Layered Crystal Structure, Color-Tunable Photoluminescence, and

Article Cite This: Inorg. Chem. 2017, 56, 12902-12913

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Layered Crystal Structure, Color-Tunable Photoluminescence, and Excellent Thermal Stability of MgIn2P4O14 Phosphate-Based Phosphors Jing Zhang,† Ge-Mei Cai,*,† Lv-Wei Yang,† Zhi-Yuan Ma,†,‡ and Zhan-Peng Jin† †

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School of Materials Science and Engineering, Education Ministry Key Laboratory of Non-ferrous Materials Science and Engineering, Central South University, Changsha 410083, Hunan, China ‡ Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan S Supporting Information *

ABSTRACT: Single-component white phosphors stand a good chance to serve in the next-generation high-power white lightemitting diodes. Because of low thermal stability and containing lanthanide ions with reduced valence state, most of reported phosphors usually suffer unstable color of lighting for practical packaging and comparably complex synthetic processes. In this work, we present a type of novel color-tunable blue−white− yellow-emitting MgIn2P4O14:Tm3+/Dy3+ phosphor with high thermal stability, which can be easily fabricated in air. Under UV excitation, the MgIn2P4O14:Tm0.02Dy0.03 white phosphor exhibits negligible thermal-quenching behavior, with a 99.5% intensity retention at 150 °C, relative to its initial value at room temperature. The phosphor host MgIn2P4O14 was synthesized and reported for the first time. MgIn2P4O14 crystallizes in the space group of C2/c (No. 15) with a novel layered structure built of alternate anionic and cationic layers. Its disordering structure, with Mg and In atoms co-occupying the same site, is believed to facilitate the energy transfer between rare-earth ions and benefit by sustaining the luminescence with increasing temperature. The measured absolute quantum yields of MgIn2P4O14:Dy0.04, MgIn2P4O14:Tm0.01Dy0.04, and MgIn2P4O14:Tm0.02Dy0.03 phosphors under the excitation of 351 nm ultraviolet radiation are 70.50%, 53.24%, and 52.31%, respectively. Present work indicates that the novel layered MgIn2P4O14 is a promising candidate as a single-component white phosphor host with an excellent thermal stability for near-UV-excited whitelight-emitting diodes (wLEDs).

1. INTRODUCTION

the advantages of high luminous efficiency, simple manufacture, comparably low cost, etc. Phosphor usually consists of a matrix or host and an activator, and sometimes a sensitizer. Most rare-earth ions with rich energy levels, as the most popular activators, can bring about various emissions over the ultraviolet−visible−near-infrared (UV-visNIR) spectral region via f-f or d-f transitions. The function of a single-component white phosphor generally relies on their interaction, especially energy transfer, between the rare-earth ions in an appropriate host. Until now, many studies related to rare-earth-doped phosphate phosphors3−9 have been conducted in search of novel single-component white phosphors, mainly due to high absorption in the UV region and structural diversity of the phosphate host. Nevertheless, few of them are suitable for commercial use, and most of these phosphors usually suffer some characteristic deficiencies, such as synthetic processing (requiring inert atmosphere protection) and emission loss or shift with

In virtue of the prominent characteristics, including high luminous efficiency, low energy consumption, high reliability, long service life and environmental friendliness, white-lightemitting diodes (wLEDs) have been regarded as the next generation of solid-state lighting sources, superseding the traditional fluorescent lamps and incandescent lamps.1 There are three main methods of mixing colors to produce white light from an LED.2 The most common implementation of the commercial wLEDs is fabricated by integrating blue LED chips with the yellow phosphor YAG:Ce (Y3Al5O12:Ce3+), but the lack of a red light component leads to a poor color rendering index (CRI). The second type of wLEDs is adopted by combining the UV LED chips with red, green, and blue (RGB) triphosphors. However, this type of wLEDs suffers from the deficient reabsorption of the emissions among the three phosphor components. In order to improve the quality of white light, a better wLED design that adopts single-component white phosphor and UV LED chips has been proposed, which has © 2017 American Chemical Society

Received: July 4, 2017 Published: October 9, 2017 12902

DOI: 10.1021/acs.inorgchem.7b01670 Inorg. Chem. 2017, 56, 12902−12913

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Inorganic Chemistry

promising host material with good thermal stability for the preparation of white phosphors.

