Interplay between Crystal Structure and Photoluminescence

Article pubs.acs.org/JPCC

Interplay between Crystal Structure and Photoluminescence Properties of β‑Ca3SiO4Cl2:Eu2+ Tae-Gon Kim,*,† Taehyung Kim,† Jaegyeom Kim,‡ Seung-Joo Kim,*,‡ and Seoung-Jae Im† †

Advanced Materials Research Center, Samsung Advanced Institute of Technology, Yongin, 446-712, Republic of Korea Department of Chemistry, Division of Energy Systems Research, Ajou University, Suwon, 443-749, Republic of Korea

‡

S Supporting Information *

ABSTRACT: The crystal structure of β-Ca3SiO4Cl2 was determined by the ab initio structure determination method based on the synchrotron powder XRD data for the first time, and the luminescence properties of a Eu2+-doped β-Ca3SiO4Cl2 phosphor were characterized. βCa3SiO4Cl2 was found to be monoclinic (space group P21/c) with the lattice parameters, a = 5.91234(1) Å, b = 10.20128(1) Å, c = 10.98866(1) Å, β = 90.3423(1)°. This structure can be considered as an intergrowth structure built up from alternating stacks of two layered sublattices, 2∞[Ca2SiO4] and 2∞[CaCl2], along the [100] direction. In this structure, the Ca atoms occupy three crystallographically distinct sites: Ca1O4Cl3, Ca2O5Cl2, and Ca3O3Cl4. The photoluminescence of the Eu2+-doped Ca3SiO4Cl2 phosphor excited at 450 nm blue light shows the 150 nm wide-band emission peaked at 635 nm with about 70% quantum efficiency. The photoluminescence properties, such as centroid shifts, crystal-field splitting, and Stokes shifts, were correlated with the crystal structure through the calculation of shared-electron populations reflecting the bond covalency between Ca and O/Cl. Effects of doping concentrations on the luminescence spectra and temperature stability were also discussed based on the inhomogeneous energy transfer property determined by the structural geometric factor.

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Green phosphor Ca3SiO4Cl2:Eu2+ with 510 nm emission has been suggested as a good candidate for UV and blue light stimulated LEDs due to its high efficiency, proper color coordinate, and good thermal stability.17,18 The crystal structure of Ca3SiO4Cl2 was initially identified by Czaya et al.19 Ca3SiO4Cl2 crystallizes in the monoclinic system, which consists of alternating layers of Ca2SiO4Cl and Ca2Cl2 with a 2:1 ratio. On the other hand, several literatures have reported different optical properties and XRD diffraction patterns for Ca3SiO4Cl2, which may be related to the polymorphic phase transition of the compound.20−24 The second modification of a Ca3SiO4Cl2:Eu2+ phosphor has been synthesized at higher temperature (∼1020 °C) than that for the former (∼900 °C). It shows an efficient orange emission peak around 620 nm with a bandwidth around 150 nm for the excitation at 450 nm, which are exceptional properties in Eu2+-doped oxide phosphors.25 Its red-shifted and broad-band emission and the blue excitation are more profitable for warm-white LED lighting than those of recently reported (Ca,Sr)7(SiO3)6Cl2:Eu2+ as a promising new phosphor showing yellow emission at 575 nm with a bandwidth around 120 nm for the excitation at 400 nm.26 However, because the crystal structure was not clear so far, the reason for the superior luminescence characteristics has not been wellunderstood. In this study, the high-temperature modification of Ca3SiO4Cl2 was isolated and the crystal structure was

