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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Bluish-White Luminescence in Rare-Earth-Free Vanadate Garnet Phosphors: Structural Characterization of LiCa3MV3O12 (M = Zn and Mg) Takuya Hasegawa,*,†,‡ Yusuke Abe,§ Atsuya Koizumi,§ Tadaharu Ueda,†,‡ Kenji Toda,§ and Mineo Sato∥ †

Department of Marine Resources Science, Faculty of Agriculture and Marine Science, Kochi University, Nankoku 783-8502, Japan Center for Advanced Marine Core Research, Kochi University, Nankoku 783-8502, Japan § Graduate School of Science and Technology, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan ∥ Department of Chemistry and Chemical Engineering, Niigata University, 8050 Ikarashi 2-nocho, Niigata 950-2181, Japan ‡

S Supporting Information *

ABSTRACT: Extensive attention has been focused toward studies on inexpensive and rare-earth-free garnet-structure vanadate phosphors, which do not have a low optical absorption due to the luminescence color being easily controlled by its high composition flexibility. However, bluish emission phosphors with a high quantum efficiency have not been found until now. In this study, we successfully discovered bluish-white emitting, garnet structure-based LiCa3MV3O12 (M = Zn and Mg) phosphors with a high quantum efficiency, and the detailed crystal structure was refined by the Rietveld analysis technique. These phosphors exhibit a broad-band emission spectra peak at 481 nm under near UV-light excitation at 341 nm, indicating no clear difference in the emission and excitation spectra. A very compact tetrahedral [VO4] unit is observed in the LiCa3MV3O12 (M = Zn and Mg) phosphors, which is not seen in other conventional garnet compounds, and generates a bluish-white emission. In addition, these phosphors exhibit high quantum efficiencies of 40.1% (M = Zn) and 44.0% (M = Mg), respectively. Therefore, these vanadate garnet phosphors can provide a new blue color source for LED devices.

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catalysis,20,21 electrochemistry,22−25 and biochemistry.26,27 For luminescent materials, vanadate compounds have attracted attention as self-activated phosphors and rare earth doping hosts.28−32 Vanadate phosphors are well-known to exhibit a highly efficient absorption of UV light and have intense emissions in the visible light region due to a charge transfer (CT) transition between V5+−O2− in the [VO4] tetrahedron with a Td symmetry.33 In recent years, various garnet structurebased vanadate phosphors have been surveyed as new rare earthfree phosphor materials.34−36 The garnet structure can accommodate a variety of chemical compositions, AIAII2M2V3O12, AI2REM2V3O12, AII5M4V6O24 (AI = alkali metals and Ag; AII = alkali earth metals; M = Mg and Zn; RE = Sc, Y, and lanthanides), and others,37−40 which exhibit high chemical and physical stabilities. Therefore, each atom in the crystal lattice can be substituted by other ions, indicating that the luminescence properties can be controlled. Indeed, the phosphors with the above compositions show various broad-

INTRODUCTION

Inorganic phosphor materials have been applied as a spectrum converter to a wide range of emitting devices, from classic cathode-ray tubes (CRT) and fluorescent lamps to white light emitting diodes (white LEDs).1−5 Recently, rare earth metals have been used in phosphor materials for emitting devices because they have many advantages, such as a high luminescence efficiency and color purity.6 In particular, Eu and Tb ions have been widely employed for solid-state lighting.1,7−10 Moreover, a pure white light can be easily obtained using these rare earth metal elements as luminescent ions.11−14 In fluorescent lamps and white LEDs, white light has been achieved by using only these elements as the luminescence centers in oxide, nitride, or sulfide lattices.2,15−17 However, rare earth ions have narrow excitation spectra due to the 4f−4f forbidden transitions, resulting in low absorbances.18 Additionally, these metals are more expensive than transition metals, which limits the dissemination of such materials.19 Therefore, rare-earth-free phosphor materials have been sought to solve these issues. Inorganic materials containing pentavalent vanadium ions have been investigated in a wide range of fields, such as © XXXX American Chemical Society

Received: November 6, 2017

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DOI: 10.1021/acs.inorgchem.7b02820 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. (Top) FE-SEM images at 1000× magnification and (bottom) EDS spectra at the cross point of the LiCa3MV3O12 (M = Zn (a) and Mg (b)) phosphors synthesized by a conventional SSR method.

