Ab Initio Structure Determination and Photoluminescent Properties of

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Ab initio structure determination and photoluminescent properties of an efficient, thermally stable blue phosphor: BaYBO :Ce 2

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Martin Hermus, Phu-Cuong Phan, and Jakoah Brgoch Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04542 • Publication Date (Web): 16 Jan 2016 Downloaded from http://pubs.acs.org on January 21, 2016

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Chemistry of Materials

Ab Initio Structure Determination and Photoluminescent Properties of an Efficient, Thermally Stable Blue Phosphor: Ba2Y5B5O17:Ce3+ Martin Hermus, Phu-Cuong Phan, and Jakoah Brgoch* Department of Chemistry, University of Houston, Houston, TX 77204-5003, USA

Abstract The crystal structure of a novel barium yttrium borate, Ba2Y5B5O17, was solved using a combination of ab initio global optimization algorithms and DFT calculations along with Rietveld refinement of high-resolution synchrotron X-ray powder diffraction data. Synthesized using a high-temperature solid-state route, the structure consists of edge- and corner-sharing Y and Ba centered polyhedra along with BO3 trigonal planes. Ba and Y occupy four crystallographically independent sites with two fully occupied by Y and two having a statistical mixture of Y and Ba. Substituting Ce3+ into the structure for Y3+ yields blue photoluminescence (λem = 443 nm) when excited with UV (λem = 365 nm) light. The emission of this new compound is efficient with an external quantum yield of 70% and is stable as a function of temperature with a quenching temperature of ≈400 K.

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1. Introduction The widespread deployment of white light LEDs is rapidly occurring due to their excellent efficiencies and long lifetimes.1-4 Continuing to improve these devices requires not only advances in the LED chips but more importantly the development of novel down-converting phosphors.4,5 There are two principal approaches currently employed for the production of white light from a monochromatic LED. The most common method combines a blue emitting (InGa)N LED and a yellow emitting phosphor such as cerium-substituted yttrium aluminum garnet (Y3Al5O12:Ce3+).2 Alternatively, a UV or near-UV LED can be fully converted through a combination of red, green, and blue emitting phosphors.5 However, these devices tend to have lower overall efficiencies due to the inherent Stokes’ losses from the phosphor conversion processes even though they can yield high quality white light. As a result, this approach requires all three phosphors (red, green, blue) to have high photoluminescence quantum yields (PLQY) to maximize the device’s efficiency. In terms of the currently used, available phosphors for this application, the number of blue emitting compounds is rather limited. BaMgAl10O17:Eu2+ and Sr3MgSi2O8:Eu2+ both show the necessary high PLQY but they also show limited temperature stability.6 BaMgAl10O17:Eu2+ shows an irreversible decrease in luminescence after heat treatment due to oxidation.7,8 Recently a blue emitting phosphate-based phosphor was discovered, RbBaPO4:Eu2+, which shows ideal luminescence properties when excited in the near-UV region.9 An alternative class of compounds that has drawn increased attention as UV excited phosphors are borates, due to their transparency in the UV, high damage threshold, and thermal stability.10-23 A borate system of particular interest for application in phosphors is the ternary phase space, BaO–Y2O3–B2O3, based on its ability to substitute a variety of rare-earth ions like Ce3+ and Eu2+ as well as the tendency to form wide band gap materials. This phase space has


