Engineering the Lattice Site Occupancy of Apatite-Structure

May 3, 2017 - A series of britholite compounds were synthesized by simultaneous introduction of trivalent La3+ and Si4+ ions into an apatite structure...
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Engineering the Lattice Site Occupancy of Apatite-Structure Phosphors for Effective Broad-Band Emission through Cation Pairing Sanjith Unithrattil, Ha Jun Kim, Kyeong Hun Gil, Ngoc Hung Vu, Van Hien Hoang, Yoon Hwa Kim, Paulraj Arunkumar, and Won Bin Im* School of Materials Science and Engineering and Optoelectronics Convergence Research Center, Chonnam National University, 300 Yongbong-dong, Buk-gu, Gwangju 500-757, Republic of Korea S Supporting Information *

ABSTRACT: A series of britholite compounds were synthesized by simultaneous introduction of trivalent La3+ and Si4+ ions into an apatite structure. The variations in the average structure, electronic band structure, and microstructural properties resulting from the introduction of cation pairs were analyzed as a function of their concentration. The effects of the structural variance and microstructural properties on the broad-band-emitting activator ions were studied by introducing Eu2+ ions as activators. For the resulting compound, which had dual emission bands in the blue and yellow regions of the spectrum, the emission peak position and strength were dependent upon the concentration of La3+−Si4+ pairs. By engineering the relative sizes of the two possible activator sites in the structure, 4f and 6h, through the introduction of a combination of trivalent La3+ and a polyanion, the preferential site occupancy of the activator ions was favorably altered. Additionally, the activator ions responsible for the lower-Stokes-shifted blue component of the emission functioned as a sensitizer of the larger-Stokes-shifted yellowemitting activators, and predominantly yellow-emitting phosphors were achieved. The feasibility of developing a white lightemitting solid-state device using the developed phosphor was also demonstrated.

1. INTRODUCTION Apatite compounds are one of the most abundant minerals and a primary source of phosphorus. These compounds are obtained from phosphorite rock and are used as a raw material for synthesizing other chemicals.1−3 Because of their biocompatibility and chemical and physical stability, apatite compounds are used in a wide variety of applications, including biomedical applications, field-emission displays, laser crystals, and phosphors.4−6 Although most naturally occurring apatite compounds are greenish in appearance, depending on their constitutions, apatites can have vivid colors and are often cut and faceted as gemstones.7 The apatite group of compounds with the general formula M(1)10−δM(2)δ(ZO4)6X2, where M(1) and M(2) can correspond to monovalent, divalent, or trivalent metal ions (Z can be, for example, P, Si, Bi, B, or V etc., and X can be one of a variety of anions, such as O2−, N3−, Cl−, F−, or OH−), have extremely diverse compositions.8 Depending on the choice of Z or the metallic cations, the X sites can remain half-occupied. The substitution of elements or the vacancy in the structure is usually manifested by changes in the lattice parameters and physical and chemical properties, depending on the nature and sizes of the substituent cations or anions.9,10 Thus, choosing an appropriate substituent can significantly modify the morphology, individual bond strengths, and distortion in the structure. Substituent elements have been reported to preferentially occupy one of the two sitesM(1) and M(2)when the substituent element concentration is relatively low.11,12 However, as the concentration of the higher- or lower-oxidation © 2017 American Chemical Society

state ion increases, the proportion of ions substituted at both sites varies substantially. By controlling the site occupancy, compounds with specific properties can be achieved. Furthermore, the occupancy of ions with multioxidation states in single Wyckoff sites without substantial alteration of the basic structure has immense potential for the design of materials for specific applications.13−15 The ratio of the oxidation states of the major and minor cations can further determine the occupancy of the 2a sites in the anion channels and could, thus, alter the C3 symmetry of the channel and significantly alter the material properties. In this work, we studied the effect of tweaking the polyanions and major cations of apatite compounds on the structural and luminescence properties of the resulting hexagonal apatite structure. The feasibility of the substitution, the phase formation of the compound, and the possibility of emission color tuning were investigated by using Eu2+ as the activator. The color-conversion mechanism involved was also investigated and elucidated through photoluminescence (PL) and absorbance measurements. The structural variance caused by the introduction of La3+−Si4+ pairs was determined through band structure analysis using density functional theory (DFT) calculations. The microstructural properties of the compound that led to the shifts in the component emission bands were also analyzed. Finally, the feasibility of using the developed Received: February 3, 2017 Published: May 3, 2017 5696