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band emission spectra at 500 to 600 nm with greenish to yellowish-white colors,41−43 but a garnet vanadate phosphor which has a bluish color emission maximum at ∼500 nm and high quantum efficiency has not been discovered. The development of the blue emission garnet vanadate phosphor with high efficiency is very important because it is possible to realize the luminescence color tuning by combination with various garnet vanadate phosphors, which exhibit yellow emission color. The luminescence behavior generated by the ligand to metal CT transition between V5+ and O2− in vanadate phosphors strongly depends on the V−O bond distance in the [VO4] tetrahedra with a Td symmetry.44 The emission energy of vanadate is inversely proportional to the bond distance of V−O. Therefore, garnet structure-based vanadate materials built using smaller ions could exhibit a bluish-colored emission. We focused on LiCa3MV3O12 (M = Zn and Mg) compounds only consisting of relatively small ions with a compact crystal lattice volume, i.e., a shorter V−O bond distance in the [VO4] tetrahedra, which have a high potential to exhibit a bluish emission. LiCa3MV3O12 (M = Zn and Mg) compounds have been recently studied in dielectric fields;45,46 nevertheless, their detailed crystal structures and luminescence properties have not been investigated. Therefore, it is necessary and important to reveal the detailed crystal structures of LiCa3MV3O12 (M = Zn and Mg) compounds and to elucidate those luminescence mechanisms. In the present study, the fundamental luminescence behavior of LiCa3MV3O12 (M = Zn and Mg) phosphors synthesized using a conventional solid-state reaction method was fully investigated by analyzing the detailed crystal structure to discover the first sample of a vanadate phosphor material with a garnet structure and bluish emission.

EXPERIMENTAL SECTION

Material Synthesis. Powder samples of the vanadate phosphor were synthesized by a conventional solid-state reaction (SSR) method. Chemical reagents, Li2CO3 (Wako Chem., Japan, 99%), CaCO3 (Wako Chem., Japan, 98%), ZnO (Wako Chem., Japan, 99%), MgO (Wako Chem., Japan, 99.9%), and V2O5 (Wako Chem., Japan, 98%) were used as the raw materials. These reagents were weighed in stoichiometric ratios and mixed with an agate mortar and pestle using acetone as the solvent. After drying, the mixtures were placed in an alumina boat and calcined at 750 °C for 6 h in air. The calcined samples were cooled down to room temperature and then crushed in an alumina mortar to create homogeneous samples. Finally, the samples were sintered at 850 °C for 6 h under atmospheric conditions. We did not use a platinum crucible to avoid the reaction between platinum and lithium. Characterization and Apparatus. The elemental analysis of the obtained phosphor samples was carried out using inductively coupled plasma-optical emission spectrometry (ICP-OES; PerkinElmer Inc., Optima 4300 DV) after dissolving the samples in 1 M of trace elemental analysis grade nitric acid. Milli-Q water (Milli-Q Biocel, > 18 MΩ−1) was used for the elemental analysis. Anal. calcd for LiCa3ZnV3O12: Li, 1.29%; Ca, 22.37%; Zn, 12.17%; V, 28.44%. Found: Li, 1.40%; Ca, 23.16%; Zn, 12.10%; V, 28.91%. Anal. calcd for LiCa3MgV3O12: Li, 1.40%; Ca, 24.23%; Mg, 4.90%; V, 30.79%. Found: Li, 1.56%; Ca, 25.14%; Mg, 5.26%; V, 31.51%. The particle morphologies of the powder samples were observed by a field-emission scattering electron microscope (FE-SEM; JEOL, JSM-6500F), and the chemical compositions were determined by energy dispersive X-ray spectrometry (EDS) using the FE-SEM electron beam. A surface was added to the samples by Pt sputtering for electron conductivity. The measurement samples were coated with platinum by a sputtering treatment to enhance the electron conductivity. Structural data of the prepared samples were collected by an X-ray diffraction (XRD) method using an X-ray diffractometer (PANalytical, X’Pert PRO) with Cu Kα radiation (λ = 1.54059 Å). The detailed crystallographic parameters were refined by a Rietveld method using a RIETAN-FP program package.47 Photoluminescence excitation and emission spectra of the powder samples were acquired at room temperature on a spectrofluorometer (Jasco Corp., FP-6500/6600) equipped with a 150 W Xe lamp as the excitation source. The quantum efficiencies (QE) were also measured B


Article

Inorganic Chemistry using the same spectrofluorometer with an optional unit for measurements. Defuse reflectance spectra were recorded using a spectrofluorometer with BaSO4 as a standard reference. The photoluminescence decay curves of the phosphor samples were recorded by a spectrofluorometer (Hamamatsu Photonics, Quantaurus-Tau C1136712) with LED irradiation at 340 nm as the excitation source.

