Luminescent Properties of Rare Earth Fully Activated Apatites, LiRE9

Jan 12, 2015 - Department of Engineering in Energy & Applied Chemistry, Silla University, Busan. 617-736, Republic of Korea. §. Busan Center, Korea B...
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Luminescent Properties of Rare Earth Fully Activated Apatites, LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb): Site Selective Crystal Field Effect Donghyeon Kim,† Doyoung Park,‡ Namgyeong Oh,‡ Jaegyeom Kim,⊥ Euh Duck Jeong,§ Seung-Joo Kim,⊥ Sungyun Kim,¶ and Jung-Chul Park*,†,‡ †

Graduate School of Advanced Engineering and ‡Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 617-736, Republic of Korea § Busan Center, Korea Basic Science Institute, Busan 618-230, Republic of Korea ⊥ Department of Chemistry, Division of Energy Systems Research, Ajou University, Suwon 443-749, Republic of Korea ¶ Hoseo University, 165 Sechul Li, Baebang Myun, Asan-city, Chungnam 336-795, Republic of Korea ABSTRACT: Novel LiCe9(SiO4)6O2 and LiTb9(SiO4)6O2 compounds have been successfully synthesized, and the site selectivity and occupancy of activator ions have been estimated including LiEu9(SiO4)6O2 compound. The rare earth (RE) fully occupied compounds, as well as the RE partially occupied congeners are required for the assessment of site selectivity of RE (activator) ions in apatitetype compounds. The splitting energies of the 6H and 4F Wycoff positions of LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds are calculated based on crystal field theory: ΔECe(6H) = 3849.3 cm−1, ΔECe(4F) = 4228.1 cm−1, ΔEEu(6H) = 3870.0 cm−1, ΔEEu(4F) = 4092.8 cm−1, ΔETb(6H) = 3637.6 cm−1, ΔETb(4F) = 4396.1 cm−1, indicating that the splitting energy for the 4F site is larger than that for the 6H site in all compounds; thus the absorption energy is higher for the 6H site. In apatite-type LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds, the Ce3+ ions predominantly occupy the 4F site associated with the absorption band around 300 nm at lower Ce3+ concentration, and then enter the 6H site associated the absorption band around 245 nm. For the Eu3+-doped compounds, the 4F site and 6H site are mixed within the charge transfer band (CTB) between 220 and 350 nm. Eu3+ ions initially preferentially occupy the 6H site (around 290 nm) at lower Eu3+ concentration and subsequently enter the 4F site (around 320 nm) with increasing Eu3+ concentration. For the Tb3+-doped compounds, the absorption due to the two different sites is mixed within f−d absorption band between 200 and 300 nm. At lower Tb3+ concentration, the Tb3+ ions enter favorably 6H site around 240 nm and then enter 4F site around 270 nm. These compounds may provide a platform for modeling a new phosphor and application in the solid-state lighting field.

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Moreover, because the unshielded 5d excited state of Ce3+ ion is strongly influenced by the crystal field, the introduction of Ce3+-activator ion into the host lattices makes it possible to monitor the influence of the host lattices on the luminescent properties. Shen et al. reported that there are two distinct crystallographic sites AI (4F site in this work) and AII (6H site in this work) for Ce3+ in the Sr2Y8−xCex(SiO4)6O2 (x = 0.01, 0.05, and 0.1) compounds, and proposed energy transfer from Ce3+ in the AI site to Ce3+ in the AII site based on the photoluminescence spectra.9 Among the rare earth ions, Eu3+ has been widely used as a red-emitting activator and has been utilized as a probe for elucidating local crystal structures.9−11 Partially Eu-doped Sr2Y8−xEux(SiO4)6O2 compounds were used to investigate the distributions of Eu3+ between 4F and 6H sites in the range of 0.05 ≤ x ≤ 5. The results showed that Eu3+ ions only occupy the 4F sites when Eu doping concentration is low (x ≤ 0.5 in Sr2Y8−xEux(SiO4)6O2), while at higher concen-

INTRODUCTION

Compounds with the apatite-type structure have been intensively studied because of their potential applications as luminescent materials.1−5 These compounds are generally expressed as M10(XO4)6Z2, where M is cation (Na+, Ca2+, La3+, etc.), XO4 is anionic group (SiO4, GeO4, etc.), and Z is anion (F−, Cl−, O2−, S2−, etc.).6 Recently, rare earth (RE)activated apatite-type silicate compounds have been drawn great attentions because of their excellent luminescent characteristics.2,4,7 Among the RE-activated apatite-type compounds, it is well-known that LiLa9(SiO4)6O2 (LLSO) compound is an interesting host lattice with two different cationic sites: one is a distorted pentagonal bipyramid by coordinating between La3+ and seven oxide ions, and the other a distorted 3-fold capped trigonal prism by coordinating between partially occupied La3+ (the site occupancy of 75%)− Li+ (the site occupancy of 25%) and nine oxide ions.8 Among the rare earth ions, Ce3+ has the simplest electron configuration, [Xe]4f1, and the allowed 4f-5d transitions exhibit strong absorption and efficient fluorescence in the ultraviolet region. © XXXX American Chemical Society

Received: August 30, 2014

A

DOI: 10.1021/ic502113a Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

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tration (0.5 < x ≤ 5), Eu3+ begin to enter the 6H sites as well.12 The luminescent properties of Tb3+ ion in different host lattices have been intensively studied. In the case of the Tb3+ activator, the transition between 4f8 and 4f75d1 levels is associated with intense absorption and emission depending on the nature of the crystal structure of host lattices.13 Chiu et al. reported that the photoluminescence spectra of the Ca(Tb1−xLax)4(SiO4)3O (0.0 ≤ x ≤ 4) compound exhibit the most intense excitation peak at about 378 nm corresponding to the f → f transition and a bright green emission peak at 541 nm; however, the site occupancy of Tb3+ ions was not discussed.14 From the viewpoint of solid-state chemistry, the concentration of activator ion is generally kept very low, that is, within the impurity level. In special cases, for example LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds, the RE ions can play the role of activators as well as elements of the host material. Therefore, at a certain concentration of the activator ion, the absorption peaks (or bands) associated with the electronic transitions of the activator ion may be concealed by strongly intensified new peaks, which leads to serious misunderstanding of the site selectivity and occupancy of activator ions, such as in apatite-type compounds with two different sites. Thus, it is necessary to evaluate the site selectivity of RE (activator) ions in the RE fully occupied compounds, as well as in the RE partially occupied congeners. In this Article, we report on the synthesis and characterization of LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds. In a series of LiRE9 (SiO4 ) 6O 2 compounds, novel LiCe9(SiO4)6O2 and LiTb9(SiO4)6O2 compounds have been successfully synthesized to estimate the site selectivity and occupancy of activator ions (Ce, Eu, and Tb). In particular, on the basis of crystal field theory, the site-selective crystal field effects are calculated in the first order perturbation and discussed.

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Article

RESULTS AND DISCUSSION Structural Characterization. The observed, calculated and difference patterns from the Rietveld refinements of LiCe9(SiO4)6O2 and LiTb9(SiO4)6O2 are shown in Figure 1.