DOI: 10.1021/acs.inorgchem.7b00310 Inorg. Chem. 2017, 56, 5696−5703

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Inorganic Chemistry

isolated PO43− tetrahedra with two cationic sites: M(1) and M(2). One cation occupying the Wyckoff position (4f) is surrounded by polyanions, whereas the other metal ion occupying the 6h position is attached to a free anion, which is not part of any of the polyanion groups present in the structure, as shown in Figure 1. Therefore, these two cation

phosphor in solid-state lighting was demonstrated by preparing prototype white light-emitting diodes (wLEDs).

2. EXPERIMENTAL PROCEDURE 2.1. Sample Preparation. Powder samples of the general formula Sr10−xLax(PO4)6−x(SiO4)xO:Eu2+ (x = 1, 2, 3, 4, and 5) were prepared by a solid-state reaction method. The starting materials were SrCO3 (99.5%, Sigma-Aldrich), Eu2O3 (99.99%, Sigma-Aldrich), SiO2 (99.9%, Kojundo), SrHPO4 (99.98%, Sigma-Aldrich), (NH4)2HPO4 (98%, Sigma-Aldrich), and La2O3 (99.99%, Sigma-Aldrich). The concentration of Eu was fixed at 30 mol % for the initial trials that addressed tuning the phosphor composition. A mixture of the stoichiometric amounts of the raw materials was ball milled for 12 h using ZrO2 with ethanol as the dispersing medium. The mixture was then dried at 65 °C for approximately 1 h. Subsequently, the dried powder was pelletized under an axial load of 1.5 ton and heated at various temperatures ranging from 1300 to 1500 °C for 4 h under a reducing atmosphere of 5% H2/95% N2 at the heating rate 5 °C min−1. The samples were allowed to cool naturally to room temperature in the furnace, pulverized, and fired under a higher reducing atmosphere (25% H2/75% N2) for the same duration. The luminescence properties of the samples were analyzed based on room-temperature PL spectra collected using a Hitachi F-4500 fluorescence spectrophotometer over the wavelength range 200−750 nm. Diffuse reflectance spectra were recorded using a Thermo Scientific Evolution 220 UV−visible spectrophotometer in the wavelength range 200−700 nm. 2.2. Structural and Optical Measurements. The structural analysis of the samples was conducted using X-ray diffraction (XRD) data obtained with Cu Kα radiation (Philips X’Pert) over the angular range 10° ≤ 2θ ≤ 100° with a step size of 0.026°. The obtained diffraction data were analyzed using the Rietveld method with the General Structure Analysis System (GSAS).16 The line shapes of the diffraction peaks were generated by pseudo-Voigt functions, and the background was refined to a first-degree polynomial. In the final run, the background coefficients, zero point, half width, pseudo-Voigt, and asymmetry parameters for the peak shape, scale factor, and unit cell parameters were refined. The band structures of the compounds were studied by performing DFT calculations in a Cambridge Serial Total Energy Package (CASTEP)17 module, and the microstructural properties were analyzed using WinPLOTR.18 2.3. Fabrication of Prototype wLEDs. Prototype wLEDs were fabricated by applying an appropriate mixture of phosphor and transparent silicone resin to the header of an InGaN LED (λmax = 395 nm). The resulting structure was then cured at 150 °C for 1 h. The optical properties of the fabricated wLED were measured in an integrating sphere under various DC forward bias conditions at room temperature.