INTRODUCTION

InGaN-based white light emitting diodes (LEDs) have attracted much attention as a lighting source for liquid crystal display and one of the next-generation illumination systems due to their high luminescence efficiency, low power consumption, excellent durability, and environmental friendliness. Recent explosive demands of research on LEDs have provided us new opportunities and challenges. A common method for fabricating white light LEDs is a combination of the blue emission from InGaN chips and the yellow emission from phosphors, such as Y3Al5O12:Ce3+ and (Ba,Sr)2SiO4:Eu2+. The color rendering index (CRI) of the white light produced by these combinations, however, is not high enough to provide sunlight-like illumination. Methods of pumping green and red phosphors with blue LEDs and of stimulating blue, green, and red phosphors with ultraviolet (UV) LEDs have been suggested as alternatives for display and general illumination light sources due to their high color gamut or high CRI or color stability.1−4 Among the numerous phosphor candidates, several limited materials, such as β-sialon:Eu2+, SrSi2O2N2:Eu2+, CaSc2O4:Ce3+, and Ca3Mg2Si3O12:Ce3+ as green phosphors and (Sr,Ca)AlSiN3:Eu2+ and Sr2Si5N8:Eu2+ as red phosphors, have been commercialized and employed in white LED lamps for highquality display and illumination.5−14 However, in spite of their superior properties, difficulties in synthesis of (oxy)nitrides, exclusive patent issues, and the resulting high costs for applications still drive the development of new oxide-based phosphors.15,16 © XXXX American Chemical Society

Received: January 9, 2014 Revised: May 7, 2014

A

dx.doi.org/10.1021/jp5002379 | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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RESULTS AND DISCUSSION Synchrotron X-ray diffraction data for β-Ca3SiO4Cl2 showing the reflection conditions (h0l, h00, 00l: l = 2n, 0k0: k = 2n) were indicative of the monoclinic space group P21/c (No. 14) with lattice parameters, a = 5.91234(1) Å, b = 10.20128(1) Å, c = 10.98866(1) Å, β = 90.3423(1)°. Liu et al. reported that the high-temperature phase of Ca3SiO4Cl2 showed a monoclinic unit cell; however, the lattice parameters suggested in their report are quite different from the present result.18 The unit cell volume of 662.8 Å3 for β-Ca3SiO4Cl2 is slightly smaller than that of the known α-Ca3SiO4Cl2 (684.2 Å3). The initial positions for Ca, Si, Cl, and O atoms were determined by the direct method and different Fourier map analysis. No additional symmetry, as tested by PLATON, was detected in this structure, and no unusual trends were found in the goodness of fit and Miller indices. Figure 1 shows the final result of Rietveld refinement based on the synchrotron powder XRD patterns of β-Ca3SiO4Cl2.

determined by using synchrotron powder X-ray diffraction data. The new modification of Ca3SiO4Cl2 will be denoted as βCa3SiO4Cl2 in distinction from the previously reported αCa3SiO4Cl2 phase. On the basis of the structural characteristics, the photoluminescence properties of the Eu2+-doped βCa3SiO4Cl2 (β-Ca3SiO4Cl2:Eu2+) were investigated.

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Article

EXPERIMENTAL SECTION

β-Ca3SiO4Cl2:Eu2+ phosphors were prepared through a twostep solid-state reaction. In the first step, CaCO3 (Aldrich, 99.99%) and SiO2 (Kojundo, 99.99%) powders were mixed together in an agate mortar with a mole ratio of 2:1. The mixture was fired at 1250 °C for 8 h in an air atmosphere to get Ca2SiO4. The obtained Ca2SiO4 powder was mixed with CaCl2· 2H2O (Aldrich, 99.99%) and Eu2O3 (Aldrich, 99.999%) in a mole ratio of 1:1.1:1.5x (x = 0, 0.00033, 0.00066, 0.001, 0.02, 0.05, and 0.1) and sintered again at 1050 °C in a reducing atmosphere of H2/N2 = 5%/95% for 8 h. After the sintering, excess CaCl2 was washed with deionized water three times and dried in a convection oven at 70 °C for 2 h. Because of the washing process in which some amounts of Eu ions might be washed away in the form of EuCl3, it cannot be excluded that the Eu concentration in the host lattice slightly deviates from the nominal composition. Synchrotron X-ray powder diffraction experiments were performed using the multiple detector system installed at the 9B beamline of the Pohang Light Source, Pohang Accelerator Laboratory in Korea. A monochromatic 1.549 Å X-ray beam was used. Diffraction data were collected in an asymmetric flatspecimen reflection geometry with a fixed angle of 7.0° at room temperature. Scanning parameters were fixed as follows: diffraction angle 2θ was 10−130°, scan step was 0.005°, and counting time was 2.5 s per step. Peaks indexing and the lattice parameters determining procedures were carried out using the TREOR software.27 A total of 1123 reflections were converted into structure factors and used as input for the program EXPO software package.28 The space group and initial structural model were determined by applying the direct method. Structure refinements of atomic positions and isotropic displacement parameters were carried out by the Rietveld method using the FULLPROF program,29 with the Thompson−Cox−Hastings pseudo-Voigt function. Manual background correction was used in the all refinements. Photoluminescence (PL) and photoluminescence excitation (PLE) were measured with a spectrophotometer (Hitachi, F7000) at room temperature and liquid nitrogen temperature 77 K, which utilized a 150 W xenon lamp as an excitation source. An electrically controlled heater was mounted on the sample holder for the spectrophotometer to analyze the dependence of PL on temperature. To evaluate the shared-electron population, the tight-binding extended Hückel (eH) calculation was performed, utilizing the Yet Another extended Hückel Molecular Orbital Package (YAeHMOP) program.30 The eH calculations are certainly less reliable for structures but offer a number of analytical tools for exploring the nature of bonding. The magnitudes of shared electrons between chemical bonds were quantitatively estimated from the Mulliken overlap population averaged with 64 k points. The eH parameters used in the computation are listed in Table S1 (Supporting Information).

