Eu2+-Activated Alkaline-Earth Halophosphates, M5(PO4)3X:Eu2+ (M

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Eu2+-Activated Alkaline-Earth Halophosphates, M5(PO4)3X:Eu2+ (M = Ca, Sr, Ba; X = F, Cl, Br) for NUV-LEDs: Site-Selective Crystal Field Effect Donghyeon Kim,† Sung-Chul Kim,⊥ Jong-Seong Bae,∥ Sungyun Kim,*,§ Seung-Joo Kim,⊥ and Jung-Chul Park*,†,‡ †

Graduate School of Advanced Engineering, Silla University, Busan 46958, Republic of Korea Center for Green Fusion Technology and Department of Engineering in Energy & Applied Chemistry, Silla University, Busan 46958, Republic of Korea § Institute of NT.IT Fusion Technology, Ajou University, Suwon 16499, Republic of Korea ∥ Busan Center, Korea Basic Science Institute, Busan 46742, Republic of Korea ⊥ Department of Chemistry, Division of Energy Systems Research, Ajou University, Suwon 16499, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Eu2+-activated M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) compounds providing different alkaline-earth metal and halide ions were successfully synthesized and characterized. The emission peak maxima of the M5(PO4)3Cl:Eu2+ (M = Ca, Sr, Ba) compounds were blueshifted from Ca to Ba (454 nm for Ca, 444 nm for Sr, and 434 nm for Ba), and those of the Sr5(PO4)3X:Eu2+ (X = F, Cl, Br) compounds were red-shifted along the series of halides, F → Cl → Br (437 nm for F, 444 nm for Cl, and 448 nm for Br). The site selectivity and occupancy of the activator ions (Eu2+) in the M5(PO4)3X:Eu2+ (M = Ca, Sr, Ba; X = F, Cl, Br) crystal lattices were estimated based on theoretical calculation of the 5d → 4f transition energies of Eu2+ using LCAO. In combination with the photoluminescence measurements and theoretical calculation, it was elucidated that the Eu2+ ions preferably enter the fully oxygen-coordinated sites in the M5(PO4)3X:Eu2+ (M = Ca, Sr, Ba; X = F, Cl, Br) compounds. This trend can be well explained by “Pauling’s rules”. These compounds may provide a platform for modeling a new phosphor and application in the solid-state lighting field.

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INTRODUCTION White-light emitting diodes (w-LEDs) have been most commonly produced using GaN-based chips coated with a yellow-emitting phosphor (YAG:Ce3+).1 However, the low efficiency and low color rendering index of w-LEDs should be improved by utilizing a near-ultraviolet (n-UV) LED chip coated with tricolor (red, green, and blue) phosphors. The development of new phosphors for w-LEDs has been intensively performed in a lot of research in the past decade. The 5d electrons of rare-earth ions such as Eu2+ and Ce3+ are known to be very sensitive to the crystal field, whereas the 4f electrons are well screened from the crystal field because of the surrounding 5s and 5p electrons.2 Eu2+ ions have been widely used as an activator that exhibits broad emission bands between UV and red spectral range depending on the environment.3−6 Fajans suggested the following rules to estimate the extent to which a cation can polarize an anion and thus induce covalent character.7 Polarization will be increased by (i) high charge and small size of the cation, (ii) high charge and large size of the © XXXX American Chemical Society

anion, and (iii) electron configuration of the cation. If the anions are sufficiently large and soft, the cation can polarize it, and in the extreme case, the cation would actually penetrate the anionic electron cloud, yielding a covalent bond. As polarization does follow some charge-to-size relationship, cations with large ionic potentials (Φ = Z+/r) have a tendency to combine with polarizable anions to yield partially covalent compounds. Eu2+ activated alkaline-earth halophosphates expressed generally as M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) are well-known for their applications as phosphor materials.8−11 The constituent cation and anion elements of the M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) crystal lattice have very different properties as presented in Table 1.7,12 In M5(PO4)3X:Eu2+ compounds, the ionic sizes of the alkaline-earth metal and halide ions at two distinct sites are different, which induces the modification of covalency associated with the variation of the luminescent Received: March 14, 2016

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DOI: 10.1021/acs.inorgchem.6b00637 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

these compounds. Furthermore, the f-factor values of the halide ions (F, Cl, and Br) as the nature of the ligands are also very different, which affects the extent of splitting of the d orbitals of activator ions. Nevertheless, many studies on M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) materials have been performed, and to the best of our knowledge, there have been no reports on the variation of the 5d → 4f transition energies of Eu2+ depending on the distinct substitution of the cation and anion sites in these materials. It might be possible to define the variation of the luminescent behaviors in Eu2+-activated Sr5−xMx(PO4)3X (M = Ca, Ba; X = F, Cl, Br) compounds providing different alkaline-earth metal and halide ions. Herein, we report on the synthesis and characterization o f E u 2 + -act ivated Sr5−xMx(PO4)3X (M = Ca, Ba; X = F, Cl, Br) phosphors. In particular, the 5d → 4f transition energies of Eu2+ are calculated using linear combination of atomic orbitals (LCAO), and their differences are discussed.