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RESULTS AND DISCUSSION

Morphology and Chemical Composition. To confirm the crystal morphology and chemical composition of the LiCa3MV3O12 (M = Zn and Mg) phosphors, the FE-SEM images and EDX spectra are shown in Figure 1. The obtained phosphor particles have a cubic-like spherical morphology and are partially aggregated by the high-temperature reaction. The particle size of the LiCa3MV3O12 (M = Zn and Mg) phosphors is in the range 5−10 μm for M = Zn and 3−7 μm for M = Mg. Generally, compounds consisting of zinc ions are sintered at lower temperatures than those with magnesium ions.37,48,49 Additionally, the particle morphology of the compounds with Zn ions changes more than that of the compounds with Mg ions at the same heating temperature, resulting in differences in the particle sizes of the phosphor samples, which is also a result of the different reactivities. The EDS data resulted in a simplified quantitative determination for the ratio of the cation mass, according to Table S1. The observed EDX spectra contained peaks of the constituent elements and of the platinum derived from the sputtering. The Li signal does not appear in the EDX because of its relatively light molecular mass.50 Thus, the chemical compositions were determined by the signals of Ca, Zn, Mg, and V. The cation ratios, Ca/M/V (M = Zn and Mg), of the LiCa3MV3O12 phosphors were estimated to be 2.7:1:2.8 for M = Zn and 2.7:1:2.9 for M = Mg. This result agrees with the elemental analysis, indicating that the obtained phosphor samples had stoichiometric chemical compositions. Crystal Structure and Phase Determination. The phases of the samples were observed by a powder X-ray diffraction (XRD) method. Figure 2a and b show the Rietveld refinement profiles for the synthesized LiCa3MV3O12 (M = Zn and Mg) phosphors. The refinements for the XRD profiles of the samples resulted in good R-factors and S values (Table 1), which indicated that the crystal structures could be successfully refined by the Rietveld method. A garnet structure is described by the general formula (A)3[B]2{C}3O12 with a cubic system structure and a space group of Ia3̅d (No. 230).38,40,51,52 The structure contains a three-dimensional dodecahedral (A) O8, octahedral [B] O6, and tetrahedral {C} O4 framework by edge and corner sharing with the polyhedra (Figure 3a and b). 53 In LiCa3MV3O12, the Ca2+ ions were in the (A) site and formed a [CaO8] dodecahedra with a Wyckoff symbol of 24c. The M2+ (Zn2+ or Mg2+) ions were in the [B] site with a 16a position, and a Li+ ion also occupied the same [B] site. Based on the ionic radius, this result is reasonable because the ionic radius of Li+ with a coordination number of 6 is 0.076 nm,54 which is similar to that of Zn2+ and Mg2+ (0.072 and 0.074 nm).54 The V5+ ions were located at the 24d position as the {C} site in the general formula to form a [VO4] tetrahedron unit. In the garnet structure with a space group of Ia3̅d (No. 230), the crystallographic site of the O atom (Wyckoff symbol; 96h) is only a generally equivalent position, and the other cation sites are at special positions; i.e., the crystal structure can be determined only by setting the position of the O atoms. The atomic parameters of the LiCa3MV3O12 phosphors are listed in Table 2. The atomic position of the O atoms in the oxide crystal

Figure 2. Observed (black cross) and calculated (red line) X-ray powder diffraction data of the LiCa3MV3O12 (M = Zn (a) and Mg (b)) phosphors prepared in this study and the difference profile (bottom blue line) between them. Bragg reflection peak positions are shown as vertical bars.

Table 1. Details of the Rietveld Refinement of LiCa3MV3O12 (M = Zn and Mg) Crystal system Space group Z a (nm) V (nm3) Rwp (%) Rp (%) S

LiCa3ZnV3O12

LiCa3MgV3O12

cubic Ia3̅d 8 1.24437(1) 1.92686(3) 4.163 3.127 1.4103

cubic Ia3̅d 8 1.24366(3) 1.92356(7) 4.263 3.151 1.4003

with the garnet structure can be estimated by the following empirical equation.52 l x = 0.0278r(A) + 0.0123r(B) − 0.0482r(C) + 0.0141 o o o o o o o o y = −0.0237r(A) + 0.0200r(B) + 0.0321r(C) o o o m + 0.0523 o o o o o o o z = −0.0102r(A) + 0.0305r(B) − 0.0217r(C) o o o o + 0.6519 n

where x, y, and z are atomic parameters of an O atom with a 96h site; (A), (B), and (C) are the ionic radii of each ion with a unit of Å in the (A)3[B]2{C}3O12 composition. Using the above equation, the atomic parameters (x,y,z) of the LiCa3MV3O12 phosphors were estimated to be (0.0374, 0.0522, 0.6556) for LiCa 3 ZnV 3 O 12 and (0.0372, 0.0520, 0.6553) for LiCa3MgV3O12. The atomic parameters of the O atom observed C


Article

Inorganic Chemistry

Table 3. Bond Distances between the Metal and Oxygen Atoms in LiCa3MV3O12 (M = Zn and Mg) bond distance (nm) bonds

LiCa3ZnV3O12

LiCa3MgV3O12

Ca1−O1 × 4 Ca1−O1 × 4 M1−O1 × 6 V1−O1 × 4

0.2438(1) 0.2521(1) 0.2148(1) 0.1693(1)

0.2437(1) 0.2527(1) 0.2144(1) 0.1688(1)

this paper, however, the Rietveld refinements revealed the crystal structure of the LiCa3MV3O12 (M = Zn and Mg) phosphors for the first time, and sufficiently reliable results for the characterization of various optical properties were obtained. Optical and Photoluminescence Properties. Figure 4 shows the diffuse reflection (DR) spectra of the LiCa3MV3O12

Figure 3. (a) Crystal structure and (b) connection of the [CaO8] dodecahedron, [MO6] octahedron, and [VO4] tetrahedron in LiCa3MV3O12 with a garnet structure, which are illustrated using the VESTA program.

