Article pubs.acs.org/IC
Discovery of a Red-Emitting Li3RbGe8O18:Mn4+ Phosphor in the Alkali-Germanate System: Structural Determination and Electronic Calculations Satendra Pal Singh, Minseuk Kim, Woon Bae Park, Jin-Woong Lee, and Kee-Sun Sohn* Faculty of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul 143-747, Korea S Supporting Information *
ABSTRACT: A solid-state combinatorial chemistry approach, which used the A−Ge−O (A = Li, K, Rb) system doped with a small amount of Mn4+ as an activator, was adopted in a search for novel red-emitting phosphors. The A site may have been composed of either a single alkali metal ion or of a combination of them. This approach led to the discovery of a novel phosphor in the above system with the chemical formula Li3RbGe8O18:Mn4+. The crystal structure of this novel phosphor was solved via direct methods, and subsequent Rietveld refinement revealed a trigonal structure in the P3̅1m space group. The discovered phosphor is believed to be novel in the sense that neither the crystal structure nor the chemical formula matches any of the prototype structures available in the crystallographic information database (ICDD or ICSD). The measured photoluminescence intensity that peaked at a wavelength of 667 nm was found to be much higher than the best intensity obtained among all the existing A2Ge4O9 (A = Li, K, Rb) compounds in the alkali-germanate system. An ab initio calculation based on density function theory (DFT) was conducted to verify the crystal structure model and compare the calculated value of the optical band gap with the experimental results. The optical band gap obtained from diffuse reflectance measurement (5.26 eV) and DFT calculation (4.64 eV) results were in very good agreement. The emission wavelength of this phosphor that exists in the deep red region of the electromagnetic spectrum may be very useful for increasing the color gamut of LED-based display devices such as ultrahigh-definition television (UHDTV) as per the ITU-R BT.2020-2 recommendations and also for down-converter phosphors that are used in solar-cell applications.
1. INTRODUCTION The importance and demand for phosphors with an emission in the red region of the spectrum have recently increased because these materials not only play a dominant role in improving the color rendering index (CRI) of commercially available white light emitting diodes (WLEDs) for general lighting applications but it also increase the color gamut for LED backlighting in display applications.1−6 In particular, a deep red emission of 650 nm or more has been of great interest in the search for an enlarged color gamut that can fulfill ITU-R BT.2020-2 recommendations.7 In addition, such a long-wavelength red phosphor can also assist in photovoltaic generation by downconverting the short-wavelength solar spectrum in solar-cell applications.8,9 In fact, very few red phosphors meet the actual requirements for practical application, and all phosphors used for commercial applications have IP-related complications. Thus, a new red phosphor, or even one containing a red emission component, is highly desired in the field of solid-state lighting and display applications. State-of-the-art Eu2+-doped red phosphors, such as CaAlSiN3:Eu2+, Sr2Si5N8:Eu2+, and Sr[LiAl3N4]:Eu2+, have shown excellent performance and meet the actual requirements for practical applications.2,10−12 However, harsh synthesis conditions, such as extremely high temperature and pressure © XXXX American Chemical Society
and strong photon reabsorption in the green or yellow spectral region due to 4f−5d transitions, can cause color changes and luminous reductions, which limit applicability. In this respect, Mn4+-doped phosphors offer advantages over rare-earth-doped phosphors due to their excellent red emission, which has gained attention in recent years.1,13−15 The sharp red emission from Mn4+ ions occurs due to a spin-forbidden 2Eg → 4A2g transition. The energy of the 2Eg excited state does not depend on the crystal-field strength.15 However, depending on the host, the spectral position of the 2Eg → 4A2g transition maxima can be detected over a wide range as a result of the covalent interaction between Mn4+ ions and their surrounding environment. Recently, fluoride-based, narrow, red-emitting phosphors,16−20 particularly K2SiF6:Mn4+ (KSF:Mn4+), have been widely accepted for the WLED in general lighting applications with high CRI, in which the Mn4+ ions lying in the octahedron exhibit the most intense excitation band located at ∼460 nm and very sharp red emission lines peaking at ∼630 nm.18−20 The focus of the present investigation was on the discovery of novel deep red phosphors with emission wavelengths that shift slightly to a longer region (up to 660 nm) in order to Received: July 1, 2016
A
DOI: 10.1021/acs.inorgchem.6b01576 Inorg. Chem. XXXX, XXX, XXX−XXX
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transferred to a specially designed sample container, a so-called combi-chem container made of alumina with the dimensions 80 × 40 × 20 mm3. The combi-chem container had 18 sample sites 8.5 mm in diameter and 16 mm in depth and was then fired at an optimized temperature of 800 °C for 4 h under an air flow (500 cc/min) in a sealed tube furnace. The total amount of raw materials at each sample site was about 0.3 g, which produced a sufficient amount of final phosphor powder available for use in any of the conventional characterizations. Each fired sample was then ground and subjected to powder X-ray diffraction (XRD) and photoluminescence (PL) analysis. In the initial screening, the powder XRD measurements were carried out using a Rigaku Miniflex 600 powder diffractometer operating at 600 W (X-ray tube) with Cu Kα radiation and fitted with a 1D silicon strip detector for fast, high-resolution scanning. The final best composition was characterized via SP-XRD. The SP-XRD measurements were conducted using a 9B high-resolution powderdiffraction beamline at the Pohang Accelerator Laboratory (PAL). The incident synchrotron X-rays were monochromatized to a wavelength of 1.4865 Å using a double-bounce Si(111) monochromator and calibrated with a SRM660a standard sample. The detector arm of the diffractometer had soller slits with an angular resolution of 2°, a flat Ge{111} crystal analyzer, an antiscatter baffle, and a scintillation detector. Data were collected in the angular 2θ range 5.0−100° with a step size of 0.01°. The emission spectra in the PL measurement were monitored at an excitation of 460 nm in a pseudo-high-throughput manner using an in-house-fabricated, continuous-wave (CW) PL system equipped with a xenon lamp.