Table 1. Properties of Cation and Anion Elements in M5(PO4)3X:Eu2+ (M = Ca, Sr, Ba; X = F, Cl, Br) element

ionic radiusa (Å, CN = 6)

Ca Sr Ba Eu O F Cl Br

1.00 1.18 1.35 1.17 1.40 1.33 1.81 1.96

electronegativityb f-factorb (in CFT) 1.00 0.95 0.89 1.01 3.44 3.98 3.16 2.96

0.99 0.90 0.78 0.72

a Data taken from ref 12. CN = coordination number. bData taken from ref 7.

properties in these compounds. Compared with alkaline-earth metal ions (Ca, Sr, and Ba), the larger difference in electronegativity and anionic size for halide ions (F, Cl, and Br) may have a greater effect on the covalent bond character in

Figure 1. Reitveld refinement results of (a) Ca5(PO4)3Cl, (b) Sr5(PO4)3F, (c) Sr5(PO4)3Cl, (d) Sr5(PO4)3Br, and (e) Ba5(PO4)3Cl. Measured data, fitted results, expected reflection positions, and the difference between measured and fitted results are expressed as red hollow circles, black lines, green vertical lines, and blue solid lines, respectively. (f) Relation between the average bond length of M−O(X) and cell volume for the compounds. B


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Inorganic Chemistry Table 2. Refinement Results of M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) compound Rwp (%) RBragg (%) Rf (%) χ2 space group lattice parameter (Å)

a b c

cell volume (Å3)

Ca5(PO4)3Cl

Sr5(PO4)3F

Sr5(PO4)3Cl

Sr5(PO4)3Br

Ba5(PO4)3Cl

13.0 8.06 9.20 0.79 P21/c 9.6365(1) 6.7718(1) 19.2720(6) 1089.12(5)

13.4 7.29 7.53 1.69 P63/m 9.7145(1)

12.7 7.34 6.52 1.49 P63/m 9.8765(1)

10.8 6.41 5.44 1.11 P63/m 9.9632(1)

14.3 8.93 9.03 0.95 P63/m 10.2665(2)

7.2850(1) 595.39(1)

7.1909(1) 607.46(1)

7.2067(1) 619.54(1)

7.6460(2) 697.92(2)

Figure 2. Environments of (a) Ca1, (b) Ca2, (c) Ca3, Ca4, and Ca5 sites in Ca5(PO4)3Cl.

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infrared (FT-IR) spectroscopy was performed using an FT-IR spectrophotometer (IRTracer-100, Shimadzu) with a resolution range of ±0.5 cm−1 by employing KBr medium (200 mg KBr + 1 mg sample). The oxidation states of the elements were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250) with a monochromatic Al Kα X-ray source (hν = 1486.6 eV) at the Busan Center of Korea Basic Science Institute (KBSI). The obtained binding energies (BEs) were calibrated with that of the adventitious carbon (C 1s) core level peak at 284.6 eV as a reference.

EXPERIMENT SECTION

Sr5−xMx(PO4)3X (M = Ca, Ba; X = F, Cl, Br) phosphors were prepared from a stoichiometric mixture of SrCO3 (or CaCO3 and BaCO3), Eu2O3, (NH4)H2PO4, and NH4F (or NH4Cl and NH4Br) under a 4% H2/Ar atmosphere at 1100 °C for 3 h. To prepare wellcrystallized phosphor compounds, two-step heating was used: in the first step, the starting chemicals except NH4X (X = F, Cl, Br) were mixed and heated at 300 °C for 10 h to ensure complete decomposition of (NH4)H2PO4. In the second step, NH4X was excessively added (the molar ratio of NH4X to the other chemicals was 2) and heated under a 4% H2/Ar atmosphere at 1100 °C for 3 h. For the homogeneous reaction, alumina boats with lids were used to achieve sufficient collision time between the other chemicals and NH4X, and to prevent unreacted NH4X from rapid outgassing. Powder X-ray diffraction (XRD) measurements of the Sr5−xMx(PO4)3X (M = Ca, Ba; X = F, Cl, Br) phosphor materials were performed using a Rigaku DMAX-2200PC X-ray diffractometer equipped with a graphite monochromator (λ = 1.5418 Å). The step scan mode was employed in the 2θ range of 10−110° with a step size of 0.02° and counting time of 5 s for each step. Structure refinements were performed using the Rietveld method using the Fullprof program with pseudo-Voigt peak shapes and refined backgrounds. 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 a slit width of 5 nm (Fluorometer FS-2, Scinco). The reflectance spectra were recorded using a UV−visible spectrophotometer (UV2600, Shimadzu) with BaSO4 as a reference. Fourier-transform

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RESULTS AND DISCUSSION Structural Characterization. The crystal structures of the compounds were resolved by analyzing their XRD patterns. The Rietveld profiles matching results presented in Figure 1a−e reveal that the single phase of each sample was obtained and all of the XRD patterns are in good agreement with previously reported structure models: ICSD No. 2789 for Ca5(PO4)3Cl,13 ICSD No. 95737 for Sr5(PO4)3F,14 ICSD No. 80084 for Sr5(PO4)3Cl,15 ICSD No. 87102 for Sr5(PO4)3Br,16 and ICSD No. 8191 for Ba5(PO4)3Cl.17 The refined structural parameters obtained from the Rietveld fitting of the XRD data are listed in Table 2. Detailed crystallographic information, including the atomic positions for the compounds is listed in Tables S1, S2, S3, and S4 (Supporting Information). Figure 1f shows the approximately linear relation between the unit cell volumes and average MO (X) bond lengths in the M5(PO4)3X (M = Ca, Sr, C


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Inorganic Chemistry

Figure 3. Environments of (a) Sr1 and (b) Sr2 sites in Sr5(PO4)3F.

Figure 4. Environments of (a) Sr1 and (b) Sr2 sites in Sr5(PO4)3Cl.

is located at the middle positions between the two Sr2 layers. The Sr2 in Sr5(PO4)3Cl has a coordination number of 8 (six O and two Cl atoms; Figure 4).18 Sr5(PO4)3Br and Ba5(PO4)3Cl are isostructural with Sr5(PO4)3Cl. The Sr2O bond length shows an increasing trend from Sr5(PO4)3F (avg. 2.59 Å) to Sr5(PO4)3Cl (avg. 2.62 Å) and Sr5(PO4)3Br (avg. 2.66 Å), whereas the Sr1O bond lengths are similar in the range of 2.66−2.69 Å. The SrX bond length in Sr5(PO4)3X gradually increases from 2.391 Å (X = F) to 3.084 Å (X = Cl) and 3.146 Å (X = Br). For Ba5(PO4)3Cl, the average Ba1O and Ba2O bond lengths are 2.821 and 2.799 Å, respectively, and the BaCl bond length is 3.226 Å. The observed bond lengths are consistent with the ionic radii. Infrared and X-ray Photoelectron Spectroscopy. Figure 5 presents the FT-IR spectra of the Sr5(PO4)3X:Eu0.02 (X = F, Cl, Br) compounds. First, in all of the IR spectra, the absorption bands at approximately 1400 cm−1 occur at precisely the same wavenumber (at 1385.5 cm−1) associated with the CO antisymmetric stretching of CO32−.19−21 Furthermore, in situ FT-IR spectroscopy was performed by Du et al. in the hydrotalcite-like compounds to study carbonate transforma-