in the refinements of LiCa3MV3O12 were well fitted to the estimated values, i.e., within +0.0052% (M = Zn) and +0.074% (M = Mg), which indicated that the obtained refinements were reasonable values. The bond distances between each cation and oxygen are summarized in Table 3. The V−O bond distance of the [VO4] tetrahedron in LiCa3ZnV3O12 was 0.1693(1) nm, which is slightly longer than that of LiCa3MgV3O12 (0.1688(1) nm). The expansion of the [VO4] volume is often observed upon the substitution of Mg2+ with Zn2+ in a garnet-structure crystal lattice, such as Ca2NaM2V3O1248,49 and Ca5M4(VO4)6 (M = Zn and Mg).39 The lattice volume expansion should be ascribed to the ionic radius of the Zn2+ ion (0.072 nm)54 and Mg2+ ion (0.074 nm).54 In previous reports,45,46 LiCa3MV3O12 (M = Zn and Mg) compounds were defined as having a garnet structure without determination of the detailed crystallographic parameters. In

Figure 4. Defuse reflectance spectra of the LiCa3MV3O12 (M = Zn and Mg) phosphors synthesized by a conventional SSR method. Inset shows the Kubelka−Munk spectra for the band gap energy calculation of LiCa3MV3O12 (M = Zn and Mg).

(M = Zn and Mg) phosphors, and the Kubelka−Munk absorption coefficient (K/S) was calculated from the observed reflectance spectra by eq 1.55 (αhυ)1/2 =

(1 − R )2 K = S 2R

(1)

where K, S, and R represent the absorption coefficient, scattering coefficient, and reflectivity, respectively. For both phosphors, a

Table 2. Atomic Coordinates for LiCa3MV3O12 (M = Zn and Mg) from the Rietveld Refinement of the X-ray Diffraction Data LiCa3ZnV3O12 atom

Wyckoff

occ.

Ca1 Zn1 Li1 V1 O1

24c 16a 16a 24d 96h

1 1/2 1/2 1 1

atom

Wyckoff

occ.

Ca1 Mg1 Li1 V1 O1

24c 16a 16a 24d 96h

1 1/2 1/2 1 1

y

z

Uiso (nm2)

0 1/2 1/2 0 0.05427(9)

3/4 0 0 3/4 0.6590(1)

0.0156(3) 0.0088(4) 0.00878 0.0035(2) 0.0150(4)

x

y

z

Uiso (nm2)

7/8 1/2 1/2 1/8 0.03996(9)

0 1/2 1/2 0 0.05369(9)

3/4 0 0 3/4 0.6589(1)

0.0154(3) 0.0100(7) 0.01005 0.0021(2) 0.0123(4)

x 7/8 1/2 1/2 1/8 0.03974(9) LiCa3MgV3O12

D


Article

Inorganic Chemistry

V5+ ion may directly form a conduction band in the band structure, and this transition process is similar to that of a semiconductor with a direct band gap, such as ZnO and ZnS.57 Under UV-light excitation at 341 nm, the PL spectra of the LiCa3MV3O12 (M = Zn and Mg) phosphors showed a bluishwhite color emission with a broad band from 370 to 700 nm with the same maximum peak at 481 nm, which had a full width at half-maximum (fwhm) of 136.1 nm (5620 cm−1) for M = Zn and 135.8 nm (5650 cm−1) for M = Mg. No clear changes in the positions of the PLE and PL wavelengths and fwhm of the emission spectra were observed for the LiCa3MV3O12 (M = Zn and Mg) phosphors. However, the LiCa3MgV3O12 phosphor showed a remarkable improvement in the emission intensity, i.e., 139%, which is as large as that of the LiCa3ZnV3O12 phosphor (Figure S1). The quantum efficiency (QE) is essential for determining the commercialization potential of phosphor materials because it gives direct information about the conversion efficiency of absorbed photons into luminescence. The QE is described by eq 2: ηext = A ·ηint (2)

strong optical absorption was observed in the UV-region from 200 to 400 nm, which was sufficiently transparent in the visiblelight region. Their optical absorptions were due to the ligand to metal CT transition between V5+−O2− in the [VO4] tetrahedra. The absorption edge of the LiCa3ZnV3O12 phosphor was observed on the reddish side compared to that of the LiCa3MgV3O12 phosphor in the DR spectra data. The band gap energies of the prepared LiCa3MV3O12 (M = Zn and Mg) phosphors were estimated to be 3.40 eV for LiCa3ZnV3O12 and 3.46 eV for LiCa3MgV3O12, respectively. Many vanadate phosphors consisting of tetrahedral [VO4] exhibit an absorption edge in the UV-light region at ∼400 nm, indicating a band gap energy of ∼3.1 eV.43,56 Figure 5 a−d show the normalized photoluminescence (PL) and photoluminescence excitation (PLE) spectra of the

where ηext represents the external QE, which means the output luminescence photon energy; A is the absorption rate at the excitation wavelength; and ηint is the internal QE, which is the most important QE value. The internal QE of the LiCa3MV3O12 (M = Zn and Mg) phosphors were 40.1% and 44.0%, respectively, and the internal and external QEs and absorption fractions of the LiCa3MV3O12 (M = Zn and Mg) phosphor excitation at 341 nm are summarized in Table 4. The Table 4. Internal and External QEs and Absorption Rates of the LiCa3MV3O12 (M = Zn and Mg) Phosphors QEs phosphors

absorption rate (%)

internal (%)

external (%)