Figure 1. (a) Unit cell representation of Sr10−xLax(PO4)6−x(SiO4)xO. Blue/pink, white, and cyan spheres correspond to La/Sr, P/Si, and O, respectively. The coordination environments of the major cations (b) M(1) and (c) M(2), respectively, at positions 4f and 6h are also shown.

sites have two different coordination environments with effective 9-fold and 7-fold coordination. Both cations are equally likely to be occupied by substituent alkali, alkali-earth, transition metal, or rare earth ions. The O atoms associated with the polyanions occupy two 6h sites and one 12i site. The lone O site 2a that is not associated with the polyanion in the structure is half-occupied in compounds with the general formula Sr10−xLax(PO4)6−x(SiO4)xO. The crystallinity of all the compounds was analyzed through SEM (Figure S1) analysis. The XRD profiles of all the synthesized compositions were well indexed to the standard (PDF# 44-0654), as confirmed by Rietveld refinement. Figure 2 shows the Rietveld refinements of the XRD profiles of compounds Sr10−xLax(PO4)6−x(SiO4)xO, where x = 1−5. Structural analyses were conducted based on the Rietveld refinements of the XRD data profiles of the compound series. For all of the compounds, reasonable Rwp and goodness of fit parameter (χ2) values were obtained. The refinement parameters are tabulated in Table 1, and the obtained lattice parameters, fractional coordinates of atoms, and bond lengths are listed in Tables 2, S1, and S2, respectively. The decreases in the unit cell parameters and unit cell volume are proportional to the x value. This behavior confirms that the solutions become iso-structural in nature upon the progressive replacement of PO43− units with SiO44− units and the substitution of Sr2+ with La3+. Because a single 6h site is shared by both P and Si, both the tricapped trigonal prismatic geometry around the M(1) cation and the pentagonal bipyramidal geometry around the M(2) cation become contracted upon the progressive replacement of PO43− with SiO44− because of the latter’s stronger covalent nature. Figure 3(a) shows a sharp decline in the unit cell parameter a = b as the replacement by PO43− increases from x = 1 to 2. However,

3. RESULTS AND DISCUSSION 3.1. Phase Formation in Oxy-Britholites. Apatite, which consists of a series of hexagonal prismatic compounds, is one of the most abundant mineral forms in the Earth’s crust and is a primary source of phosphorus. Indeed, because of their peculiar structural chemistry, these compounds can allow numerous substitutions, and possible substituents include metal cations from all three groups of the periodic table. Apatite can also accommodate a wide variety of polyanions with different charges, and configurations, such as SiO44−, CO32−, and SO44−. When the naturally occurring PO43− is partially replaced by SiO44−, the resulting compounds are referred to as britholites. Although most substituents occupy a relatively small fraction of the host sites, a few can occupy large amounts of host sites or even result in a solid solution. This trait of apatite compounds is useful for designing materials for specific application in various scientific fields. Apatite compounds crystallize in a hexagonal space group, P63/m. The basic structure consists of 5697

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Table 2. Unit Cell Parameters and Unit Cell Volume of Sr10−xLax(PO4)6−x(SiO4)xO; (x = 1 to 5)a sample x x x x x

= = = = =

1 2 3 4 5

a = b (Å) 9.73854 9.71523 9.71627 9.71352 9.71185

(17) (16) (19) (15) (13)

c (Å) 7.2798 7.2554 7.2484 7.2412 7.2403

(2) (2) (3) (2) (17)

V (Å3) 597.91 593.06 592.61 591.69 591.41

(2) (2) (2) (2) (2)

a

The numbers in parentheses are the estimated standard deviations of the least significant figures.

Figure 3. Evolution of lattice parameters as a function of x in Sr10−xLax(PO4)6−x(SiO4)xO: (a) a = b-axis, (b) c-axis, and (c) volume. Figure 2. (a−e) Rietveld refinement patterns of Sr10−xLax(PO4)6−x(SiO4)xO using XRD. The black circles represent the observed intensities, and the solid line reflects the calculated intensities. A difference (obs−cal) plot is shown underneath. The top panel presents the calculated Bragg’s positions.

Table 1. Rietveld Refinement Parameters of Sr10−xLax(PO4)6−x(SiO4)xO (x = 1 to 5) by XRDa formula

Sr10−xLax(PO4)6−x(SiO4)xO x=1

radiation 2θ (degree) T/K symmetry space group Z RP Rwp χ2

4.96% 7.76% 3.58

x=2

x=3

x=4

x=5

5.04% 7.63% 3.10

Cu Kα 10−100 295 hexagonal P63/m 4 3.84% 5.64% 1.89

4.49% 7.17% 2.88

4.3% 6.89% 2.56

a

Constraints: xSr1 = xLa1; ySr1 = yLa1; zSr1 = zLa1; xSr2 = xLa2; ySr2 = yLa2; zSr2 = zLa2; xP = xSi; yP = ySi; zP = zSi; LaSr2 = 2/3LaSr1.

