of the spectrum very complicated. From theoretical analysis, the symmetry and frequency of the most intense vibration in the Raman spectrum are Ag = 537 cm-1 which corresponds to the vibrational mode of the six-membered AlO4 tetrahedron ring.14 The low-frequency region of the Raman spectrum of energy less than 400 cm-1 is related to mixed vibrations appeared due to the Sr–O and Sr–F pair interactions while the high-frequency region over the range of 600-1000 cm-1 cover the AlO4 vibrations.42 The maximum phonon energy of this host has been found to be 537 cm−1 which is smaller than the similar hosts like Ca2Al3O6F (572 cm−1) and Sr3AlO4F (700 cm−1).14, 43 The low phonon energy of the host is favorable as the multi-phonon decay rate of rare-earth ions strongly depends on the phonon energy of the host.44 The band structure was calculated to study the electronic structure of Sr2Al3O6F, and the results are shown in Fig. 6 (a). The top level of the valence band and the bottom of the conduction band are located at the same Γ point of the Brillouin zone, indicating that the Sr2Al3O6F is a direct bandgap compound with a bandgap of approximately 4.76 eV (Fig. 6 (a)). The value of bandgap estimated from the band structure is very close to its experimental value of 5.02 eV estimated from the DRS of Sr2Al3O6F (discussed later). Additionally, the upper levels of the valence band are nearly flat, whereas the lower levels of the conduction band are dispersive, signifying that the mobility of the electron is higher than the mobility of the hole.45 To get more insight into the electronic structure of Sr2Al3O6F, pDOS (projected density of states) and total DOS (density of states) were also calculated and the results are figured in Fig. 7 (b). The inner core valence band at about −31.0 eV is derived from the Sr 4s and 5s states whereas the deeper valence band between -21.0 to -13.0 eV is derived from the F 2s, O 2s and Sr 4p states with a small contribution of Al 3s and 3p states (Fig. 6 (b)). The narrowness of the band indicates a high

11

degree of localization of these electronic states.45 The top of the valence band is dominated by the O 2p and F 2p states between −5.0 to 0.0 eV with a small contribution from the Al 3s and 3p states. The conduction band from 6.0 eV to about 11.0 eV is formed due to the Sr 4p states and Al 3s and 3p states with a small contribution of O 2p states. In Fig. 7 (a), the DRS of the undoped and Eu3+ doped Sr2Al3O6F samples are comprised of a narrow absorption band ranging from 200 to 250 nm with λmax = 230 nm, which can be entitled as the charge transfer band (CTB). According to the above DFT calculations, the valence band contains F and O state along with minor Al states. While the conduction band contains Sr and Al states, and fewer O states. The CTB is therefore attributed to the charge transfer from the levels of O2- levels situated in the valence band to the levels of Al3+ situated in AlO4]5− groups of the conduction band. The incorporation of Eu3+ ions in the host matrix shifts the CTB towards the shorter wavelength (λmax = 220 nm). Using the Tauc relation,46 the bandgap of the Sr2Al3O6F has been estimated to be 5.02 eV, which increased to 5.11 eV upon (x = 0.1) doping (inset of Fig. 7 (a)). The elevated electropositive nature of Eu3+ ions than that in Sr2+ might be responsible for this blue shift. The PL excitation as well as the emission spectra of Sr2Al3O6F sample, is depicted in Fig. 7 (b). Excitation spectrum recorded by monitoring λem = 414 nm contains a broadband ranging from 225 to 335 nm with λmax = 243 nm, which is deconvoluted and fitted with two Gaussian curves peaking at 243 and 283 nm. While the emission spectrum recorded under λex = 243 nm consists of a broadband ranging from 350 to 550 nm with λmax = 414 nm. Both the excitation as well as emission peaks are attributed to CTB (O2- - Al3+) in [AlO4]5- group. A similar band in this region is also reported earlier by Itou and co-workers.47 The position of this band is governed by the charge transfer from Al to the O, which strongly depends on the Al atoms coordination environment.