Ba; X = F, Cl, Br) compounds. Ca5(PO4)3Cl is characterized by a monoclinic unit cell containing five crystallographically distinct Ca atoms. Figure 2 shows the coordination spheres of the Ca atoms. The Ca1 and Ca2 atoms are coordinated with nine O atoms to form irregular CaO9 polyhedra with average bond lengths of 2.59 and 2.55 Å, respectively (the bond lengths are listed in Table S3). The Ca3, Ca4, and Ca5 are coordinated with six O atoms (CaO, avg. 2.51 Å) and one Cl atom (CaCl, avg. 2.82 Å) to exhibit a distorted pentagonal bipyramidal geometry. The Cl atom links to three surrounding CaO6 groups, forming a trimeric Ca3O18Cl group.13 Sr5(PO4)3F has hexagonal symmetry. In this structure, there are two different sites occupied by Sr atoms. The Sr1 is located on a 3-fold axis and coordinated with nine O atoms (SrO, avg. 2.69 Å). The Sr2 exhibits a distorted pentagonal bipyramidal geometry with one F atom (SrF, 2.39 Å) and six O atoms (SrO, avg. 2.59 Å; Figure 3 and Table S4).14 The Sr5(PO4)3Cl structure is closely related to that of Sr5(PO4)3F. The structural difference between Sr5(PO4)3Cl and Sr5(PO4)3F is the relative position of the halogen atom and Sr atom; the F atom lies on the same z coordinate as the Sr atom, whereas the Cl atom in Sr5(PO4)3Cl D


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Inorganic Chemistry

Figure 6. Eu 4d XPS spectra of reference compounds, Eu2O3, EuCl3, and EuCl2.

Figure 5. Infrared spectra of Sr5(PO4)3X:Eu0.02 (X = F, Cl, Br) phosphor.

XPS spectra of EuCl2, two new main bands appeared at 128.4 and 134.1 eV and are associated with Eu2+ 4d5/2 and Eu2+ 4d3/2, respectively.29−31 The spin−orbital splitting values for the Eu3+ and Eu2+ species are 5.7 eV, which is in good agreement with the previously reported value (5.7 eV).26 It is well-known that the Eu2+ species are thermodynamically unstable based on the standard reduction potential (Eu3+/Eu2+ = −0.36 V vs. standard hydrogen electrode (SHE)).32 Thus, it is evident that the Eu2+ species in the EuCl2 crystal lattice are destined to be easily oxidized by oxidizing agents, such as H2O and O2, particularly under X-ray irradiation in the XPS measurements. Based on the Eu 4d binding energies of the reference compounds, the XPS 3d binding energies of the reference compounds could be precisely assigned as shown in Figure 7: Eu3+ 3d5/2 (1133.8 eV)

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tions during adsorption and desorption of CO2. They reported that the gaseous CO2 molecules were chemisorbed on various active sites (highly basic metal-bound unsaturated oxygen atoms), forming unidentate, bidentate, and bridged carbonates with an absorption band at 1385 cm−1. Thus, it is evident that the absorption band at 1385.5 cm−1 in our study is a good internal standard to correctly examine the variation of the wavenumber of the IR modes in these compounds. As observed in Figure 5, the bands at approximately 590 and 560 cm−1 are associated with the ν4 bending vibration of the PO mode, and the 950 cm−1 band is assigned to the ν1 symmetric PO stretching vibration. The strong bands around 1030 and 1070 cm−1 correspond to the triply degenerate ν3 antisym23,24 metric PO stretching vibration of the PO3− As 4 groups. observed in Figure 5, in particular, the absorption band maxima at approximately 1070 cm−1 shift toward a higher wavenumber moving from Br to F in the Sr5(PO4)3X:Eu0.02 (X = F, Cl, Br) compounds, whereas no considerable change is observed in the other bands. The Rietveld analysis presented in Table 2 reveals that the unit cell volumes of the Sr5(PO4)3X:Eu0.02 (X = F, Cl, Br) compounds are reduced along with the series of halide ions (619.54 Å3 for Br, 607.46 Å3 for Cl, and 595.39 Å3 for F). In the series of Sr5(PO4)3X:Eu0.02 (X = F, Cl, Br) compounds, it is interesting that the wavenumbers of the absorption modes generally shift to higher frequencies, which may be attributed to the shrinkage of the unit cell volumes associated with the shortening of the SrX bond length upon moving from Br to F.25 To examine the valence state of the Eu ions in the Sr5(PO4)3Cl:Eu compound, XPS measurements were performed. All the XPS spectra were fitted after a Shirley background correction, and their binding energies were determined. Figure 6 presents the Eu 4d XPS spectra of the reference compounds, Eu2O3 (Eu3+), EuCl3 (Eu3+), and EuCl2 (Eu2+). The Eu 4d XPS spectra of the three reference compounds reveal the characteristic Eu3+ binding energies between 132 and 145 eV: Eu3+ 4d5/2 (134.4 eV) and Eu3+ 4d3/2 (140.0 eV) for Eu2O3, Eu3+ 4d5/2 (137.3 eV) and Eu3+ 4d3/2 (142.9 eV) for EuCl3, and Eu3+ 4d5/2 (136.7 eV) and Eu3+ 4d3/2 (142.3 eV) for EuCl2.26−28 The large difference in the Eu3+ 4d binding energies between Eu2O3 and EuCl3 is due to the different ligand atoms (O and Cl). In particular, for the Eu 4d