achieve a larger color gamut and thereby to fulfill ITU-R BT.2020-2 recommendations.7 In this regard, Mn4+-doped A− Ge−O (A = Li, K, Rb,) systems such as Li2Ge4O9,21 LiNaGe4O9,22,23 Rb2Ge4O9, K2Ge4O9,24,25 and MGe4O9 (M = Sr, Ba)26 have recently been discovered and extensively investigated as suitable phosphor hosts for deep red emissions in the desired wavelength region under UV and blue excitation. Although the chemical formulas of these host compounds look very similar, the crystal structures are different. The crystal structure largely depends on the size of the monovalent cations at the A site and on the Ge−O bond distance. The crystal structure of three alkali-tetragermanates A2Ge4O9 (A = Na, K, and Rb), Na2Ge4O9, K2Ge4O9, and Rb2Ge4O9, are trigonal in the P3c̅ 1 space group while Li2Ge4O9 is orthorhombic in the P21ca space group.27,28 The crystal structure of the LiNaGe4O9 system on the other hand belongs to the orthorhombic system in the Pcca space group.29,30 Thus, there is an enormous scope for obtaining a new phosphor host in the alkali-germanates system depending upon a suitable search for one or a combination of alkali metal ions at the A site. Recently, solid-state combinatorial synthesis based on heuristic optimization techniques has proven to be a very effective tool for the discovery of novel phosphors for application in WLEDs, and our group has successfully discovered several novel phosphors using these techniques.31−38 In the present study, a similar approach was adopted in the search for a novel red-emitting phosphor host in the alkali-germanate system composed of the A−Ge−O (A = Li, K, Rb) system and doped with a small amount of Mn4+ ions. All the phases obtained during the synthesis were qualitatively and quantitatively characterized using powder X-ray diffraction, and the luminescent behavior was systematically studied using photoluminescent measurements. The above-described solidstate combinatorial chemistry process successfully led to the discovery of a novel phosphor in the Li−Rb−Ge−O system with the chemical formula “Li3RbGe8O18:Mn4+”. This chemical formula was based on a complete crystal structural analysis using direct methods and on subsequent Rietveld refinement using high-resolution synchrotron powder X-ray diffraction (SP-XRD) data. The crystal structure of the novel phosphor was found to be trigonal in the P3̅1m space group. As far as we could ascertain, this phosphor appears to be a novel one because neither the composition nor the crystal structure in the P3̅1m space group exists in either the International Centre for Diffraction Data (ICDD) or the Inorganic Crystal Structure Database (ICSD). Although the novelty issue has been confusing in both the academic and industry fields, we define the term “novel phosphor” as a phosphor the crystal structure of which is not isostructural with any of structures types listed in the ICSD database. The measured emission intensity of the novel (Li3RbGe8O18:Mn4+) phosphor was also found to be much higher than the best available intensity among all the existing A2Ge4O9 (A = Li, K, Rb) compounds in the alkaligermanates system.
3. RESULTS AND DISCUSSION 3.1. Combinatorial Chemistry Search Technique for the Discovery of a Novel Phosphor. A combinatorial chemistry search technique was adopted for the discovery of a novel red-emitting phosphor in the A−Ge−O system where the “A” site consisted of one or more alkali metal (K, Li, Rb) ions. The inspiration for the search of a novel phosphor in this system was influenced by the recent development of phosphor host compositions in the alkali-tetragermanates of the A2Ge4O9:Mn4+ (A = K, Li, Na, Rb) family that emits a red color under blue excitation.39,21−25 The structures of K2Ge4O9, Na2Ge4O9, and Rb2Ge4O9 were all identical,27 but Na2Ge4O9 could not be synthesized in the ambient environment and, instead, required high pressure for complete synthesis.40 In this regard, we employed K, Li, and Rb only on the “A” site, although the structure of Li2Ge4O9 differed from the other three. The combinatorial chemistry search began with the ternary diagram shown in Figure 1, and consisted of three cations (K, Li, and Rb) on the A site of the A2Ge4O9:Mn4+ formula unit with the molar ratio of A:Ge fixed as 2:4, and the first 21 compositions were prepared simultaneously. This composition search space was then further expanded by varying the A:Ge ratio in five proportions (2:4, 2.25:4, 2.25:4, 2.5:4, and 3:4)
2. EXPERIMENTAL SECTION The commercially available starting powder materials, Li2CO3 (SigmaAldrich, 99%.), K2CO3 (Junsei, 99.5%), Rb2CO3 (Alpha Aesar, 99%), GeO2 (Alpha Aesar, 99.99%), and MnCO3 (Sigma-Aldrich, 99.99%), were used for the synthesis. A given amount of raw materials was weighed and mixed manually using an agate mortar and pestle. The Mn4+ concentration was initially fixed at 0.005 mol during the combinatorial screenings. The mixed raw materials were then
Figure 1. Design of the phosphor−composition search space. B
DOI: 10.1021/acs.inorgchem.6b01576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. Prismatic diagram showing different phases of the ternary compositions obtained after XRD analysis of the synthesized compounds and actual sample photos taken under a 365 nm UV lamp. The pie graph located at each site represents a rough measure of the phase fraction, and each color in the pie graph represents a unique compound. The range of measured emission intensity is also shown in different colors.