upon further increases, this parameter remained nearly constant except for a slight decline for x = 5. In contrast, the decline in the unit cell parameter c was nearly monotonic throughout the process. 3.2. Dual Band Emission and Preferential Occupation by Cation Pairs. The excitation and emission spectra of Sr10−xLax(PO4)6−x(SiO4)xO for x = 1 to 5 are shown in Figure 4(a). All of the excitation spectra were recorded at the peak emission wavelength of the corresponding emission spectra,

Figure 4. (a) Excitation and emission spectra of the Sr10−xLax(PO4)6−x(SiO4)xO:Eu2+ under 400 nm excitation, (b) the diffuse reflectance spectra, and (c) the Tauc’s plot of compounds with different values of x. The optical band gaps estimated from the tangents to the inflection point are also shown.

and the emission spectra of all the compositions were recorded at 400 nm. The excitation and emission spectra can be 5698

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Inorganic Chemistry attributed to the 4f−5d transitions of the Eu2+ ions. The excitation spectra of all the compositions contained a host absorption band positioned at approximately 320 nm and two other distinguishable absorption bands at roughly 380 and 410 nm for x = 1; these peaks shifted toward progressively higher wavelengths as the composition varied. These bands are a result of the electronic transition of Eu2+ from a 4f 7 ground state to a 4f65d excited state.19 The relative strengths of the host absorption and activator absorption were found to vary from ∼1 to ∼2 for x = 1 to 5 as increasing amounts of ligand O2− became attached to the relatively electronegative SiO44− group. The relative intensities of the host and activator absorption bands were 1.75, 1.85, and 1.9 respectively, for x = 2, 3, and 4. These findings were further verified by the diffuse reflectance spectra of the compounds, which are shown in Figure 4(b). In all the compounds, except for that with x = 1, the absorption bands were found to be above ∼4.5 eV and to exhibit an increasing trend as the composition varied. Two absorption peaks could be assigned to the charge-transfer bands and were observed to shift progressively as the polyanion radicals were varied. The optical band gaps of the samples estimated from the Tauc’s plots20 shown in Figure 4(c) also exhibited an increasing trend, with values of 3.28, 4.46, 4.54, 5.13, and 5.02 eV for x = 1, 2, 3, 4, and 5, respectively. All the emission spectra were found to contain asymmetric bands arising from the multiple emission centers in the structure. Additionally, all the emission spectra appeared to be more or less similar, irrespective of the excitation source. To distinguish between the direct excitation of the activators occupying the two emission centers in the structure, the excitation was fixed at 400 nm for all subsequent analyses in this work. The asymmetrical emission spectra of the compounds were resolved (Figure S2) into two Gaussian peaks centered in the blue and yellow regions, as shown in Figure 5(a). The individual Gaussian components were determined to originate from the two crystallographically distinct sites in the structure. The activator ion occupying the Sr(1) site, which has 9-fold coordination, gives rise to the blue component, whereas that occupying the site with 7-fold coordination produces the yellow emission band. This

conclusion was verified using the empirical relation proposed by van Uitert, given as21 V E (cm−1) = Q [1 − ( )1/ V × 10−(nEar)/80] 4

(1)