Figure 7. Eu 3d XPS spectra of reference compounds, Eu2O3, EuCl3, and EuCl2.

and Eu3+ 3d3/2 (1163.7 eV) for Eu2O3; Eu3+ 3d5/2 (1136.4 eV) and Eu3+ 3d3/2 (1166.1 eV) for EuCl3; and Eu3+ 3d5/2 (1135.9 eV), Eu3+ 3d3/2 (1165.7 eV), Eu2+ 3d5/2 (1126.0 eV), and Eu2+ 3d3/2 (1156.0 eV) for EuCl2. It is reasonable that the Eu2+ species in the EuCl2 compound is prone to being easily oxidized under X-ray irradiation, as observed in Eu 3d and Eu 4d XPS spectra. Notably, the differences in the binding energies of Eu2+ and Eu3+ in Eu 3d and Eu 4d are estimated to be E


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Inorganic Chemistry approximately 9.9 and 8.3 eV, respectively. This difference is most likely due to the principal quantum number (n). The nucleus of an atom more strongly attracts nearby electrons with lower n value, indicating that the electrons nearby the nucleus have a higher binding energy. Thus, the difference in binding energies between Eu2+ and Eu3+ in Eu 3d (approximately 9.9 eV) is somewhat higher than that in Eu 4d (8.3 eV), which guarantees that the assignments of binding energies in the Eu 3d and 4d region are perfectly logical. To examine the valence state of the Eu species in the Sr5(PO4)3Cl:Eu compound, XPS measurements were performed, and the results are presented in Figure 8. As the PL emission behavior of Sr5(PO4)3Cl:Eu2+ 0.05

Figure 9. Excitation and emission spectra of M5(PO4)3Cl:Eu2+ 0.02 (M = Ca, Sr, Ba) phosphors.

Ba in the M5(PO4)3Cl:Eu0.02 phosphor materials. The correlation between the crystal field splitting and shape and size of the polyhedron is given in the following equation:36,37 Dq = 3Ze 2r 4 /5R5

(1)

where Dq represents the crystal field strength, Z represents the valence of the anion ligand, e represents the charge, r represents the radius of the frontier d wave function, and R represents the bond length between a center ion and ligands. In general, the crystal field strength decreases with increasing bond length in the order of Ca, Sr, and Ba. Therefore, the decrease in the crystal field strength could result in the blue shift for the 5d → 4f transition of Eu2+ from Ca to Ba in M5(PO4)3Cl:Eu0.02 phosphor materials. The interpretation of the blue shift of the emission peak maxima in M5(PO4)3Cl:Eu0.02 (M = Ca, Sr, and Ba) phosphor materials is plausible and coincident with those previously reported.38,39 However, the r factor in eq 1 should be considered to precisely define the crystal field splitting because the approximation (Dq ∝ 1/R5) is oversimplified. Additionally, if there are crystallographically distinct sites in the host lattices, it is difficult to simply apply eq 1 because of the many factors affecting the strength of the crystal field splitting. As observed in Figure 10, the peak maxima of the emission

Figure 8. Eu 3d XPS and PL spectra of Sr5(PO4)3Cl:Eu0.05 phosphors.

(top right) exhibits blue emission with a maximum intensity of approximately 444 nm, it is evident that the valence state of the Eu species is perfectly Eu2+. However, the mixed valence state, Eu3+/Eu2+ is observed in the Eu 3d XPS spectrum (top left), which may be attributed to the difference of the energy source, i.e., UV irradiation (in PL) and X-ray irradiation (in XPS). As mentioned in the discussion of Figures 6 and 7 in the XPS measurements, the Eu2+ species are destined to be easily oxidized in the presence of H2O and O2 under X-ray irradiation. In contrast, the PL emission behavior of the 3+ Sr 5(PO 4 ) 3 Cl:Eu 0.05 compound synthesized under an O2 atmosphere (bottom right) exhibits red emission with a maximum intensity of approximately 613 nm, indicating that the valence state of the Eu species (bottom left) is predominantly Eu3+. Valuable information was obtained from the XPS measurements; it is very difficult to obtain fully reduced Eu2+ 3d and 4d XPS spectra in Sr5(PO4)3Cl:Eu2+ phosphor materials because of the thermodynamic instability of the Eu2+ species in the XPS measurement. Photoluminescence Spectra. The excitation and emission spectra of the M5(PO4)3Cl:Eu0.02 (M = Ca, Sr, and Ba) phosphors are presented in Figure 9. The excitation spectra of the M5(PO4)3Cl:Eu0.02 phosphors monitored at the wavelength maxima in the emission spectra consist of broad bands between 250 and 420 nm, which may be attributed to the 4f → 5d transition of Eu2+.33−35 The emission spectra monitored at 367 nm excitation show broad bands with peak maxima between 430 and 460 nm: 454 nm for Ca, 444 nm for Sr, and 434 nm for Ba. It is remarkable that the peak maxima are blue-shifted along with the series of alkaline-earth metal ions, Ca → Sr →

Figure 10. Excitation and emission spectra of Sr5(PO4)3X:Eu2+ 0.02 (X = F, Cl, Br) phosphors. F


