Article pubs.acs.org/cm
Tunable Optical Properties and Increased Thermal Quenching in the Blue-Emitting Phosphor Series: Ba2(Y1−xLux)5B5O17:Ce3+ (x = 0−1) Martin Hermus, Phu-Cuong Phan, Anna C. Duke, and Jakoah Brgoch* Department of Chemistry, University of Houston, Houston, Texas 77204, United States S Supporting Information *
ABSTRACT: The preparation of cerium-substituted barium lutetium borate, Ba2Lu5B5O17:Ce3+, is achieved using high temperature solid state synthesis. This compound crystallizes in the Ba2Y5B5O17-type structure and shows an efficient blue emission (λmax = 447 nm) when excited by UV-light (λex = 340 nm) with a photoluminescent quantum yield near 90%, a fast luminescence decay time (423 K,26 which is considered the normal operating temperature for most LED packages. The best opportunity to modify a cerium-substituted inorganic phosphor’s optical properties is through atomic substitution via the formation of a solid solution. For example, the complex silicate series (Ba1−ySry)9(Sc1−δYδ)2Si6O24:Ce3+ (y = 0−1; δ = 0−1) shows highly tunable luminescence depending on the composition with the maximum emission wavelength (λmax) shifting by more than 20 nm.27,28 In the garnet and inverse garnet series, Y3(Al1−zGaz)5O12:Ce3+ (x = 0−1) and Mg3(Y1−yGdy)2(Ge1−zSiz)3O12:Ce3+ (y = 0−1, x = 0, 1), respectively, the optical properties change following variations in the Ce−O bond lengths arising from the change in composition.29 It is also possible to tune the optical properties and chemical stability by preparing a solid solution as demonstrated in Sr2Ba(AlO4F)1−x(SiO5)x:Ce3+ (x = 0−1), where increasing x red-shifts the emission spectrum as well as limits moisture sensitivity.30−32 In all of these examples, the formation of the solid solution involves chemical substitution of isovalent ions with similar ionic radii and preferred coordination environments.5,33 Thus, changing the optical properties of Ba2Y5B5O17:Ce3+ can occur through chemical substitution; the most likely candidates are substitution of Sr2+ for Ba2+ or Gd3+, Sc3+, or Lu3+ for Y3+. Here, the discovery of a new barium lutetium borate, Ba2Lu5B5O17:Ce3+, is reported along with the preparation of the solid solution, Ba2(Y1−xLux)5B5O17:Ce3+ (x = 0−1), to optimize the optical properties. The compounds form nearly phase pure products that maintain the Ba2Y5B5O17-type structure across the range of x. The partial substitution of the rare-earth ions by Ce3+ yields an efficient, thermally stable blue emitting phosphor when excited by UV light. Varying Lu3+ and Y3+ content causes a slight shift in the emission and negligable changes to the excitation bands. More importantly, atomic substitution leads to an improvement in the PLQY as well as the thermal stability, with a 49 K increase in the quenching temperature across the solid solution. These results support that the optical properties of this borate-based phosphor series are well suited for incorporation in a UV-LED based device in conjunction with an efficient green and red phosphor to produce a high quality white solid state light.
function was used to model the background. All crystal structures were visualized using VESTA.37 2.3. Steady-State Temperature and Time Dependent Photoluminescence. Steady-state photoluminescent spectra were collected at room temperature and between 80 and 500 K on a PTI fluorescence spectrophotometer with a 75 W xenon arc lamp for excitation and a Janis cryostat (VPF-100) to control the temperature. 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 Spectralon-coated integrating sphere (150 mm diameter, Labsphere) and exciting at 340 nm. The PLQY was determined based on the method of de Mello et al.38 The time-gated luminescence decay measurements were collected using a Horiba DeltaFlex Lifetime System with a NanoLED N-360 nm LED (λex = 363 nm). A total measurement length of 800 ns was measured with a repetition rate of 1 MHz and a 10 ns delay. 2.4. Density Functional Theory. Calculations were carried out using the Vienna ab initio Simulation Package (VASP) with pseudopotentials employing the projector augmented wave method (PAW) and the generalized gradient approximation parametrized by Perdew, Burke, and Ernzerhof.39−42 A cutoff energy of 500 eV was employed,and a 6 × 4 × 2 Γ-centered Monkhorst−Pack k-point grid was used. To find the electronic ground state of the structures, the atomic positions, lattice parameters, and unit cell volumes were allowed to relax with an electronic convergence criterion of 1 × 10−8 eV until the residual forces were 1 × 10−6 eV/Å. Stress tensor calculations were carried out on the fully relaxed structures with an energy cutoff of 600 eV, while the elastic moduli and ΘD were approximated using the quasi-harmonic Debye model.43−45 The band gaps (Eg) were calculated using a screened hybrid functional, HSE06,46 with a cutoff energy of 450 eV and a 4 × 2 × 2 Γ-centered Monkhorst−Pack k-point grid.