Figure 1. Rietveld refinement of the synchrotron powder XRD profiles of the high-temperature monoclinic modification β-Ca3SiO4Cl2. The phase includes Ca2SiO4 and an orthorhombic unknown phase as impurities. Measured data, fitted results, expected reflection positions, and the difference between measured and fitted results are expressed as black open circles, red solid lines, green vertical lines, and blue solid lines, respectively.

The sample includes a small amount (∼5%) of impurities of Ca2SiO4 and an orthorhombic unknown phase. Taking into account the unit cell dimensions (a ≈ 11.63 Å, b ≈ 10.29 Å, c ≈ 5.57 Å, and V ≈ 666 Å), the orthorhombic phase can be an additional polymorphic form of Ca3SiO4Cl2. The structural characterization of the orthorhombic phase is in progress. Final values of the atomic coordinates, equivalent isotropic displacement parameters, and the reliability factors of the refinement are given in Table 1. The selected bond distances are summarized in Table 2. The correctness of structure determination was confirmed by comparing theoretical valence and bond valence sum (BVS) calculations from the observed bond distances. The estimated BVSs of all the ions are close to their expected valence states, indicating their reasonable coordination environments. In the structure of β-Ca3SiO4Cl2, all atoms occupy the general positions (Wyckoff position 4e). Ca atoms are positioned at three different sites that would be B



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Table 1. Structural Parameters of High-Temperature Monoclinic Modification β-Ca3SiO4Cl2 Determined by Rietveld Refinement Based on Synchrotron Powder XRD Dataa atom

site

x

y

z

Uiso (Å2)

BVS

Ca1 Ca2 Ca3 Si O1 O2 O3 O4 Cl1 Cl2

4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

0.0660(3) 0.4332(3) 0.2580(3) 0.4314(5) 0.5711(9) 0.2752(9) 0.2712(8) 0.609(1) 0.7765(4) 0.0479(4)

0.7923(2) 0.1148(2) 0.4456(2) 0.2601(3) 0.1112(6) 0.2150(5) 0.2255(5) 0.1471(5) 0.3968(2) 0.5575(2)

0.1259(2) 0.1234(2) 0.1167(2) 0.3701(3) 0.9167(5) 0.7517(5) 0.4821(5) 0.3275(5) 0.1018(2) 0.3554(2)

0.0214(7) 0.0123(6) 0.0183(7) 0.017(8) 0.016(2) 0.014(2) 0.009(2) 0.019(2) 0.0246(8) 0.0259(9)

1.74 1.78 2.05 4.49 −2.00 −2.19 −2.17 −1.97 −0.92 −0.93

Space group: P21/c (No. 14), Z = 4, a = 5.91234(1) Å, b = 10.20128(1) Å, c = 10.98866(1) Å, β = 90.3423(1)°, V = 662.752(1) Å3, Rwp = 13.7%, RBragg = 8.6%, Rf = 5.8%, χ2 = 3.02.

