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Highly Luminous and Thermally Stable Mg-Substituted Ca2−xMgxSiO4:Ce (0 ≤ x ≤ 1) Phosphor for NUV-LEDs

Donghyeon Kim,† Daeseong Lim,‡ Hyeonjeong Ryu,‡ Jungjun Lee,‡ Sung Il Ahn,‡ Bong Soo Son,§ Seung-Joo Kim,*,§ Chang Hae 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 § Department of Energy Systems Research and Department of Chemistry, Ajou University, Suwon 16499, Republic of Korea ∥ Advanced Materials Division, Korea Research Institute of Chemical Technology (KRICT), 141, Gajeong-ro, Yuseong-gu, Daejeon 34114, Republic of Korea

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‡

S Supporting Information *

ABSTRACT: Blue-emitting Ca2−xMgxSiO4:Ce (0.0 ≤ x ≤ 1.0) phosphors were successfully synthesized and characterized. Rietveld refinement revealed that four main phases exist within the solid-solution range of CaO-MgOSiO2, namely, β-Ca2SiO4 (Mg (x) = 0.0), Ca14Mg2(SiO4)8 (Mg (x) = 0.25), Ca3Mg(SiO4)2 (Mg (x) = 0.5), and CaMgSiO4 (Mg (x) = 1.0). The variation of the IR modes was more prominent with increasing Mg2+ content in the Ca2−xMgxSiO4 materials. The sharing of O atoms of the SiO4tetrahedra by the MgO6-octahedra induced weakening of the Si−O bonds, which resulted in the red shift of the [SiO4] internal modes and appearance of a Mg−O stretching vibration at ∼418 cm−1. Raman measurement revealed that the change of the Ca−O bond lengths because of the Mg2+-substitution directly reflected the frequency shift of the Si−O stretching-Raman modes. Notably, the thermal stability of Ca2−xMgxSiO4:Ce (Mg (x) > 0.0) phosphors was superior to that of β-Ca2SiO4:Ce (Mg (x) = 0.0) as confirmed by temperature-dependent photoluminescence (PL) measurements. This indicated that Mg2+ ions play an important role in enhancement of the thermal stability. In combination with the results from PL and electroluminescence (EL), it was elucidated that the luminous efficiency of Ca2−xMgxSiO4:Ce (Mg (x) = 0.1) was approximately twice as much as β-Ca2SiO4:Ce (Mg (x) = 0.00), directly indicating a “Mg2+-substitution effect”. The large enhancements of PL, EL, and thermal stability because of Mg2+-substitution may provide a platform in the discovery of more efficient phosphors for NUV-LEDs.

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rare-earth ion (Nd3+, Pr3+, Sm3+, Eu3+, Eu2+, Ce3+, Tb3+, and Yb3+).7,9,14 Particularly, Ca2SiO4 has attracted considerable attention because of various crystal structures, such as γ (orthorhombic), β (monoclinic), αL′(orthorhombic), αH′ (orthorhombic), and α-phase (trigonal/hexagonal).16 The Ce3+ ion is a well-known activator that effectively emits from blue to yellow light in diverse host lattices, associated with the allowed 4f−5d transitions of Ce3+.5,17−19 Interestingly, the Cedoped β-Ca2SiO4 and γ-Ca2SiO4 phosphor with different crystal structures exhibit blue and yellow emission, respectively.17,20,21 It is presumed that the subtle structural difference between two phases induces modifications in the local symmetry as well as covalency of the CaOn-polyhedra, which results in variation of the PL property. Thus, the Mg2+-substituted alkaline-earth

INTRODUCTION In recent years, white light-emitting diodes (w-LEDs) have been developed by the combination of a blue-emitting InGaN LED chip and yellow-emitting Y3Al5O12:Ce (YAG:Ce) phosphor. The YAG:Ce phosphor exhibits several advantages, such as long lifetime, lower energy consumption, higher reliability, and environmentally friendly composition for LED applications.1−5 However, it exhibits a low color rendering index (CRI) of w-LEDs because of weak emission in the red spectral region.6 To overcome this disadvantage, it is necessary to develop new phosphor materials. Therefore, several new classes of phosphors such as silicate, sulfide, oxy-nitride, and nitride phosphors have been developed.7−15 Among the various newly developed materials, it is well-known that the alkalineearth orthosilicate phosphors show good UV or blue light excitation and emission for w-LEDs, expressed as M2SiO4:RE, where M is the alkaline-earth ion (Ca2+, Sr2+, and Ba2+), or RE © 2017 American Chemical Society

Received: May 9, 2017 Published: September 26, 2017 12116

DOI: 10.1021/acs.inorgchem.7b01166 Inorg. Chem. 2017, 56, 12116−12128

Article

Inorganic Chemistry

Figure 1. XRD patterns of the synthesized (a) Ca2−xCexSiO4 (0.01 ≤ x ≤ 0.05) and (b) Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 1.00) materials as a function of Ce and Mg content, respectively. phases present in a mixture were calculated from the following equation:27

orthosilicate phosphors have generated significant research interest because Mg belongs to the alkaline-earth metal group even though there is little difference in the ionic radius between Mg2+ and Ca2+, i.e., 0.89 and 1.12 Å (at coordination number (CN) = 8), respectively. Park et al. reported that the emission intensity of Mg2+-codoped Sr2−x−yBaxMgySiO4:Eu phosphor was higher than that of the Sr2−xBaxSiO4:Eu phosphor, indicating that the Mg2+ ion plays a role in the enhancement of the luminous efficiency.22 Lu et al. reported that the emission intensity of the Ca2Si2O2N:Eu phosphor was enhanced by the partial substitution of Mg2+ for Ca2+ ions in this system.23 Fukuda et al. also reported that the Mg2+-doped BaTiO3:Pr3+ phosphor exhibited strongly enhanced red emissions.24 Yu et al. reported that the emission intensity of the Mg2+-doped Ba0.84Mg0.06Al2S4:Eu0.102+ phosphor was 2.5 times higher than that of Mg2+-undoped material.25 Although the Mg2+ doping effect on the enhancement of PL property has been described, the variation of the PL and the crystal structure with the varying Mg2+ content has not been reported for the wide range of CaO−MgO−SiO2 solid-solutions, to the best of our knowledge. Herein, we report the synthesis and characterization of Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 1.00) phosphors, and discuss the variation of the PL property and crystal structures with the Mg2+-substitution rate.