increasing temperature. Regarding the latter, the so-called “thermal quenching” (TQ) still is a key challenge for groping for novel single-component white phosphors with good thermal stability and high luminescence. TQ is often ascribed to the nonradiative relaxation of excited electrons to the ground state of the activator. With regard to most phosphors with a single activator, TQ can be well-explained using a configurational coordinate diagram in which the excited luminescent center is thermally activated through phonon interaction and then relaxed through the crossing point between excited and ground states.10−12 In fact, TQ also is dependent on structural rigidity of the host, including rigid bonding networks and bond strength, which could substantially minimize the emission loss with increasing temperature.13 Very recently, the zero-thermalquenching behavior of blue-emitting phosphor Na3−2xSc2(PO4)3:xEu2+ was reported by Wang’s group and Kim’s group, respectively.14,15 They explained the compensation of emission losses due to the Na+ disordering nature by nonequivalence doping, phase transformation of host and consequent energy transfer from defect level, leading to radiative recombination. On every account, exploring a novel phosphate host with rigid bonding networks, containing disordering cationic, and receptive diverse activators favoring energy transfer could initiate the generation of color-tunable phosphors with excellent TQ behavior. Recently, we have targeted the MgO−In2O3−P2O5 ternary system for exploration of new phosphor host materials, based on the following considerations. First, magnesium and indium prefer to co-occupy the same lattice site and form a highly disordered structure, because of their similar ionic radius and coordination properties in the oxide. Second, both magnesium and indium have similar ionic radius and coordination properties to most rare-earth ions, which facilitate the substitution of different rare-earth dopants. Moreover, indium can form flexible coordination geometries in inorganic oxides, namely, InO4 tetrahedra, InO5 trigonal bipyramids, InO6 octahedra, InO7 pentagonal bipyramid, or InO8 dodecahedra. The incorporation of indium cations into phosphates can create various topologies and crystal fields, thus offering great opportunities to design new host compounds, where the rare-earth dopants may exhibit novel luminescence properties. In this contribution, the first magnesium indium phosphate, MgIn2P4O14, was observed and confirmed in the MgO−In2O3− P2O5 system. Based on crystallographic analysis using powder Xray diffraction data, the crystal structure of this compound is determined with novel layered structure in which Mg and In atoms co-occupy one special site 4e and one general site 8f. Taking into account our previous research experience,16,17 the blue-emitting Tm3+ ions and yellow-emitting Dy3+ ions are chosen and focused on the codoped activators through effective energy transfer to fabricate white-light emitting inorganic phosphors. Recently, Tm3+/Dy3+-doped single-component whit e phospho rs, such as YAl 3 ( B O 3 ) 4 : Tm 3 + and YAl3(BO3)4:Dy3+ (see ref 18) and Ca9Y(PO4)7:Tm3+ and Ca9Y(PO4)7:Dy3+ (see ref 19), exhibit preferable color-tunable properties in white-light-emitting diodes (wLEDs). The composition- and temperature-dependent luminescent properties of Tm3+- and Dy3+-activated MgIn2P4O14 phosphors have been systematically investigated by means of structural analysis, photoluminescence excitation (PLE), and emission (PL) spectrum, vacuum ultraviolet (VUV) excited luminescence properties, decay lifetime, and high-temperature luminescence. Present results indicate that MgIn2P4O14 is deemed to be a