12

Fig. 8 (a) is showing the PL excitation spectra of Eu3+ doped Sr2Al3O6F recorded at 613 nm emission. The spectra consist of several sharp excitation lines peaking at 361, 380, 393, 412 and 464 nm due to the intra f–f Eu3+ transitions 7F0 → 5D4, 7F0 → 6G2–6 / 5L7, 7F0 → 5L6, 7F0 → 5

D3 and 7F0 → 5D2 respectively.48 Besides Eu3+ transitions, a broad band of CTB is also observed.

This is because the Eu3+ ions might be interacting with O2- ions strongly owing to which the mixing probability of 4f-orbitals of Eu3+ ions and 2p orbitals of O2- ions have been increased.48 Fig. 8 (b) shows the PL emission spectra of Eu3+ doped Sr2Al3O6F samples recorded at 280 nm wavelength of excitation. In the spectral range of 570–720 nm, the 5D0 → 7FJ (J = 0 - 4) emission transitions of Eu3+ ions are denoted in Fig. 8 (b). The broad blue emission centered at 430 nm is the CTB of the host sample as discussed earlier. Fig. 8 (c) and (d) exhibits the PL emissions of Eu3+ doped Sr2Al3O6F phosphors recorded at 393 and 464 nm, respectively. All the emission spectra display two main peaks; strong red and orange emission peaks centered at 615 and 594 nm, respectively. Additionally, the hypersensitive 5D0 → 7F2 (615 nm) electric dipole transition is having higher intensity than the 5D0 → 7F1 (594 nm) transitions of a magnetic dipole in nature. This outcome indicates that Eu3+ ions are mainly situated in the sites having low symmetry.48 However the extent of the splitting of the spectral lines is very small suggesting that the field possessing only slightly low symmetry.48 This further confirmes that the Eu3+ ions occupied least non-centrosymmetric sites 6c of the all other sites. Also, the presence of the 5D0 → 7F0 transition at 578 nm further corroborates that the Eu3+ ions are situated in the low symmetric sites.49 The difference of the asymmetric ratio [I (5D0 → 7F2) / I (5D0 → 7F1)] with the Eu3+ ion concentration is shown in Table S1 and Table S2. Higher the value of asymmetry ratio, the higher would be the color purity.48 The Eu3+ emission intensities linearly increased up to the Eu3+ concentration of x = 0.07, above that the PL emission decreased owing to the effect of

13

concentration quenching, which might be activated by the non-radiative energy transfer among the adjoining Eu3+ ions. Furthermore, the asymmetric ratio in the present phosphor is not much varied with the change of Eu3+ content since the Eu3+ local environment is independent of the doping concentrations. To find the concentration quenching mechanism, the distance among two adjacent Eu3+ ions recognized as the critical distance (Rc) has been calculated via the Blasse’s formula:50 R ≈2

/ π

(7)

where symbols have their usual meanings.52 For the optimized system, V = 1985.35 Å, N = 12 and the XC = 0.07. The RC value is calculated to be 8.26 Å, which is larger than 5.0 Å demonstrating that the quenching might be occurred because of the multi-polar interaction.48 Further, the nature of multi-polar interaction was analyzed using the Dexter’s formula:51

=

( )!/"

(8)

where k and β are constants and θ is approximately equal to 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. 9, was found to be −2.15. The value of θ was found to be 6.45, which is approximately equal to 6. Thus, the existence of concentration quenching effects is might be due to the dipole-dipole interactions.51 The Judd–Ofelt (J-O) spectroscopic parameters Ωt (2 and 4) are also calculated to understand the influence of Eu3+ content on the site symmetry and luminescence behavior of Sr2Al3O6F: Eu3+ phosphors.52,

53

The details of JO parameters and their calculation

methodologies are provided in the supporting information, and the estimated values are tabulated in Table S2 and Table S3. The values of estimated asymmetric ratio and Ω2 are sufficiently high