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Wi =

Siρi Vi2 ∑j Sjρj V j2

(1)

where Wi(%) is the weight fraction, Si is the scale factor, Vi is the unitcell volume, ρi is the density of phase i, and subscript j includes all phases present. Fourier-transform infrared (FT-IR) spectra were obtained on a FTIR spectrophotometer (IRTracer-100, Shimadzu) in the range 400− 2000 cm−1 using a KBr medium (KBr 200 mg + sample 1 mg) with a resolution range of ±0.5 cm−1. Raman spectra were recorded at room temperature on Jobin-Yvon LabRam HR spectrometer. The samples were excited with a 532 nm Nd:YAG laser. The photoluminescent excitation and emission spectra were collected using a Fluorometer (FS-2 model, Scinco) with a 150 W xenon lamp under an operating voltage of 350 V. The temperature-dependent luminescent properties were measured on a PSI model-150 spectrophotometer equipped with a temperature controlled electric furnace. The diffuse reflectance spectra were recorded using UV−visible spectrophotometer (UV2600, Shimadzu) with BaSO4 as a reference.

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RESULTS AND DISCUSSION Phase Identification. Figure 1a shows the powder-XRD patterns of Ca2−xCexSiO4 (x = 0.00−0.05) phosphor. The XRD results confirmed the high purity of the monoclinic structure (β-form) with the space group P21/n (No. 14).17,28 All observed peaks of Ca2SiO4:Ce phosphors were found to be consistent with the JCPDS file (83-0463), and impurity phases were not detected. Thus, it is evident that the Ce3+ activator ions were well-stabilized into the Ca2SiO4 crystal lattice via partial substitution for Ca2+ ions. The XRD patterns of Ca2−xMgxSiO4:Ce (0.0 ≤ x ≤ 1.0) phosphors are presented in Figure 1b. The XRD patterns reveal that the materials have the same crystal structure as β-Ca2SiO4 when the Mg2+ content is below Mg (x) = 0.10. Furthermore, the unit-cell volumes of the single phases of Ca1.98−xMgxSiO4:Ce0.02 (x = 0.00, 0.02, and 0.05) gradually decreased with increasing Mg2+ content as presented in Figure 2, most likely because of the smaller ionic radius of Mg2+ (0.72 Å for CN = 6) in comparison with that of Ca2+ (1.12 Å for CN = 8).29 When the Mg2+ content was

EXPERIMENTAL SECTION 2+

Mg -substituted Ca2SiO4:Ce phosphors were prepared from a stoichiometric mixture of CaCO3, SiO2, CeO2, Li2CO3, and Mg(OH)2 under a reducing atmosphere (4% H2/Ar) at 1300 °C for 6 h, followed by cooling the samples to room temperature in the furnace. Li2CO3 was added to compensate for the charge imbalance caused by substitution of Ce3+ for Ca2+. Powder X-ray diffraction measurements were performed using an X-ray diffractometer equipped with a graphite monochromator (DMAX-2200PC, Rigaku). A step scan mode was employed in a 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 by the Rietveld method with the Fullprof program.26 The diffraction profiles were fitted with a pseudo-Voigt peak function and manually selected background points. The weight fractions of the 12117

DOI: 10.1021/acs.inorgchem.7b01166 Inorg. Chem. 2017, 56, 12116−12128

Article

Inorganic Chemistry

listed in Table 1. On the basis of the Rietveld refinement results, it was evident that the pristine β-Ca2SiO4 (Mg (x) = 0.0) compound exhibits a pure single phase with the space group of P21/n (No. 14). However, for the Mg (x) = 0.1 composition, the two phases of β-Ca2SiO4 and Ca14Mg2(SiO4)8 were mixed with weight percentage contributions of 69.4% and 30.6%, respectively. A single phase of Ca14Mg2(SiO4)8 was obtained for the Mg (x) = 0.25 composition, while two phases of Ca14Mg2(SiO4)8 and Ca3Mg(SiO4)2 were mixed in weight percentages of 86.9 and 13.1%, respectively, for the Mg (x) = 0.3 composition. In the case of the Mg (x) = 0.5 composition, a single phase of Ca3Mg(SiO4)2 was obtained. However, for the Mg (x) = 0.7 composition, two phases were mixed: i.e., Ca3Mg(SiO4)2 (67.5%) and CaMgSiO4 (32.5%). Finally, for the Mg (x) = 1.0 composition, two phases of Ca3Mg(SiO4)2 (9.9%) and CaMgSiO4 (90.1%) were found to be mixed with each other. On the basis of the identified phases, the ternary CaO−MgO−SiO2 phase diagram of Ca2−xMgxSiO4 (0.0 ≤ x ≤ 1.0) synthesized at 1300 °C (4% H2/Ar) is presented in Figure 4. Comparison of the Crystal Structures. The crystal structure of the Ca2−xMgxSiO4 (0.0 ≤ x ≤ 1.0) evolves into four forms, as shown in Figure 5, depending on the amount of Mgsubstitution. The description of each structure is given in the literature, and this work mainly focuses on the local structures around Si, Mg, and Ca atoms. The Si atoms have tetrahedral coordinates, and the Mg atoms have octahedral coordinates in these structures, while the coordination number of Ca atoms varies from 6 to 10 depending on the structure. The structure of β-Ca2SiO4 consists of isolated SiO4-tetrahedra and two crystallographically distinct Ca atoms coordinated by seven and eight O atoms. In the structure of Ca14Mg2(SiO4)8, the MgO6 octahedra are linked by four different SiO4-tetrahedra to form a

Figure 2. Unit-cell volume of Ca2−xMgxSiO4 depending on the Mg2+ion content.

further increased above Mg (x) = 0.10, the crystal structure of the pristine β-Ca2SiO4 transformed into the following distinct structures depending on the Mg2+ content: Ca14Mg2(SiO4)8 (JCPDS 36-0399), Ca3Mg(SiO4)2 (JCPDS 35-0591), CaMgSiO4 (JCPDS 35-0590). To precisely estimate the phase transformation of Ca2−xMgxSiO4 (0.0 ≤ x ≤ 1.0) materials, Rietveld refinements were performed on the basis of the powder XRD patterns (Figure 3). The initial structure models for the compounds were taken from the ICSD database (no. 245075 for β-Ca2SiO4, no. 9828 for Ca14Mg2(SiO4)8, no. 26002 for Ca3Mg(SiO4)2, no. 202287 for CaMgSiO4). The reliability factors and bond lengths are listed in Table S1 and Table S2, respectively. The phase compositions for the mixed phases are

Figure 3. Rietveld refinement profiles of Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 1.00) phosphor materials: (a) Mg (x) = 0.0, (b) Mg (x) = 0.1, (c) Mg (x) = 0.25, (d) Mg (x) = 0.5, (e) Mg (x) = 0.7, and (f) Mg (x) = 1.0. 12118