2. EXPERIMENTAL SECTION 2.1. Sample Preparations. Pure MgIn2P4O14 polycrystalline sample and a series of Tm3+/Dy3+ single- or co-doped phosphors Mg(In1−xTmx)2P4O14 (MgIn2P4O14:Tmx, x = 0.01, 0.03, 0.04, 0.05, 0.07, 0.09 and 0.11), Mg(In1−yDyy)2P4O14 (MgIn2P4O14:Dyy, y = 0.01, 0.03, 0.04, 0.05, 0.07, 0.09 and 0.11), and Mg(In1−x‑yTmxDyy)2P4O14 (MgIn2P4O14:TmxDyy, x = 0.005−0.05, y = 0.005−0.05) were synthesized using conventional high-temperature solid-state reaction methods. First, different stoichiometric amounts of starting materials MgO (99.99%), In2O3 (99.99%), NH4H2PO4 (99.95%), Tm2O3 (99.99%), and Dy2O3 (99.99%) were ground in an agate mortar for 30 min to form homogeneous mixtures. Then, the mixtures were heated in corundum crucibles at 600 °C for 12 h to get the precursors. Subsquently, the precursors underwent two 24-h calcinations at 1000 °C after being ground for each time. Finally, the products were gradually cooled to room temperature in the furnace and were ground again into homogeneous powders for the following analysis. 2.2. Characterization. The phase compositions of the samples were characterized via powder X-ray diffraction (XRD) technique through an X-ray diffractometer (Rigaku, Model D/MAX-2500). Data for crystal structure determination and refinement was collected on a PANalytical powder X-ray diffractometer (X’Pert Pro). The chemical compositions of the samples were measured by inductively coupled plasma−atomic emission spectrometry, using a PerkinElmer ICP/6500 spectrometer. The nominal and the measured compositions of MgIn2P4O14 and the representative Tm3+/Dy3+ single- and co-doped MgIn2P4O14 samples determined from ICP-AES method are given in Table S1 in the Supporting Information. The measured atomic ratios of the samples are almost-ideal atomic ratios. Infrared (IR) spectrum in the wavenumber range of 400−2000 cm−1 was obtained at room temperature via a Nicolet 6700 infrared spectrophotometer with a KBr pellet as standards. Differential thermal analysis (TG-DTA) was performed on a Netzsch Model STA 449C apparatus with a heating rate of 10 K/min. Photoluminescence excitation and emission spectra at room temperature were obtained on a fluorescence spectrophotometer (Hitachi, Model F-7000) equipped with a 150 W Xe lamp as the excitation source. Temperature-dependence luminescence spectra (25−300 °C) were measured by adopting the heating apparatus (Tianjin Orient Koji Co., Ltd., TAP ≈ 02). The vacuum ultraviolet (VUV) spectra at temperatures ranging from 27 K to room temperature were obtained at Beamline 3B1B in Beijing Synchrotron Radiation Facilities (BSRF). Time-resolved emission decay behaviors and the quantum efficiency were evaluated on a Model FLS920n spectrometer (Edinburgh Instruments, Ltd., England) with Model μF900 lamps as the excitation source and a Model R928-PA photo multiplier as the signal detector.

3. RESULTS AND DISCUSSION 3.1. Crystal Structure and Characterization of MgIn2P4O14. 3.1.1. Crystal Structure of MgIn2P4O14. Based on the powder XRD data acquired in slow-scan mode, the crystal structure of the new compound MgIn2P4O14 was solved by using the charge flipping method of the Jana2006 program20 and refined by using the Rietveld method of the FullProf_Suite program.21 In the process of refinement, a total of 64 parameters were refined, including 19 profile parameters and 45 structure parameters. The final agreement factors converged to RB = 4.47%, Rp = 2.08%, Rwp = 2.68%, and S = 1.29, indicating that the determined structure should be quite credible. The detailed Rietveld refinement parameters and crystal structure data of MgIn2P4O14 are listed in Table 1. The atomic parameters of MgIn2P4O14 are given in Table S2 in the Supporting Information. MgIn2P4O14 crystallizes in the monoclinic system with the space group C2/c (No. 15) and lattice parameters a = 9.8733(3) Å, b = 12903


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Inorganic Chemistry Table 1. Details of Retvield Refinement Result of MgIn2P4O14 parameter

value

formula radiation type 2θ range step scan time per step structure determination refinement No. of refined parameters RB RP RWP S symmetry space group a b c β volume Z ρcalc

MgIn2P4O14 Cu Kα 10.00°−129.98° 0.02° 2s charge-flipping method Rietveld method 64 4.47% 2.08% 2.68% 1.29 monoclinic C 2/c (No.15) 9.8733(3) Å 8.7595(2) Å 12.5931(3) Å 108.546(2)° 1032.56(5) Å3 4 3.850 g cm−3

Figure 1. Final Rietveld refinement pattern for MgIn2P4O14. The observed intensities, calculated intensities, and the difference plot are represented by the red hollow circle, black solid line, and blue solid line, respectively. The positions of all Bragg reflections are marked with the green bars. The inset is the projective view of MgIn2P4O14 cell along the b-axis, with the closed tetrahedrons representing the PO4 groups.