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Inorganic Chemistry spectra for Sr5(PO4)3X:Eu0.02 (X = F, Cl, Br) are red-shifted along with the series of halide ions, F → Cl → Br (437 nm for F, 444 nm for Cl, and 448 nm for Br). As indicated in Table 1, the halide ions as the anionic ligands have very different properties. In particular, the f factor value as the nature of the ligands, which is one of several factors affecting the extent of splitting of d orbitals in CFT, is different; this tendency is very closely related to the spectrochemical series Br− < Cl− < F−.7 The strongest crystal field ligand (F−) makes the emission maximum shift to the longest wavelength, and the weakest one (Br−) makes the shortest one, based on the spectrochemical series. Therefore, one might expect that the order of the emission wavelength is Sr5(PO4)3F:Eu0.02 > Sr5(PO4)3Cl:Eu0.02 > Sr5(PO4)3Br:Eu0.02. However, the emission spectra, as presented in Figure 10, are quite the opposite of the expectation based on CFT. Coordination field analysis of the fluorescence spectra of LaOX:Eu3+ (X = Cl, Br, and I) phosphors was studied using the double-sphere coordination point-charge field model.40 According to the report, the crystal field parameters (Bkm) in the series of LaOX:Eu3+ (X = Cl, Br, and I) compounds showed a change in the strength of the coordination field effect depending on the ionic radius and the electronegativity of X− anions. Crystal field and free-ion analysis for Eu3+ ion in the series of LaOBr−LaOCl−LaOF host lattice were reported by Wang et al.,41 and revealed that the crystal field strength decreases along the series of ligands, Br → Cl → F, which is contrary to the spectrochemical series.7 Kim et al. reported that the splitting energies of 5d orbitals were calculated in the LaOX:Eu2+ (X = Cl, Br, and I) phosphor materials and compared with the experimental PL data.42 They mentioned that the calculated splitting energies of 5d orbital increased along the series of ligands, Cl → Br → I in the LaOX:Eu2+ phosphor materials, which was quite the opposite of one based on spectrochemical series (Cl− > Br− > I−) but was matched well with the experimental emission spectra. From the previous reports, it is noteworthy that the crystal field strength decreases when the interatomic distance decreases, which probably depends on the ionic radius of halide ions in the series of LaOX:Eu phosphor materials. Thus, the previous results on the crystal field strength of the Eu-activator ions in LaOX (X = F, Cl, Br, I) host lattice warrant further examination of theoretical calculation in M5(PO4)3X:Eu2+ (M = Ca, Sr, Ba; X = F, Cl, Br) materials. The maximum emission intensities of Sr5(PO4)3X:Eu2+ phosphors (X = F, Cl, Br) as a function of Eu concentration are depicted in Figure 11. The PL intensities of the Sr5(PO4)3F:Eu2+ phosphors (Figure 11a) increase as the Eu concentration increases until the maximum intensity at x = 0.02 is observed and then decrease. For Sr5(PO4)3Cl:Eu2+ (Figure 11b) and Sr5(PO4)3Br:Eu2+ (Figure 11c) phosphor materials, the Eu contents at the maximum emission intensity are the same as that for the Sr5(PO4)3F:Eu2+ phosphor. These results indicate that the optimum Eu concentration in the Sr5(PO4)3X:Eu2+ (X = F, Cl, Br) phosphor is 0.02. To examine the partial substitution effect of alkaline-earth metal and halide ions on the luminescent property, Sr5−xMx(PO4)3Cl:Eu2+ 0.02 (M = Ca, Ba) and Sr5(PO4)3X1−yFy:Eu2+ (X = Cl, Br) were 0.02 synthesized and characterized. All the emission spectra were deconvoluted by Gaussian fitting. As all the emission bands correspond to the 5d → 4f transition of Eu2+ activator ions, it is believed that there are distinct crystallographic sites of Eu2+ occupied in the Sr5−xMx(PO4)3Cl:Eu2+ 0.02 (M = Ca, Ba) crystal lattice. In Sr5−xCax(PO4)3Cl:Eu2+ 0.02 (x = 0 ∼ 5) compounds, the emission peak maxima are gradually red-shifted with increasing

Figure 11. Variation of normalized PL emission intensity in Sr5(PO4)3X:Eu2+ (X = F, Cl, Br) phosphors as a function of Eu concentration. Sr5(PO4)3F:Eu2+ (a), Sr5(PO4)3Cl:Eu2+ (b), and Sr5(PO4)3Br:Eu2+ (c).

Ca content, as indicated in Figure 12, which is in agreement with Figure 9. After Gaussian fitting, three sub-bands are

Figure 12. Emission spectra of Sr5−xCax(PO4)3Cl:Eu2+ 0.02 phosphor as a function of Ca content. Ca = 0 (a), Ca = 1(b), Ca = 3 (c), Ca = 5 (d).

observed from Ca = 1 to Ca = 5 because the parent compound, Ca5(PO4)3Cl, has five crystallographically distinct Ca atoms compared with the two crystallographically distinct Sr atoms for Sr5(PO4)3Cl as depicted in Figure 2 and Figure 3. It is presumed that the number of crystallographically distinct sites could not give the same number of sub-bands in the emission spectrum because the sub-bands with small energy difference could be overlapped and buried. For the Sr5−xBax(PO4)3Cl:Eu2+ 0.02 (x = 0 ∼ 5) compounds, there is no considerable change in the maximum emission wavelength from Ba = 1 to Ba = 3; however, severe tailing is observed for Ba = 3, as depicted in Figure 13, which most likely indicates that the mixed alkaline-earth metal ions (Sr and Ba) induce the local symmetry around the Eu2+ activator ion to change because of the largest ionic radius of Ba2+ (1.18 Å at CN = 6). In the 2+ Ba5(PO4)3Cl:Eu0.02 (Ba = 5) compound, the maximum emission wavelength is blue-shifted. In the G


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Inorganic Chemistry

Figure 15. Emission spectra of Sr5(PO4)3Br1−yFy:Eu2+ 0.02 phosphor as a function of F content. F = 0 (a), F = 0.5 (b), F = 1 (c).

Figure 13. Emission spectra of Sr5−xBax(PO4)3Cl:Eu2+ 0.02 phosphor as a function of Ba content. Ba = 0 (a), Ba = 1 (b), Ba = 3 (c), Ba = 5 (d).