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Inorganic Chemistry Table 1. Phase Identification of Ca2−xMgxSiO4 (0.00 ≤ x ≤ 1.00) Materials Using Rietveld Refinement compd phase space group wt percent (%) compd

Mg (x) = 0.0

Mg (x) = 0.10

β-Ca2SiO4 P21/n (No. 14) 100 Mg (x) = 0.25

phase space group wt percent (%) compd

Ca14Mg2(SiO4)8 P2nn (No. 34) 100

phase space group wt percent (%)

Ca3Mg(SiO4)2 P21/a (No. 14) 67.5

β-Ca2SiO4 P21/n (No. 14) 69.4 Mg (x) = 0.30 Ca14Mg2(SiO4)8 P2nn (No. 34) 86.9

Ca3Mg(SiO4)2 P21/a (No. 14) 13.1

CaMgSiO4 Pmnb (No. 62) 32.5

Ca3Mg(SiO4)2 P21/a (No. 14) 9.9

Mg (x) = 0.70

Ca14Mg2(SiO4)8 P2nn (No. 34) 30.6 Mg (x) = 0.50 Ca3Mg(SiO4)2 P21/a (No. 14) 100 Mg (x) = 1.00 CaMgSiO4 Pmnb (No. 62) 90.1

Figure 5. Structural views of the four phases detected in the Ca2−xMgxSiO4 (0.00 ≤ x ≤ 1.00) system: (a) β-Ca2SiO4 (Mg (x) = 0.0), (b) Ca14Mg2(SiO4)8 (Mg (x) = 0.25), (c) Ca3Mg(SiO4)2 (Mg (x) = 0.5), (d) CaMgSiO4 (Mg (x) = 1.0). The large and small spheres represent Ca and O atoms, respectively. The polyhedral geometries of MgO6 and SiO4 are depicted by orange and blue color, respectively.

Figure 4. Ternary CaO−MgO−SiO2 phase diagram of Ca2−xMgxSiO4 (0.0 ≤ x ≤ 1.0) synthesized at 1300 °C (4% H2/Ar): (a) Mg (x) = 0.00 (Ca2SiO4), (b) Mg (x) = 0.10 (Ca2SiO4 and Ca14Mg2(SiO4)8), (c) Mg (x) = 0.25 (Ca14Mg2(SiO4)8), (d) Mg (x) = 0.30 (Ca14Mg2(SiO4)8 and Ca3Mg(SiO4)2), (e) Mg (x) = 0.50 (Ca3Mg(SiO4)2), (f) Mg (x) = 0.70 (Ca3Mg(SiO4)2 and CaMgSiO4), (g) Mg (x) = 1.0 (Ca3Mg(SiO4)2 and CaMgSiO4).

chain that runs parallel to the a-axis, and the eight distinct Ca atoms constitute the CaOn-polyhedra (n = 8 and 10). In Ca3Mg(SiO4)2, the MgO6 octahedra are linked at every oxygen corner by different SiO4-tetrahedra to form slabs parallel to the (100) plane. The two oxygen corners in each SiO4-tetrahedron are shared with the different MgO6 octahedra. The Ca atoms occupy three distinct sites with coordination numbers of 8 and 9. The structure of CaMgSiO4 consists of the MgO6 octahedra sharing edges with one another to form ribbons parallel to the (001) plane. The ribbons linked with the SiO4-tetrahedra through corner- and edge-sharing. As the Mg content in Ca2−xMgxSiO4 varies, the local structure around the Ca atom changes considerably. The coordination numbers (CNs) of Ca atoms vary from (7, 8) in β-Ca2SiO4 to (8, 10) in Ca14Mg2(SiO4)8, to (8, 9) in Ca3Mg(SiO4)2, and finally to 6 in CaMgSiO4, as shown in Figure 6. The average Ca−O bond distance corresponding to each CN also spreads over a quite different range depending on the structure. Evidently, it is very difficult to obtain a pure single phase in this range because diverse phases exist in the solid-solution range. Generally, it is well-known that the green-emitting M2SiO4:Eu2+ (M = Ca, Sr,

Figure 6. Coordination numbers and the average Ca−O bond distances for the phases with different Mg content in Ca2−xMgxSiO4 (Mg (x) = 0.0, 0.25, 0.50, 1.0).

Ba) phosphors are suitable for UV-LEDs because of their high luminous efficacy and short decay time under UV-light excitation.30 On the basis of the ionic radii of alkaline-earth metals, i.e., the ionic radii of Mg2+ (0.89 Å), Ca2+ (1.12 Å), Sr2+ 12119

DOI: 10.1021/acs.inorgchem.7b01166 Inorg. Chem. 2017, 56, 12116−12128

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Inorganic Chemistry (1.26 Å), and Ba2+ (1.42 Å) for CN = 8, it is evident that the Mg2+-substitution should be performed in the Ca2SiO4 host lattice better than Sr2SiO4 (or Ba2SiO4) because the difference in the ionic radius between Mg2+ and Ca2+ is smaller than that between Mg2+ and Sr2+ (or Ba2+). Thus, it is interesting to elucidate the optical properties as well as the crystal structures in the CaO−MgO−SiO2 solid-solution range, particularly for the Ca2−xMgxSiO4:Ce (0.0 ≤ x ≤ 1.0) phosphor materials. Spectroscopic Characterization: IR Spectra. Figure 7 shows IR modes of the Ca2−xMgxSiO4 materials. The

O−Si−O bending modes are somewhat modified, most likely because of the change of local symmetry of the SiO4-matrix. There are four modes that are observed in the CaMgSiO4 (Mg (x) = 1.0 composition) sample, consistent with the previously reported results:38−41 590 cm−1 (Si−O bending), 484 cm−1 (Si−O bending), 435 cm−1 (Si−O bending and Mg−O stretching), 418 cm−1 (Mg−O stretching). Notably, the IR bending modes between 565 and 595 cm−1 are progressively shifted to the higher frequencies with increasing Mg2+substitution rate. This trend could be explained by the difference in the linkage patterns of the SiO4-tetrahedra and MgO6-octahedra in the four phases. As shown in Figure 5, the Ca14Mg2(SiO4)8 (Mg (x) = 0.25) compound contains a chain structure formed from the linkage of SiO4 and MgO6-groups, while the Ca3Mg(SiO4)2 (Mg (x) = 0.50) compound includes the 2-dimensional slabs built from the double-linkage of SiO4and MgO6-groups. Furthermore, the CaMgSiO4 (Mg (x) = 1.0) compound consists of the SiO4−MgO6 slabs more tightly connected through corner- and edge-sharing. Thus, the observation that the IR modes between 565 and 595 cm−1 are shifted to higher frequencies, and become more intense with increasing Mg2+-substitution rate, reveals that this mode may be mainly assigned to O−Si−O bending and partly to the vibration of the MgO6 octahedra (the coupling of the O−Si−O bending and MgO6 lattice vibration).39,41 It is also worth mentioning that, in the olivine mineral Mg2SiO4, the IR mode at approximately 605 cm−1 is considered as a characteristic of the olivine phase.39,41 Spectroscopic Characterization: Raman Spectra. The Raman spectra of the Ca2−xMgxSiO4 samples are presented in Figure 8. The intense Raman modes of β-Ca2SiO4 (Mg (x) = 0