where E is the position of the Eu2+ emission peak, Q is the energy of the lower d-band edge of the free Eu2+ ion (Q = 34000 cm−1), V is the valency of the Eu2+ ion (V = 2), n is the number of anions in the immediate shell around the Eu2+ ion, Ea is the electron affinity of the anion atom in eV, and r is the radius of the host cation replaced by the Eu2+ ion (Å). Here, r is the ionic radius, and values of 1.21 and 1.10 Å were chosen for the seven- and nine-coordinated Sr/La ions; Ea, the electron affinity of the coordination environment, was set at 1.65 eV for both sites. The obtained values of the energy were 20,266 and 17,826 cm−1, respectively, in agreement with the experimental values. The individual Gaussian peaks of the composite emission bands were observed to vary in intensity and peak position as the composition changed. The lower-Stokes-shifted band located in the blue region of the spectra red-shifted as the concentration of SiO4 increased. Simultaneously, the largerStokes-shifted band located in the yellow region of the spectra blue-shifted, as shown in Figure 5(b). The emission intensity of the blue component decreased as the composition changed from phosphate to silicate radicals. The decrease in the relative intensity of the blue component was accompanied by an increase in the relative intensity of the yellow component of the deconvoluted emission bands. The progressive blue and red shifts of the constituent Gaussian peaks with analogous crystalline environments originated from the drastic change in the bond length of the Eu2+ ligand. However, the blue shift of the yellow emission component was much more rapid than the average compression or elongation of the metal ligand separation in the compounds. Although the compressive and expansive trends of the metal−ligand separation corresponding to both Sr sites were opposite, as shown in Figure S3, the peak shift appears to be consistent. Another important observation is that the intensity of the blue component of the emission band, which was red-shifted, decreased rapidly as the x value increased. If the relative position of the Eu2+ excitation levels with respect to the bottom of the conduction band is significantly low, it could result in photoionization of the excitonic electron, and a decline in emission intensity could be expected. To understand this phenomenon, the variation in the band structure as the x value increased was studied. DFT calculations based on the local density approximation method using a Perdew−Burke−Ernzerhof functional were employed to estimate the electronic band structures of the compound series. The band structures of the compounds with different x values are shown in Figure 6. Both the valence and conduction bands vary significantly as the composition was altered. The bottom of the conduction band flattens as the x value increases, whereas the split states in the valence band join together for x = 3 and split off again as x increases further. The electronic band gaps of the compounds were found to initially increase as the x value increased. The largest band gap was obtained for the compound corresponding to x = 3, and thereafter, the electronic band gap decreased. The band gap was 4.31 eV for x = 1, approached a maximum of 5.30 eV for the compound corresponding to x = 3, and then decreased to 4.69 eV upon approaching x = 5. This behavior is against the generally observed phenomenon of the band gap increasing as

Figure 5. (a) Deconvolution of the emission peaks of Sr10−xLax(PO4)6−x(SiO4)xO:Eu2+ into blue and yellow components. (b) The red shift and blue shift of the component blue and yellow emission bands as a function of x. 5699

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directions. The peak broadening was deconvoluted to both the size and strain effects as a function of 2θ. By plotting β cos θ/λ vs sin θ/λ, the microstrain can be estimated from the line slope, and the inverse of the y-intercept gives the crystallite size. The analysis was performed on three families of planes, viz., the 0k0, 0ll, and hh0 planes. The Williamson−Hall plots of the compounds corresponding to the x values of 1, 3, and 5 are shown in Figure 7, and the obtained values are tabulated in

Figure 7. Williamson−Hall plot of the Sr10−xLax(PO4)6−x(SiO4)xO compounds corresponding to x = 1, 3, and 5 based on the XRD pattern showing the lattice strain variation. Figure 6. Electronic band structure of the Sr10−xLax(PO4)6−x(SiO4)xO compounds corresponding to x = 1, 2, 3, 4, and 5 presented in (a), (b), (c), (d), and (e), respectively, and (f) the density of states of the x = 5 composition.

Table S4. For all three compounds, the variation of 4 sin θ with β cos θ is linear, and the slopes are positive, indicating that the nature of the strain is expansive. Additionally, the increase in x considerably increased the microstrain. Therefore, although it shrinks the lattice in general, the substitution of the La3+−Si4+ pair for the Sr2+−P5+ pair has an expansive tendency locally at Sr(1) sites. This observation, in addition to the downward shift of the valence band, confirms the effect of the microstrain on the structure. The introduced La3+ is smaller in size than Sr2+, whereas Si4+ is much larger than P5+ and bears an additional formal charge. Furthermore, additional trivalent ions introduced into the britholite lattice will not be evenly distributed in the Sr(1) lattice sites but will instead be distributed in the Sr(1) and Sr(2) lattice sites in a ratio of 3:1, as assumed in the refinement. This behavior is substantiated by the studies conducted by Hughes et al.,24 where lanthanide ions, such as La3+, Ce3+, and Pr3+, were reported to be slightly overbonded in the Sr(2) sites and, thus, to prefer the Sr(1) sites. However, studies on fluoro-apatites and apatite structures in general suggest that this ratio will vary as the overall La/Sr ratio increases. Therefore, the increase in the La3+ content in the major cation sites may not be linearly distributed, resulting in shear strain in the lattice. This behavior can be explained by the slight initial expansion of the Sr(1) polyhedra and the corresponding bond length of Sr(1) at x = 1 and the smooth inflection thereafter. In contrast, the Sr(2) polyhedral volume initially decreases up to x = 3 and smoothly increases thereafter. The tetrahedra in which the relatively small minor cations are replaced by larger Si4+ ions continued to expand. The estimated lattice parameters of the compounds also reflect lattice shrinkage without inflection, justifying the increase in the strain. In the britholite structure, when La3+−Si4+ pairs are introduced, the average size of the Sr(1) site decreases because