From the variation of the emission peaks in Table 3, it is further substantiated that the modification of the electronic structure of the Eu2+-activator ion can be induced by the different cations and anions. The trend in the emission peaks from the dual site substitution warrants further theoretical calculation study on the dual site substitution effect. Figure 16 presents the diffuse reflectance spectra of Sr5(PO4)3F and Sr5(PO4)3F:Eu2+ x (x = 0.005, 0.01, 0.02, and 0.03). When Eu2+ ions are doped into the Sr5(PO4)3F host lattice, the broad absorption bands are manifested in the range of 230−420 nm. The broad absorption bands at 230 and 420 nm in Sr5(PO4)3F:Eu2+ phosphors may be associated with the 4f7 → 4f65d transition of Eu2+, because the absorption bands of the Sr5(PO4)3F host lattice are absent in this region. The diffuse reflectance spectra of Sr5(PO4)3F:Eu2+ x coincide well with the excitation spectra as observed in Figure 10. The luminescent behavior of the Sr5(PO4)3X:Eu2+ 0.02 (X = Cl, Br) phosphors was compared with a commercial (Ca,Sr,Ba,Mg)5(PO4)3Cl:Eu2+ (CSBMP:Eu2+) phosphor obtained from Nichia Corp., Japan. The excitation and emission spectra of the Sr5(PO4)3X:Eu2+ 0.02 (X = Cl, Br) and CSBMP:Eu2+ phosphor are presented in Figure 17. The emission spectra under the excitation at 367 nm are the same; for the Sr5(PO4)3Br: Eu2+ 0.02 phosphor, the symmetric emission band is centered at 448 nm with a full width at halfmaximum (fwhm) of 35 nm; for the CSBMP:Eu2+ phosphor, an asymmetric band is centered at 447 nm with a fwhm of 50 nm. The difference in the emission spectra between the two phosphors may be attributed to the modification of the crystal field around Eu2+ due to the different crystal symmetry and constituent atoms. The emission intensity of the Sr5(PO4)3Br: Eu2+ 0.02 phosphor is approximately 1.4 times higher than that of the commercial CSBMP:Eu2+ phosphor. The Commission International de l’Eclairage (CIE) coordinates of Sr5(PO4)3Br:Eu2+ 0.02 obtained under UV light at 365 nm are x = 0.141 and y = 0.025, and those for CSBMP:Eu2+ are x = 0.152 and y = 0.131, as shown in the inset of Figure 17, which indicates that the two compounds are blue-emitting phosphor materials. Theoretical Consideration for the Site Selectivity of Eu2+. The emission spectra of M5(PO4)3X:Eu2+ compounds were compared with the theoretical calculations using a linear combination of atomic orbital (LCAO) theory.43 Light emission is due to the outermost electron of the Eu2+ ion,

Sr5(PO4)3X1−yFy:Eu2+ 0.02 (X = Cl, Br) compounds, the emission peak maxima are progressively blue-shifted with increasing F content, as observed in Figure 14 and Figure 15. Additionally,

Figure 14. Emission spectra of Sr5(PO4)3Cl1−yFy:Eu2+ 0.02 phosphor as a function of F content. F = 0 (a), F = 0.5 (b), F = 1 (c).

the two sub-bands are observed after Gaussian fitting because the Sr5(PO4)3X1−yFy (X = Cl, Br) materials have two crystallographically distinct Sr atoms, i.e., Sr1 and Sr2, as depicted in Figure 3 and Figure 4. The evolution of the emission wavelength corroborates the idea that the electronic structure of the Eu2+ activator ion is subtly distorted depending on the concentration of the substituted cations and anions in 2+ Sr5−xMx(PO4)3X1−yFy:Eu0.02 (M = Ca, Ba; X = Cl, Br) compounds. The maximum peak positions and peak areas (%) after deconvolution depending on the cationic and anionic substitution are summarized in Table 3. The emission peak maxima are regularly changed as a function of the substitution rate of cations and anions in these compounds, indicating that the individual ionic species are homogeneously introduced into the dual site of the M5(PO4)3X host lattice, as desired. Furthermore, the multisub bands after deconvolution indicate that there are many sites available for the Eu2+ (activator ion). H


Article

Inorganic Chemistry Table 3. Luminescent Properties of M5(PO4)3X:Eu2+ Compounds as a Function of Cation and Anion Content M = Ca

M = Ba

Sr5−xMx(PO4)3Cl:Eu0.02

emission peak maximum, nm

after deconvolution, nm (peak area, %)


x=0 x=1 x=2

444 447 449

443 (56.3), 456 (43.7) 442 (39.5), 457 (44.5) 484 (16.0) 443 (37.0), 459 (45.5) 486 (17.5)

444 443 442

x=3

451

444 (34.1), 460 (49.5) 484 (16.4)

444

x=4

453

444 (27.5), 459 (54.1) 478 (18.4)

442

x=5

454

447 (29.9), 460 (48.8) 474 (21.3) X = Cl

434

Sr5(PO4)3X1‑yFy:Eu0.02


y y y y y

= = = = =

0 0.25 0.50 0.75 1

444 444 443 439 437

after deconvolution, nm (peak area, %) 443 442 441 437 434

(56.3), (59.0), (57.3), (56.0), (58.6),

456 457 456 452 451

after deconvolution, nm (peak area, %) 443 (56.3), 456 440 (39.2), 456 437 (26.8), 453 505 (17.3) 436 (21.1), 455 510 (17.1) 434 (20.7), 451 502 (17.1) 430 (40.2), 445 X = Br


(43.7) (41.0) (42.7) (44.0) (41.4)

(43.7) (34.9) 483 (25.9) (24.0) 476 (31.9), (25.5) 479 (36.3), (26.9) 473 (35.3), (37.0) 468 (22.8)

after deconvolution, nm (peak area, %)

448 447 445 438 437

445 445 442 435 434

(58.2), (58.9), (56.9), (53.8), (58.6),

459 459 457 450 451

(41.8) (41.1) (43.1) (46.2) (41.4)

surrounding cations and anions, the Hamiltonian of the outermost electron for the quantum wave equation can be written as H = H0 + V

(2)

where H0 is the Hamiltonian for the central charge plus one outermost electron and V is the crystal field potential by nearby ions. The explicit forms of H0 and V are given as H0 = −

Z e2 ℏ2 2 ∇ − c ,V= 2μ r

∑ i

Zie 2 |R⃗ i − r |⃗

(3)

In eq 3, μ is the reduced electron mass, Zc is the central Eu charge number, and Zi and R⃗ i are the charge number and position vector of the ith ion, respectively. The eigenenergy equation is

Figure 16. Diffuse reflectance spectra of Sr5(PO4)3F:Eu2+ as a function of Eu concentration.