14

suggesting the presence of asymmetric environment at the vicinity of Eu3+ site (Table S24 and Table S3). Meanwhile, the values of Ω2 and Ω4 does not vary systematically with the change in Eu3+ concentrations. The value of Ω2 is highest for the Sr1.93Al3O6F: Eu0.07 whereas the highest asymmetric ratio is observed in the case of Sr1.95Al3O6F: Eu0.05. This implies that the intensity of the 5D0 → 7F2 transition becomes weak at the cost of the 5D0 → 7F1 transition.53 The other radiative properties which include radiative transition probabilities (A0-4), total radiative transition probability (AT), branching ratios β(ψJ) and the stimulated emission cross-section σ(λp) are also found to be very high suggesting the potentiality of Sr2Al3O6F: Eu3+ phosphor in optical device at room temperature. The CIE (X, Y) illustration exhibiting the coloring chromaticity of the emission spectra of Eu3+-doped Sr2Al3O6F phosphors at λex = 393 and 464 nm excitations are shown in Fig. 10. The calculated CIE coordinates of the doped phosphors are situated within the red-orange area, and the emission color also does not vary appreciably with Eu3+ concentrations. The color purity (CRI) and the CCT values were further evaluated via the methods mentioned in ref. [54] and ref. [55], respectively. The calculated color coordinates (x, y), CCT, CRI and color purity values under 393 and 464 nm excitations for all the obtained phosphors are tabulated in Table 3. The red color purity has been reached up to 99.8% when Sr2Al3O6F: Eu3+ is co-doped with Ba2+ ions, indicating the formation of high-quality red-emitting phosphor. Also, the CCT values range from 1084 to 1303 K for different Eu3+ ion concentrations which are < 3200 K indicating the present phosphors can be useful as warm red light sources under UV light for display/lamp applications. Meanwhile, the emission and excitation nature of Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor are compared with those of the Sr1.93Al3O6F: Eu0.07 and Sr1.9Al3O6F: Eu0.1 phosphors (Fig. 11 (a)). Due to the higher stability of the Ba2+ co-doped sample as suggested by the BVS approach, the

15

emission intensities including the emitting regions of the Sr1.8Al3O6F: Ba0.1/Eu0.1 red phosphor are higher than that of the Sr1.9Al3O6F: Eu0.1 phosphor and even higher than the optimized Sr1.93Al3O6F: Eu0.07 phosphor. Moreover, the expansion of lattice due to the incorporation of larger Ba2+ ions for smaller Sr2+ ions will increase the distance between each Eu3+ ions. Consequently, the radiative recombination of individual Eu3+ ions enhanced owing to the reduction of inter-ionic energy transfer among Eu3+ ions. The emission of Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor is again observed under the excitation of 393 nm and compared with the famous Y2O3:Eu3+ red phosphor (Fig. 11 (b)). The results, depicted in Fig. 11 (b), indicate that the PL emission intensity of Sr1.8Al3O6F: Eu0.1/Ba0.1 is much better than that of the commercial sample. Meanwhile, the PL emission of Sr1.8Al3O6F: Ba0.1/Eu0.1 is also observed under the blue excitation of 464 nm (inset for Fig. 11 (b)) to check the compatibility of this phosphor for blue-LEDs. The sharp and intense red-orange peaks due to the emission of Eu3+ ions are consistent with the results of the PL excitation spectra shown in Fig. 11 (a). The red-orange color intensity of Sr1.8Al3O6F: Ba0.1/Eu0.1 upon blue excitation can be visible by the naked eye (Inset of Fig. 11 (b)). We need to mention that the influence of Ba on the structure needs an elaborate study and therefore this research will be continued, and the results will be communicated as a separate article after a careful evaluation. Fig. 12 (a) exhibits the room temperature electroluminescence (EL) spectrum of the Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor at an excitation of 280 nm UV-LED operated at 500 mA current. Inset-I in Fig.12 (a) plots the normalized intensity of Sr1.8Al3O6F: Ba0.1/Eu0.1 emission at various temperatures under 280 nm excitations. Though emission intensity showed a decreasing trade with the rising of temperature, its value at 150 °C is estimated to be around 72% of that recorded at room temperature, approximately. Therefore, this phosphor has adequate thermal