Figure 1. Rietveld refinement profiles of (a) LiCe9(SiO4)6O2 and (b) LiTb9(SiO4)6O2. Observed (open circles), calculated (line), and difference (bottom line) patterns are shown together with Bragg positions.

EXPERIMENTAL SECTION

LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) compounds have been prepared from appropriate mixtures of Li2CO3, La2O3, CeO2, Eu2O3, Tb4O7, and SiO2 under air (or 4% H2−Ar) atmosphere at 1300 °C for 6 h. The crystal structures of LiRE9(SiO4)6O2 (RE = La, Ce, Eu and Tb) series were investigated by structural refinement using X-ray powder diffraction data. Powder X-ray diffraction measurements of LiRE9(SiO4)6O2 were carried out using a Rigaku DMAX-2200PC Xray diffractometer equipped with a graphite monochromator (λ = 1.5418 Å). Step scan mode was employed in the 2θ range 10−110° with a step size of 0.02° and counting time of 5 s for each step. Structure refinements were carried out by the Rietveld method using the Fullprof program15 with pseudo-Voigt peak shapes and refined backgrounds. The photoluminescence (PL) spectra were measured at room temperature using a fluorescent spectrophotometer with a 150 W xenon lamp under an operating voltage of 350 V with the slit width of 5 nm (Fluorometer FS-2, Scinco). The reflectance spectra were recorded using UV−visible spectrophotometer (UV-2600, Shimadzu) with BaSO4 as a reference. Fourier-transform infrared spectroscopy (FT-IR) was performed using an FT-IR spectrophotometer (IRTracer100, Shimadzu) with the resolution range of ±0.5 cm−1 by employing KBr medium (KBr 200 mg + sample 1 mg). Raman spectra were recorded at room temperature in the spectral range 200−1200 cm−1 using the BRUKER RFS 100/S Raman spectroscopy system with Nd:YAG laser (excitation at 1064 nm). The resolution was 1 cm−1 for the excitation of 1064 nm. The oxidation states of the elements were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) at Busan Center of Korea Basic Science Institute (KBSI). The obtained binding energies (BEs) were calibrated with that of adventitious carbon (C 1s) core level peak at 284.6 eV as a reference.

The powder diffraction patterns of LiCe9(SiO4)6O2 and LiTb9(SiO4)6O2 show the hexagonal symmetry with the lattice parameters: a = 9.6183(3) Å, c = 7.0816(3) Å for LiCe9(SiO4)6O2, a = 9.3737(13) Å, c = 6.7992(11) Å for LiTb9(SiO4)6O2. The lattice parameters and the systematic absences revealed that the structure of LiRE9(SiO4)6O2 (RE = Ce and Tb) is isotypic with that of the previously reported LiRE9(SiO4)6O2 (RE = La, Pr, Nd, Sm, Eu, Gd, and Er).16 Although the isostructures, LiRE9(SiO4)6O2, have been investigated, the Ce and Tb analogues of this family have not been reported so far. For Rietveld refinement, the structural parameters of LiLa9(SiO4)6O2 were adopted as an initial model.16 In this refinement procedure, it was assumed that Li atoms occupy only 4F sites and the RE atoms (RE = Ce or Tb) occupy 4F, as well as 6H, sites. The refinement results and the comparison with LiRE9(SiO4)6O2 (RE = La and Eu) are summarized in Table 1. The final structural parameters (refined atomic positions and isotropic temperature factors for all atoms) are listed in Table 2. Selected interatomic distances from the refined crystal structure are presented in Table 3. The structure of LiRE9(SiO4)6O2 can be described as a hexagonal oxyapatite structure with a general composition, Li[REI]3[REII]6(SiO4)6O2.17−19 In this structure, the tetrahedral [SiO4]4− ions are approximately hexagonal-close packed, giving B

DOI: 10.1021/ic502113a Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

rise to interstitial sites that form channels through the structure parallel to the c-axis. The one-dimensional channels are occupied by the Li, RE, and O atoms (Figure 2a). The REI ions in the 4F Wickoff sites are coordinated by nine O atoms, forming a tricapped trigonal-prismatic geometry (Figure 2b). The REII ions in the 6H Wickoff sites are coordinated by seven oxygen atoms (one O1, one O2, four O3, and one O4). The REII−O bond lengths range from 2.2 to 2.8 Å, leading to an irregular polyhedron (Figure 2c). XRD patterns of LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) synthesized at 1300 °C for 6 h in air atmosphere (or 4% H2−Ar for Ce and Tb) show a similarity in their Bragg positions as indicated in Figure 3, which reveals that all compounds belong to the family of isomorphous ones. As depicted in Figure 4, the unit cell parameter a and c are gradually decreased on moving La to Tb, which is consistent with the variation in the ionic radius of RE3+

Table 1. Refinement Results of LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) Rwp (%) RBragg (%) Rf (%) χ2 space group lattice parameter (Å) a c cell volume (Å3)

La

Ce

Eu

Tb

10.8 5.69 6.17 2.62 P63/m (No. 176)

14.7 6.15 5.36 4.47 P63/m (No. 176)

12.4 7.93 6.49 2.42 P63/m (No. 176)

10.1 7.23 5.92 2.57 P63/m (No. 176)

9.6835(13) 7.1523(11) 580.82(14)

9.6183(3) 7.0816(3) 567.36(3)

9.4321(14) 6.8757(12) 529.74(14)

9.3737(13) 6.7992(11) 517.38(13)

Table 2. Refinement Atomic Positions for LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) LiLa9(SiO4)6O2

LiCe9(SiO4)6O2

LiEu9(SiO4)6O2

LiTb9(SiO4)6O2

1/3 2/3 −0.0040(5) (0.75/0.25) 1.10(5)

1/3 2/3 −0.0023(6) (0.75/0.25) 1.07(5)

0.0110(2) 0.2433(2) 1/4 1 0.64(3)

0.0061(3) 0.2392(2) 1/4 1 0.70(3)

0.4017(7) 0.3403(8) 1/4 1 0.4(14)

0.4051(7) 0.3734(8) 1/4 1 0.7(14)

0.316(2) 0.495(2) 1/4 1 0.7(2)

0.329(2) 0.491(2) 1/4 1 1.0(2)

0.610(2) 0.480(2) 1/4 1 0.7(2)

0.600(2) 0.475(2) 1/4 1 1.0(2)

0.3401(9) 0.260(10) 0.071(11) 1 0.7(2)

0.336(10) 0.245(10) 0.069(11) 1 1.0(2)

0 0 1/4

0 0 1/4 1 1.0(2)

I

x y z site occupancies (RE/Li) Beq

1/3 2/3 −0.0034(5) (0.75/0.25) 1.29(4)

x y z site occupancies Beq

0.0147(2) 0.2462(13) 1/4 1 0.91(2)

x y z site occupancies Beq

0.4061(7) 0.3735(7) 1/4 1 1.2(13)

x y z site occupancies Beq

0.334(15) 0.492(14) 1/4 1 1.2(14)

x y z site occupancies Beq

0.601(15) 0.473(14) 1/4 1 1.2(14)

x y z site occupancies Beq

0.3422(8) 0.2532(9) 0.0708(9) 1 1.2(14)

x y z site occupancies Beq

0 0 1/4 1.2(14)