a

Figure 2a illustrates the crystal structure of β-Ca3SiO4Cl2. A single SiO4 tetrahedral unit does not share its oxygen with other SiO4 units, which is a typical feature of orthosilicate compounds. An intriguing point is the fact that β-Ca3SiO4Cl2 can be regarded as an intergrowth-type compound built up from two layered sublattices, 2∞[Ca2SiO4] and 2∞[CaCl2], that are stacked alternately along the [100] direction. If the two different sublattices are denoted by A and B, respectively, the stacking sequence is A-B-A-B-A-B-.... The structure of βCa3SiO4Cl2 is distinct from that of the low-temperature modification (α-Ca3SiO4Cl2) reported previously. As shown in Figure 2b, the structure of α-Ca3SiO4Cl2 also consists of two kinds of layered sublattices. One is the 2∞[Ca2SiO4Cl] sublattice in which Cl− and SiO44− anions form a cubic close-packed arrangement and Ca2+ cations occupy octahedral sites, forming a distorted NaCl-type structure. The other is the 2∞[Ca2Cl2] sublattice, which is formed by binding of Ca2+ and Cl− ions along the bc plane. The layers are stacked in such way that two 2∞[Ca2SiO4Cl] layers alternate with one 2∞[Ca2Cl2] layer. The stacking sequence in αCa3SiO4Cl2 is A′-B′-A″-A′-B′-A″-.... Figure 3 illustrates the crystal structure of β-Ca3SiO4Cl2 and the coordination spheres of three different Ca atoms. Ca atoms are coordinated with O and Cl atoms to form irregular polyhedra, Ca1O4Cl3, Ca2O5Cl2, and Ca3O3Cl4. The environment of Ca2+ ions is a crucial factor to determine the luminescent property of substituted Eu2+ ions. As shown in Table 2, three Ca sites are coordinated with two types of anion layers with different bond-distance ranges. The average distances of Ca3−O (2.281 Å, CN = 3) and Ca3−Cl (3.005 Å, CN = 4) are the shortest among the corresponding bonds in three polyhedra. The effective size of the Ca2 site is smaller

Table 2. Bond Distances and the Calculated Electron Overlap Populations between Ca and O/Cl in β-Ca3SiO4Cl2 cation

anion

bond length (Å)

Ca1

O1 O2 O3 O4 Cl1 Cl1 Cl2

2.409(6) 2.432(6) 2.412(5) 2.476(6) 3.300(3) 3.306(3) 2.796(3)

Ca2

O1 O1 O2 O3 O4 Cl2 Cl2

2.347(6) 2.418(6) 2.427(6) 2.443(6) 2.489(6) 2.914(3) 3.130(3)

Ca3

O2 O3 O4 Cl1 Cl1 Cl1 Cl2

2.212(6) 2.289(6) 2.283(6) 2.895(3) 2.899(3) 3.106(3) 3.125(3)

shared-electron population

total

total

total

0.11 0.08 0.10 0.08 0.13 0.12 0.33 0.95 0.18 0.09 0.11 0.09 0.09 0.29 0.20 1.05 0.22 0.19 0.19 0.29 0.28 0.21 0.14 1.52

substituted by aEu2+ ion. O and Cl occupy four and two different sites, respectively.

Figure 2. Schematic structures of the (a) high-temperature modification, β-Ca3SiO4Cl2, and (b) low-temperature modification, α-Ca3SiO4Cl2. Yellow, blue, red and purple spheres represent Ca, Si, O, and Cl, respectively. A tetrahedron represents the SiO4 group. C



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Figure 3. Ca-centered polyhedra in β-Ca3SiO4Cl2. Structures (a)−(c) are Ca1O4Cl3, Ca2O5Cl2, and Ca3O3Cl4 polyhedra, respectively. Yellow, red, and purple spheres represent Ca, O, and Cl, respectively.