16

stability which can be useful for lighting applications. Other than that, the corresponding PL digital image of the 280 nm UV-LED excited phosphor and its chromaticity coordinates (0.62, 0.35) and CRI (95.0) signifying its suitability as a pure red-emitting component in phosphorbased LED systems (Inset-II of Fig. 12 (a)). Meanwhile, a white-LED device is also fabricated with the combination of Sr1.8Al3O6F: Ba0.1/Eu0.1 and commercial YAG: Ce3+ phosphors coated on a blue-LED chip having excitation peak at around 460 nm and their EL spectrum is shown in Fig. 12 (b). The resultant CIE coordinates approached towards even more bright white light [(0.38, 0.36) → (0.33, 0.36)] because of the incorporation of our red-emitting Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor. Their digital images demonstrating bright and cool white light are shown in the inset of Fig. 12. The white-LED, compiled with conventional YAG: Ce3+ and blue LEDs of 460 nm emission exhibited the CIE around (0.29, 0.37), CCT around 7241 K and the CRI of 68. However, the CCT, CIE and CRI values are estimated to be around 5510 K, (0.33, 0.36) and 80.5, respectively, due to the incorporation of Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor as a red component with YAG: Ce3+. The results suggested the potential applications of our optimized sample for cool white-LEDs.

Conclusions Undoped and Eu3+-doped Sr2Al3O6F microcrystalline powder phosphors having hexagonal structure were developed via the solid-state route. The refined XRD pattern and stability of the crystal structure were confirmed from the results of valence bond sums and global instability index. Furthermore, the elemental composition of Sr2Al3O6F was also confirmed by XPS analysis. The theoretical bandgap calculated using DFT was 4.76 eV, which is close to the experimental value of 5.02 eV estimated using DRS. Results of PL emission recorded at λex =

17

393 and 464 nm suggested that the optimum sample is Sr1.8Al3O6F: Ba0.1/Eu0.1 which showed high red color purity (>95%), suggesting that it could be the ideal red component for whiteLEDs. A white-LED fabricated in combination with the optimized Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor and commercial yellow phosphor showed bright white emission with a CRI of 80.5%, CCT of 5510 K, and CIE coordinates of (0.33, 0.36). All the research outcome suggesting that the present phosphors are suitable enough for making cool white-LEDs.

Acknowledgements This research work is supported by the Council of Scientific & Industrial Research (CSIR), Government of India financially through the Focused Basic Research under 4M theme (4MFBR, Project no. MLP0033). We thank Dr. Kaustabh Kumar Maiti, CSIR-NIIST, Thiruvananthapuram for the Raman measurements.

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Figures with captions

25

Sr2Al3O6F

Intensity (a.u.)

Intensity (a.u.)

# SrAl2O4

# 26

27

28

#

29

2θ

30

31

32

33

Ca2Al3O6F (JCPDS = 17-0107)

20

40

2θ

60

80

Fig. 1: XRD pattern of the product Sr2Al3O6F from the reaction of SrCO3, Al2O3 and SrF2. Inset shows the presence of minor impurity of monoclinic SrAl2O4.

26

Fig. 2: Rietveld refinement of the XRD pattern of the Sr2Al3O6F 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. The Structure of hexagonal Sr2Al3O6F using the DIAMOND 3 program is shown as an inset.

27

O KLL F KLL

F 1s

O Loss

O 1s Sr Loss Sr 3s

Sr 3p

Sr 3d Sr Loss

O 2s Al 2p

Sr 4s Al 2p

Intensity (a.u.) 0

200

400

600

800

Binding Energy (eV) Fig. 3: Survey XPS spectrum of hexagonal Sr2Al3O6F sample.

28

1000

(a)

Sr1 3d5/2

Intensity (a.u.)

(b)

Sr 3d

Sr2 3d5/2

Al 2p

Sr1 3d3/2 Sr2 3d3/2

130

132

134

136

70

72

O 1s

(c)

536

682

74

76

78 F 1s

Intensity (a.u.)

(d)

138

528

530

532

534

684

686

688

Binding Energy (eV)

Binding Energy (eV)

Fig. 4: XPS core-level spectrum of (a) Sr 3d, (b) Al 2p, (c) O 1s and (d) F 1s of Sr2Al3O6F sample.