Figure 7. FT-IR spectra of Ca2−xMgxSiO4 (0.00 ≤ x ≤ 1.00): (a) Mg (x) = 0.00, (b) Mg (x) = 0.25, (c) Mg (x) = 0.50, (d) Mg (x) = 0.70, (e) Mg (x) = 1.0.

absorption bands around 1400 cm−1 in all materials were found at exactly the same wavenumber (at 1384.9 cm−1), which is associated with the C−O antisymmetric stretching of the CO32− anion adsorbed on the surface.31−33 Thus, we assigned the absorption peak at 1384.9 cm−1 to an internal standard to assess the wavenumber of the IR modes in the samples. As the chemical bond between Si and O is stronger than that between Ca (or Mg) and O, the internal vibrations of SiO4-tetrahedra are approximately independent from the lattice vibrations and prominent in the region 400−1000 cm−1. β-Ca2SiO4 shows the [SiO4] internal modes associated with the Si−O stretching modes34−36 between 800 and 1100 cm−1 and the O−Si−O bending modes37,38 between 400 and 600 cm−1. When the x value increases in the range from Mg (x) = 0 to 0.5, the stretching modes shift to higher frequencies, i.e., 844.8−995.3 cm−1 (Mg (x) = 0.0), 850.6−1014.6 cm−1 (Mg (x) = 0.25), 885.3−1020.3 cm−1 (Mg (x) = 0.5), and then, however, shift back to lower frequencies in the range Mg (x) = 0.5−1.0, i.e., 829.4−1012.6 cm−1 (Mg (x) = 0.7), and 827.5−996.3 cm−1 (Mg (x) = 1.0). The variation of the O−Si−O bending modes depending on the Mg2+-substitution rate (x) is somewhat different from that of the Si−O stretching modes. The O−Si− O bending modes of β-Ca2SiO4 are characterized by three peaks (538.1, 520.8, and 510.3 cm−1) superimposed within a narrow range, indicating that the SiO4-tetrahedra have a regular environment because the Si atom is occupied only one crystallographic site and the SiO4-tetrahedra are surrounded only by the Ca2+ ions. With increasing Mg2+-substitution, the

Figure 8. Raman spectra of Ca2−xMgxSiO4 (0.00 ≤ x ≤ 1.00): (a) Mg (x) = 0.00, (b) Mg (x) = 0.25, (c) Mg (x) = 0.50, (d) Mg (x) = 0.70, (e) Mg (x) = 1.00.

composition) at 844, 858, and 977 cm−1 are assigned to the stretching modes of the SiO4-groups.42,43 As the Mg2+ content increases from 0.0 to 0.25 and 0.5, the Raman modes slightly shift to higher frequencies. The observed Raman modes at 818−950 cm−1 for CaMgSiO4 (Mg (x) = 1.0), which are consistent with the previously reported values,43,44 however, are of lower frequencies in comparison with the modes observed at 869−1008 cm−1 for Ca3Mg(SiO4)2 (Mg (x) = 0.5). The Raman 12120

DOI: 10.1021/acs.inorgchem.7b01166 Inorg. Chem. 2017, 56, 12116−12128

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Inorganic Chemistry Table 2. Frequencies of IR and Raman Modes in Ca2−xMgxSiO4 (0.00 ≤ x ≤ 1.00) Compounds Si−O stretching (cm−1) compd β-Ca2SiO4 (Mg (x) = 0.00) Ca14Mg2(SiO4)8 (Mg (x) = 0.25) Ca3Mg(SiO4)2 (Mg (x) = 0.50) Ca3Mg(SiO4)2 (67.5 wt %) and CaMgSiO4 (32.5 wt %) (Mg (x) = 0.70) Ca3Mg(SiO4)2 (9.9 wt %) and CaMgSiO4 (90.1 wt %) (Mg (x) = 1.00) a

IR

Raman

Si−O bending (cm−1) IR

Raman

995, 899, 845 1015, 982, 920, 890, 851 1020, 939, 885

977, 896, 858, 844 989, 890, 880, 870, 854

538, 521, 510 569, 517, 419a

553, 538, 515 567, 524

1008, 918, 908, 884, 869

576, 541, 528

1013, 964, 939, 885, 829 996, 950, 885, 828

1008, 950, 918, 908, 884, 869, 851, 818 950, 899, 851, 818

590, 536, 515 435,b 419a 592, 517, 435,b 419a 594, 519, 435,b 419a

575, 537, 528, 513 587, 535, 510

Mg−O stretching. bSi−O bending + Mg−O stretching.