MgIn2P4O14 (listed in Table S4 in the Supporting Information) were calculated using the following equation: 8.7595(2) Å, c = 12.5931(3) Å, â = 108.546(2)°, V = 1032.56(5) Å3, and Z = 4. In the asymmetric unit of MgIn2P4O14, there exist 11 crystallographically independent atom sites. All of the sites except only one special site 4e, which is co-occupied by Mg1 and In1 atoms (denoted as M1), belong to general sites 8f. The metal cations Mg2 and In2 share a general site 8f as well (denoted as M2). M1 and M2 are coordinated by six O atoms. Every P atom is connected with four O atoms to form a PO4 tetrahedron. Two adjacent PO4 can further construct a P2O7 group by sharing an O atom at a common vertex. The final refinement pattern of MgIn2P4O14 is shown in Figure 1. The inset of Figure 1 is the projective view of the MgIn2P4O14 cell along the b-axis, with the closed tetrahedrons representing the PO4 groups. As can be seen from the projective view along the c-axis in Figure 2a, the three-dimensional framework of MgIn2P4O14 has a layered structure that is composed of alternating anionic and cationic layers. Two adjacent [MnO4n+2]∞ layers are bridged by the vertexes of P2O7 groups. Each cationic layer comprises of countless parallel wavy chains [MnO4n+2] interlinked by P2O7 groups through corner-sharing (Figure 2b). In the structure, P2O7 groups play dual roles, being interchain parts in one [MnO4n+2]∞ layer and intrachain parts in the next adjoining [MnO4n+2]∞ layer. The [MnO4n+2] chain is formed by edgesharing MO6 octahedra running along the [101] direction with the repeated basic unit of −[M2O6-M1O6−M2O6]− (see Figure 2c). The projection of MgIn2P4O14 along the c-axis direction is shown in Figure S1 in the Supporting Information, demonstrating long hollow tunnels surrounded by MO6 octahedra and PO4 tetrahedron in this structure. In this structure, the distances between adjacent intrachain metal cations are 3.558 and 3.423 Å for M2−M1 and M2−M2, respectively. M1−O and M2-O distances range from 2.053(9) to 2.281(9) Å and from 2.044(10) to 2.219(8) Å, respectively. Representative bond lengths and angles in MgIn2P4O14 are listed in Table S3 in the Supporting Information. Based on Brown’s bond-valence theory,22 the bond valence (Vcalc) of each atom

N

Vi ,calc. =

⎛ r0 − rij ⎞ ⎟ 0.37 ⎠

∑ exp⎜⎝ j=1

(1)

where Vi,calc is the sum of the bond valence for atom i, rij is the distance between atom i and its bonding atom j, and r0 is a constant for a given ion pair (1.693, 1.902, and 1.617 Å for Mg− O, In−O, and P−O, respectively22). Obviously, all those calculated results are in agreement with the theoretical oxidation states, within reasonable error ranges, which indicate that the structure is legitimate. 3.1.2. Infrared Spectrum of MgIn2P4O14. In order to further confirm the coordination surroundings of P−O in the MgIn2P4O14 structure, the IR spectrum for MgIn2P4O14 was measured at room temperature and displayed in Figure S2 in the Supporting Information. Within the wavenumber range of 2000−400 cm−1, the IR spectrum exhibits several vibration bands with the characteristic of basic anionic unit P2O7.23,24 At high frequencies, the bands located at 1279 and 1259 cm−1 are attributed to the asymmetric stretching vibration of PO2 (iaś (PO2)), while the symmetric stretching vibration of PO2 (iś (PO2)) generates the peaks at ∼1127 and 1078 cm−1. At middle frequencies, the bands at wavenumbers of 956 and 900 cm−1 can be assigned to the asymmetric stretching vibration of POP linkages (iaś (POP)). As for the band at 741 cm−1, it should belong to the POP symmetric stretching vibration (iś (POP)). At low frequencies, the strong absorption within the 450−700 cm−1 range icorresponds to the PO2 asymmetrical deformation vibration (äas(PO2)). 3.1.3. DTA Curve of MgIn2P4O14. To clarify the thermal stability of the newly discovered ternary compound MgIn2P4O14, the DTA curve of MgIn2P4O14 from room temperature to 1200 °C was collected and presented in Figure S3 in the Supporting Information. It can be seen that the sample continues to release heat slowly and constantly, as the temperature increases from ∼430 °C to 1150 °C. This can be ascribed to the continuously improving degree of crystallinity, which is a process of liberating 12904


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Figure 2. (a) Representation of MgIn2P4O14 projected along the b-axis direction. (b) Projected view of the selected [MnO4n+2]∞ layer of panel (a). (c) Infinite [MnO4n+2] chains formed by M1O6 and M2O6 tetrahedrons through sharing edges running along the [101] direction and the fundamental building blocks, namely, P2O7 and M3O14.