with a fixed Mn4+ concentration at 0.05 mol %. Thus, a total of 21 × 5 = 105 different compositions were synthesized in the process. Our primary objective in the screening was to look for either type-III or type-IV novel phosphors with acceptable luminescence properties in the given search space. The details of the nomenclature for “type-III and type-IV” have been described in our previous reports.31,36 Powder XRD and photoluminescence (PL) analysis for all the 105 synthesized compositions were carried out extensively, and the results are depicted in Figure 2. All the phases shown in Figure 2 were characterized by considering the strongest reflections of the XRD patterns. In Figure 2, various phases that formed during the synthesis are shown by unique colors in the pie diagram arranged in the triangular shape under the XRD in the first column. The actual photograph taken under 365 nm UV appears in the second column, and the measured intensity appears in the third column. The color notation for each phase and the numerical range of the measured PL intensity are denoted by the specific colors shown on the left side of the figure. The results of five different A:Ge molar ratios are shown in five different rows in Figure 2. This figure shows that, in addition to the formation of well-known phases like Li2Ge4O9, Li2Ge7O15, Li2GeO3, and (K, Rb)2Ge4O9 in the given compositional search space, the formation of another two unknown phase types also took place and are christened “unknown 1” and “unknown 2”, as shown by the red and gray colors, respectively, in the pie diagram. These were christened unknown types because the powder XRD pattern of these phases could not be matched with any of the available XRD databases such as ICDD or ICSD. It is also clearly evident from Figure 2 that the unknown type phase was never formed as a pure single phase but always accompanied one or more types of minor impurity phases. Thus, more precise compositional search techniques and/or a more precise synthesis procedure is required to obtain a single phase of the unknown phase. It is noteworthy that all of the phases that are formed under the compositional search space emit only a red color, although with slight variations in emission intensity under 365 nm UV excitations. The measured PL intensity was found to be
maximum for the unknown-1 phase compared with all the other phases formed during synthesis. The compositions of the unknown-1 phase in the ternary search space were found mostly on or toward the line joining Li−Rb in the ternary diagram. Our interest was mainly focused on the discovery of a novel phase and not on the phases that already existed in the crystallographic database. Therefore, the well-known phases that appeared as pure single phase or sometimes showed better luminescence properties than the unknown phase were not considered for further study. This speculation led to a dramatic reduction in the processing composition search space during the combinatorial chemistry search. The cation composition at the A site was then limited to only Li and Rb ions, since the use of K ions resulted in the formation of mostly well-known types of phases. After the necessary information regarding the compositions of the unknown phase was obtained, particle swarm optimization (PSO) techniques41 were adopted for further fine-tuning of the compositional search space so as to achieve better luminescent properties. 3.2. Particle Swarm Optimization-Assisted Combinatorial Chemistry Search (PSOCMS). The PSO is a heuristic optimization technique, so-called metaheuristics, the origin of which was inspired by the social behavior of swarms.41 Similar to a genetic algorithm (GA), the PSO is also a populationbased search method, which is a zero-order, non-calculus-based method (no gradients are needed), and it works inherently better in a continuous decision parameter space; the details of the PSO technique have been discussed in our previous work.34,36 For PSO execution we have designed a quaternary search space consisting of Li2CO3−Rb2CO3−GeO2−MnO2 only, which is graphically represented as a prismatic shape shown in Figure 3. The PSO iteration was executed on the set of 18 completely random compositions. The phosphor compositions were regarded as major decision parameters, and the objective function for use in the PSO were the PL and XRD peak intensities of each phosphor. The most significant point of our PSO process was that the objective function was evaluated experimentally by the actual measurement of XRD and PL for actual samples, rather than by a mathematical C
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Ge9O20:Mn4+. However, this processing composition had nothing to do with the actual stoichiometry, but was simply our starting composition for the synthesis. It is customary that the actual stoichiometry differs from the processing composition in the synthesis of almost all inorganic compounds. It should be noted that the PSOCMS adopted both the PL intensity and the approximate phase purity parametrized by XRD as objective functions in a systematic manner, and involved no structural details. Thus, an extensive structural study using high-resolution SP-XRD data was implemented to precisely determine the crystal structure and true crystal composition of the optimal sample discovered from the PSOCMS, while the preliminary examinations regarding the various phase formations were done using laboratory XRD data. Indexing of the SP-XRD data for the determination of the unitcell dimensions followed by a space-group search based on the extinction symbol and a direct method for the atomic positions were sequentially performed to identify the complete crystal structure of the best unknown composition. Rietveld refinement was then finally carried out using the structural parameters obtained from the direct method. In the process of crystal structure determination, the Treor42 program was first employed to index the synchrotron powder XRD data, and the results were further verified using the DICVOL program.43 The indexing of the SP-XRD resulted in a hexagonal unit cell with the following lattice parameters: a = 9.4473(1) Å, b = 9.4473 (1) Å, c = 4.6604(1) Å, α = 90°, β = 90°, and γ = 120°. The figures of merit, M(20) and F(20), for the indexed structure were obtained as 428 and 426, respectively, and the 2θ difference between the positions of the observed and calculated peaks was found to be less than 0.003°. After analyzing the hkl values obtained from the indexing results, the extinction symbol was found to P_ _ _. On the basis of this information, it was difficult to comment on
Figure 3. Quaternary composition search space (Rb2CO3−GeO2− Li2CO3−MnO2) used for PSOCMS.