spectra for the material with Mg (x) = 0.7 consist of the modes of the mixed phases, Ca3Mg(SiO4)2 and CaMgSiO4 (67.7 and 32.5 wt %, respectively). The frequency variation of the Raman modes is quite similar to that of the Si−O stretching modes in IR spectra. The frequencies of IR and Raman modes are summarized in Table 2. The variation of stretching modes in the IR and Raman spectra can be explained by the structural transformation induced by Mg2+-substitution. In the orthosilicate structure, an O atom that bonds with a Si atom also bonds with other Me (Me = Ca or Mg) atoms, represented as Si4+··· O···Me2+. Hence, the covalency of the Si−O bond is influenced by the nature of the Me−O bonds; the weaker the Me−O bond is, the greater the strength of the Si−O bond is. From the CN of Ca atoms and the bond lengths listed in Table S2 and Figure 6, the average Ca−O bond lengths can be compared: 2.504, 2.652, 2.582, and 2.362 Å for β-Ca2SiO4 (Mg (x) = 0.0), Ca14Mg2(SiO4)8 (Mg (x) = 0.25), Ca3Mg(SiO4)2 (Mg (x) = 0.5), and CaMgSiO4 (Mg (x) = 1.0), respectively. The relatively long and weak Ca−O bonds in the compositions of Mg (x) = 0.25 and Mg (x) = 0.5 likely strengthen the adjacent Si−O bonds, which results in the shift of the [SiO4] internal modes to higher frequencies. On the other hand, the short and strong Ca−O bond with CN = 6 in CaMgSiO4 (Mg (x) = 1.0) weakens the adjacent Si−O bonds. The different electronegativities for Ca (1.00) and Mg (1.31) may also affect the covalency of Si−O bonds. As the electronegativity of the Me atom increases, the Me−O bond becomes less covalent, which makes the Si−O stretching modes shift to higher frequencies. The lowest frequencies of the IR and Raman stretching modes observed for CaMgSiO4 can be understood as a result due to both effects involving the electron induction and the geometric modifications due to the large amount of Mg2+-substitution. Photoluminescent Properties of the Phosphors: At Low Mg2+-Substitution rates. Figure 9 presents the PL spectra of Ce3+-doped β-Ca2SiO4 phosphors as a function of the Ce 3+ concentration. The excitation spectra of βCa2SiO4:Ce3+ phosphors monitored at 427 nm exhibit the broad bands between 240 and 400 nm, which are associated with the 4f−5d transition of Ce3+. The emission spectra monitored under excitation at 360 nm exhibit the broad bands centered at 427 nm, which can be attributed to the allowed 4f05d1−4f15d0 transition of Ce3+.45−47 To examine the emission spectra in more detail, the emission band of the Ca2−xCexSiO4 (Mg (x) = 0.01) phosphor was deconvoluted by Gaussian fitting, as indicated in the inset of Figure 9. From the Gaussian fitting, it was found that the emission band centered at 427 nm consists of two Gaussian bands: one centered at 415 nm (24 096 cm−1) and the other at 445 nm (22 471 cm−1), which indicates that there are two distinct Ca-sites available for Ce3+-

Figure 9. Excitation and emission spectra of Ca2−xCexSiO4 phosphors as a function of Ce content.

activator ions. This explanation of the emission band in the Ca 2−x Ce x SiO 4 (Ce (x) = 0.01) phosphor is further substantiated when the emission band is compared with the structural characteristics of the compound. In β-Ca2SiO4, there are two distinct Ca sites (Ca1 and Ca2) coordinated by seven and eight oxygen atoms of the isolated SiO4-tetrahedra, respectively. Furthermore, the average Ca−O bond distances of Ca1 and Ca2 site are determined from Rietveld refinements to be 2.514 and 2.495 Å, respectively. The correlation between the crystal field splitting and the shape and size of the polyhedron is given by the following equation:48,49 Dq = 3Ze 2r 4 /5R5

(2)

where Dq, Z, e, r, and R represent the crystal field strength, valence of the anionic ligand, charge, radius of the frontier d wave function, and bond length between the central metal ion and ligands, respectively. Assuming that Z, e, and r are constant in eq 2, it can be concluded that Dq is approximately proportional to 1/R5. Thus, if R has a small value, Dq is large and the emission band is red-shifted. According to eq 2 and the emission spectrum (the inset of Figure 9), it is evident that the Ce3+-activator ions preferably enter the Ca2 site (average Ca− O bond length = 2.495 Å) and the energy splitting of 5d orbitals of Ce3+ ions is larger in this site compared with that in the Ca1 site (average Ca−O bond length = 2.514 Å). Therefore, the Gaussian bands centered at 415 and 445 nm can be assigned to the Ca1 and Ca2 sites, respectively. Additionally, the optimal Ce3+ concentration is estimated to be 12121

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Inorganic Chemistry Ce = 0.01, as shown in Figure 9. To estimate the effect of Mg2+substitution on the PL properties, Ca2−xMgxSiO4:Ce (Mg (x) = 0.00, 0.02, 0.05, and 0.10) phosphors were synthesized and characterized. Figure 10 presents the excitation and emission

absorption bands can be observed around 320 nm. From a comparison of spectrum c with spectrum d in Figure 11, it is found that the strong absorption band between 350 and 450 nm is newly observed in the Ca1.88Mg0.10SiO4:Ce0.02 phosphor (d), which is consistent with the intensified excitation spectrum of the phosphor (see Figure 10), most likely due to the Mg2+substitution effect. Photoluminescent Properties of the Phosphors: High Mg2+-Substitution Rates. The remarkable PL enhancement via Mg2+-substitution warrants further examination for the phosphors with higher Mg 2+ -substitution rates. Thus, Ca2−xMgxSiO4:Ce0.02 (Mg (x) = 0.30, 0.50, 0.70, and 1.00) phosphors were synthesized and characterized. Figure 12 shows

Figure 10. Excitation and emission spectra of Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 0.10) phosphors as a function of Mg content.

spectra of these materials. As shown in the inset of Figure 10, it was found that Mg2+-substitution has no effect on the PL properties of these materials (at Ce (x) = 0.01). However, with increasing the Ce3+ content up to 0.02, highly intensified emission spectra were observed by Mg2+-substitution in these materials: the spectrum at Mg (x) = 0.10 was approximately twice as intense as that at Mg (x) = 0.00. Figure 11 presents the diffuse reflectance spectra of (a) β-Ca 2 SiO 4 , (b) βCa2SiO4:Ce0.02, (c) Ca2−xMgxSiO4 (Mg (x) = 0.10), and (d) Ca2−xMgxSiO4:Ce0.02 (Mg (x) = 0.10). The host lattice of βCa2SiO4 exhibits a strong absorption around 250 nm, probably associated with a nonradiative process. When the Ce3+-activator ions are introduced into the β-Ca2SiO4 host lattice, the broad

Figure 12. Excitation and emission spectra of Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 1.00) phosphors as a function of Mg content.

the excitation and emission spectra of these phosphors. It was found that these phosphors did not display particularly higher luminosity relative to the Mg (x) = 0.10 composition, although the Mg2+-substitution effect on the enhancement of the emission intensity was prominent: the emission intensity of the material with Mg (x) = 0.05 was approximately 1.8 times higher than that of β-Ca2SiO4 (Mg (x) = 0.00), and 1.6, 1.7, 1.7, and 1.3 times higher for Mg (x) = 0.25, 0.30, 0.50, and 0.70, respectively. However, the maximum emission wavelengths of these phosphors dramatically changed depending on the Mg2+-substitution rate, 427 nm (Mg (x) = 0.10), 397 nm (Mg (x) = 0.25), 394 nm (Mg (x) = 0.30), 375 nm (Mg (x) = 0.50), 380 nm (Mg (x) = 0.70), and 419 nm (Mg (x) = 1.00), most likely due to the distortion of the CaOn-polyhedra via the introduction of the MgO6 octahedra. It is noteworthy that the Ce3+ activator ions preferably enter the Ca sites because of the “size effect”; i.e., the ionic radius of the Mg2+ ion (0.72 Å for CN = 6) is smaller than that of both Ca2+ (1.12 Å for CN = 8) and Ce3+ (1.14 Å for CN = 8), and the latter two ions are quite similar. This proposition is further substantiated when we compare the maximum emission wavelength with the average Ca−O bond distance determined from the Rietveld measurements. As presented in Table S2, the average Ca−O bond distances of the four main phases changed as a function of the Mg2+ content in the Ca2−xMgxSiO4 (0.0 ≤ x ≤ 1.0) composition: 2.504, 2.652, 2.582, and 2.362 Å for β-Ca2SiO4 (Mg (x) = 0.00), Ca14Mg2(SiO4) 8 (Mg (x) = 0.25), Ca3Mg(SiO4)2 (Mg (x) = 0.50), and CaMgSiO4 (Mg (x) = 1.00), respectively. The magnitude of splitting of the d orbitals of the Ce3+-activator ion by ligands (O2− ions) is approximately