Hψ = (H0 + V )ψ = Eψ

(4)

In this article, the numerical ψ is constructed by linear combination of atomic orbitals. The eigenfunctions of Hamiltonian H0 are known analytically as ⟨nlm|rθϕ⟩ = Rnl(r) Ylm(θ,ϕ) where r, θ, and ϕ are radial distance and two angles in spherical coordinates, and n, l, and m are principal, angular, magnetic quantum numbers, respectively. Rnl(r) and Ylm(θ,ϕ) are radial and angular eigenfunctions of hydrogen-like atom, respectively. Since the wave function ψ is for the outermost electron of the Eu2+ ion associated with 4f−5d transition, the basis atomic orbitals are chosen as eigenfunctions of H0. Written as the linear combination of basis functions

ψ=

∑ ajϕj (5)

j

where ϕj is the eigenfunction of H0 with eigenvalue Ej. Substituting eq 5 into 4, one gets

Figure 17. Excitation and emission spectra of Sr5(PO4)3X:Eu2+ 0.02 (X = Cl, Br) and a commercialized phosphor, (Ca,Sr,Ba,Mg)5(PO4)3Cl:Eu2+ (CSBMP:Eu2+) (obtained from Nichia Corp., Japan).

∑ (Ej + V )ajϕj = E ∑ ajϕj

and the variation of the transition energy of the Eu2+ ion by surrounding ions causes shift of the emission wavelength. With point charge approximation of the central Eu atom and

j

j

(6)

Multiplying ϕ*i on both sides of eq 6 and integrating yields I


Article

Inorganic Chemistry

Ba) compounds can be roughly classified by two distinct sites (A and B). The A sites are surrounded by nine oxygens (Ca1 and Ca2, Sr1, Ba1 site) and the B sites surrounded by two chlorines and six oxygens (Sr2, Ba2 site, one chlorine for Ca3, Ca4, and Ca5 site). Evidently, the exclusive occupation of Eu2+ in the fully oxygen-coordinated A sites (Ca1 and Ca2, Sr1, Ba1 site) is plausible because the calculated 5d → 4f transition energies of Eu2+ progressively increased, which is coincident with the blue shift of the emission peak maxima from Ca to Ba as presented previously in Figure 9. For the Sr5(PO4)3X:Eu2+ (X = F, Cl, Br) compounds (unit in cm−1):

∑ (Ejδij + Vij)aj = Eai , ∑ ((Ej − E)δij + Vij)aj = 0 j

j

(7)

where

Vij =

∫

ϕi*Vϕj

(8)

Equation 7 holds for each i, and total N equations become matrix eigenvalue equation

MA = EA

(9)

with M of Hermitian matrix size N × N and A of column vector with elements ai. Mij = Ejδij + Vij

(10)

Ai† = ai

(11)

35866.3, 35541.5, 34939.4 (F, Cl, Br in Sr1 site) 12272.0, 7027.6, 7782.5 (F, Cl, Br in Sr2 site)

In the PL experiment, the maximum emission peaks of Sr5(PO4)3 X:Eu were located at 437, 444, and 448 nm, corresponding to 22883, 22522, and 22321 (cm−1) for X = F, Cl, and Br, respectively. According to the calculation results, the exclusive occupation of Eu2+ in the Sr1 sites is plausible because the calculated 5d → 4f transition energies of Eu2+ are gradually decreased going from F to Br, which is in good agreement with the red shift of the emission peak maxima along with F → Cl → Br presented previously in Figure 10. Among several factors, the geometry of ligands and metal−ligand bond distance have an important effect on the magnitude of the crystal field splitting (Dq). As observed in Table S4, the Sr1−O bond lengths are strongly affected by Sr2−O bond lengths in the Sr5(PO4)3X (X = F, Cl, Br) compounds because two distinct Sr sites are connected via oxygen atoms. The partial substitution of the halides for oxide ions in the Sr2 sites would induce a structural stress which can be relieved by the reverse change of the Sr1−O bond lengths, as observed in Table S4: the avg. Sr1−O bond lengths (2.693 Å for F, 2.682 Å for Cl, 2.660 Å for Br) and the avg. Sr2−O bond lengths (2.585 Å for F, 2.619 Å for Cl, 2.655 Å for Br). This trend can be well explained by the bond competition for the bond character of Sr1[O−Sr2−X− Sr2−O]Sr1. According to the calculated emission energies, it is evident that the splitting energy is higher in the Sr2 sites than Sr1 sites because the magnitude of splitting energy of the Eu2+ 5d orbital is inversely proportional to that of the emission energy, most likely implying that the emission energy is mainly dependent on the Sr−O bond length. In combination with the experimental PL data and theoretically calculated emission energies, it points to the conclusion that the Eu2+ ions preferably enter the Sr1 sites (the fully oxygen-coordinated sites), and the emission energies are gradually red-shifted from F to Br in the Sr5(PO4)3X:Eu2+ (X = F, Cl, Br) compounds. It is well-known that “Pauling’s rule 3” is based on the fact that cations prefer to maximize their distance from other cations in order to minimize electrostatic repulsion. For this reason, it is presumed that the Eu2+-activator ions preferably enter the fully oxygen-coordinated sites with the higher CN = 9 (for Ca1, Ca2, Sr1, and Ba1) compared with the halogen-coordinated ones with the lower CN = 7 or 8 (CN = 7 for Ca3, Ca4, Ca5 in Ca5(PO4)3Cl and Sr2 in Sr5(PO4)3F or CN = 8 for Sr2 in Sr5(PO4)3Cl and Ba2 in Ba5(PO4)3Cl). As indicated in Tables S3 and S4, the fact that the average bond lengths (M−O) in the fully oxygen-coordinated sites are somewhat longer than those in the halogen-coordinated ones warrants that the Eu2+activator ions prefer the fully oxygen-coordinated sites to the halogen-coordinated ones. Additionally, the polyhedrons in the fully oxygen-coordinated sites are more regular than those of