3. RESULTS AND DISCUSSION 3.1. Formation of the Ba2Lu5B5O17:Ce3+ Inorganic Phosphor. The synthesis of the target compound, Ba2Lu5B5O17:Ce3+ was achieved as a phase pure product according to laboratory (Cu Kα) powder X-ray diffraction by using a 1 mol % deficit of BaCO3. Loading on stoichiometry yielded minor Lu2O3 and LuBO3 impurities. A Rietveld refinement was subsequently conducted using high intensity, high resolution synchrotron X-ray powder diffraction data, shown in Figure 1. The refinement illustrates excellent agreement between the collected data and the structural model. Table 1 provides the refinement details, and Table 2 contains the atomic coordinates, isotropic displacement parameters, and occupancies.
2. EXPERIMENTAL SECTION 2.1. Synthesis. The solid solution, Ba2(Y1−xLux)4.75Ce0.25B5O17 (x = 0, 0.25, 0.50, 0.75, 1), was synthesized by mixing, in an agate mortar and pestle, BaCO3 (Johnson Matthey, 99.99%), Y2O3 (Alfa Aesar, 99.9%), Lu2O3 (Alfa Aesar, 99.99%), CeO2 (Sigma-Aldrich, 99.995%), and H3BO3 (Sigma-Aldrich, 99.999%) in their stoichiometric ratios for all reactants, except for BaCO3, which was added with 1 mol % deficit to prevent the formation of Lu2O3 and LuBO3 impurities and obtain phase pure products. Pellets of the reactant mixtures were placed in alumina crucibles (AdValue Tech.) and fired in a tube furnace for 12 h at 1200 °C with heating and cooling rates of 3 °C/min under a reducing atmosphere of 5% hydrogen and 95% nitrogen gas. 2.2. X-ray Diffraction. The samples were checked for phase purity by X-ray powder diffraction on a PanAnalytical X’Pert powder diffractometer using Cu Kα radiation (λ = 1.54183 Å). Additionally, synchrotron X-ray powder diffraction data were collected at a temperature of 100 K with a calibrated wavelength of 0.414174 Å (beamline 11-BM, Advanced Photon Source, Argonne National Laboratory).34 LeBail and Rietveld refinements were conducted using the GSAS package.35,36 The peak shapes were modeled using a pseudo-Voigt function with Finger−Cox−Jephcoat asymmetry to correct for axial divergence at low angles. A shifted-Chebyshev
Figure 1. Rietveld refinement plot of Ba2Lu5B5O17:Ce3+ synchrotron powder X-ray diffraction data. The experimental data are black; the Rietveld fit is in yellow, and the difference curve is in blue. 5268
DOI: 10.1021/acs.chemmater.7b01416 Chem. Mater. 2017, 29, 5267−5275
Article
Chemistry of Materials
containing 13% Y3+ on the 8d site. Interestingly, mixing of Ba2+ and Lu3+ is not observed on the equivalent Ba-site in Ba2Lu5B5O17. The final refined composition is therefore Ba2.17(1)Lu4.83(1)B5O17, referred to herein as Ba2Lu5B5O17. The crystal structure, illustrated in Figure 2, contains lutetium and barium centered polyhedra as well as boron
Table 1. Rietveld Refinement Data of Ba2Lu5B5O17:Ce3+ Using 11-BM Synchrotron Radiationa refined formula
Ba2.17(1)Lu4.83(1)B5O17
radiation type; λ (Å) 2θ range (deg) T (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 RF2 χ2
synchrotron (11-BM); 0.414174 0.5−50.0 100 Pbcn; 4 17.25233(5) 6.56961(2) 12.890904(5) 1461.07(1) 6.679 1469.14 0.0604 0.0279 1.626
a
Further details of the crystal structure investigation including the CIF may be obtained from FIZ.47
Figure 2. Representation of the crystal structure of Ba2Lu5B5O17:Ce3+ obtained by Rietveld refinement with different polyhedral units highlighted. B is blue, O is orange, Lu is gray, and Ba is red. Cerium is omitted from the refinement due to its low substitution concentration.
Ba2Lu5B5O17:Ce3+ crystallizes isostructurally to the parent compound, Ba2Y5B5O17, in space group Pbcn (No. 60).24 The starting model for the refinement was thus based on the Ba2Y5B5O17-type structure and was initialized with the Ba site fully occupied, and the Y sites occupied by Lu. Ce3+ was omitted from the refinement due to its low substitution concentration (5%). The converged refinement revealed a 45.84 Å3 (3%) decrease in the unit cell volume compared to that of the Y3+ analogue, in accordance with the smaller ionic radius of Lu3+. Additionally, the refinement indicated only minor shifts (