model. Therefore, we handled compositions in the continuous range when the PSO was implemented. This implies that we could reach an optimum composition with no constraints in a continuous search space when the PSO implementation was completed on the basis of a stopping criterion. The results obtained after each PSO execution in terms of PL intensity and the XRD pattern for the best composition of each swarm are shown in Figure 4. It is evident from this figure that the PL intensity gradually increases as we move from the first to the third swarm. The XRD pattern of the third swarm contains an almost clean phase with very small impurity peaks and the highest PL intensity compared with the previous two swarms. The concentration of Mn4+ that resulted in the best sample with the highest PL intensity was ∼0.005 mol, which was used during the third PSO execution. The best composition showing the strongest PL intensity was then subjected to extensive structural analysis using SP-XRD. 3.3. Structural Determination of the Discovered Phosphor. The optimum processing composition obtained from the PSOCMS execution was revealed to be Li3Rb-
Figure 4. Results of particle swarm optimization used in the combinatorial materials search (PSOCMS) execution are shown in the prismatic diagram. The emission intensity and the XRD of the best compositions obtained after each PSOCMS execution are shown below each. D
DOI: 10.1021/acs.inorgchem.6b01576 Inorg. Chem. XXXX, XXX, XXX−XXX
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from the direct method and the fixed position of the Li ion as the initial model. In the refinements, a pseudo-Voigt function and a linear interpolation between the set background points with refinable heights were used to define the profile shape and the background, respectively. Parameters, such as scale factor, zero correction, background, half-width parameters, the mixing parameters, lattice parameters, positional coordinates, and thermal parameters were varied in the course of refinement. It was found necessary to use anisotropic peak broadening45 in the refinements of SP-XRD patterns. Figure 5 shows a full-
the exact crystal system of the indexed structure because this extinction symbol belongs to both trigonal and hexagonal crystal systems. The possible space groups available under this extinction symbol are P3, P3̅, P321, P3m1, P3̅m1, P312, P31m, and P31̅ m in the trigonal and P6, P6̅, P6/m, P622, P6mm, P6̅2m, and P6/mmm in the hexagonal crystal systems. It should be noted that some impurity peaks with very weak intensities were also present in the SP-XRD pattern. The impurity peaks, however, could be easily excluded during the indexing process, because the strongest intensity peaks of the impurity phase were very distinct and well-separated from the peaks of the main phase. The impurity phases arose mainly due to the presence of some unreacted raw materials in the final product or to the formation of some other well-known complexes in very small fractions along with the main phase. The impurity phase present in a small amount could be indexed with the GeO2 (raw material) phase having a tetragonal structure in the P42/mnm space group while other impurity phases present in negligibly small amounts could be indexed with the Li2GeO3 and Li2Ge4O9 phases with orthorhombic structures in the Cmc21 space group and trigonal structures in the P3̅c1 space group, respectively. Lebail refinement was then carried out using the Fullprof program44 for all 15 of the space groups in the trigonal as well as hexagonal crystal systems in order to provide clues on the actual crystal system. The Lebail refinement ended up with a nearly similar value of χ2 for all 15 space groups and, thus, provided no significant clues regarding the true crystal system. The Check group program that was available with the Fullprof package was then employed for the search of possible space groups, but this approach also ended up with a similar figure of merit. A direct method was then employed one-by-one for all 15 available space groups, and the results were systematically analyzed by trial-and-error with the true space group being judged on the basis of the agreement factor and the quality-of-fit between the observed and the calculated data. Among all the available 15 space groups, only P3̅1m space group No. 162 in the trigonal crystal system resulted in a better agreement between the observed and calculated XRD pattern and with an acceptable value for agreement factors along with reasonable arrangements for the octahedral and tetrahedral networks. Using the direct method, the atomic position