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already been systematically investigated24 identifying six quaternary compounds: BaYB9O16 (P2/m),25 Ba3YB3O9 (α-phase P63cm26 and β-phase R3ത )27, Ba3Y2B4O12,28 BaY3B3O10 (monoclinic),24 Ba3Y2B6O15 (‫ܽܫ‬3ത),29 and Ba3YB9O18 (P63/m).30 Very little is known about many of these phases and only a few borates in this system have also been investigated as phosphors. For example, Ba3YB3O9:Ce3+ (R3ത ) is a UV excited, blue emitting phosphor albeit with rather low PLQY.31 Additionally, co-doping Ba3Y2B4O12 with Ce3+ and Tb3+ produces a tunable emission from deep blue to yellow-green by varying Tb3+ content.32 These results indicate there is excellent potential for novel phosphor development form this system. In this contribution we present the ab initio determination of the crystal structure of a novel barium yttrium borate, Ba2Y5B5O17, discovered while continuing the systematic investigation of this phase space. The crystal structure of the new compound was solved through a combined experimental and computational approach. After using heuristic based global-optimization algorithms to obtain a preliminary structural model a subsequent density functional theory optimization led to a crystal structure, which was confirmed as the correct structure through phonon calculations and Rietveld refinement. Partial substitution of the rare-earth ion by Ce3+, e.g., Ba2(Y4.75Ce0.25)B5O17, yields an efficient, thermally stable blue emitting phosphor when excited by UV light. The optical properties of this new compound are ideal for incorporation into a UV-based device in conjunction with an efficient green and red phosphor to produce a high quality white solid state light.

2. Experimental Section Synthesis Ba2Y5B5O17 was prepared according to the reaction: 2 BaCO3 + 2.5 Y2O3 + 5 H3BO3 Ba2Y5B5O17 + 2CO2↑ + 7.5 H2O↑ by grinding BaCO3 (Johnson Mathey, 99.99 %), Y2O3 (Alfa Aesar, 99.9 %), and H3BO3 (Sigma-Aldrich, 99.999 %) in their stoichiometric ratios with an agate mortar.



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Ba2(Y4.75Ce0.25)B5O17 was prepared accordingly by substituting Y2O3 partially by CeO2 (Sigma-Aldrich, 99.995 %). Pressed pellets of the reactants were placed in alumina crucibles (AdValue Tech.) on sacrificial powder to prevent direct contact with the crucible. They were fired in a tube furnace for 24 h at 1200 °C under a reducing (5% H2/95% N2) atmosphere with heating and cooling ramps of 3 °C/min. X-Ray-Diffraction Phase purity was confirmed by X-ray powder diffraction on a PanAnalytical X'Pert powder diffractometer using CuKα radiation (1.54183 Å). Additionally, synchrotron X-ray powder diffraction data were collected at a temperature of 100 K using beamline 11-BM at the Advanced Photon Source, Argonne National Laboratory with a calibrated wavelength of 0.414161 Å.33 The diffraction pattern of the unknown product was indexed using the TREOR90 program.34 Rietveld analysis was conducted using the GSAS package.35,36 The background was modelled by a shifted Chebyshev function, while the peak shapes were modelled using a pseudo-Voigt function with Finger−Cox−Jephcoat asymmetry to correct for axial divergence at low angles. DFT-Calculations The electronic structure of the synthesized phases were calculated using density functional theory within the Vienna ab initio Simulation Package (VASP)37,38 to determine the lowest energy crystal structure. Plane-wave basis sets and projectoraugmented-wave (PAW) pseudopotentials were used.39 Exchange and correlation were treated with the generalized gradient approximation (GGA) in the parameterization of Perdew, Burke, and Ernzernhof (PBE).40 A cut-off energy of 500 eV was set and a 6×4×2 Γcentered Monkhorst-Pack k-point grid yielding 24 irreducible k-points41 was used for the integration within the Brillouin zone. For the structural relaxation forces, stress tensors, atomic positions, unit cell shapes, and unit cell volumes were allowed to relax with a convergence criterion of 10–6 eV. The convergence criterion for the electronic relaxation was set to 10–8 eV. The phonon dispersion of Ba2Y5B5O17 was calculated with PHONOPY,42 which