29

Fig. 5: Raman spectrum of undoped Sr2Al3O6F sample.

30

Fig. 6: (a) Calculated band structure of Sr2Al3O6F and (b) calculated partial and total DOS diagrams for Sr2Al3O6F.

31

25

(a) Absorbance (a.u.)

(α hυ )2 x 103 (eV/cm)2

20 15 10 5

5.02 eV

0 4.25

5.11 eV

4.50

4.75 5.00 5.25 Photon Energy (eV)

Sr2Al3O6F 200

300

700

800

λex = 243 nm

Intensity (a.u.)

(b)

5.75

Sr2Al3O6F: Eu

400 500 600 Wavelength (nm)

λem = 414 nm

5.50

250

300

350 400 450 Wavelength (nm)

500

Fig. 7: (a) DRS of the Eu3+ doped and undoped Sr2Al3O6F samples. Inset shows their bandgap estimation using the Tauc relation. (b) PL excitation (λem = 414 nm) and emission (λex = 243 nm) spectrum of Sr2Al3O6F sample.

32

D0 - F4

480

Eu 0.01 Eu 0.03 Eu 0.05 Eu 0.07 Eu 0.10

500

(d)

7 5

600

700

Eu 0.01 Eu 0.03 Eu 0.05 Eu 0.07 Eu 0.10

5

D0 - F 3

5

7

7 7

D0 - F 0

5

7 5

5

D1 - F2

7

D1 - F1

7 5

7 5

5

D0 - F 3

D0 - F0

D0 - F 1

5

7

7

D 0 - F4

5

400

7

440

D0 - F 2

400

D0 - F3

7 5

360

D0 - F 1

Intensity (a.u.)

7

F0 - D3

7

320

7

(c)

Eu 0.01 Eu 0.03 Eu 0.05 Eu 0.07 Eu 0.10

5

280

D0 - F2

240

5

5

F0 - D2

6

Eu 0.01 Eu 0.03 Eu 0.05 Eu 0.07 Eu 0.10

200

D0 - F1

5

F0 - G2-6 , L7

7

7

5

F0 - D 4

Intensity (a.u.)

5

5

7

5

7

7

λex = 280 nm

(b)

D0 - F 2

F0 - L6

λem = 613 nm

(a)

570

600 630 Wavelength (nm)

660

550

600 650 Wavelength (nm)

700

Fig. 8: (a) PL excitation spectra of Sr2-xAl3O6F: xEu3+ (x = 0.01 to 0.10) phosphors at λem = 613 nm. PL emission spectra of Sr2-xAl3O6F: xEu (x = 0.01 to 0.10) phosphors at (b) λex = 280 nm, (c) λem = 393 nm and (d) λex = 464 nm.

33

20 Slope = 2.15

ln(I/X)

PL intensity (a.u.)

5.1

15

4.8 4.5 4.2

-2.6

-2.4 ln(X)

-2.2

10

5 0.00

0.04

0.08

0.12

0.16

0.20

3+

Eu conc. Fig. 9: 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 464 nm.

34

Fig. 10: CIE (X, Y) coordinate diagram of Sr2-xAl3O6F: xEu3+ (x = 0.01 to 0.10) and hexagonal Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphors for the emission spectra recorded at (a) λex = 394 nm and (b) λex = 464 nm.

35

(a)

λex = 310 nm

λemi = 613 nm

Intensity (a.u.)

Eu0.07 Eu0.1 Eu0.1/Ba0.1

200 250 300 350 400 450 540560580600620 640660 Wavelength (nm)

(b)

Eu0.1/Ba0.1 3+ Comm Y2O3: Eu

λex = 464 nm

Intensity (a.u.)