of the heaviest “Rb” ion was first determined via a difference Fourier map using the G-Fourier program available with the Fullprof package followed by the positions of the other lighter atoms such as germanium (Ge) and oxygen (O) ions. The Rb ion occupied a special position (0, 0, 0) at the 1a site, while the Ge ions occupied two Wyckoff positions: Ge1 (2x, x, 0) at the 6j site and Ge2 (1/3, 2/3, 0) at the 2c site. Oxygen also occupied two Wyckoff positions: O1 (x, y, z) at the 12l site and O2 (x, 0, z) at the 6k site. Since it was not possible to determine the exact position of the light Li atom using this method by employing XRD data, a crystal chemistry approach was adopted to find its position. On the basis of the occupancy sites of the various cations and anions, it was found that anion content was 3 units in excess of the cations and, thus, three more Li ions may be required to maintain the electrical neutrality. The Wyckoff sites available to accommodate three Li ions in the crystallographic table for the space group P3̅1m were either 3e or 3f. Among the available Wyckoff sites, 3f was chosen on the basis of fulfillment of the bond length conditions and the quality of the Rietveld fit. Full-pattern Rietveld refinement was then finally carried out using the atomic positions obtained
Figure 5. Observed (dots), calculated (red line), and difference (blue line) profiles for some selected peak profiles obtained after full-pattern Rietveld refinement of Li3RbGe8O18:Mn4+ using a trigonal structure in the P3̅1m space group along with the impurity GeO2 phase in the tetragonal structure in the P42/mnm space group. The vertical tick marks in the first and second lines, respectively, from the top denote the positions of Bragg reflections for the Li3RbGe8O18:Mn4+ phase and impurity GeO2 phases. The inset shows the zoomed portion of the XRD pattern, which depicts the strongest reflections of the impurity phases Li2Ge4O9 and Li2GeO3 marked with a triangle (Δ) and an asterisk (*), respectively.
pattern Rietveld refinement fit using a trigonal structure in the P31̅ m space group along with only one of the impurity phases, GeO2. A difference profile that is almost flat with very good values of the agreement factors (Rp = 7.12, Rwp = 9.33, Rexp = 3.99, and χ2 = 5.47) shows a very good fit between the observed and calculated profiles. The value of the χ2 in the current Rietveld refinement seems somewhat higher because some of the known impurity phases such as Li2Ge4O9 and Li2GeO3 are present in the XRD pattern, and these were not considered in the full-pattern Rietveld refinement. The peak intensities of these impurity phases were very weak and lying almost in the background and could be observed only when magnified on a very large scale. The peaks due to impurity phases with the strongest reflections are marked with a triangle (Δ) and an asterisk (*) for Li2Ge4O9 and Li2GeO3, respectively, in the magnified portion shown in the inset of Figure 5. The values of structural parameters such as atomic positions, thermal displacement parameters, and the site occupancy factor obtained after the Rietveld refinement are given in Table 1. The final stoichiometry of the discovered phosphor was estimated to be Li3RbGe8O18:Mn4+ on the basis of the final structural refinement. This discovered phosphor was classified as type-IV, since neither its composition nor its structure exists in any of the crystallographic databases (ICDD or ICSD). The final composition (Li3RbGe8O18:Mn4+) of the phosphor differs slightly from the processing composition Li3RbGe9O20:Mn4+ as obtained from the PSOCMS execution, because some of the GeO2 was precipitated out as an impurity phase and thereby the effective Ge content was reduced in the final composition. E
DOI: 10.1021/acs.inorgchem.6b01576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Table 1. List of Fractional Atomic Coordinates, Isotropic Displacement Parameters Uiso (Å2) and the Site Occupancy Factor (SOF) Obtained after the Full-Pattern Rietveld Refinement of Li3RbGe8O18:Mn4+ Using SP-XRD Data atom
Wyckoff site
x/a
y/b
z/c
U (Å2)
SOF
Rb Ge1 Ge2 O1 O2 Li
1a 6j 2c 12l 6k 3f
0.00000 0.39691(7) 0.66667 0.5058(3) 0.2630(4) 0.50000
0.00000 0.19848(7) 0.33333 0.3318(3) 0.00000 0.00000
0.00000 0.50000 0.00000 0.2362(6) 0.3662(8) 0.00000
0.0221(4) 0.00349(17) 0.0026(3) 0.0040(7) 0.0144(13) 0.06017
1.00 1.00 1.00 1.00 1.00 1.00
a Compound name: Li3RbGe8O18:Mn4+. Wavelength (λ): 1.4865 Å. Space group: P3̅1m, No. 162. Z: 1. Lattice parameters: a = b = 9.44549(3) Å, c = 4.66191(2) Å; α = β = 90°, γ = 120° (Rp = 7.12, Rwp = 9.33, Rexp = 3.99, χ2 = 5.47).