than that of Ca1 due to the shorter distance of Ca2−Cl (3.021 Å, CN = 2) than that of Ca1−Cl (3.130 Å, CN = 3) despite the same average distances of Ca1−O (2.434 Å, CN = 4) and Ca2−O (2.434 Å, CN = 5). In short, the average bond distances between Ca and the coordinated anions decrease in the order of Ca1, Ca2, and Ca3. To correlate the coordination structures with the luminescence parameters, the bond covalency between Ca and O/Cl was quantitatively evaluated by calculating the Mulliken overlap population through the extended Hückel method. The estimated shared-electron populations in Ca1O 4 Cl 3 , Ca2O5Cl2, and Ca3O3Cl4 polyhedra as a function of bond distances are plotted in Figure 4 (exact values are given in the

Figure 5. Excitation and emission spectra of β-Ca3SiO4Cl2:0.05Eu2+ phosphors at room temperature. (a) Excitation spectrum for the emission at 650 nm, and the emission spectra excited at 300, 350, 400, 450, and 500 nm. (b) Excitation spectra for the emissions at 500, 550, 600, 650, 700, and 750 nm, and the emission spectra excited at 400 nm. Positions of excitation or emission wavelengths are indicated by solid arrows. Inset: color coordinates of the emission spectra in (a) βCa3SiO4Cl2:Eu2+, Y3Al5O12:Ce3+ (YAG), and (Sr,Ca)AlSiN3:Eu2+ (SCASN).

150 nm bandwidth for the 450 nm excitation, arising from the 4f6d1 ↔ 4f 7 transition in doped Eu2+ ions. The red-shifted and broad-band emission excited at 450 nm is a suitable feature for the warm white LED illumination with a high color rendering index. The quantum efficiency (QE) of β-Ca3SiO4Cl2:0.05Eu2+ was estimated as 70% through the comparison of the integrated PL intensity excited at 450 nm with that of commercialized (Sr,Ca)AlSiN3:Eu2+ with QE = 85% (Figure S1, Supporting Information). As the excitation wavelength increases from 300 to 350, 400, 450, and 500 nm, emission peaks correspondingly change from 620 to 623, 626, 635, and 655 nm (Figure 5a). Excitation spectra also shift to longer wavelength for the change of emission position to longer wavelength, as shown in Figure 5b. The change of excitation spectral shapes comes from the change of the relative intensities of three bands around 360, 430, and 480 nm. The luminescence variation at different excitations and emissions was caused by the multiluminescence centers with different electronic environments in a single phosphor.13,31 Three humps in excitation spectra around 350, 430, and 480 nm would correspond to three types of Ca2+ sites in β-Ca3SiO4Cl2.

Figure 4. Correlation between the bond distances and the calculated electron overlap populations between Ca and O/Cl in β-Ca3SiO4Cl2. Values for Ca1O4Cl3, Ca2O5Cl2, and Ca3O3Cl4 polyhedra are expressed as blue squares, green circles, and orange triangles, respectively. Fitted lines for the values and Ca−O and Ca−Cl bonds are expressed as dotted and dashed lines, respectively.

fourth column in Table 2). The shared-electron populations of Ca−O and Ca−Cl bonds could be linearly correlated with the bond distances regardless of the types of involved polyhedrons as fitted with dotted and dashed lines in Figure 4, respectively. That is, the bond covalency is mainly determined by the bond distances rather than other factors, such as coordination numbers, symmetry, and O/Cl ratio, in this structure. This is consistent with the accordance between the increasing order of the calculated shared-electron populations in Ca1O4Cl3, Ca2O5Cl2, and Ca3O3Cl4 polyhedra and the decreasing order of the average bond distances. PL emission and excitation spectra of β-Ca3SiO4Cl2:0.05Eu2+ at room temperature are shown in Figure 5. β-Ca3SiO4Cl2: 0.05Eu2+ emits yellowish orange light with a 635 nm peak and a D



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To investigate the multiluminescence more closely, we measured the photoluminescence properties at 77 K, as shown in Figure 6. The Eu2+-doping concentration was very low as