RE /Li, 4F 1/3 2/3 −0.0015(8) (0.75/0.25) 1.47(7) REII 6H 0.0129(3) 0.2432(2) 1/4 1 1.03(3) Si, 6H 0.4036(9) 0.3685(9) 1/4 1 0.9(2) O1, 6H 0.328(2) 0.494(2) 1/4 1 1.3(2) O2, 6H 0.592(2) 0.469(2) 1/4 1 1.3(2) O3, 12i 0.347(12) 0.256(13) 0.065(14) 1 1.3(2) O4, 2e 0 0 1/4 1 1.3(2) C

0.7(2)

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Inorganic Chemistry Table 3. Selected Bond Lengths and Bond Angles of LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) LiLa9(SiO4)6O2 REI−O1 (×3) REI−O2 (×3) REI−O3 (×3)

2.479(11) 2.489(10) 2.875(8)

REII−O1 REII−O2 REII−O3 (×2) REII−O3 (×2) REII−O4

2.804(11) 2.482(17) 2.445(6) 2.596(6) 2.3160(16)

Si−O1 Si−O2 Si−O3 (×2)

1.618(19) 1.633(14) 1.632(8)

O3−Si−O3 O1−Si−O3 O2−Si−O3 O1−Si−O2

103.5(6) 111.3(12) 109.5(8) 111.4(16)

LiCe9(SiO4)6O2 bond length (Å) of REI (4F)−O 2.418(15) 2.500(14) 2.817(12) bond length (Å) of REII (6H)−O 2.774(14) 2.51(2) 2.406(10) 2.598(10) 2.280(2) bond length (Å) of Si (6H)−O 1.69(3) 1.570(18) 1.611(11) bond angles (deg) of O−Si−O 108.6(10) 110.0(17) 108.3(12) 110(2)

LiEu9(SiO4)6O2

LiTb9(SiO4)6O2

2.392(12) 2.412(12) 2.837(9)

2.362(11) 2.412(11) 2.813(9)

2.630(12) 2.347(2) 2.355(8) 2.543(7) 2.2451(17)

2.755(11) 2.374(19) 2.333(7) 2.391(7) 2.215(2)

1.65(2) 1.701(15) 1.525(9)

1.587(2) 1.583(14) 1.613(9)

107.2(8) 107.8(13) 110.3(9) 113.3(17)

99.5(7) 111.6(13) 111.0(9) 111.6(17)

Figure 3. Powder XRD patterns of RE-fully occupied LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) compounds.

Figure 2. Projection of the crystal structure of crystal structure of LiRE9(SiO4)6O2 along to the c-axis direction (a). Representations of REI-O polyhedron (b) and REII-O polyhedron (c). Green sphere: REI atom. Yellow sphere: REII atom. Red sphere: O atoms.

ions: La3+ (1.216 Å), Ce3+ (1.196 Å), Eu3+ (1.12 Å), Tb3+ (1.095 Å) for CN = 9.20 Theoretical Calculation of Crystal Field Effect.21 The spectral features of the absorption spectra of LiRE9(SiO4)6O2 (RE = Ce, Eu, Tb) are theoretically estimated using crystal field theory (CFT). The RE3+ ions are believed to play dominant roles in the LiRE9(SiO4)6O2 absorption spectra. These ions occupy two different sites, one surrounded by 7 oxygens (6H site) and the other surrounded by 9 oxygens (4F site).9,16 For a lower level to 5d level transition, the different crystal field would generate different 5d level splittings. The approximate absorption energy change will be Eabs → Eabs − 0.5ΔEspl

Figure 4. Variation of unit-cell parameters of LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) compounds.

and the differences in the 5d splittings can be estimated by CFT calculation. In the present calculation, the RE3+ ion is approximated as 4e+ central charge and outermost electron. Thus, the Hamiltonian for the CFT calculation becomes

(1) D

DOI: 10.1021/ic502113a Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry H = H0 + HC

RTb52(r ) = 1.6261 × 1034 exp( −1.26576 × 1010r )r 2

(2)

(21 − 1.7721 × 1011r + 3.2043 × 1020r 2)

where H0 is the Hamiltonian for 4e+ central charge plus one outermost electron and HC is the crystal field potential by nearby oxygens. In LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds, the atomic positions are referred to CIF data obtained from Rietveld analyses. The unperturbed Hamiltonian is set as H0, and HC is treated as perturbation. The explicit forms of H0 and HC are given as H0 = −

4e 2 ℏ2 2 , HC = ∇ − 2m r

∑ i

Y2m(θ , φ) =

(5)

where R52(r) is written in MKS units and is the associated Legendre polynomial of (2,m). The energy splitting due to the crystal field is calculated in first order perturbation as the difference between the maximum and minimum of ⟨5,2,m|HC| 5,2,m⟩. In the case of LiRE9(SiO4)6O2 (RE = Ce, Eu, and Tb), the energy splitting calculation for the 6H and 4F sites yield

(3)

where m is the mass of electron and Zi and R⃗ i indicate the charge number and position vector of ith ion, respectively. For 6H and 4F Wycoff sites, H0 are the same but HC are different depending on the configuration of nearby oxygen ion. Notably, one nearby oxygen in a 6H site is a free oxygen and has an effective charge of −1.79.12 The other effective charge numbers Zc and Zi s are determined through electronegativity considerations. The electronegativity is a measure of the tendency of an atom in a molecule to attract electrons. With the assumption that an electron wave function in a bonding contains a mixture of ionic and covalent character, Ψmolecule = aΨcovalent + bΨionic wave function form is written. The percentage of ionicity is estimated by the following empirical formula22 for two atoms with Pauling electronegativity χA and χB, % of ionic character = 16|χA − χB | + 3.5 |χA − χB |2

5(2 − m)! exp(imφ)P2m(cos θ ) π (2 + m)!