LiCa3ZnV3O12 LiCa3MgV3O12

90.4 88.2

40.1 44.0

36.3 38.8

LiCa3MV3O12 (M = Zn and Mg) phosphors have QE values that are equal to or higher than those of other garnet vanadate phosphors, such as Ca2KZn2V3O12 (19.2%),36 Ca5Zn3.92In0.08(V0.99Ta0.01O4)6 (40.8%), and Ca5M4(VO4)6 (M = Mg and Zn; 41.6 and 15.9%),39 as listed in Table 5. Nakajima et al. revealed that the QE of vanadate phosphors inversely depends on the distance between the nearest vanadium−vanadium bond in the lattice.28 A shorter V−V

Figure 5. PLE (black solid line (a and c)) and PL spectra (red solid line (b and d)) of the LiCa3MV3O12 ((a and b) M = Zn and (c and d) M = Mg) phosphors synthesized by conventional SSR method. Gauss deconvolutions of the PLE and PL spectra of the phosphors excited at 341 nm. Dotted orange and cyan lines are Gauss components for the PLE and PL spectra, respectively. Insets show the photographs of the emitting phosphor samples under the UV-light at 365 nm. (e) PLE and PL mechanism in [VO4]3− tetrahedra with Td symmetry in the garnet structural LiCa3MV3O12 (M = Zn and Mg) phosphors.

Table 5. Comparison of Emission Peak Positions, V−O Distances, IQEs, and V−V Distances of the Garnet Vanadate Phosphors

LiCa3MV3O12 (M = Zn and Mg) phosphors. The PLE spectra were monitored at the emission peak at 481 nm and showed a broad band from 220 to 400 nm with a peak at 341 nm from the CT transition of V5+−O2− → V4+−O− in the [VO4]3− tetrahedra with a Td symmetry. A remarkable difference was not observed in the PLE spectra of the phosphors, which was in good agreement with the DR spectra. Additionally, an excitation edge was located at the same optical position as that in the DR spectra. On the basis of the obtained results, a 3d orbital of the

a

E

phosphors

λem (nm)

V−O distance (nm)

IQE (%)

V−V distance (nm)

LiCa3ZnV3O12

481

0.1693(1)

40.1

0.3810

LiCa3MgV3O12

481

0.1688(1)

44.0

0.3808

Ca5Zn4(VO4)6 Ca5Mg4(VO4)6 Ca2KZn2V3O12 CZIVTa

550 530 591 546

0.1753 0.1727 0.1685 0.1732

15.9 41.6 19.2 40.8

0.3813 0.3806 0.3794 0.3821

ref this work this work 35,39 35,39 36 35

Ca5Zn3.92In0.08(V0.99Ta0.01O4)6. DOI: 10.1021/acs.inorgchem.7b02820 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Table 6. Peak Wavelength and Energy of the PLE and PL Bands from the Gaussian Deconvolution LiCa3ZnV3O12

LiCa3MgV3O12

band

transition

peak position (nm)

energy (cm−1)

energy (eV)

peak position (nm)

energy (cm−1)

energy (eV)

Ex1 Ex2a Ex2b Em1 Em2

1

A1 → T2 1 A1 → 1T1 1 A1 → 1T1 3 T2 → 1A1 3 T1 → 1A1

264 326 350 470 524

37 900 30 700 28 600 21 300 19 100

4.70 3.80 3.54 2.64 2.37

257 309 344 469 522

38 900 32 400 29 100 21 300 19 200

4.82 4.01 3.60 2.64 2.38

1

tetrahedral [VO4] in the LiCa3MV3O12 (M = Zn and Mg) phosphors was created (Figure 5 e) on the basis of the above optical property results. In the CT transition of the tetrahedral [VO4], the electrons on the 1T1 and 1T2 excited levels move to the 3T2 and 3T1 metastable states with a nonradiative transition and then back to the 1A1 ground state via a radiative process, i.e., an emissive transition.33 The Stokes shifts cause thermal quenching at lower temperatures, including room temperature.39 Therefore, the magnitude of the nonradiative energies between 1T2 and 1T1 and the Stokes shift strongly depends on the internal QE value. A higher QE value can be obtained at a lower nonradiative transition energy. Since the LiCa3MgV3O12 phosphor has a smaller magnitude of energy due to the nonradiative transition compared with that of the LiCa3ZnV3O12 phosphor, the increase in the internal QE of the LiCa3MgV3O12 phosphor is related to the decrease in the nonradiative energy compared to that of the LiCa3ZnV3O12 phosphor (Table 4). Usually, the luminescence lifetime scale is determined by the luminescence ions due to the spin or Laporte tolerances of the electron transitions. Mn2+, Eu3+, and Tb3+ ions with 3d−3d and 4f−4f forbidden intratransitions exhibit long luminescence lifetimes (>1 ms), and phosphor materials containing these luminescence ions can easily cause quenching due to luminescence saturation.10 Therefore, it is difficult for these phosphors to be used for high-power excitation and in nextgeneration optical devices. Figure 6 shows the luminescence