The Mn4+ ion has an ionic radius that is comparable to that of the Ge4+ ion and was believed to randomly occupy the positions of Ge ions in the crystal structure. The crystal structure of Li3RbGe8O18 viewed in the [001] direction and constructed using VESTA for a three-dimensional visualization of crystal, volumetric, and morphology data46 is shown in Figure 6. In the crystal structure, Ge ions located at
structure for the DFT calculation was constructed using the lattice parameters obtained from the Rietveld refinement of the experimental SP-XRD data. The exchange correlation potential in the DFT calculation was based on a generalized gradient approximation (GGA), as parametrized by Perdew, Burke, and Ernzerhof (PBE),47 and was employed in the Vienna ab initio simulation package (VASP5.3).48−51 The projector augmented wave (PAW) potentials52 along with a cutoff energy of 500 eV and 4 × 4 × 7 k-mesh were adopted using the Monkhorst− Pack scheme.53 No spin polarization was included in the present investigation. The structural relaxation was based on the PBE functional and was implemented with atomic positions and lattice parameters that were permitted to vary, while the symmetry remained unchanged. After completion of the structural relaxation under the above-described scheme, values for the band structure and the density of states (DOS) were also calculated under the same conditions. As a result of the GGA-PBE calculation, the band gap energy was calculated to be 2.72 eV. When compared with the experimental data, this band gap was found to be much lower than the experimentally measured optical band gap (5.26 eV). Figure 7 shows the diffuse reflectance spectra for Li3RbGe8O18
Figure 6. Polyhedral representation of the crystal structure of Li3RbGe8O18:Mn4+ viewed along the [001] direction.
two different Wyckoff sites form two types of a polyhedron. The Ge1 located at the 6j site is connected with four oxygen ions forming a tetrahedron while the Ge2 at the 2c site are connected with six oxygen ions forming an octahedron. Two different Ge−O bond distances exist in the tetrahedra, Ge−O1 (1.691(3) Å) and Ge−O2 (1.770(4) Å), while all six of the Ge−O bonds are identical (1.871(3) Å) in the octahedra. The three tetrahedra in the unit cell are connected by common corners to form a crankshaft-like structure similar to that obtained in the case of Li2Ge4O9 with an orthorhombic structure in the P21ca space group.28 The Rb ions located in the larger voids are coordinated with six oxygen ions with an average bond length of 3.014(3) Å, while Li ions located in relatively smaller voids are also coordinated with six oxygen ions, albeit with different Li−O bond lengths: four Li−O1 at distances ∼1.956(3) Å and two Li−O2 at distances ∼2.815(4) Å. Due to the smaller size of the Li ions and the larger dimensions of the voids in which they reside, these could have shown anisotropic displacement parameters. However, the displacement parameters of the Li ions were considered isotropic and were fixed at an optimum value during Rietveld refinement. 3.4. Density Functional Theory Calculation. The ab initio calculation was based on density function theory (DFT) and verified the host structure of a newly discovered oxynitride phosphor, Li3RbGe8O18. As mentioned earlier, the crystal structure of this phosphor in P3̅1m symmetry is novel, and therefore, structural verification is required. An input model
Figure 7. Diffuse reflectance spectra of Li3RbGe8O18 (LRG) with Mn4+ (red line) and without Mn4+ (black line). Inset shows the {F(R) hν }2 vs energy (hν) plot; the straight line in the plot intersects the energy axis at 5.26 eV.
and Li3RbGe8O18:Mn4+, and the inset to this figure shows a method for obtaining the optical band gap energies. The optical band gap for Li3RbGe8O18 was evaluated as 5.26 eV using the intersection of a straight line with the energy axis in the {F(R) hν}2 versus energy (hν) plot shown in the inset of Figure 7. The Kubelka−Munk equation was used to calculate the absorbance, F(R), from the diffuse reflectance data.54−56 The Mn4+-activated sample also exhibited a similar band gap energy, but several valleys were observed in the longer-wavelength region due to the absorption by the Mn4+ activator, and as such, F
DOI: 10.1021/acs.inorgchem.6b01576 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry these are not shown in Figure 7. Since all of the band gap energies calculated using GGA-PBE were far lower than the experimental values, we had to employ a more reliable calculation scheme such as either a DFT + U approach57−60 or a hybrid functional method.61−63 In this regard, we adopted a hybrid functional involving a nonlocal Fock exchange (HSE06) for further calculation to obtain a more realistic band gap energy.61−63 The GGA + U calculation gave smaller values for the band gap energy for various Ueff values in comparison with the HSE approach that was used for Li3RbGe8O18. A similar result was reported in the case of Ge metal.64 The final band gap energy was calculated as 4.64 eV when the HSE06 functional was employed. Although this value is still slightly lower than the experimental data shown in Figure 7, a clear improvement is obvious when compared with the GGA-PBE calculation. The HSE06 functional has proven to be highly effective in calculating the exact band gap energy of semiconductors, but also is known to fail significantly for large-gap insulators such as Li3RbGe8O18.65 However, the HSE06 hybrid functional remains the only substantial option by considering the computational expenses, although a complementary option, for instance an HSE+U approach,66 could be appropriate for Li3RbGe8O18. The band structure and total DOS obtained after the ab initio calculation are shown in Figure 8.