Pm2

Zie 2 |R⃗ i − r |⃗

1 2

ΔE X = ⟨5, 2, m|HC|5, 2, m⟩max − ⟨5, 2, m|HC|5, 2, m⟩min (6) −1

ΔECe(6H) = 3849.3 cm , ΔECe(4F) = 4228.1 cm

−1

ΔE Ee(6H) = 3870.0 cm−1, ΔE Eu(4F) = 4092.8 cm−1 ΔE Tb(6H) = 3637.6 cm−1, ΔE Tb(4F) = 4396.1 cm−1

The estimated absorption energy difference between 6H site and 4F site would be −0.5(ΔERE(6H) − 0.5ΔERE(4F)); thus −0.5(ΔECe(6H) − ΔECe(4F)) = 189.4 cm−1 −0.5(ΔE Eu(6H) − ΔE Eu(4F)) = 111.4 cm−1 −0.5(ΔE Tb(6H) − ΔE Tb(4F)) = 379.2 cm−1

(4)

(7)

These results show that the 4F site has more splitting in all cases; thus, the absorption energy of the 6H site is higher. Infrared and Raman Spectroscopy. Figure 5 presents the IR spectra of the LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb)

For covalent bond, the electron is shared between atoms and for ionic bond, the electron is transferred to one atom. Effective charges are calculated by 0.5 (% of covalent character) + (% of ionic character). The Pauling electronegativities are 0.98, 1.2, 1.12, 1.1, 3.44, 1.9 for Li, Eu, Ce, Tb, O, Si, respectively. For O atoms, there are two cases. In one case, the O atom is surrounded by two REI atoms, one REII atom and one Si atom, and in the other case, it is surrounded by one REI atom, two REII atoms, and one Si atom. For LiCe9(SiO4)6O2 case, the Ce effective charge is 2.34, at 6H site the O effective charges are 1.52, 1.52, 1.51, 1.79 for O1, O2, O3, O4 (free oxygen), at 4F site the O effective charges are 1.52, 1.52, 1.51 for O1, O2, O3. For LiTb9(SiO4)6O2 case, the Tb effective charge is 2.35, at 6H site the O effective charges are 1.526, 1.526, 1.516, 1.79 for O1, O2, O3, O4 (free oxygen), at 4F site the O effective charges are 1.526, 1.526, 1.516 for O1, O2, O3. For LiEu9(SiO4)6O2 case, the Eu effective charge is 2.30, at 6H site the O effective charges are 1.52, 1.52, 1.50, 1.79 for O1, O2, O3, O4 (free oxygen), at 4F site the O effective charges are 1.52, 1.52, 1.50 for O1, O2, O3. The eigenfunction of H0 is written as ⟨nlm|rθϕ⟩ = Rnl(r) Ylm(θ,ϕ) where r, θ, ϕ are radial distance and two angles in spherical coordinate; n, l, and m are principal, angular, magnetic quantum numbers. Rnl(r) and Ylm(θ, ϕ) are referred to as radial and angular wave functions, and their explicit forms for 5d (n = 5, l = 2) orbital are

Figure 5. Infrared spectra of LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) compounds.

R Ce52(r ) = 1.6096 × 1034 exp( −1.2621 × 1010r )r 2

compounds and that of SiO2 (fired at 1300 °C with α-quartz) and α-quartz. The maximum absorption band at 1090 cm−1 in the spectra of α-quartz is associated with Si−O asymmetrical stretching vibrations, those at 800 and 780 cm−1 with Si−O symmetrical stretching vibrations, that at 696 cm−1 with Si−O symmetrical bending vibrations, and those at 510 and 460 cm−1

(21 − 1.7669 × 1011r + 3.1858 × 1020r 2) REu52(r ) = 1.546 × 1034 exp( −1.2476 × 1010r )r 2 (21 − 1.7466 × 1011r + 3.1130 × 1020r 2) E

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Inorganic Chemistry with Si−O asymmetrical bending vibrations.23 After firing αquartz at 1300 °C, the IR pattern becomes largely consistent with that of α-cristobalite with a notable new absorption at 620 cm−1 corresponding to Si−O asymmetrical bending vibrations.24 Rietveld analysis of the compound obtained after firing αquartz at 1300 °C reveals a mixed phase comprising α-quartz (10% in weight) and α-cristobalite (90% in weight). In the LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) structure, the SiO44− complex anions are not only linked, but are also surrounded by RE3+ ions, and the chemical bond distance of Si−O (mean value = 1.613 Å) is shorter than that of RE−O (mean value = 2.524 Å). Thus, the [SiO4] internal modes may be predominantly manifested in the IR spectral range of 1200− 400 cm−1. RE3+ ions occupy 4F and 6H sites in which they are surrounded by a number of O2− ions from SiO44− ions. As a consequence of the chemical bonding, (RE3+···[O−Si−O]4··· RE3+), the Si−O bonds are weakened, which results in a shift of the [SiO4] internal modes of LiRE9(SiO4)6O2 to lower wavenumber compared with those of SiO2 (fired at 1300 °C) and α-quartz. In the series of LiRE9(SiO4)6O2 compounds, it is very interesting that the wavenumbers of the absorption modes generally shift to higher frequencies on moving from La to Tb, which may be attributed to the contraction of the RE−O and Si−O bond lengths corresponding to the shrinkage of the unitcell volumes. The IR absorption bands and average bond lengths for the LiRE9(SiO4)6O2 compounds are indicated in Table 4. Figure 6 shows the Raman spectra for LiRE9(SiO4)6O2

(RE = La, Ce, Eu, and Tb) compounds at room temperature. The spectra are well consistent with those of the related silicate apatite samples in literature.25−27 The bands below the 350 cm−1 region are assigned to external lattice modes and the bands above 350 cm−1 range are all assigned to internal modes of SiO4 tetrahedral units. The significantly intense bands detected at 857−878 cm−1 are associated with symmetric (νs) stretching mode, those at 920−950 cm−1 to asymmetric (νas) stretching modes, respectively. The bands appearing in ranges of 391−426 and 530−535 cm−1 are assigned to the symmetric (δs) and asymmetric (δas) bending modes of SiO4, respectively. On moving La to Tb-containing compound, the band maxima shift toward higher wavenumbers. This phenomenon agrees well with the IR result. Photoluminescence Spectra. Figure 7 shows the excitation and emission spectra of LiLa9−xCex(SiO4)6O2 (x =

Table 4. IR Absorption Bands and Average Bond Lengths for LiRE9(SiO4)6O2 Compounds La average RE−O bond length (Å) average Si−O bond length (Å) Si−O antisymmetric stretching (cm−1) Si−O antisymmetric bending (cm−1)

Ce

Eu

Tb

2.576 2.549 2.497 2.472 1.629 1.621 1.601 1.599 912, 975 917, 979 925, 989 937, 993

Figure 7. Excitation and emission spectra of LiLa9−xCex(SiO4)6O2 (x = 3, 6, 9) compounds.

489, 541 494, 548 501, 557 505, 567

3, 6, and 9) compounds synthesized as a function of the Ce3+ concentration. Due to the low intensity of the blue emissions, the excitation and emission spectra for three compounds were monitored by setting the slit width of the fluorescence spectrophotometer to 10 nm. As shown in the absorption band of LiCe9(SiO4)6O2 compound with Ce3+ fully occupied two sites (4F and 6H), two absorption bands (at 245 and 300 nm) are clearly seen, which may be ascribed to the 4f−5d transition of Ce3+.28 As the Ce3+ content is decreased from x = 9 to 3, the intensity of the band centered at 300 nm is increased. The emission spectra of LiLa9−xCex(SiO4)6O2 (x = 3, 6, and 9) under excitation at 300 nm reveal that three compounds are the blue emitting phosphor materials with emission peak maxima around 400 nm. Excitation at 245 nm exhibit also the same PL behavior, as indicated in the inset of Figure 7. The excitation and emission spectra of LiLa9−xCex(SiO4)6O2 doped with very low concentrations of Ce3+ (0.01 ≤ x ≤ 0.15) are shown in Figure 8. The absorption bands at 245 nm in the spectra of the x = 3, 6, and 9 compounds (Figure 7) are completely absent in the excitation spectra of the 0.01 ≤ x ≤ 0.15 compounds monitored at 385 nm, and broad bands with peak maxima at 295 nm appeared in the spectra of the latter, which possibly indicates that in the very low dopant concentration range, the Ce3+ ions predominantly occupy one site associated with the absorption

Figure 6. Raman spectra of LiRE9(SiO4)6O2 (RE = La, Ce, Eu, and Tb) compounds. The band positions for the SiO4 symmetric bending (δs) modes and symmetric stretching (νs) modes are represented.