The N eigenvalues of this matrix M are the eigenenergies of eq 4, and the N eigenvectors determine aj coefficients of N eigenwave functions ψ in eq 5. The basis wave functions are 1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s in the orbital filling order. Each basis function has distinct n, l, m quantum number (spin ignored), and the total number N of basis functions is 44. Due to the advances of computer technology, direct threedimensional numerical integration is now possible. Explicit ligand potential V in eq 3 as well as hydrogen-like atom eigenfunctions ϕj are substituted in eq 8 and directly calculated without modification of forms. Central charge values of Zc and Zi are determined with electronegativity considerations to match with more realistic settings and location of ions are determined by experimental data. Mathematica version 10 is used for numerical calculations. The sites to be calculated are as follows. The Ca5(PO4)3Cl compound with monoclinic phase has five distinct sites available for Eu2+ ions, which we denote as Ca1, Ca2, Ca3, Ca4, and Ca5. The Sr5(PO4)3F compound has two distinct sites available for Eu2+ ions, one surrounded by nine oxygens (Sr1) and another surrounded by one fluorine and six oxygens (Sr2). The Sr5(PO4)3Cl compound has two different sites available for Eu2+ ions, one surrounded by nine oxygens (Sr1) and another surrounded by two chlorines and six oxygens (Sr2). The Sr5(PO4)3Br compound has two distinct sites available for Eu2+ ions, one surrounded by nine oxygens (Sr1) and another surrounded by two bromines and six oxygens (Sr2). The Ba5(PO4)3Cl compound has two different sites available for Eu2+ ions, one surrounded by nine oxygens (Ba1) and another surrounded by two chlorines and six oxygens (Ba2). After matrix element calculation and diagonalization, it is found that the energy gap from the 37th state to the 36th state is most likely corresponding to the 5d → 4f transition of Eu2+ in our model calculation. Comparison with experimental data is shown below. For the M5(PO4)3Cl:Eu2+ (M = Ca, Sr, Ba) compounds (unit in cm−1): 32444.8, 27866.3, 30682.4, 27768.1, 29829 (Ca1, Ca2, Ca3, Ca4, Ca5 site) 35541.5, 7027.6 (Sr1, Sr2 site) 39902.2, 29886 (Ba1, Ba2 site)

In the PL experiment, the maximum emission peaks of M5(PO4)3Cl:Eu2+ were located at 454, 444, and 434 nm, corresponding to 22026, 22522, and 23041 (cm−1) for M = Ca, Sr, and Ba, respectively. The M5(PO4)3Cl:Eu2+ (M = Ca, Sr, J


Article

Inorganic Chemistry Notes

the halogen-coordinated ones because M−X bond lengths are greatly changed from F to Br compared with nearly constant M−O bond lengths in the fully oxygen-coordinated sites, for example, 2.391 Å for F (Sr−F bond length), 3.084 Å for Cl (Sr−Cl bond length), and 3.146 Å for Br (Sr−Br bond length) in the Sr5(PO4)3X compounds. For this reason, it is presumed that the Eu2+ ions preferably enter the fully oxygen-coordinated sites with the more regular polyhedrons. As previously discussed, the M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) compounds providing different alkaline-earth metal and halide ions have many distinct sites available for Eu2+-activator ions. Furthermore, a part of the energy absorbed by phosphor materials is lost via a nonradiative process, and the remaining energy is released via a radiative process, which is related to the emission spectra of phosphor materials. Thus, errata may be inevitable when comparing experimental PL data and theoretical calculation results because of the nonradiative process of phosphor materials. Thanks to the theoretical calculation using LCAO, however, the valuable information on the site selectivity of Eu2+-activator ions was obtained in these compounds.

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS This work was partially supported by the National Research Foundation (NRF) of Korea (Grant no. 2009-0094046).

■

CONCLUSIONS Eu2+-activated M5(PO4)3X (M = Ca, Sr, Ba; X = F, Cl, Br) compounds providing different alkaline-earth metal and halide ions were successfully synthesized and characterized. In these compounds with many distinct sites available for activator ions (Eu2+), the site selectivity and occupancy of Eu2+ ions were estimated based on the theoretical calculation using LCAO and compared with the experimental PL results. For the M5(PO4)3Cl:Eu2+ (M = Ca, Sr, Ba) compounds, the emission peak maxima were gradually blue-shifted along the series of alkaline-earth metals, Ca → Sr → Ba (454 nm for Ca, 444 nm for Sr, and 434 nm for Ba). Furthermore, the Sr5(PO4)3X:Eu2+ (X = F, Cl, Br) compounds exhibited a red-shift of the emission peak maxima along the series of halides, F → Cl → Br (437 nm for F, 444 nm for Sr, and 448 nm for Ba). Careful examination of the PL spectra and theoretical calculations revealed that the activator ions (Eu2+) more preferably enter the fully oxygencoordinated sites than the halogen-coordinated ones in the M5(PO4)3X:Eu2+ (M = Ca, Sr, Ba; X = Cl, Br, I) compounds. This trend can be well explained by “Pauling’s rule”, which is matched well with the fact that the average bond lengths (M− O) in the fully oxygen-coordinated sites (CN = 9) are somewhat longer than those in the halogen-coordinated ones (CN = 7 or 8). These compounds may provide a platform for modeling a new phosphor and application in the solid-state lighting field.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b00637. Refinement atomic positions, selected bond lengths, and bond angles for Ca5(PO4)3Cl, Sr5(PO4)3F, Sr5(PO4)3Cl, Sr5(PO4)3Br, and Ba5(PO4)3 (PDF)

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Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. K


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Eu2+-Activated Alkaline-Earth Halophosphates, M5(PO4)3X:Eu2+ (M

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