Figure 11. Diffuse reflectance spectra of (a) β-Ca2SiO4, (b) βCa2SiO4:Ce0.02, (c) Ca2−xMgxSiO4 (Mg (x) = 0.10), and (d) Ca2−xMgxSiO4:Ce0.02 (Mg (x) = 0.10). 12122

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Inorganic Chemistry proportional to 1/R5, where R represents the bond length between the central metal ion and ligands (see eq 2). On the basis of the relationship between the crystal field splitting and the bond length of the polyhedra, the blue and red shifts of the maximum emission wavelengths can be easily explained. For the β-Ca2SiO4:Ce phosphor (Mg (x) = 0.00, average Ca−O bond length = 2.504 Å), the maximum emission wavelength is observed at 427 nm. With increasing Mg2+ content up to 0.3, the mixed phases of Ca14Mg2(SiO4)8 (86.9 wt %) and Ca3Mg(SiO4)2 (13.1 wt %) with the average Ca−O bond lengths of 2.652 and 2.582 Å, respectively, were formed. The fact that the average Ca−O bond length for the material with Mg (x) = 0.3 is slightly longer than that for the material with x = 0.0 directly reflects the smaller crystal field splitting of the Ce3+ d orbitals in the former compared with that in the latter, which explains the blue shift of the maximum emission wavelength with increasing Mg2+ content from 0.0 to 0.3. With a further increase of the Mg2+ content up to 0.5, the single phase of Ca3Mg(SiO4)2 (for Mg (x) = 0.5) with the average Ca−O bond length 2.582 Å and the maximum emission wavelength at 375 nm was obtained. At the Mg (x) = 0.7 composition, the mixed phases of Ca3Mg(SiO4)2 (67.5 wt %) and CaMgSiO4 (32.5 wt %) were obtained with the average Ca−O bond lengths of 2.582 and 2.362 Å, respectively. The maximum emission wavelength at Mg (x) = 0.7 (380 nm) is slightly red-shifted relative to that at Mg (x) = 0.5 (375 nm) because of the CaMgSiO4 (32.5 wt %) phase formed at Mg (x) = 0.7 composition with the average Ca−O bond length of 2.362 Å. Finally, mixed phases were obtained at Mg (x) = 1.0, i.e., Ca3Mg(SiO4)2 (9.9 wt %) and CaMgSiO4 (90.1 wt %) with the average Ca−O bond lengths of 2.582 and 2.362 Å, respectively. Thus, it is evident that the maximum emission wavelength at Mg (x) = 1.0 (419 nm) is red-shifted relative to that at Mg (x) = 0.7 (380 nm), primarily depending on the Ca−O bond length in the CaOn-polyhedra. Remarkably, the average Ca−O bond length of Ca14Mg2(SiO4)8 (2.652 Å for Mg (x) = 0.25) is slightly larger than that of Ca3Mg(SiO4)2 (2.582 Å for Mg (x) = 0.50), but its emission band maximum is red-shifted relative to the Ca3Mg(SiO4)2 (Mg (x) = 0.50). This observation is quite the opposite of the trend based on eq 2. The discrepancy between the two materials could be explained by the siteselectivity of the Ce3+-activator ion. The selected bond length data (see Table S2) reveal that the Ca14Mg2(SiO4)8 phase consists of eight distinct Ca2+ sites with 8- and 10-coordination possibilities available for the Ce3+-activator ions. Therefore, it is necessary to correlate the emission band wavelength with the average Ca−O bond length of the CaOn-polyhedra because the emission band wavelength of the Ca14Mg2(SiO4)8 phase reflects the magnitude of the crystal field splitting depending on the site-selectivity of the Ce3+ activator ions. The estimated average Ca−O bond lengths of the eight distinct CaOn-polyhedra are 2.721 (Ca1), 2.731 (Ca2), 2.531 (Ca3), 2.491 (Ca4), 2.751 (Ca5), 2.661 (Ca6), 2.691 (Ca7), and 2.771 Å (Ca8). Among them, it is presumed that the Ce3+-activator ions favorably enter the Ca3O8 and Ca4O8 site with the average Ca−O bond length of 2.531 and 2.491 Å, respectively. This explanation warrants the emission band maximum at approximately 397 nm. As shown in Figure 13, the Commission International de I’Eclairage (CIE) coordinates of Ca2−xMgxSiO4:Ce reveal the color tunability of these phosphors. It should be noted that the Ce3+-activator ions are very sensitive to the crystal field in the Ca2+-sites of the CaOn-polyhedra. Evidently, the Mg2+ ions substituted for Ca2+ ions induce modifications in the local

Figure 13. CIE chromaticity of Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 1.00) phosphors monitored under 365 nm UV-light: (a) Mg (x) = 0.05, (b) Mg (x) = 0.10, (c) Mg (x) = 0.30, (d) Mg (x) = 0.50, (e) Mg (x) = 0.70, (f) Mg (x) = 1.00.

symmetry as well as covalency of the CaOn-polyhedra, which results in the variation of the luminescent properties in these phosphors. Temperature-Dependent Emission Spectra. Temperature-dependent emission spectra of Ca2−xMgxSiO4:Ce (Mg (x) = 0.00, 0.05, 0.10) phosphors under 365 nm excitation were measured between room temperature and 200 °C, and are presented in Figure 14. Insets a−c of Figure 14 show the variations of the emission intensity during the heating/cooling cycle. The emission intensities return nearly to the same value after the cooling process. For the Mg (x) = 0 phosphor, the emission intensities are gradually reduced with increasing temperature, as previously reported.8,50 Finally, the emission intensity at 200 °C corresponds to approximately 78% relative to that at room temperature. Remarkably, the Mg2+-substituted Ca2−xMgxSiO4 phosphor is thermally more stable and maintains a high temperature emission intensity corresponding to 86% (Mg (x) = 0.05) and 88% (Mg (x) = 0.10) of the emission intensity at room temperature. A possible factor that alters thermal stability is the structural strain induced by the Mg2+substitution because the ionic radius of Mg2+ (0.89 Å) is smaller than that of Ca2+ ion (1.12 Å). The strain can be estimated with the Williamson−Hall (W−H) method based on XRD profile analysis.51 In this method, the peak broadening is assumed to be caused by small crystallite size and the lattice strain due to crystal deformation. The total peak width at half-maximum intensity, βhkl, is the sum of the size broadening, βD, and the strain broadening, βs: βhkl = βD + βs