F

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intense. At higher Ce3+ concentrations (x = 3, 6, and 9), very broad and intense absorption bands are observed between 230 and 450 nm. For x = 3 and 6 compounds, small sub-bands are observed in the visible range. In particular, for the Ce3+ fully occupied compound (x = 9) in RE sites, there is an intense absorption in the visible region as the body color of LiCe9(SiO4)6O2 compound is pale-goldenrod. The presence of Ce4+ ions in LiLa9−xCex(SiO4)3O2 (x = 3, 6, 9) compounds can be confirmed by XPS analysis. Figure 10 presents Ce 3d

Figure 8. Excitation and emission spectra of LiLa9−xCex(SiO4)6O2 (x = 0.01−0.12) compounds.

band around 300 nm; thus, the other site corresponds with the absorption around 245 nm. The emission spectra under excitation at 295 nm exhibit the blue emission broad bands centered at 385 nm, which is ascribed to the allowed 4f05d1− 4f15d0 transition of Ce3+.28 The maximum PL intensity is achieved at x = 0.12, followed by a decrease with increasing Ce3+ concentration due to concentration quenching. The reflectance spectra of the LiLa9‑xCex(SiO4)6O2 compounds are shown in Figure 9. The LiLa9(SiO4)6O2 host absorbs energy in

Figure 10. Ce 3d XPS spectra of CeO 2 , CeCl 3 , and LiLa9−xCex(SiO4)6O2 (x = 3, 6, 9) compounds. All the XPS spectra were fitted after a Shirley background correction.

XPS spectra of LiLa9−xCex(SiO4)3O2 (x = 3, 6, 9) and standard compounds (Ce(IV)O2 and Ce(III)Cl3). All the XPS spectra were fitted after a Shirley background correction and their binding energies are determined as shown in Table 5. The Ce Table 5. Ce 3d XPS Binding Energy and Ce4+ Content of LiLa9−xCex(SiO4)3O2 (x = 3, 6, 9) Compounds compound CeO2

Figure 9. Diffuse reflectance spectra of LiLa9−xCex(SiO4)6O2 (x = 0.01−9) compounds. CeCl3

the λ < 220 nm region, which implies that there is no host absorption corresponding to valence-to-conduction band transition in the n-UV region. Compared with the absence of the host absorption band, the broad absorption bands between 230 and 450 nm can be assigned to the transition from 4f15d0 to 4f05d1 of the Ce3+ ions. The profile of the absorption spectra is changed markedly as the concentration of Ce3+ in the LiLa9−xCex(SiO4)6O2 compounds is increased. For Ce3+ concentrations between 0.02 and 0.12, two broad absorption bands are apparent: one centered at 245 nm and the other at 325 nm. Notably, as the Ce3+ concentration increased within the range of 0.02 to 0.12, the intensity of the two bands is increased, and in particular, the band at 325 nm become more

Ce = 3

Ce = 6

Ce = 9

G

spin− orbit Ce 3d5/2 Ce 3d3/2 Ce 3d5/2 Ce 3d3/2 Ce 3d5/2 Ce 3d3/2 Ce 3d5/2 Ce 3d3/2 Ce 3d5/2 Ce 3d3/2

binding energy (eV) 882.7, 889.0, 898.5

Ce(III,IV) 100% Ce(IV)

901.0, 907.8, 917.0 882.9, 886.9

100% Ce(III)

901.1, 905.4 881.9(III), 883.4(IV), 885.9(III), 899.1(III) 900.5(III), 901.4(IV), 904.5(III) 908.4(IV), 917.1(IV) 882.2(III), 883.4(IV), 886.0(III), 898.8(IV) 900.6(III), 901.6(IV), 904.5(III), 908.4(IV), 917.1(IV) 882.3(III), 883.5(IV), 886.0(III), 889.0(IV), 898.9(IV) 900.9(III), 901.8(IV), 904.7(III), 908.4(IV), 917.1(IV)

5.8% Ce(IV)

8.1% Ce(IV)

15.5% Ce(IV)

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Inorganic Chemistry 3d XPS spectra of CeO2 is representative of the Ce4+ oxidation state,29−31 which reveals a pure CeO2. For the Ce 3d XPS spectra of the standard compound CeCl3, there is no peak around 917 eV which is the fingerprint of Ce4+ species,31,32 indicating that the oxidation state of Ce ion in CeCl3 compound is the Ce3+ of nearly 100%. The Ce 3d binding energies of CeCl3 are well coincident with those as previously reported.32,33 On the basis of the Ce 3d binding energies of the standard compounds (CeO2 and CeCl3), the integral area of the Ce4+−Ce 3d with respect to the total Ce 3d area could be transformed into the percentage of Ce 4 + in LiLa9−xCex(SiO4)3O2 (x = 3, 6, 9) compounds.34 As shown in Figure 10 (the Ce = 9 spectra in bottom), the peaks (2, 4, 5,7,9, and 10) correspond to the Ce4+−Ce 3d states. The percentages of LiLa9−xCex(SiO4)3O2 (x = 3, 6, 9) compounds are calculated at 5.8% (Ce = 3), 8.1% (Ce = 6), 15.5% (Ce = 9). It is remarkable that the percentage of Ce4+ is gradually increased with higher Ce content in LiLa9−xCex(SiO4)3O2 (x = 3, 6, 9) compounds. It is presumed that the partial oxidation of Ce3+ to Ce4+ is ascribed to the cation defects or the surface oxidation. From XPS measurement, the valuable information was obtained; it is very difficult to form the perfect Ce3+ oxidation state and the Ce3+ species are predominantly occupied in LiLa9−xCex(SiO4)3O2 compounds. It should be noted that there are two absorption bands in Ce 3+ -fully occupied LiCe9(SiO4)6O2 compound as shown in Figure 7: the high energy absorption band (HEB) at 245 nm and the low energy absorption band (LEB) at 300 nm. As mentioned above, the apatite structure (P63/m space group) has two different crystallographic sites for cation: a nine-coordinated 4F site (C3 point symmetry) and a seven-coordinated 6H site (Cs point symmetry). It is well-known that the crystal field splitting in a tetrahedral field (CN = 4) is intrinsically smaller than that in an octahedral field (CN = 6) because there are only two-thirds as many ligands and they have a less crystal field effect on the d orbitals compared to that in an octahedral field. The pointcharge model predicts that for the same metal ion and ligands, Δt = (4/9) Δo,35 where Δ is the extent of the splitting of d orbitals, and the subscripts t and o denote tetrahedral and octahedral, respectively. By considering the extent of the crystal field splitting between a tetrahedral field (CN = 4) and an octahedral field (CN = 6), it is evident that the splitting of 4F site (CN = 9) is larger than that of 6H site (CN = 7) in LiLa9−xREx(SiO4)6O2 (RE = Ce, Eu, and Tb) compounds, which is consistent with the splitting energies calculated based on CFT. Thus, the site selectivity of Ce3+ ions in the LiLa9−xCex(SiO4)6O2 compounds could be determined by comparing the calculated splitting energies and PL spectra. The HEB and LEB bands at 245 and 300 nm in the absorption spectrum of the LiCe9(SiO4)6O2 compound (Figure 7) definitively correspond to the 6H and 4F site, respectively. Thus, initially, at lower Ce3+ concentrations, the Ce3+ ions preferentially enter the 4F sites and eventually fill the 6H sites with increasing Ce3+ concentration, which is confirmed by the calculated splitting energies of the two different sites: 4F site (4228.1 cm−1) and 6H site (3849.3 cm−1) corresponding to the absorption at longer wavelength (300 nm) and at shorter wavelength (245 nm), respectively. The excitation and emission spectra of LiLa9−xEux(SiO4)6O2 (x = 1, 3, 6, and 9) compounds are shown in Figure 11. The broad bands in the range of 200− 350 nm are attributed to the Eu3+−O2− charge transfer band (CTB). The sharp peaks from 350 to 500 nm correspond to the intra4f transitions of Eu3+. At higher Eu3+ concentration, the