Table 3. Emission Peaks, Centroid Shifts, Crystal-Field Splitting, and Stokes Shifts of Subphotoluminescence from Three Activation Sites in High-Temperature Monoclinic Modification β-Ca3SiO4Cl2:Eu2+ Phosphors

I II III

emission peak (eV)

centroid shift, Ec (eV)

crystal-field splitting, Ecfs (eV)

Stokes shift, ΔS (eV)

2.66 2.36 1.97

0.41 0.59 0.63

1.19 1.30 1.58

0.37 0.56 0.67

splitting, and Stokes shifts, as summarized in Figure 7. Since the centroid shift is determined by covalency between Eu2+ and

Figure 6. Excitation and emission spectra of β-Ca3SiO4Cl2: 0.00033Eu2+ phosphors at 77 K. Emission bands are fitted with three Gaussian curves (dashed lines), and the excitation bands corresponding to the fitted emissions are also acquired. The pairs of the fitted emission peaks and the excitation bands are separately plotted in (b)−(d). Figure 7. Energy levels of red shifts (D), centroid shifts (Ec), crystalfield splitting (Ecfs), and Stokes shifts (ΔS) of subphotoluminescence from three activation sites in β-Ca3SiO4Cl2:0.05Eu2+ phosphor.

0.00033 for minimizing the contribution of energy transfer on the spectral shape.25 Three peaks were clearly measured in an emission spectrum excited at 300 nm, and subemission bands peaked at 467, 525, and 629 nm could be extracted through fitting with Gaussian functions.32 Excitation spectra corresponding to the subemissions were measured for the emissions at 465, 530, and 700 nm indicated as blue, green, and orange arrows, respectively (Figure 6a). Especially, the emission at 700 nm was selected to minimize the contributions of the second emission peaked at 525 nm to the third excitation band. Each fitted emission and the corresponding excitation spectrum were paired and considered as a single-site photoluminescence as expressed in Figure 6: (b) (luminescence I, blue curves), (c) (luminescence II, green curves), and (d) (luminescence III, orange curves). Photoluminescence parameters, such as red shift (D), centroid shifts (Ec), crystal-field splitting (Ecfs), and Stokes shift (ΔS), of each subluminescence were estimated from the extracted spectra. Red shifts were acquired from the difference between the energy of the 5d level in a free Eu2+ ion (4.22 eV) and the lowest excitation energy among the five d levels.25,31 The latter was determined from the mirror image relationship between the emission and the excitation spectra with their cross point as a center of the symmetry.31 Centroid shifts were estimated from the average energy of the excitation spectrum through the equation ∫ f(E)E dE/∫ E dE, where f(E) is the excitation intensity as a function of energy.32 Crystal-field splitting was approximated to be the difference between the highest and the lowest excitation energy levels. The position of the highest energy band was taken from the fitting of the excitation spectra with five Gaussian functions, in which the parameters for the lowest-energy sub-band were fixed as the already-acquired values. Stokes shifts were extracted from the difference between the lowest excitation and the emission energies. The estimated values of luminescence parameters from three subluminescences are given in Table 3. The decrease of emission energy from I to II to III is attributed to the increase of centroid shifts, crystal-field

O2−/Cl−, the increase of centroid shifts can be matched to the increase of shared-electron populations reflecting the bond covalency as 0.95, 1.05, and 1.52 for Ca1, Ca2, and Ca3, respectively, in Table 2. This estimation is based on the assumption that the trend of covalent character between O 2p and Ca 3d is sustained in the bonds between O 2p and Eu 5d. The crystal-field splitting is proportional to the inverse square of Eu2+−O2−/Cl− average bond distances in inorganic phosphors.33 Hence, it is reasonable that the crystal-field splitting increases with the decrease of the average bond distances in the sequence of Ca1, Ca2, and Ca3 sites. This is consistent with the trend of centroid shift, because the shorter bond distances would be favorable for orbital overlap or electron sharing. Stokes shift also changes from 0.37 to 0.56 to 0.67 eV for the luminescences I, II, and III, respectively. The increase of Stokes shift is attributed to the considerable increase of phonon energy despite the decrease of the Huang−Rhys parameter. (Extraction of the involved phonon energy and the Huang−Rhys parameter is explained in the Supporting Information.) Since the Huang−Rhys parameter would decrease and the coupled phonon energy increases with the increase of a material’s stiffness in the harmonic potential oscillation model, the increase of Stokes shift is reasonably in accordance with the increase of the stiffness of hosts, and it is known that the stiffness is proportional to the covalency between Eu2+ and host anions.34 Consequently, the Stokes shift might also depend on the bond covalency in the doping sites. To sum up, the decrease of emission energy from I to II to III is basically caused by the decrease of bond lengths and the increase of the sharedelectron population in the sequence of Ca1, Ca2, and Ca3. It is noteworthy that luminescence III shows the emission peaked at 629 nm, the exceptionally red-shifted value for oxideE