(3)

The size broadening is related by the Scherrer equation: βD = kλ/(D cos θ), where D is the crystallite size, λ is the wavelength of the X-rays, and k is the shape factor (k ≈ 0.9 for a cubic crystal). The strain broadening is expressed by ε = 4βs/tan θ, where ε is either maximum tensile or compressive strain. Using these relations, we get the W−H equation: βhkl cos θ /λ = k /D + 4ε sin θ /λ

(4)

The plot βhkl cos θ/λ against sin θ can give crystallite size that corresponds to the y-intercept and strain due to lattice deformation that can be calculated from the slope of graph. 12123

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Figure 14. Temperature-dependent emission spectra of Ca2−xMgxSiO4:Ce (Mg (x) = 0.00, 0.05, 0.10): (a) Mg (x) = 0.00, (b) Mg (x) = 0.05, (c) Mg (x) = 0.10, and (d) variation of emission intensities of the three phosphors. The emission intensity of Ca2−xMgxSiO4:Ce at room temperature is adjusted to 100%. The insets of parts a−c indicate heating/cooling cycles of emission spectra.

induced tensile stress by the compressive stresses due to the substitution of small Mg2+ ions for large Ca2+ ions. Comparison with a Commercialized Phosphor and Electroluminescence. The luminescence behavior of the Ca1.88Mg0.10SiO4:Ce0.02 phosphor was compared with that of a commercial Y2SiO5:Ce phosphor obtained from the Phosphor Technology Co., Ltd., England, as shown in Figure 16. The excitation spectra of the two phosphors are very similar to each

The W−H plots show that the stain (4ε) increases from −0.015% for β-Ca2SiO4 (Mg (x) = 0.0) to −0.035% for Ca2−xMgxSiO4 (Mg (x) = 0.1) (Figure 15). The negative values

Figure 15. Williamson−Hall plots of Ca2−xMgxSiO4 (Mg (x) = 0.0, 0.1).

indicate that the strain is compressive. As the temperature increases, generally, a solid lattice expands and undergoes the tensile stress. Under the tensile stress, the lattice vibrations and/or the formation of defects occur more easily to promote nonradiative relaxation in phosphors.52−54 The enhanced PL intensity and the thermal stability of Ca2−xMgxSiO4 (Mg (x) = 0.1) may be attributed to the compensation of the thermally

Figure 16. Excitation and emission spectra of Ca2−xMgxSiO4:Ce0.02 (Mg (x) = 0.10) and a commercial Y2SiO5:Ce phosphor (obtained from Phosphor Technology Co., Ltd., England). 12124

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the Mg2+-substitution effect may be explained in two different ways. The first explanation is that the distinct Ca sites available for Ce3+-activator ions are formed depending on the Mg2+substitution rate: two distinct Ca sites for β-Ca2SiO4 (Mg (x) = 0.0), eight distinct Ca sites for Ca14Mg2(SiO4)8 (Mg (x) = 0.25), three distinct Ca sites for Ca3Mg(SiO4)2 (Mg (x) = 0.5), and one Ca site for CaMgSiO4 (Mg (x) = 1.0). It should be mentioned that, for the CaMgSiO4 (Mg (x) = 1.0) structure, there is only one Ca site, involving the more symmetric CaO6octahedra. On the contrary, in Ca14Mg2(SiO4)8 (Mg (x) = 0.25) and Ca3Mg(SiO4)2 (Mg (x) = 0.5) phases, the MgO6octahedra are partially connected with CaOn-polyhedra and SiO4-tetrahedra, sharing their O atoms, leading to the more severely distorted CaOn-polyhedra. The irregularities of CaOnpolyhedra can be confirmed by the widespread distributions of the Ca−O bond distances in the Mg (x) = 0.25 and Mg (x) = 0.5 phases (see Figure 4). The Ce3+-activators stabilized in the CaOn-polyhedra are affected by ligand fields (by O2− ions), and the lowering of local symmetry may force the total eigenfunction of the activator to be distorted, which results in the increased dipole moment. Thus, the enhanced PL intensity after Mg2+-substitution may be explained by the following eq 5:55

other except for the difference in their excitation intensity. Furthermore, the emission spectra monitored under excitation at 360 nm show somewhat different shape and intensity, which can be attributed to the different crystal symmetry and arrangement of atoms. The emission intensity of the Ca1.88Mg0.10SiO4:Ce0.02 phosphor is approximately 1.5 times higher than that of the commercial Y2SiO5:Ce phosphor. Thus, it is evident that the Ca1.88Mg0.10SiO4:Ce0.02 phosphor material is a good candidate as a blue-emitting NUV-LED. The Commission International de I’Eclairage (CIE) coordinate of Ca1.88Mg0.10SiO4:Ce0.02 and commercial Y2SiO5:Ce under UVlight at 365 nm are x = 0.152, y = 0.045 and x = 0.159, y = 0.125, respectively, as shown in Figure 17. According to CIE

Im → n ∝ |Mm → n|2

(Em > En)

(5)

where Im→n and Mm→n are the emission intensity and dipole moment, respectively. It should be mentioned that this explanation warrants further study on direct evidence of the enhanced emission intensity in phosphor materials after Mg2+substitution, for example, the theoretical calculation of the electric dipole transition probability by LCAO, which is beyond the scope of the present study. The second factor, which affects the PL property mainly for low Mg2+-substitution rate (0.0 ≤ x ≤ 0.1), is the strain-effect induced by the substitution of ions with different sizes. Williamson−Hall plots based on the XRD profile indicate that the strain increases from −0.015% for βCa2SiO4 (Mg (x) = 0.0) to −0.035% for Ca2−xMgxSiO4 (Mg (x) = 0.1) (see Figure 15). The negative values indicate that the strain is compressive. As the temperature increases, generally, a solid lattice expands and undergoes tensile stress. Under the tensile stress, the lattice vibrations and/or the formation of defects occur more easily to promote nonradiative relaxation in phosphors.52−54 The enhanced PL intensity and the thermal stability of Ca2−xMgxSiO4 (x = 0.1) may be attributed to the compensation of the thermally induced tensile stress by the compressive stresses due to the substitution of small Mg2+ ions for large Ca2+ ions. Additionally, the origin of the color tunability in the Ca2−xMgxSiO4:Ce (0.0 ≤ x ≤ 1.0) phosphors was previously discussed.