Figure 11. Excitation and emission spectra of LiLa9−xEux(SiO4)6O2 (x = 1, 3, 6, 9) compounds.

absorption bands centered at 293 nm are less intense than the peaks at 395 nm (λem = 613 nm): I395/I293 = 2.5 for x = 1, 2.9 for x = 3, 5.4 for x = 6, and 5.7 for x = 9. The increase in the relative intensity ratio (I395/I293) as the Eu3+ concentration increases from x = 1 to 9 is an indicator of the site selectivity of the Eu3+ ions for the 6H versus 4F sites. The PL properties of the Eu3+-fully occupied LiEu9(SiO4)6O2 compound are compared with those of the LiLa9−xEux(SiO4)6O2 (0.05 ≤ x ≤ 0.30) compounds, as shown in Figure 12. At lower Eu3+

Figure 12. Excitation and emission spectra of LiLa9−xEux(SiO4)6O2 (x = 0.05, 0.18, 0.30) compounds.

concentrations, the absorption bands centered at 293 nm were more intense than those at 395 nm (λem = 613 nm): I293/I395 = 3.0 for x = 0.05, 1.9 for x = 0.18, and 1.5 for x = 0.30. The variation of the relative intensity ratio (I293/I395) reveals that at lower Eu3+ concentration, the Eu3+ ions preferentially occupy a certain site associated with the absorption band of either the 6H or 4F site in the LiLa9−xEux(SiO4)6O2 compound. The respective emission spectra (Figure 12) monitored under UV irradiation at 293 or 395 nm show no appreciable differences, except in the emission intensity. Figure 13 shows the relative intensities of emission spectra monitored at peak maxima of CTB (293 nm) and intra4f transitions (395 nm) as a function H

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Figure 13. Relative emission intensities of LiLa9−xEux(SiO4)6O2 compounds as a function of Eu content.

Figure 14. Diffuse reflectance spectra of LiLa9−xEux(SiO4)6O2 (x = 0.05−9) compounds.

of the Eu3+ concentration in LiLa9−xEux(SiO4)6O2 compound. It is evident that the emission monitored at CTBs is higher than that at intra4f transitions at Eu3+ concentrations below x = 0.30. The emission intensities of intra4f transitions are rapidly increased, and the highest intensity is observed at x = 2. Moreover, the emission intensities of intra4f transitions are decreased at x > 2 because of the concentration quenching effect. It should be noted that the Eu3+ ions in the 4F and 6H sites both contribute to the overall absorption intensities of the intra4f transitions whether Eu3+ ions enter 4F or 6H site. Thus, spectral evidence of the site selectivity of the Eu3+ ions might be observed in the CTB region rather than from the intra4f transition because the subtle difference of the crystal field splitting between 4F and 6H sites is reflected in the CTB. It is well-known that the CT energy is decisively determined by the position of the valence band associated with the 5d energy level of RE3+ and the valence electron energy level of the ligand because 4f orbitals are effectively shielded by the outer filled 5s2 and 5p6 orbitals. The calculated splitting energies of the two different Eu3+ sites are estimated to be 4092.8 cm−1 for the 4F site and 3870.0 cm−1 for the 6H site. Thus, it is presumed that there is mixing of the LEB (4F site) and HEB (6H site) within the CTB between 220 and 350 nm. Careful examination of the excitation spectra (Figure 11) indicates that there are at least two CTB centers (one around 290 nm (HEB) and the other in the shoulder region around 320 nm (LEB)). Interestingly, as shown in Figure 11, at higher Eu3+ concentration from x = 1 to 9, the relative intensity ratio of the two CTB centers (IHEB/ ILEB) is decreased: IHEB/ILEB = 1.7 for x = 1, 1.5 for x = 3, 1.1 for x = 6, 1.0 for x = 9. However, the intensities of the CTBs are less intense than those of the intra4f transitions. At lower Eu3+ concentration from x = 0.30 to x = 0.05, the relative intensity ratio of the two CTB centers (IHEB/ILEB) is increased, as shown in Figure 12: IHEB/ILEB = 2.2 for x = 0.30, 2.5 for x = 0.18, 3.0 for x = 0.05. The intensities of the CTBs are more intense than those of the intra4f transitions, which is opposite to that at higher Eu3+ concentration from x = 1 to 9. This spectral evolution in the absorption region provides valuable information on the site selectivity of Eu3+ in apatite-type phosphor materials. The reflectance spectra of LiLa9−xEux(SiO4)6O2 compounds are shown in Figure 14. The LiLa9(SiO4)6O2 host absorbs energy in the λ < 220 nm region, which implies that there is no host absorption corresponding to valence-to-conduction band transition in the n-UV region. By