the lattice contracts, as represented by the band gap deformation potential.22 αV =

∂Eg

(2) ∂ ln V where V is the unit cell volume. The bandwidths of all the compositions were sufficiently large so that they did not directly influence the excitation and emission of high-energy bands from the Sr(1) sites. The excitation energy used in this work (λex = 400 nm) is substantially lower than the band energy gaps of all the compounds. Therefore, the rapid decrease in the emission intensity and the eventual absence of the blue emission component can be assumed to be uninfluenced by the relative positions of the 5d levels of Eu2+ occupying Sr(1) sites. To investigate the simultaneous red shift of the blue component and blue shift of the yellow component of the emission bands as the composition varied, the microcrystalline properties of the three compounds corresponding to x = 1, 3, and 5 were analyzed. Williamson−Hall analysis23 allows the contributions of the crystalline domain size and microstrain to XRD peak broadening to be separated. The crystallite strain induces a variation in the X-ray peak width and is proportional to tan θ. The peak width also varies with the crystallite size but is inversely proportional to cos θ. Therefore, the effect of the lattice strain on the peak broadening was analyzed by applying a uniform deformation model, a modified form of Williamson− Hall analysis, to the XRD data. In this approach, the lattice strain is assumed to be uniform in all crystallographic 5700

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Inorganic Chemistry of the smaller size of the introduced cations. The corresponding introduction of Si4+ into the smaller P5+ site simultaneously expands the polyanion tetrahedra. The newly introduced Si4+ will largely be associated with La3+, thereby maintaining charge neutrality at local levels. However, when neighboring cationic sites are successively replaced with smaller La3+, the average size of the lattice site decreases, and eventually, the broad-bandemitting Eu2+ ions are ionized to Eu3+, which no longer emits the blue component. The line emission characteristic of Eu3+ ions was consistently observed in all the analyzed samples. In contrast, the introduction of Si4+ increases the size of the tetrahedra, and therefore, to release the lattice strain, the Sr/ La(1)−O bond length decreases. Similarly, at Sr(2) sites, which largely remain associated with smaller PO4− tetrahedra, a portion (1/3) is occupied by trivalent ions. Therefore, this site continues to expand at the expense of the Sr(1) site, and the emission shifts toward the blue region of the spectra. The relative changes in the metal−ligand bond length are tabulated in Table S3. As the bond lengths of the compounds from x = 1 to 5 change, the distributions of ligands around the central cations become increasingly spherical. This behavior is evident from the calculated electron density distributions of the end members shown in Figure 8. The electron density distribution,

Figure 9. (a) PL excitation and emission spectra of Sr5La5(PO4)(SiO4)5O:yEu2+ for different values of the activator concentration y, the relative intensity of (b) blue and (c) yellow components in the emission band, and the peak emission wavelength of (d) blue and (e) yellow components as a function of the activator concentration y.