distance means a stronger interaction between the tetrahedral [VO4]. In the case of the Ca5M4(VO4)6 (M = Mg and Zn) phosphors,39 the QEs are very different, whereas the differences in the QEs of the LiCa3MV3O12 phosphors are small because the V−V bond distances are not larger than those in Ca5M4(VO4)6 (0.38129 nm for M = Zn and 0.38062 nm for M = Mg). The higher QE of the LiCa 3 MgV 3 O 12 phosphor than the LiCa3ZnV3O12 phosphor was ascribed to the shorter V−V bond distance (0.3810 nm for LiCa3ZnV3O12 and 0.3808 nm for LiCa3MgV3O12). However, the absorption rate at 341 nm UVlight excitation for the LiCa3MgV3O12 phosphor was lower than that of LiCa3ZnV3O12 phosphor. This result agreed with the differences in the reflectance spectra for these phosphors (see Figure 4). Generally, the higher absorption rate is ascribed to a larger particle size due to a longer optical path. The smaller particle and morphology size of the LiCa3MgV3O12 phosphor relative to that of the LiCa3ZnV3O12 phosphor (see Figure 1) causes a decrease in the absorption rate. The PLE and PL spectra of the phosphors were deconvoluted using a Gaussian function (Figure 5) to elucidate the detailed optical properties of the LiCa3MV3O12 (M = Zn and Mg) phosphors. The deconvoluted peak positions are listed in Table 6. The energy levels of the tetrahedral [VO4] group consist of a ground state, 1A1, and excited levels, 1T1, 1T2, 3T1, and 3T2. The excitation spectra of the LiCa3MV3O12 phosphors were deconvoluted to obtain one Ex1 band and two Ex2 bands corresponding to 1A1 → 1T2 and 1A1 → 1T1 electronic transitions, respectively.33,36 The energy gaps between the deconvoluted bands, Ex1 and Ex2a, in the LiCa3MV3O12 (M = Zn and Mg) phosphors were 7200 and 6500 cm−1, respectively, which are equal to the energy gap between 1T1 and 1T2.36 The PL spectra were composed of two Gaussian deconvoluted broad bands of Em1 and Em2 due to the CT transition between the O2− 2p orbital and the V5+ 3d orbital in the [VO4] tetrahedral unit, which correspond to the 3T2 → 1A1 and 3T1 → 1 A1 transitions, respectively. The energy positions of the 3T2 and 3 T1 levels were estimated to be 21 300 and 19 100 cm−1 for LiCa3ZnV3O12 and 21 300 and 19 200 cm−1 for LiCa3MgV3O12, respectively. Generally, the energy gap between the transition states of the 3T2 and 3T1 emission levels are estimated to be ∼2500 cm−1,36,39,48 which indicates that the energy gaps of the LiCa3MV3O12 phosphors are reasonable values for the vanadate phosphors. The Stokes shifts, ΔS, are related to the energy gap between the excitation edge and the emission band and reveal the luminescence mechanism and thermal property of phosphor materials. The ΔS values of the LiCa3MV3O12 (M = Zn and Mg) phosphors are 7300 and 7800 cm−1, respectively, which are similar to the ΔS values of other garnet vanadate phosphors with high quantum efficiencies, such as Ca 5 Zn 3 . 9 2 In 0 . 0 8 (V 0 . 9 9 Ta 0 . 0 1 O 4 ) 6 (7650 cm − 1 ) 3 5 and Ca2KZn2(VO4)3 (6330 cm−1).36 A schematic model of the energy diagram for the excitation and emission processes of the

Figure 6. PL decay curves of the LiCa3MV3O12 (M = Zn and Mg) phosphors under excitation at 340 nm and monitored at 481 nm.

decay curves of the LiCa3MV3O12 (M = Zn and Mg) phosphors under 340 nm LED excitation irradiation and monitoring the emission center at 481 nm. The decay curves consist of two decay components with a biexponential function, which well fit the second-order exponential eq 3:58 F


Article

Inorganic Chemistry

ij t yz ij t yz I(t ) = I0 + A1 expjjj− zzz + A 2 expjjj− zzz j τz j τ z (3) k 1{ k 2{ where I(t) and I0 are the luminescence intensity at time t and the initial intensity, respectively, A1 and A2 are constants, and τ1 and τ2 are the luminescence lifetimes of the exponential components. The A and τ values calculated from eq 3 are shown in Table S2. Additionally, average lifetime constants (τ*) are obtained using the A and τ values according to eq 4.58 τ* =