considering the absence of the host absorption band, the broad absorption bands between 230 and 350 nm can be assigned to the Eu3+−O2− CTB, and the sharp lines between 360 and 600 nm correspond to the intra4f transitions of Eu3+: 7F0 → 5D4 (360 nm), 7F0 → 5G3 (381 nm), 7F0 → 5L6 (394 nm), 7F0 → 5 D3 (413 nm), 7F0 → 5D2 (464 nm), 7F0 → 5D1 (532 nm), which coincides well with the excitation spectra (Figure 11 and 12). On the basis of the bond length data of LiEu9(SiO4)6O2 compound (Table 3), the average Eu−O bond length at the 6H site (2.431 Å) is somewhat shorter than that at the 4F sites (2.547 Å). Moreover, the Eu3+ ions in the 6H sites are bonded to the free oxygen O(4) with the bond length of 2.245 Å. On the basis of the effect of the cation size on the site selectivity, Eu3+ ions preferentially occupy the 6H sites because the ionic radius of Eu3+ (1.12 Å for CN = 9) is smaller than that of La3+ (1.216 Å for CN = 9),20 which is in good accordance with Blasse’s study.36 Thus, considering the shorter bond lengths (including the free oxygen O(4)) for the 6H sites and the smaller ionic radius of Eu3+ as mentioned above, it is presumed that CT in the 6H sites occupied by Eu3+ ions is more effective than that in the 4F sites because of the stronger covalency of the 6H sites. It should be noted that in the CT process, the transfer of one electron from O2− to Eu3+ changes 4f6 configuration (Eu3+, ground state 7F0) into 4f7 configuration (Eu2+, ground state 8S). It should also be mentioned that the formation of half-filled 4f7 subshells by the CT process results in greater stabilization (more exchange energy) because of the symmetrical nature of these subshells. Thus, the intra4f transitions of Eu3+ in the 6H sites are less probable and exhibit lower intensities than that of Eu3+ in the 4F sites. It is therefore concluded that first of all, Eu3+ ions preferentially occupy the 6H sites at lower Eu3+ concentration and eventually enter the 4F sites with increasing Eu3+ concentration, which is confirmed by the calculated splitting energies of the two different sites: 6H site (3870.0 cm−1) and 4F site (4092.8 cm−1). The excitation and emission spectra of the LiLa9−xTbx(SiO4)6O2 (x = 3, 6, and 9) compounds are shown in Figure 15. The broad absorption bands in the range of 200−300 nm are attributed to the 4f8− 4f75d of Tb3+ and the overlapped peaks between 300 and 400 nm correspond to the 4f−4f transitions of Tb3+. With increasing Tb3+ concentration from x = 3 to 9, the overall absorption intensities are decreased and the emission spectra show also the same tendency to decrease. In particular, the I

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1.2 for x = 0.35, 1.1 for x = 0.40. On the basis of the increase in I240/I270 with decreasing Tb3+ concentration, it is clearly deduced that at lower Tb3+ concentration, the Tb3+ ions preferentially occupy the sites that absorb around 240 nm and subsequently enter the sites that absorb around 270 nm. The calculated splitting energies of the two different Tb3+ sites are estimated to be 4396.1 cm−1 for the 4F site and 3637.6 cm−1 for the 6H site. It is therefore presumed that the LEB (4F site) and HEB (6H site) because of the two different sites are located within the f−d absorption band between 200 and 300 nm. Figure 17 shows the relative intensities of the emission

Figure 15. Excitation and emission spectra of LiLa9−xTbx(SiO4)6O2 (x = 3, 6, 9) compounds.

relative intensity ratio of the 4f−4f transition at 375 nm versus the 4f−5d transition around 240 nm, If−f,375/If−d,240 is considerably changed: If−f,375/If−d,240 = 1.4 for x = 3, 2.0 for x = 6, 3.0 for x = 9. At lower Tb3+ concentration from x = 0.08 to 0.40 (Figure 16), If−d,240/If−f,375 is estimated as 2.4 for x = 0.08,

Figure 17. Relative emission intensities of LiLa9−xTbx(SiO4)6O2 compounds as a function of Tb content.

maxima of the f−d transition band (240 nm) and 4f−4f transition (375 nm) as a function of the Tb3+ concentration in LiLa9−xTbx(SiO4)6O2 compound. It is clear that the emission of the f−d transition is more intense than those of the 4f−4f transitions for Tb3+ concentrations below x = 3. At lower Tb3+ concentration, emission from the 240 nm absorption centers increases rapidly with increasing Tb+ concentration, and the highest intensity is observed at x = 0.35; there is a subsequent decrease in the intensity of the emission from these centers at x ≥ 0.40 because of the concentration quenching effect. The reflectance spectra of LiLa9−xTbx(SiO4)6O2 compounds are shown in Figure 18. By considering the absence of the host (LiLa9(SiO4)6O2) absorption band, the broad absorption bands between 200 and 300 nm can be assigned to the f−d absorption of Tb3+; the overlapping small peaks (at 302, 318, 338, 351, 375, and 484 nm) are exactly consistent with 4f−4f absorptions of Tb3+. The luminescent behavior of LiLa9−xREx(SiO4)6O2 (RE = Ce0.12, Eu2.00, Tb0.30) phosphors were compared with commercial Y2SiO5:Ce (obtained from Phosphor Technology in England), LaPO4:(Ce,Tb) (Nichia in Japan), and Y2O3:Eu (Nichia in Japan) phosphor. Their emission spectra are shown in Figure 19. The difference in the excitation and emission spectra between the two phosphors with the same activator ion may be ascribed to the modification of the crystal field around activator ion due to the different crystal symmetry and arrangement of atoms. The relative emission intensity of LLSO:Ce0.12 is about 52% compared to a commercial Y2SiO5:Ce; for LLSO:Eu2.00 about 69% compared to a commercial Y2O3:Eu; for LLSO:Tb0.30 about 20% compared to a commercial LaPO4(Ce,Tb). The Commission Interna-

Figure 16. Excitation and emission spectra of LiLa9−xTbx(SiO4)6O2 (x = 0.08−0.40) compounds.

1.8 for x = 0.18, 1.4 for x = 0.35, 1.5 for x = 0.40. With increasing Tb3+ concentration, If−d,240/If−f,375 is decreased, whereas the value at x = 0.40 is increased compared to that at x = 0.35 because of the concentration quenching effect at x > 0.35. This fact reveals that at lower Tb3+ concentration, Tb3+ ions preferentially enter the sites around 240 nm, and then begin to enter the other sites, which is consistent with the relative intensity ratio at higher Tb3+ concentration (x = 3, 6, and 9). Careful examination of the 4f8−4f75d absorption band between 200 and 300 nm (Figure 16) indicates that there are at least three f−d absorption centers (around 240, 260, and 275 nm). In this study, two main absorption centers are determined as there are two different crystallographic sites, that is , the 6H and 4F site: one around 240 nm, the other around 270 nm (half region between 262 and 275 nm). The relative intensity ratio, I240/I270 of the f-d absorption band at lower Tb3+ concentration is estimated to be I240/I270 = 2.0 for x = 0.08, 1.4 for x = 0.18, J

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Figure 18. Diffuse reflectance spectra of LiLa9−xTbx(SiO4)6O2 (x = 0.08−9) compounds. Figure 20. CIE chromaticity for LiLa9−xCex(SiO4)6O2 (x = 0.12, monitored at 300 nm), LiLa9−xEux(SiO4)6O2 (x = 0.30 at 300 nm), LiLa9−xTbx(SiO4)6O2 (x = 0.35 at 254 nm) compound.