observed. Additionally, the blue component of the emission peak appeared to completely vanish as the doping concentration of the activator ions increased. The complete absence of the blue emission from compounds with higher activator concentrations cannot be assumed to be attributable to photoionization because the electronic band gap of the host composition and the optical band gap of the moderately doped compounds are significantly high. The above observations can be understood as resulting from three phenomena that are typical of broad-band-emitting activator ions: the preferential occupancy of activator ions, energy transfer among activator ions, and the typical concentration-quenching phenomenon. The preferential occupation of the Sr(1) site by the activator ions gives rise to a substantial blue emission component. As the concentration of the activator ions increases, the occupancy of the Sr(2) site increases, leading to an increase in the yellow emission component. Additionally, the energy transfer from the activator ions at the Sr(1) sites to those at the Sr(2) sites, the efficiency of which increases as the concentration of the acceptor ions increases (here, the Eu2+ ions at the Sr(2) sites) approaches unity. Furthermore, higher activator concentrations at the seven-coordinate Sr(2) site lead to the complete absence of Eu2+ emission from the Sr(1) sites. Therefore, the Eu2+ ions at the Sr(1) sites function as a sensitizer, transferring energy to the yellow-emitting activator ions. Figure S4 shows the deconvolution of the emission bands of the selected compositions for different activator concentrations. The corresponding relative intensities of the component emission peaks and the red shifts of the constituent Gaussian bands are shown in Figure 9(c). The emission intensity of the blue components of the emission bands progressively decreased, whereas the yellow component exhibited typical concentrationquenching behavior. The corresponding energy-transfer

Figure 8. Electron density distributions in the unit cells of the compositions corresponding to x = 1 and 5 along the (001) plane presented in (a) and (c), respectively, and along the (100) plane presented in (b) and (d), respectively.

which can be taken as an indication of the local symmetry around the Sr(2) sites, tends to be spherical, whereas the corresponding distribution around the Sr(1) sites remains nearly constant in the structure. Therefore, the introduction of La3+−Si4+ imparts a lower crystal field split of the activator ions present in the Sr(2) sites, and the resultant component emission is blue-shifted. A fairly similar but opposite behavior was observed in the emission properties of the compounds when the concentration of the activator was varied in one of the compositions discussed above. Figure 9 shows the PL excitation (PLE) and PL spectra of the selected composition Sr5La5(PO4)(SiO4)5O:yEu2+ (x = 5) for different activator concentrations. Increasing the activator concentration strengthened the yellow emission component of the composition at the expense of the blue component. Unlike when the La3+−Si4+ pair was varied, the shift of the blue emission component was negligible. In contrast, a significant red shift of the yellow component was 5701

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Inorganic Chemistry efficiency calculated based on the relative intensity of the activator emissions from both the activator sites is shown in Figure S5(a). Eventually, this phenomenon leads to a symmetric emission band typical of Eu2+ emission from a single emission center. The yellow component of the emission peak was found to be red-shifted, whereas the blue component does not exhibit a noticeable shift in the peak emission wavelength. The discrepancy in the observation of the concentration quenching of the blue component, which is shown in Figure 9(b), arises from the fact that the additional activator ions prefer to occupy the Sr(2) sites rather than the Sr(1) sites. Therefore, the phenomenon cannot be identified to have reached the critical concentration. In contrast, the occupancy of the yellow-emitting Sr(2) sites continues to increase, and the typical concentration-quenching phenomenon arises. Based on some previous reports on the occupancy of trivalent ions in the Sr(1) and Sr(2) sites of apatite structures, which maintains a 1:3 ratio between the 4f and 6h sites, the approximate concentrations of the activator at each site can be estimated to be 0.054. As a result of concentration quenching, the excitation energy at the Sr(2) sites may not be relayed through the Sr(1) sites because of the energy difference between the excited states of the activator ions. Therefore, a modified Blasse’s relation is used to calculate the critical distance for concentration quenching,25 ⎡ ⎤1/3 3V ⎢ ⎥ Rc ≈ 2 ⎢⎣ 4πXcEu(2)N ⎥⎦

Figure 10. EL spectra of the InGaN LED + phosphors under various forward bias currents (indicated). (a) InGaN (λmax = 395 nm) + Sr5La5(PO4) (SiO4)5O:0.1Eu2+ phosphor. Inset shows a fabricated wLED with and without bias current. (b) CIE chromatic coordinates of the device under various forward bias currents. The Planckian locus line and the points corresponding to color temperatures of 3500 and 6500 K are indicated.