(A1τ12 + A 2 τ2 2) (A1τ1 + A 2 τ2)

where I0 and I(T) are the luminescence intensities of the phosphors at the initial temperature (room temperature) and different temperatures T, respectively, A is a constant, Ea is the activation energy, and k is the Boltzmann constant (8.617 × 10−5 eV K−1). The activation energies, Ea, were equal to the slope of the ln[I0/I(T) − 1] vs [kT]−1 plot obtained from the above equation, and the Ea values of the LiCa3MV3O12 (M = Zn and Mg) phosphors were 0.3602 and 0.3568 eV, respectively. The magnitude of activation energy value is strongly related to a nonradiative transition via thermal process according to eq 6: i E y α = s expjjj− a zzz k kT {

(4)

The average luminescence lifetimes τ* of the LiCa3MV3O12 (M = Zn and Mg) phosphors were 6.28 and 12.4 μs, respectively. Similarly, the τ* value of the Ca5Mg4(VO4)6 garnet phosphor was larger than that of the corresponding Ca5Zn4(VO4)6 garnet phosphor.39 Generally, the internal QE ηint is directly proportional to the luminescence lifetime, τ, indicating that a shorter lifetime should depress the internal QE. The results agree with the difference in the QEs for LiCa3MV3O12 (M = Zn and Mg) (Table 4). Nonradiative relaxation occurs via two processes, thermal activation and the release of multiple phonons. In particular, the relaxation process of the vanadate phosphors mostly occurs via the thermal activation process due to their simple excited levels. Generally, the luminescence intensity of phosphor materials gradually decreases as the nonradiative transition probability increases with the thermal energy of the heating process.59,60 The thermal dependence of the PL and PLE spectra for the LiCa3MV3O12 (M = Zn and Mg) phosphors was measured from 25 to 150 °C under UV-light excitation at 341 nm (Figures S2 and S3). The relative PL and PLE intensities of both samples drastically decreased as the temperature increased (Figure 7),

(6)

in which α represents a nonradiative transition probability with thermal activation and s is a frequency factor (s−1). Two kinds of nonradiative transition exist: thermal activation and multiple phonons. Although LiCa3MgV3O12 has a higher internal QE than the LiCa3ZnV3O12, the thermal activation type relaxation tends to occur. Therefore, it is speculated that the nonradiative relaxation of LiCa3MgV3O12 is more affected by multiple phonons compared to that of LiCa3ZnV3O12. The slight difference in the PL spectra of the two vanadate phosphors, LiCa3MV3O12 (M = Zn and Mg), leads to luminescence color changes. We determined the luminescence colors by calculating the Commission International de I’eclairage (CIE) coordinates for the two samples. The calculated CIE coordinates (x,y) were (0.2346,0.3234) for LiCa3ZnV3O12 and (0.2293,0.3135) for LiCa3MgV3O12 (Figure 8).

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CONCLUSION Vanadate phosphors with a garnet structure, LiCa3MV3O12 (M = Zn and Mg), were synthesized by a conventional solid-state reaction method. The Rietveld refinement analysis of the XRD data revealed that the LiCa3MV3O12 (M = Zn and Mg) phosphors have a cubic structure with a space group of Ia3̅d, and they are composed of tetrahedral [VO4] units with V−O bond lengths that are shorter than those in other garnet vanadates. The LiCa3MV3O12 (M = Zn and Mg) phosphors exhibit a bluish-white emission with a peak maximum at 481 nm under UV-light irradiation excitation at 341 nm. The conventional garnet vanadate phosphors had emitted only from the green to yellow region. However, the LiCa3MV3O12 (M = Zn and Mg) phosphors realized the expansion of the luminescence region to the blue region, leading to their great potential in application to future photodevices with a new concept. This unique blue emission is due to the shorter V−O bond distance compared to that in conventional garnet-structure vanadate phosphors. The high internal quantum efficiencies of the LiCa3MV3O12 (M = Zn and Mg) phosphors were 40.1% and 44.0%, respectively. This is attributed to the short neighboring V−V bond distances in the lattice, leading to stronger interactions between the [VO4] tetrahedron units. The luminescence mechanism was created by a Gaussian deconvolution of the luminescence excitation and emission spectra. Although the developed vanadate phosphors have poor thermal stability, these phosphors could be suitable for the new LED system “remote phosphor”, which is a technology of placing a phosphor layer at a distance from the LED chip, not the conventional LED devices. The emission colors (CIE coordinates) of the LiCa3MV3O12 (M = Zn and Mg) phosphors are (x,y) = (0.2346,0.3234) for M = Zn and (0.2293,0.3135) for M = Mg. This result indicates that a high-

Figure 7. (a) Temperature dependence of the PL and (b) PLE intensities of the LiCa3MV3O12 (M = Zn and Mg) phosphors. Inset shows the Arrhenius fitting of the emission intensities of the LiCa3MV3O12 (M = Zn and Mg) phosphors and the calculated activation energies for thermal quenching.