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based phosphors.25 As discussed above, the long wavelength emission comes from the aggregated contribution of large centroid shift, crystal-field splitting, and Stokes shift. The Ca3 site has highly covalent and short chemical bonds enough to push the emission peak over 620 nm. This characteristic can be correlated with the mixed anion coordination of O and Cl. In spite of the longer bond distances than O, Cl ions significantly contribute to the electron sharing with the cation (61, 47, and 61% of total shared-electron population for Ca1, Ca2, and Ca3 sites) due to the smaller electronegativity (3.16) than that of O (3.44).35 Accordingly, the two-layer coordination with O (inner layer) and Cl (outer layer) would be favorable for the large coordination numbers without the sacrifice of electron sharing. This is the advantage of oxychlorides compared with pure oxides with the strict trade-off relation between coordination numbers and electron sharing. Consequently, the maximized compactness of Ca3O4Cl4 assisted by mixed-anion coordination might be the reason for the long-wavelength emission. The largely red-shifted emission over 620 nm in Sr3Al2O5Cl2:Eu2+ can be also explained with the same reason.36 The emission spectra of β-Ca3SiO4Cl2:Eu2+ as a function of Eu2+-doping concentration were obtained by the excitation at 360 nm, where all activation centers at Ca1, Ca2, and Ca3 in βCa3SiO4Cl2:Eu2 can be excited, as shown in Figure 8. As the

0.02Eu2+ smaller than the critical concentration 0.05Eu2+, where the average energy transfer rate is equal to the radiative decay rate. This difference could be explained with the inhomogeneous energy transfer rates between various activator sites. The energy transfer rates were evaluated with the angular class model derived by Vásquez, which correlates the energy transfer with the sublattice structures of doping sites.32,38 On the basis of the geometric factors (Ω) shown in Table S4 (Supporting Information), the estimated effective energy transfer rates between different-type sites (Ca1 → Ca2, Ca1 → Ca3, and Ca2 → Ca3) are about 2 or 3 times higher than those between the same-type sites (Ca1 → Ca1, Ca2 → Ca2, and Ca3 → Ca3). This explains that the change of spectral shape caused by the energy transfer between crystallographically different sites can be found at the lower doping concentration than the critical concentration, where the energy transfer occurs between all types of doping sites. Figure 9 shows the temperature dependence of the emission of the β-Ca3SiO4Cl2:0.05Eu2+ phosphor excited at 350, 440,

Figure 9. (a) Temperature dependence of photoluminescence emission of β-Ca3SiO4Cl2:0.05Eu2+ excited at 350, 440, and 490 nm. (b−d) The normalized emission spectra excited at 350, 440, and 490 nm, respectively, with varying temperatures. Inset shows the fitted activation energy (ΔE) for the thermal quenching. Figure 8. Emission spectra of β-Ca3SiO4Cl2:Eu2+ phosphors under 350 nm excitation with varying Eu2+ concentrations (0.00033, 0.00066, 0.001, 0.02, 0.05, and 0.1) at room temperature. Emission spectrum of β-Ca3SiO4Cl2:0.00033Eu2+ is deconvoluted into three Gaussian subpeaks (dashed lines). Inset shows the relative emission intensities as a function of Eu2+ substitution.