Figure 9. Comparison of the emission spectra of various alkaligermanates at λex = 460 nm.
Figure 10. Comparative plot for the (a) excitation and (b) emission spectra of Li3RbGe8O18:Mn4+ (LRG: Mn4+) and K2SiF6:Mn4+ (KSF: Mn4+) phosphors. Figure 8. Band structure and total DOS of Li3RbGe8O18 calculated using the hybrid functional (HSE06).
emission peak is based on the 2Eg → 4A2g transition of the Mn4+ ions. The differences in the emission wavelengths stem from the fact that the 2Eg → 4A2g transition is singularly dependent on the covalency of the Mn4+−ligand bonding and is fairly independent of the strength of the crystalline field in a solid. The covalent character of the metal−ligand bonding, which determines the energy of the 2Eg → 4A2g emission transition of the Mn4+ ions, can be altered by changing the constitution (cations and anions) of the host lattice.20−22 Even though the emission intensity of the LRG:Mn4+ phosphor was smaller by comparison with the KSF:Mn4+ phosphor, it was much higher than those of other alkali-germanates systems. It is believed that the emission intensity could be further improved by precisely monitoring the synthesis conditions. In Figure 10a the emission spectrum lies in a very deep red region (667 nm), for which the eye is less sensitive. This may not be very useful for general lighting, but it may be very useful for a display application by providing a wider color gamut in modern ultrahigh-definition television (UHDTV) according to the ITU-R BT.2020-2 recommendations.7 UHDTV provides viewers with an enhanced visual experience primarily by having a wide field of view both horizontally and vertically with appropriate screen sizes relevant to usage at home and in public
3.5. Photoluminescence Measurements. For comparative studies of the PL properties of alkali-germanates A2Ge4O9:Mn4+ (A = Li, K), which were prepared separately under conditions similar to those of the new compound (Li3RbGe8O18), PL measurements were performed at λex = 460 nm, and their emission intensity plot is shown in Figure 9. As this figure shows, the emission intensity of the newly discovered phosphor was much higher than the best intensity obtained among all the existing A2Ge4O9 (A = Li, K, Rb):Mn4+ phosphors. Further, in order to compare PL properties with a well-known red emission phosphor, KSF:Mn4+, the emission spectra under an excitation wavelength of 460 nm for both the KSF:Mn4+ phosphor and the newly discovered Li3RbGe8O18:Mn4+ (LRG:Mn4+) phosphor are shown in Figure 10a, while the PLE spectrum obtained by monitoring at λem = 667 nm is shown in Figure 10b. The PLE spectrum exhibited two broad bands centered at ∼325 and ∼460 nm, which can be assigned to the 4A2 → 4T1 and 4A2 → 4T1 transitions of Mn4+ ions, respectively. The emission peak of the KSF:Mn 4+ phosphor lay at 631 nm while that of the LRG:Mn4+ phosphor was found in the deep red region at 667 nm. The sharp red G
DOI: 10.1021/acs.inorgchem.6b01576 Inorg. Chem. XXXX, XXX, XXX−XXX
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(4) Chen, W. T.; Sheu, H. S.; Liu, R. S.; Attfield, J. P. Cation-SizeMismatch Tuning of Photoluminescence in Oxynitride Phosphors. J. Am. Chem. Soc. 2012, 134, 8022−8025. (5) Im, W. B.; George, N.; Kurzman, J.; Brinkley, S.; Mikhailovsky, A.; Hu, J.; Chmelka, B. F.; DenBaars, S. P.; Seshadri, R. Efficient and Color-Tunable Oxyfluoride Solid Solution Phosphors for Solid-State White Lighting. Adv. Mater. 2011, 23, 2300−2305. (6) Shang, M.; Li, C.; Lin, J. How to Produce White Light in a SinglePhase Host? Chem. Soc. Rev. 2014, 43, 1372−1386. (7) Recommendations ITU-R BT.2020-2, Parameter Values for UltraHigh Definition Television Systems for Production and International Programme Exchange; International Telecommunication Union, 2015. (8) Trupke, T.; Green, M. A.; Würfel, P. Improving Solar Cell Efficiencies by Down-Conversion of High-Energy Photons. J. Appl. Phys. 2002, 92, 1668−1674. (9) Van der Ende, B. M.; Aarts, L.; Meijerink, A. Near-Infrared Quantum Cutting for Photovoltaics. Adv. Mater. 2009, 21, 3073− 3077. (10) Xie, R.-J.; Hirosaki, N.; Suehiro, T.; Xu, F.-F.; Mitomo, M. A. Simple, Efficient Synthetic Route to Sr2Si5N8:Eu2+-Based Red Phosphors for White Light-Emitting Diodes. Chem. Mater. 2006, 18, 5578−5583. (11) Piao, X.; Machida, K.