Figure 17. CIE chromaticity of Ca2−xMgxSiO4:Ce0.02 (Mg (x) = 0.10) and a commercial Y2SiO5:Ce phosphor (obtained from Phosphor Technology Co., Ltd., England).

coordinates, it is evident that the Ca1.88Mg0.10SiO4:Ce0.02 material emits still deeper blue light than a commercial Y2SiO5:Ce phosphor. To estimate the potential of phosphor materials for NUV-LEDs, the phosphor powder−silica disc assembly was loaded onto the 365 nm emitting InGaN LED caps. The electroluminescence (EL) spectra were obtained under forward bias currents from 10 to 50 mA. Figure 18 shows the EL spectra of Ca2−xMgxSiO4:Ce (x = 0.00, 0.05, 0.10) and a commercially available Y2SiO5:Ce phosphor. UV-light of ∼365 nm emitted from the InGaN LED was absorbed by the phosphors and down-converted into the intensive wide bandemitting light. As the luminous output has a similar rising tendency and the position and shape of the EL bands do not change considerably, it is evident that the phosphor materials exhibit a stable EL property. Furthermore, when the PL spectra (in Figure 10) are compared with the EL spectra (in Figure 18), it can be seen that PL is not equivalent to EL. The discrepancy between PL and EL originates from the two distinct techniques related to the different excitation principles and instruments: PL can be attributed to light excitation and EL to electronic excitation.8 Remarkably, the EL intensity of Ca2−xMgxSiO4:Ce (Mg (x) = 0.10) is approximately 3 times higher than that of βCa2SiO4:Ce (Mg (x) = 0.00) and 2.5 times higher relative to that of a commercial Y2SiO5:Ce phosphor, most likely due to the “Mg2+-substitution effect. Feasible Factors for the Enhanced PL Intensity and Thermal Stability via Mg2+-Substitution. Regarding the origin of the enhanced PL intensity as well as the color tunability in the Ca2−xMgxSiO4:Ce (0.0 ≤ x ≤ 1.0) phosphors,

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CONCLUSIONS In this work, blue-emitting Ca2−xMgxSiO4:Ce (0.00 ≤ x ≤ 0.10) phosphors were successfully synthesized and characterized. Rietveld refinements revealed that four main phases existed in the solid-solution range of CaO−MgO−SiO2, namely, β-Ca2SiO4 (Mg = 0.0), Ca14Mg2(SiO4)8 (Mg (x) = 0.25), Ca3Mg(SiO4)2 (Mg (x) = 0.5), and CaMgSiO4 (Mg (x) = 1.0). The introduction of MgO6-octahedra with sharing of the O atoms of the SiO4-tetrahedra changed the covalency and local symmetry of the CaOn-polyhedra compared to the pristine β-Ca2SiO4:Ce (Mg (x) = 0.0) phosphor. From the IRspectroscopic measurements, it was found that the Si−O 12125

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Figure 18. EL spectra of Ca2−xMgxSiO4:Ce (Mg (x) = 0.00, 0.05, 0.10) and a commercialized Y2SiO5:Ce phosphor (obtained from Phosphor Technology Co., Ltd., England): (a) Mg (x) = 0.10, (b) Mg (x) = 0.05, (c) Mg (x) = 0.00, (d) Y2SiO5:Ce. The emission lines in the inset to part a correspond to UV-light (365 nm) of bare LED chip.

responsible for the large enhancement of PL, EL, and thermal stability and may provide a platform for the discovery of new and more efficient phosphors for solid-state lighting devices.

stretching and bending vibration modes were considerably modified with increasing the Mg2+-substitution rate in the composition: (i) For the Si−O stretching vibrations, the absorption maxima were slightly red-shifted. (ii) For the Si−O bending vibrations, the absorption bands were clearly separated, and a new mode corresponding to Mg−O stretching was observed at 418 cm−1, arising most likely because of the bonding structure of Ca2+···[O−Si−O]44−···Mg2+. Raman measurements revealed that the change of the Ca−O bond lengths in the CaOn-polyhedra directly reflects the modification of the bond strength of the SiO4-groups, which eventually resulted in the frequency shift of the Si−O Raman stretching frequencies. Furthermore, the variation of the PL properties was more prominent with increasing Mg2+ content in the Ca2−xMgxSiO4:Ce phosphor materials. A highly luminous phosphor material was obtained at the Mg (x) = 0.1 composition, with an emission band that is approximately twice as intense relative to that of the β-Ca2SiO4:Ce phosphor. The color tunability was found to be dependent on the Mg2+substitution rate; the maximum emission intensity changed with increasing Mg2+ content, 427 nm (Mg (x) = 0.00) → 427 nm (Mg (x) = 0.10) → 397 nm (Mg (x) = 0.25) → 394 nm (Mg (x) = 0.30) → 375 nm (Mg (x) = 0.50) → 380 nm (Mg (x) = 0.70) → 419 nm (Mg (x) = 1.00). The trend of the color tunability could be well explained by the correlation between the crystal field splitting and the geometry of the CaOnpolyhedra as expressed in eq 2. From the temperaturedependent photoluminescence (PL) measurements, it was elucidated that the thermal stability of the Ca2−xMgxSiO4:Ce (Mg (x) > 0.00) phosphors was superior to that of βCa2SiO4:Ce (Mg (x) = 0.00). This was direct evidence of an “Mg2+-substitution effect”. The “Mg2+-substitution effect” is

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01166. Reliability factors, lattice parameters, and selected bond lengths of Ca2−xMgxSiO4 (0.00 ≤ x ≤ 1.00) materials using Rietveld refinement (PDF) Accession Codes

CCDC 1551978−1551979 and 1552197−1552200 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (S.-J. Kim). *E-mail: [email protected] (J.-C. Park). ORCID

Jung-Chul Park: 0000-0002-0573-018X Notes

The authors declare no competing financial interest. 12126

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ACKNOWLEDGMENTS This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (Grant 2017R1D1A1B03034550). One of the authors, S.-J. Kim, acknowledges that this work was partially supported by C1 Gas Refinery Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (2015M3D3A1A01064899).

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DOI: 10.1021/acs.inorgchem.7b01166 Inorg. Chem. 2017, 56, 12116−12128