phosphors combined with commercial InGaN chips (λmax = 395 nm) on plates. These wLEDs were operated under various bias currents ranging from 20 to 120 mA at room temperature. The EL spectra show distinct emission bands centered at 395 and 565 nm. The narrow band centered at 395 originates from the LED chip, whereas that centered at 565 nm is the Stokesshifted broad-band emission from the phosphor, which ranges from 450 to 700 nm. The average color-rendering index (CRI) of the combination with a single component phosphor was found to be approximately 65, with a correlated color temperature (CCT) of 3811 K. Thus, the emission of the wLED is not perfect white light and the properties can be improved up to the commercial standard by incorporating phosphors with blue and red components, as done in industry. Figure 10(b) shows the color coordinates on an International Commission on Illumination (CIE) chromaticity diagram for various operating currents ranging from 20 to 120 mA, and the corresponding coordinates are shown in Table S5. The color coordinates of the fabricated wLED remain approximately the same on the CIE chromaticity diagram, regardless of the variation in the operating current.

(3)

where V is the volume of the unit cell, N is the number of host cations in the unit cell, and XcEu(2) is the critical concentration of the activator ion at the Sr(2) sites. The critical concentration was calculated to be 7.78 Å, which exceeds the typical critical distance of 5 Å. The mechanism underlying the concentration quenching was analyzed using the relation26 I k = x 1 + β(x)θ /3

(4)

where k and β are constants and θ assumes values of 6, 8, and 10 for dipole−dipole, dipole−quadrupole, and quadrupole− quadrupole interactions, respectively. Figure S5(b) shows a plot of the log (I/x) vs log(x) relation, and the slope of the linear fit was found to be −1.92 for Sr5La5(PO4)(SiO4)5O:0.1Eu2+. Thus, a dipole−dipole interaction is the mechanism underlying the concentration quenching of Sr5La5(PO4)(SiO4)5O:0.1Eu2+ phosphors. Since the excitation and emission spectra of Sr5La5(PO4)(SiO4)5O:0.1Eu2+ overlap significantly (Figure S5(c)), the critical distance was also analyzed using Dexter’s theory.27 Rc6 = 0.63 × 1028

4.8 × 10−16PA E4

∫ fs (E)fA (E)dE

4. CONCLUSIONS The overall Stokes shift of the Eu2+ emission band and the relative strength of the near-ultraviolet (UV)-blue absorption of the britholite host composition were successfully tuned. Tuning of the emission bands was achieved by engineering the preferred site occupancy of the apatite structure by introducing La3+−Si4+ pairs, which are of different sizes and bear different formal charges than the substituent cation pair Ca2+−P5+. The variation in the electronic band structures of the compounds resulting from the introduction of La3+−Si4+ pairs was found to be favorable for large-Stokes-shifted broad-band emission. Introducing cation pairs caused lattice shrinking because of the simultaneous expansion and compression of two major cation polyhedra. The apparent lattice strain in the compound resulting from the introduction of cations of varying sizes

(5)

where PA is the oscillator strength of the transition, which was taken as 0.01 for Eu2+ ions; E is the energy corresponding to the maximum overlap, which was determined to be 2.54 eV; and the spectral overlap integral derived from the normalized PLE and PL spectra was 4.02 × 107 eV−1. The critical distance was calculated to be 18.5 Å, which exceeds the typical critical distance of 5 Å. Figure 10 presents the electroluminescence (EL) spectra of the wLED fabricated using Sr5La5(PO4) (SiO4)5O:0.1Eu2+ 5702

DOI: 10.1021/acs.inorgchem.7b00310 Inorg. Chem. 2017, 56, 5696−5703

Article

Inorganic Chemistry

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induces significant shifts in the component emission bands. Because of the lattice contraction caused by the introduction of the polyanions, the charge density distribution around the emission centers varied significantly.

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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.7b00310. Rietveld refinement results, bond lengths, polyhedral volume, lattice strain parameters, CIE color coordinates, deconvolution of PL emission bands, and energy transfer analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +82-62-530-1715. Fax: +82-62-530-1699. E-mail: [email protected]. ORCID

Sanjith Unithrattil: 0000-0001-9072-7163 Won Bin Im: 0000-0003-2473-4714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and future Planning (2017R1A2B3011967, 2016R1E1A2020571).

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REFERENCES

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DOI: 10.1021/acs.inorgchem.7b00310 Inorg. Chem. 2017, 56, 5696−5703