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is based on the ab initio force constant method (Modified Parlinski-Li-Kawazoe method).43 To construct the force constant matrix the Hellmann-Feynman forces were calculated by VASP using a 2×1×1 supercell, in which symmetry independent atoms are separately displaced from their equilibrium position by ±0.01 Å. Photoluminescence Room temperature and temperature dependent photoluminescent spectra were collected on a Horiba Fluoromax-4 fluorescence spectrophotometer with a 150 W xenon arc lamp for excitation and a Janis cryostat (VPF-100) to control the temperature between 77 K and 500 K. Each sample was mixed into silicone resin (GE Silicones, RTV615) and deposited on a quartz substrate (Chemglass). The absolute external quantum yield was determined by placing the samples inside a SpectralonTM-coated integrating sphere (150 mm diameter, Labsphere) and exciting at 365 nm. The PLQY was calculated based on the method of de Mello et al.44

3. Results and Discussion 3.1 Ab-initio structure solution of Ba2Y5B5O17 During the exploratory syntheses for novel phosphors in the BaO-Y2O3-B2O3 phase space, a synthesis of the reported composition BaY3B3O10 was attempted following the reported reaction conditions.24 The products diffractogram contained reflections that could not be indexed based on the previously reported monoclinic cell of BaY3B3O10. Instead, the Power Diffraction File (ICDD-PDF) showed a partial match of the product to a compound with a different composition, Ba2Y5B5O17 (PDF No. 00-056-0113),45 as well as a number of unidentified peaks. A subsequent synthesis loading the nominal composition of the major phase in the first attempt (Ba2Y5B5O17) produced a pure phase product that was readily indexed with an orthorhombic unit cell that could be unequivocally assigned as Pbcn (No.



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60) based on extinctions. The lattice parameters were refined using the LeBail method and found to be a = 17.3946(5) Å, b =6.6586(2) Å, c =13.0486(4) Å. Given that products obtained from these synthetic methods are fine powders and no other information regarding the crystal structure of Ba2Y5B5O17 is known, a structural solution was sought using only powder diffraction data. Employing the open source ab initio structure determination program FOX,46 “parallel tempering” global-optimization algorithms are ideal to optimize the crystal structure and fit the calculated diffractogram to experimental data.47-49 The starting crystal structure for the FOX input (Figure 1, left side) maintained the nominal loaded composition while the number of formula units per unit cell was set to be Z = 4 based on the LeBail refined volume of the unit cell and the reported densities of related borates. To simplify the model and limit the degrees-of-freedom in the system, boron and oxygen atoms were initially constructed as rigid trigonal planar units, a common structural arrangement in these compounds. After multiple (10) FOX optimization runs using 107 iterations in each optimization, a credible crystal structure was obtained. This structure showed an atomic arrangement and distances that are plausible for a borate; however, it showed some obvious flaws. For example, some of the BO3 units switched boron and oxygen creating oxygen-centered polyhedral. Due to the small difference in scattering power of B and O, especially in the vicinity of heavy atoms like Ba and Y, such small errors can be expected. Manually correcting these errors generated a sound structural model. To improve this structural model further, ab initio calculations based on DFT were conducted using VASP. A complete, free relaxation of the unit cell (cell shape and volume) and atomic positions combined with tight convergence criteria produced a structure in its lowest energy ground state. The VASP optimized crystal structure closely resembled the FOX crystal structure. The obtained unit cell has slightly larger lattice parameters (a = 17.645 Å, b = 6.703 Å, c = 13.104 Å) compared to the experimental indexing, as expected from a GGA


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calculation. Figure 1 also shows the optimized crystal structure. The FOX + VASP crystal structure was also confirmed to be dynamically stable by calculating the phonon dispersion curves. The absence of imaginary modes in the phonon density of states as calculated by PHONOPY, shown in Figure 2, for this phase indicate it is likely a valid crystal structure. Figure 2 also decomposes the phonon spectrum into the partial phonon DOS. The vibrations with low wavenumbers (long wavelengths) arise mainly from vibrations of the heavy atoms Ba and Y, while B and O vibrations are present at higher wavenumbers.

Figure 1: The input for the FOX structure determination is shown on the left side. The right side shows the structural solution from FOX after applying minor manual improvements to ensure boron centered polyhedra and subsequent VASP optimizations.