λex = 393 nm

Eu0.1/Ba0.1

500

450

525

475

550

575

500

600

525

625

650

550

675

575

600

625

650

675

Wavelength (nm)

Fig. 11: (a) PL excitation and emission spectra of Sr1.8Al3O6F: Ba0.1/Eu0.1, Sr1.93Al3O6F: Eu0.07 and Sr1.9Al3O6F: Eu0.1 phosphors recorded at λem = 613 nm and λex = 310 nm, respectively. (b) Comparative PL emission spectra of Sr1.8Al3O6F: Ba0.1/Eu0.1 and commercial Y2O3:Eu3+ red phosphors recorded at λex = 393 nm. PL emission of Sr1.8Al3O6F: Ba0.1/Eu0.1 recorded at λex = 464 nm is shown as an inset. The digital image showing bright red-orange light is also shown in the inset.

36

Fig. 12: (a) EL spectrum of Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor recorded with a UV LED at λex = 280 nm. Inset-I shows the PL emission intensity of Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor at various temperatures. Inset-II shows the CIE and the digital image of the bright red emitting Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphor with the 280 nm LED. (b) EL spectrum of the mixture of Sr1.8Al3O6F: Ba0.1/Eu0.1 and commercial YAG: Ce3+ phosphors coated on a blue LED chip. Inset shows the CIE and the digital image of corresponding cool white light.

37

Tables Table 1: Lattice prameters of hexagonal Sr2Al3O6F Composition

Sr2Al3O6F

Structure

Hexagonal

Space group

R-3 (# 148)

a [Å]

17.8232 (1)

c [Å]

7.2168 (0)

3

V [Å ]

1985.35 (1)

Z

12 3

ρcalc [g/cm ] χ2

1.796

Rp(%)

0.0410

Rwp (%)

0.0523

GOF (S) Number of data points

1.34 0.0167113°/99.695 sec per step 4787 (2θ = 10-90º)

Number of parameters

60

Temperature

298 K

3.72521

Step size/ Step time

Table 2: Atomic parameters estimated after the final cycle of refinement. Atom site

x

y

z

S.O.F.

Uiso

Sr1 Sr2 Al1 Al2 O1 O2 O3 O4 F

0.22441 0 0.59193 0.64615 0.59934 0.14020 0.56537 0.60910 0.34883

0.41330 0 0.45454 0.05007 0.53244 0.11063 0.08397 -0.05445 0.56526

0.07579 0.26187 0.29550 0.11780 0.43294 0.10560 0.13138 0.21453 0.18927

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.66667

0.0252 0.0274 0.0188 0.0215 0.0051 0.0152 0.0253 0.0141 0.0519

18f 6c 18f 18f 18f 18f 18f 18f 18f

38

Table 3: The CCT, CRI, CIE (x, y), and color purity of the emission spectra of Sr2-xAl3O6F: xEu3+ (x = 0.01 to 0.10) and Sr1.8Al3O6F: Ba0.1/Eu0.1 phosphors under 393 and 464 nm excitation. Eu3+ Conc.

CCT (K)

CRI

CIE

Color Purity (%)

(x) 393 nm

464 nm

393 nm

464 nm

393 nm

464 nm

393 nm

464 nm

0.01

1105

1153

35

54

(0.622, 0.343)

(0.628, 0.359)

89.4

96.5

0.03

1148

1218

37

57

(0.602, 0.336)

(0.618, 0.365)

81.3

95.0

0.05

1094

1098

33

48

(0.600, 0.320)

(0.636, 0.354)

76.1

97.2

0.07

1141

1197

37

54

(0.623, 0.352)

(0.622, 0.364)

92.4

95.2

0.1

1099

1303

33

64

(0.631, 0.349)

(0.615, 0.381)

94.2

99.1

Eu0.1/Ba0.1

1084

1089

32

34

(0.637, 0.351)

(0.642, 0.357)

96.5

99.8

39

Highlights •

Sr2Al3O6F in hexagonal symmetry was synthesized by using a solid-state method.

•

Sr2Al3O6F was refined by the Rietveld refinement for the first time.

•

Electro-optical properties of Sr2Al3O6F were studied by density functional theory.

•

Sr2Al3O6F: Eu3+ phosphors showed intense red emission under UV and blue lights.

•

A cool white-LED was developed using a yellow phosphor and the optimized phosphor.

Declaration of Competing Interest The authors of this paper is declaring that they have no recognized competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.