but a marked difference in the PL spectral shapes between the phosphor samples was not observed. The activation energy (Ea) for a nonradiative relaxation process can be expressed by eq 5.61 I0 I (T ) = E 1 + A exp − kTa (5)

( )

G


Article

Inorganic Chemistry

Figure 8. (a) CIE coordinates under 365 nm UV light for the LiCa3MV3O12 (M = Zn and Mg) phosphors. (b) Magnification of the coordinate range for 0.2 ≤ x ≤ 0.3 and 0.3 ≤ y ≤ 0.4. (3) Feldmann, C.; Justel, T.; Ronda, C. R.; Schmidt, P. J. Inorganic Luminescent Materials: 100 Years of Research and Application. Adv. Funct. Mater. 2003, 13, 511−516. (4) Ozawa, L.; Itoh, M. Cathode Ray Tube Phosphors. Chem. Rev. 2003, 103, 3835−3855. (5) Schubert, E. F.; Kim, J. K. Solid-State Light Sources Getting Smart. Science 2005, 308, 1274−1278. (6) Qin, X.; Liu, X.; Huang, W.; Bettinelli, M.; Liu, X. LanthanideActivated Phosphors Based on 4f-5d Optical Transitions: Theoretical and Experimental Aspects. Chem. Rev. 2017, 117, 4488−4527. (7) Kim, S. W.; Toda, K.; Hasegawa, T.; Uematsu, K.; Sato, M. Color Tuning of Oxide Phosphors. In Phosphors, Up Conversion Nano Particles, Quantum Dots and Their Applications; Liu, R.-S., Ed.; Springer: 2016; pp 219−246. (8) Lin, C. C.; Meijerink, A.; Liu, R. Critical Red Components for Next-Generation White LEDs. J. Phys. Chem. Lett. 2016, 7, 495−503. (9) George, N. C.; Denault, K. A.; Seshadri, R. Phosphors for SolidState White Lighting. Annu. Rev. Mater. Res. 2013, 43, 481−501. (10) Setlur, A. A. Phosphors for LED-Based Solid-State Lighting. Electrochem. Soc. Interface 2009, 18, 32−36. (11) Setlur, A. A. Sensitizing Eu3+ with Ce3+ and Tb3+ to Make Narrow-Line Red Phosphors for Light Emitting Diodes. Electrochem. Solid-State Lett. 2012, 15, J25−J27. (12) Wen, D.; Shi, J. A Novel Narrow-Line Red Emitting Na2Y2B2O7:Ce3+,Tb3+,Eu3+ Phosphor with High Efficiency Activated by Terbium Chain for near-UV White LEDs. Dalton Trans. 2013, 42, 16621−16629. (13) Jiao, M.; Guo, N.; Lü, W.; Jia, Y.; Lv, W.; Zhao, Q.; Shao, B.; You, H. Tunable Blue-Green-Emitting Ba3LaNa(PO4)3F: Eu2+,Tb3+ Phosphor with Energy Transfer for near-UV White LEDs. Inorg. Chem. 2013, 52, 10340−10346. (14) Zhang, X.; Zhou, L.; Pang, Q.; Shi, J.; Gong, M. Tunable Luminescence and Ce3+→Tb3+→Eu3+ Energy Transfer of BroadbandExcited and Narrow Line Red Emitting Y2SiO5:Ce3+, Tb3+, Eu3+ Phosphor. J. Phys. Chem. C 2014, 118, 7591−7598. (15) Jia, D.; Wang, X.-j. Alkali Earth Sulfide Phosphors Doped with Eu2+ and Ce3+ for LEDs. Opt. Mater. 2007, 30, 375−379. (16) Xie, R. J.; Hirosaki, N. Silicon-Based Oxynitride and Nitride Phosphors for White LEDs-A Review. Sci. Technol. Adv. Mater. 2007, 8, 588−600. (17) Zeuner, M.; Pagano, S.; Schnick, W. Nitridosilicates and Oxonitridosilicates: From Ceramic Materials to Structural and Functional Diversity. Angew. Chem., Int. Ed. 2011, 50, 7754−7775.

purity white emission can be achieved by building a solidsolution LiCa3MV3O12 and conventional yellow-emitting vanadate phosphor with a garnet structure. As mentioned above, the LiCa3MV3O12 (M = Zn and Mg) phosphors presented by this study are the first report of bluish-white emission in garnet-structure vanadate phosphors with a high quantum efficiency. We conclude that these phosphors can be key materials as a blue-color source in a solid-solution of garnetstructure vanadate materials.

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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.7b02820.

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EDS elemental analysis, comparison of the PL and PLE spectra, calculated lifetime data, temperature dependent PL and PLE spectra and schematic configuration coordinate diagram for LiCa3MV3O12 (M = Zn and Mg) phosphors (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Takuya Hasegawa: 0000-0002-6170-5632 Tadaharu Ueda: 0000-0001-6797-5716 Notes

The authors declare no competing financial interest.

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