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Figure 2: Total and partial phonon density of states of the VASP optimized Ba2Y5B5O17 crystal structure.

3.3 Structure Refinement and Description The combined FOX + VASP optimized crystal structure of Ba2Y5B5O17 was supplied as starting model for a Rietveld refinement using synchrotron X-ray powder diffraction data. Refining the atomic positions, isotropic displacement parameters, and profile parameters yield an excellent agreement between experimental data and structural model, as shown in Figure 3. The refinement data and resulting atomic positions are listed in Tables 1 and 2, respectively. During the refinement of the fully ordered crystal structure, anomalous isotropic displacement factors were obtained for two of the cation sites, Ba1 and Y2. Statistically mixing Ba and Y on these positions satisfied the refinement allowing the refinement to converge. One of the positions, Ba1/Y1, Wyckoff site 8d, is a majority Ba (86.4(4)% Ba / 13.6% Y) whereas the second site, Y2/Ba2, Wyckoff site 4c, is a majority of Y (79.7(5)% Y / 20.3% Ba). Statistical mixing between Ba and Y is not unprecedented and is observed in other Ba-Y-Borates like Ba2.555Y1.445B3O9, where Ba and Y share all crystallographic sites.31 Including this disorder, a refined formula of Ba1.93(2)Y5.07(2)B5O17 is obtained, indicating there 8 ACS Paragon Plus Environment

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is a slight yttrium excess in the phase but there is an overall agreement with the loaded composition. Table 1: Rietveld refinement data of Ba1.93(2)Y5.07(2)B5O17 using synchrotron radiation. Further details of the crystal structure investigation including the CIF may be obtained from FIZ.50 formula Radiation type, λ / Å 2θ range / ° Temperature / K Space group, Z Lattice parameters a/Å b/Å c/Å V / Å3 Calculated density / g cm–3 Formula weight / g mol–1 Rp / % Rwp / %

Ba1.93(2)Y5.07(2)B5O17 Synchrotron (APS 11-BM) , 0.414161 0.5 – 50 100 K Pbcn (No. 60), 4 17.38257(5) 6.65299(3) 13.03035(5) 1506.91(2) 4.592 1041.86 0.0764 0.0963

Table 2: Atomic Coordinates, isotropic displacement parameters and occupancies as determined by Rietveld refinement of 11-BM synchrotron X-ray diffraction data. Atom Wyck. 8d Ba1 8d Y1 4c Y2 4c Ba2 8d Y3 8d Y4 8d B1 8d B2 4c B3 8d O1 8d O2 8d O3 8d O4 8d O5 8d O6 8d O7 8d O8 4c O9

x

y

z

Uiso / Å2

0.33608(3) 0.33608(3) 1/2 1/2 0.18196(3) 0.01652(4) 0.1444(4) 0.6592(5) 0 0.1994(3) 0.1428(3) 0.0821(2) 0.6849(3) 0.5980(3) 0.7008(3) 0.9448(3) 0.0405(3) 1/2

0.02357(7) 0.02357(7) 0.4125(2) 0.4125(2) 0.0410(1) 0.7613(1) 0.495(2) 0.453(2) 0.651(2) 0.3888(6) 0.7119(6) 0.4616(6) 0.4234(6) 0.3433(6) 0.5826(6) 0.9409(6) 0.2433(6) 0.9415(9)

0.18479(4) 0.18479(4) 3/4 3/4 0.41299(5) 0.47990(5) 0.4305(7) 0.8592(7) 1/4 0.4053(4) 0.3943(4) 0.4970(4) 0.7567(4) 0.8831(4) 0.9131(4) 0.5876(4) 0.6786(4) 1/4

0.0042(2) 0.0042(2) 0.0035(3) 0.0035(3) 0.0005(2) 0.0026(2) 0.014(2) 0.014(2) 0.014(2) 0.0033(4) 0.0033(4) 0.0033(4) 0.0033(4) 0.0033(4) 0.0033(4) 0.0033(4) 0.0033(4) 0.0033(4)