-I.; Horikawa, T.; Hanzawa, H.; Shimomura, Y.; Kijima, N. Preparation of CaAlSiN3:Eu2+ Phosphors by the Self-Propagating High-Temperature Synthesis and Their Luminescent Properties. Chem. Mater. 2007, 19, 4592−4599. (12) Tsai, Y.-T.; Chiang, C.-Y.; Zhou, W.; Lee, J.-F.; Sheu, H.-S.; Liu, R.-S. Structural Ordering and Charge Variation Induced by Cation Substitution in (Sr,Ca)AlSiN3:Eu Phosphor. J. Am. Chem. Soc. 2015, 137, 8936−8939. (13) Brik, M. G.; Camardello, S. J.; Srivastava, A. M. Influence of Covalency on the Mn4+2Eg→4A2g Emission Energy in Crystals. ECS J. Solid State Sci. Technol. 2015, 4, R39−R43. (14) Wang, B.; Lin, H.; Xu, J.; Chen, H.; Wang, Y. CaMg2Al16O27:Mn4+-based Red Phosphor: A Potential Color Converter for High-Powered Warm W-LED. ACS Appl. Mater. Interfaces 2014, 6, 22905−22913. (15) Brik, M. G.; Srivastava, A. M. On the Optical Properties of the Mn4+ Ion in Solids. J. Lumin. 2013, 133, 69−72. (16) Jin, Y.; Fang, M.-H.; Grinberg, M.; Mahlik, S.; Lesniewski, T.; Brik, M. G.; Luo, G.-Y.; Lin, J. G.; Liu, R.-S. Narrow Red Emission Band Fluoride Phosphor KNaSiF6:Mn4+ for Warm White LightEmitting Diodes. ACS Appl. Mater. Interfaces 2016, 8, 11194−11203. (17) Wei, L.-L.; Lin, C. C.; Wang, Y.-Y.; Fang, M.-H.; Jiao, H.; Liu, R.-S. Photoluminescent Evolution Induced by Structural Transformation Through Thermal Treating in the Red Narrow-Band Phosphor K2GeF6:Mn4+. ACS Appl. Mater. Interfaces 2015, 7, 10656− 10659. (18) Takahashi, T.; Adachi, S. Mn4+-Activated Red Photoluminescence in K2SiF6 Phosphor. J. Electrochem. Soc. 2008, 155, E183−E188. (19) Kasa, R.; Adachi, S. Red and Deep Red Emissions from Cubic K2SiF6:Mn4+and Hexagonal K2MnF6 Synthesized in HF/KMnO4/ KHF2/Si Solutions. J. Electrochem. Soc. 2012, 159, J89−J95. (20) Radkov, E. V.; Grigorov, L. S.; Setlur, A. A.; Srivastava, A. M. Red Line Emitting Phosphor Materials for Use in LED Applications. United States Patent Application Publication US20060169998A1, 2006. (21) Kunitomo, J.; Suzuki, R.; Takahashi, Y.; Miyazaki, T.; Terakado, N.; Fujiwara, T. Red-Emissive Mn-Doped Li2Ge4O9 Phase Synthesized via Glass-Ceramic Route. J. Ceram. Soc. Jpn. 2014, 122, 725−727. (22) Li, P.; Tan, L.; Wang, L.; Zheng, J.; Peng, M.; Wang, Y. Synthesis, Structure, and Performance of Efficient Red Phosphor LiNaGe4O9:Mn4+ and Its Application in Warm WLEDs. J. Am. Ceram. Soc. 2016, 99, 2029−2034. (23) Omel’chenko, K. S.; Khmelenko, O. V.; Panchenko, T. V.; Volnyanskii, M. D. Photoluminescence of Manganese-Doped LiNaGe4O9 Crystals. Phys. Solid State 2014, 56, 751−756.
places. The ITU-R BT.2020-2 recommendation specifies various UHDTV image system parameters for production and international program exchange. According to one of its recommendations for a wider color gamut, phosphors with deep red emissions are highly desirable, and the LRG:Mn4+ phosphor would meet those requirements. In addition, the emission of LRG:Mn4+ phosphors lying in deep red regions would also be useful in solar-cell applications because of the ability to down-convert energy.
4. CONCLUSIONS A solid-state combinatorial chemistry approach has led to the discovery of a novel deep red phosphor, Li3RbGe8O18:Mn4+ (LRG: Mn4+), in the alkali-germanate system. The crystal structure of the novel host compound obtained after extensive structural analysis, which included indexing, Lebail refinement, direct methods, and Rietveld refinement, was found to be trigonal in the P3̅1m space group. The emission intensity of the discovered phosphor was much higher than the best intensity obtained among all the existing A2Ge4O9 (A = Li, K, Rb) compounds. The ab initio calculation based on density function theory (DFT) calculation verified the discovery of this novel structure by exhibiting good agreement between the theoretical and experimental values of band gap energy. The emission spectrum of the LRG:Mn4+ phosphor peaks in the very deep red region (667 nm), which would be applicable for use in UHDTV applications according to the ITU-R BT.2020-2 recommendation and as a down-converting phosphor for solar cells.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b01576. Crystal structure details (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by Creative Materials Discovery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and Future Planning (2015M3D1A1069705).
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REFERENCES
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