ΔECe(6H) = 3849.3 cm−1, ΔECe(4F) = 4228.1 cm−1 ΔE Eu(6H) = 3870.0 cm−1, ΔE Eu(4F) = 4092.8 cm−1 ΔE Tb(6H) = 3637.6 cm−1, ΔE Tb(4F) = 4396.1 cm−1

Decreasing the Ce 3+ content from x = 9 to 3 in LiLa9−xCex(SiO4)6O2 results in a decline in the intensity of the absorption band centered at 245 nm, whereas that of the band at 300 nm is increased, especially in the case of the LiLa3Ce6(SiO4)6O2 compound. At very low Ce3+ concentration (0.0 ≤ x ≤ 0.15), the absorption bands at 245 nm are completely undetected and broad bands with a peak maximum at 295 nm are appeared, which possibly indicates that in the very low Ce3+ concentration range, the Ce3+ ions initially predominantly occupy only the site associated with the absorption band around 300 nm, and eventually the other site with an absorption band around 245 nm is occupied. On the basis of the calculated splitting energies, the HEB (at 245 nm) and LEB (at 300 nm) absorptions are definitively assigned to the 6H and 4F site, respectively. Therefore, at lower Ce3+ concentrations, the Ce3+ ions preferentially fill the 4F sites and then enter the 6H sites with increasing Ce3+ concentration. Eu3+ ions in both sites contribute to the overall absorption intensities of the intra4f transitions. Thus, spectral evidence of the site selectivity of Eu3+ ions might be derived from the CTB region rather than the intra4f transitions because the subtle differences in the crystal field splitting of the 4F and 6H sites are reflected in the CTB. It is presumed that the absorptions of the LEB (4F site) and HEB (6H site) because of the two different sites are mixed within the CTB between 220 and 350 nm. Thus, Eu3+ ions preferentially occupy the 6H sites at lower Eu3+ concentration and then enter the 4F sites with increasing Eu3+ concentration. The increase in I240/I270 with decreasing Tb3+ concentration clearly confirms that at lower Tb 3+ concentration, the Tb3+ ions favorably occupy the sites that absorb around 240 nm and then enter the sites that absorb around 270 nm. It is therefore proposed that the LEB (4F site) and HEB (6H site) due to the two different sites are located

Figure 19. Emission spectra of LiLa9−xREx(SiO4)6O2 (RE = Ce, Eu, Tb) and commercialized phosphors; Y2SiO5:Ce (obtained from Phosphor Technology in England), LaPO4(Ce,Tb) (Nichia in Japan), Y2O3:Eu (Nichia in Japan).

tional de I’Eclairage (CIE) coordinates for LiLa9−xREx(SiO4)6O2 (RE = Ce0.12, Eu0.30, and Tb0.35) under UV irradiation at 300 and 254 nm, are x = 0.137, y = 0.037, x = 0.643, y = 0.356, x = 0.241, y = 0.6460, respectively as shown in Figure 20. On the basis of the CIE coordinates, it is clear that the LiLa9−xREx(SiO4)6O2 (RE = Ce0.12, Eu0.30, and Tb0.35) compounds emit blue, red, and green, respectively.

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CONCLUSIONS

Novel LiCe9(SiO4)6O2 and LiTb9(SiO4)6O2 compounds have been successfully synthesized, and the site selectivity and occupancy of activator ions have been estimated including LiEu9(SiO4)6O2 compound. On the basis of crystal field theory, the splitting energy calculations of the 6H and 4F sites of LiRE9(SiO4)6O2 (RE = Ce, Eu, Tb) compounds are estimated as follows: K

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(21) Kim, D.; Park, S.; Kim, S.; Kang, S. G.; Park, J. C. Inorg. Chem. 2014, 53, 11966. (22) House, J. E.; Inorganic Chemistry; Elsevier Inc.: Canada, 2008. (23) Hlavay, J.; K. Jonas, K.; S. Elek, S.; Inczedy, J. Clays Clay Miner. 1978, 26, 139. (24) Pires, A. M.; Davolos, M. R. Chem. Mater. 2001, 13, 21. (25) Lucazeau, G.; Sergent, N.; Pagnier, T.; Shaula, A.; Kharton, V.; Marques, F. M. B. J. Raman. Spectrosc. 2007, 38, 21. (26) Zhang, F. X.; Lang, M.; Zhang, J. M.; Cheng, Z. Q.; Liu, Z. X.; Lian, J.; Ewing, R. C. Phys. Rev. B 2012, 85, 214116. (27) An, T.; Orera, A.; Baikie, T.; Herrin, J. S.; Piltz, R. O.; Slater, P. R.; White, T. J.; Sanjuán, M. L. Inorg. Chem. 2014, 53, 9416. (28) Lai, H.; Bao, A.; Yang, Y.; Tao, Y.; Yang, H.; Zhang, Y.; Han, L. J. Phys. Chem. C 2008, 112, 282. (29) Wagner, D. C.; Riggs, W. M.; Davis, L. E.; Moulder, J. F. Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer: Minneapolis, MN, 1979; p 132. (30) Wang, Z.; Quan, Z.; Lin, J. Inorg. Chem. 2007, 46, 5237. (31) Renaudin, G.; Dieudonné, B.; l’Avignant, D.; Mapemba, E.; ElGhozzi, M.; Fleutot, S.; Martinez, H.; Č erný, R.; Dubois, M. Inorg. Chem. 2010, 49, 686. (32) Shen, Y.; Huang, Y.; Zheng, S.; Guo, X.; Chen, Z.; Peng, L.; Ding, W. Inorg. Chem. 2011, 50, 6189. (33) Pal, N.; Cho, E. B.; Kim, D.; Jaroniec, M. J. Phys. Chem. C 2014, 118, 15892. (34) Trovarelli, A. Catalysis by Ceria and Related Materials; World Scientific Publishing: London, 2002. (35) Huheey, J. E.; Keiter, E. A.; Keiter, R. L. Inorganic Chemistry; Harper Collins College Publishers: New York, 1993. (36) Blasse, G. J. Solid State Chem. 1975, 14, 181.

within the f−d absorption band between 200 and 300 nm. The site selectivity of Ce3+, Eu3+, and Tb3+ in LiLa9−xREx(SiO4)6O2 (RE = Ce, Eu, Tb) compounds is estimated considering the excitation/emission spectra, theoretical calculation of crystal field effect, and effective ionic radii of the three lanthanide ions. The theoretically calculated splitting energies support the PL behaviors depending on the site preferences of activator ions with the variation of the activator ion concentration. The effective ionic radii of activator ions reflect also the site preferences between 4F and 6H site with respect to their ionic radii. The Ce3+ ions with the largest ionic radius are probable to occupy 4F site with the larger volume polyhedrons at lower concentration, while at lower concentration the Eu3+ and Tb3+ with the smaller ionic radius enter preferentially 6H site with the smaller volume polyhedrons. These compounds could provide a platform for modeling a new phosphor and application in the solid-state lighting field.

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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 One of the authors, S. J. Kim, acknowledges support from the National Research Foundation of Korea (Grant 20100013089).

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REFERENCES

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DOI: 10.1021/ic502113a Inorg. Chem. XXXX, XXX, XXX−XXX