Occ. 0.864(4) 0.136(4) 0.797(5) 0.203(5) 1 1 1 1 1 1 1 1 1 1 1 1 1 1

The crystal structure obtained from the Rietveld refinement is shown in Figure 4. The structure consists of edge- and corner-sharing Y and Ba centered polyhedra, which build a



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three-dimensional net. There are two crystallographically independent, fully occupied Y3+ sites and a third Y3+ site that is shared with Ba2+. These coordination polyhedra form a distorted octahedron around atom Y3, a distorted capped trigonal prism around atom Y4, and a distorted pentagonal bipyramid around the mixed Y2/Ba2 site, as depicted in Figure 4. The Y-O distances within the yttrium polyhedra range between 2.21 Å to 2.48 Å, while the distances in the partly Ba2+ containing polyhedra around Y2/Ba2 are longer (2.33 Å to 2.49 Å) due to the larger ionic radii of Ba2+ compared to Y3+. Ba2+ mainly occupies one crystallographic position and is mixed with Y3+ (18.8 %) in a 10-coordinate, highly distorted polyhedra. The calculated bond valence sum (BVS) of the Ba1/Y1 crystallographic site is 2.03 indicating optimal bonding. Calculating the BVS for the Y-centered polyhedra suggest optimal bonding for Y3 (BVS = 2.99), slight over-bonding for Y4 (BVS = 3.21), and underbonding on Y2/Ba2 (BVS = 2.41). The Ba/Y-O distances here range between 2.65 Å and 3.09 Å. All of the boron atoms are three-coordinated by oxygen forming slightly distorted trigonal planar units.

Figure 3: 11-BM Synchrotron X-ray powder diffraction data of Ba2Y5B5O17:Ce3+ with the resulting Rietveld refinement shown. The experimental data are shown in black, the Rietveld fit is shown in yellow, and the difference curve is shown in blue.


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Figure 4: Representation of the crystal structure of Ba2Y5B5O17 obtained by Rietveld refinement with the BO3 units highlighted. The crystallographically independent cation coordination environments are also shown.

3.4 Photoluminescent Properties Substituting 5% of Ce3+ for Y3+ in the structure, e.g., Ba2(Y4.75Ce0.25)B5O17, leads to photoluminescence when excited with UV light ranging from 250 nm to 375 nm. The excitation spectrum contains three distinctive peaks (λex = 265 nm, 308 nm, and 350 nm) as shown in Figure 5a. The emission spectra collected at all three excitation maxima are the same indicating these peaks likely arise from excitations between the Ce3+ 4f orbitals to multiple Ce3+ excited states (5d orbitals). This is somewhat surprising given the expected multiple emission peaks from the four crystallographically independent Y-centered polyhedra; however, these four peaks cannot be resolved even at 80 K. The emission spectrum, also depicted in Figure 5a when λex = 308 nm, shows a rather broad emission band with a maximum centered at 443 nm and a FWHM of approximately 100 nm. Fitting the emission data to two Gaussian curves with maxima at 424 nm and 464 nm indicates a separation of 2006 cm–1 corresponding to the spin-orbit coupled 2D3/2 2F5/2 and 2D3/2 2

F7/2 transitions. The CIE chromaticity coordinates calculated from the emission spectrum

highlights the blue emission of this compound when excited with UV (λex = 308 nm) light, as 11 ACS Paragon Plus Environment