and 490 nm corresponding to the excitations of Eu2+ ions mainly at Ca1, Ca2, and Ca3 sites, respectively. Because of the active energy transfer between the different Eu2+ sites at the concentration of 0.05, the temperature stability of the phosphor would be dominated by luminescence with the lowest energy, i.e., the emission from Eu2+ at the Ca3 site. Therefore, in spite of the different excitation wavelengths, thermal stabilities and the barrier energy for thermal quenching are almost constant, as shown in Figure 9. The luminescence intensity decreased to 50% of the initial values at 180 °C and to 20% at 250 °C. The large temperature drop might be attributed to the large Stokes shift of the luminescence as 0.67 eV or the small barrier energy for thermal quenching as ∼0.37 eV of the luminescence from the Ca3 site.39

concentration of Eu2+ increases from 0.00033 to 0.00066, 0.001, 0.02, 0.05, and 0.1, the emission peaks shift from 497 to 560, 595, 608, 609, and 610 nm. The similar intensities of the deconvoluted three peaks in the emission of 0.00033Eu2+doped β-Ca3SiO4Cl2 (Figure 8) indicate that Eu2+ ions might be evenly distributed in three types of Ca sites.37 A considerable shape change of emission spectra by the slight increase of doping concentration from 0.00033 to 0.001 occurs through the energy transfer of Ca1 → Ca2, Ca1 → Ca3, and Ca2 → Ca3. The disappearance of the emission intensity around 500 nm in the samples with the concentrations of 0.02, 0.05, and 0.1 implies that the energy transfer becomes more dominant than the radiative decay at Ca1 and Ca2 sites at the concentrations. The energy transfer dominance begins at the concentration of

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CONCLUSIONS High-temperature modification of Ca3SiO4Cl2 was isolated, and its crystal structure was solved on the basis of the synchrotron powder XRD data. The structure is built up from alternating stacks of two layered sublattices, 2∞[Ca2SiO4] and 2∞[CaCl2], along the [100] direction. In the unit cell, there are three types F



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of Ca sites, Ca1O4Cl3, Ca2O5Cl2, and Ca3O3Cl4, which can be doped with an activator Eu2+ ion. From the photoluminescence spectra of Eu2+-doped Ca3SiO4Cl2 measured at 77 K, threetypes of subphotoluminescence were separated and their luminescence factors, such as centroid shifts, crystal-field splitting, and Stokes shifts, were evaluated. The values increase with the bond covalency determined by the shared-electron populations, which is strongly correlated with the bond distances between Ca and O/Cl: the shorter bond distances, the higher shared-electron populations. It is noteworthy that the third emission at 629 nm is an exceptionally red-shifted one in oxide-based phosphors, which might come from the compact mixed-ligand coordination by O and Cl with the short bond distances without the sacrifice of coordination number. The red shift of the emission from green to yellowish orange over 600 nm with increasing the Eu2+-doping concentrations was attributed to the active energy transfer between Eu2+ ions in the different crystallographic sites as Ca1 → Ca2, Ca1 → Ca3, and Ca2 → Ca3. The temperature stability was dominated by the Stokes shift of the luminescence from the Ca3 site with the lowest luminescence energy because of the strong energy transfer to the Ca3 site.

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ASSOCIATED CONTENT

S Supporting Information *

Parameters of eH calculation, photoluminescence intensities of β-Ca3SiO4Cl2:Eu2+ and (Sr,Ca)AlSiN3:Eu3+, extraction of the Huang−Rhys parameter and the coupled phonon energy, estimation of energy transfer rates based on the angular class model are shown in Table S1, Figure S1, Table S2, and Tables S3 and S4, respectively. Further details of the crystal structure of β-Ca3SiO4Cl2 are provided as a CIF file. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +82-31-280-8127 (T.-G.K.). *E-mail: [email protected]. Tel: +82-31-219-2661 (S.-J.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the contributions of Dr. DoCheon Ahn and Dr. Kihong Kim for synchrotron XRD measurement. One of the authors, S. J. Kim, acknowledges support from the Priority Research Centers Program through the National Research Foundation of Korea (No. 20090094046).

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REFERENCES

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Interplay between Crystal Structure and Photoluminescence

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