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shown in Figure 5b. Such a blue shifted emission is common for Ce3+ based borate systems. For example, alkaline earth borates like M3YB3O9:Ce3+ (M = Ba, Sr) show a comparable blue emission with λex = 313 nm and λem = 422 nm, respectively.31 The color tunable phosphor Ba3Y2B4O12:Ce3+,Tb3+ also shows a deep blue luminescence on the Tb poor side.32 The large energetic transition between the 4f and 5d orbitals of Ce3+ are due to a weak nephelauxetic effect, which indicates a weak field coordination environment as expected for alkaline earth borates. The PLQY was determined to be 70 % at room temperature (≈300 K), which is highly efficient and is higher than any other measured Ba-Y borate. For comparison, the already mentioned Ba3YB3O9:Ce3+ has only a PLQY of 16 %.31 The origin of this high PLQY in this phase is likely due to the highly connected edge- and corner-sharing Y and Ba centered polyhedra in the crystal structure in combination with the structurally rigid BO3 trigonal planar units.51-53


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Figure 5: a) Excitation and emission (λex = 308 nm) spectra at room temperature of Ba2Y5B5O17: Ce3+. b) The blue emission of Ba2Y5B5O17: Ce3+ shows CIE coordinates of (0.159, 0.155).

Of particular importance in the development of new phosphor for incorporation in white LEDs lighting is their stability with respect to thermal quenching. As these solid state lights operate and the device temperature elevates, the optical properties of the phosphor can change dramatically compared to the LED chip.54 Thus, materials that are robust to temperature are required for incorporation in future high-powered devices. Figure 6 shows the relative integrated photoluminescence intensity as a function of temperature for Ba2Y5B5O17:Ce3+. Increasing the temperature from 80 K to 500 K causes small changes in the emission spectrum, most notably it broadens as well as blue-shifts following a decrease in crystal field splitting.5 A decrease of crystal field splitting is expected with increasing temperatures due to an elongation of the bond lengths within the Ce-centered polyhedra. The emission intensity is also rather stable as a function of temperature with the integrated intensity decreasing by ≈15 % from low to room temperature. The thermal quenching 13 ACS Paragon Plus Environment


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temperature (T50), or the temperature at which 50 % of the integrated intensity is lost, occurs at 403 K. Although this does not reach the performance of some industry standard blue phosphors like BaMgAl10O17:Eu2+, which only decreases by ≈20 % between room temperature and 500 K,55 Ba2Y5B5O17:Ce3+ still shows respectable temperature dependence.

Figure 6: Temperature dependent measurement of the emission spectrum of Ba2Y5B5O17: Ce3+, showing a blue-shift with increasing temperature (top) and a quenching temperature of T50 = 403 K.

4. Conclusion The crystal structure of a novel barium yttrium borate, Ba2Y5B5O17, was solved using a combination of ab initio structure solution methods and density functional theory calculations. The crystal structure was confirmed and further refined by a Rietveld refinement of synchrotron X-ray powder diffraction data. The structure consists of a three dimensional network of Y and Ba-centered polyhedral units as well as trigonal planar BO3 units. While yttrium is found exclusively on two crystallographic sites, Barium is on two sites mixed with yttrium. Investigation of the photoluminescent properties shows that Ba2(Y4.75Ce0.25)B5O17 is an efficient blue emitting phosphor when excited with UV light. The photoluminescence is highly efficient (PLQY = 70 %, λex = 365 nm) at room temperature while the temperature


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dependent luminescence measurements show this new borate is capable of operating at temperatures approaching 400 K with minimal impact on the materials performance. Thus, Ba2Y5B5O17:Ce3+ is a notable phosphor for application in UV based solid-state lights and should be explored further.

5. Acknowledgements The authors thank the Department of Chemistry and the Division of Research at the University of Houston for providing generous startup funds. M. H. gratefully acknowledges the Eby Nell McElrath Postdoctoral Fellowship for financial support. This work was also supported by the R. A. Welch Foundation through the TcSUH Robert A. Welch Professorship in High Temperature Superconducting (HTSg) and Chemical Materials (E-0001). The research presented here used the Maxwell/Opuntia Cluster(s) operated by the University of Houston and the Center for Advanced Computing and Data Systems (CACDS). Use of the Advanced Photon Source at Argonne National Laboratory was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. 6. Author Information Corresponding author *Email: [email protected] (J. Brgoch)



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