Figure 3. (a) Photoluminescence excitation and emission spectra of representative phosphor MgIn2P4O14:Tm0.04. (b) Plot of the emission peak intensity (458 nm) versus Tm doping concentration (x). (c) Plot of log(I/x) vs log(x).

energy. When the temperature reaches ∼1152 °C, MgIn2P4O14 starts to melt and reaches the highest melting rate at ∼1183 °C. No extra phase transformation was observed throughout the heating process.

3.2. Synthesis of Tm3+/Dy3+-Doped MgIn2P4O14 Phosphors. Considering the similar valence, similar coordination in the oxide and effective ionic radii of Dy3+ (r = 0.91 Å) and Tm3+ (r = 0.87 Å) onto In3+ (r = 0.81 Å), it is expected that RE ions 12905


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Figure 4. Photoluminescence excitation and emission spectra of Dy single-doped phosphors (MgIn2P4O14:Dy0.04 as representative).

would be preferred to occupy the In3+ sites in the MgIn2P4O14 host. A series of Tm3+ and Dy3+ single- and co-doped MgIn2P4O14 phosphors were synthesized. The phase assemblage and structure of as-prepared phosphors were investigated by analyzing their XRD patterns. Figure S4 in the Supporting Information depicts the XRD patterns of several representative Tm3+/Dy3+-doped MgIn2P4O14 phosphors. As can be seen from Figure S4, XRD patterns of the representative phosphors show good consistency with the pattern of MgIn2P4O14 and no impurities are detected, implying that the doping ions Tm3+ and Dy3+ are completely dissolved into the In3+ sites of MgIn2P4O14 lattice without inducing any significant changes in the crystal structure. 3.3. Photoluminescence Properties of Tm3+ /Dy3+ Single-Doped MgIn2P4O14 Phosphors. 3.3.1. UV-Excited Luminescence Properties of MgIn2P4O14:Tm Phosphors. The excitation and emission spectra of representative phosphor MgIn2P4O14:Tm0.04 are shown in Figure 3a. Monitored by the characteristic emission of Tm3+ at 458 nm, a predominant peak at 355 nm corresponding to the f−f transition 3H6 → 1D2 was observed.25 Under the excitation of 355 nm radiation, the emission spectrum exhibits a strong peak at 458 nm (1D2 → 3F4) and a weaker one at 481 nm (1G4 → 3H6), both of which belong to the characteristic emissions of Tm3+ ions that originated from f−f inner shell transitions. Figure 3b shows the curve of emission peak intensity versus the Tm3+ ion doping concentration. As the Tm3+ doping concentration x increases, the emission intensity first rises and then declines, peaking at x = 0.04, indicating a quenching effect when x > 0.04. According to Blasse’s report, the mean volume for per activator ion at quenching concentration can be written as26−28 3 4 4 ⎛R ⎞ V = πr 3xcN = π ⎜ c ⎟ xcN 3 3 ⎝2⎠

where xc is the critical concentration of the activator ion, N represents the number of lattice sites that can be occupied by activator ions in unit cell, and V is the volume of the unit cell. The critical transfer distance (Rc) of MgIn2P4O14:Tm0.04 is calculated to be 16.02 Å, with xc, N, and V taking the values of 0.04, 12, and 1032.56 Å3, respectively. This value is much larger than the critical transfer distance limitation (5 Å) between adjacent activator ions for the concentration quenching mechanism of exchange interaction.29 Accordingly, it cannot be the explanation for the quenching phenomenon. Considering that the overlapped region between excitation and emission spectra of phosphor MgIn2P4O14:Tm is limited, radiation reabsorption would not occur significantly in this case. Thereby, it is plausible to presume that resonant multipolar interaction should be the concentration quenching mechanism. To further clarify this deduction, Dexter-Schulman’s theory30 was utilized. The relationship between emission intensity (I) and activator concentration (x) can be expressed by the following equation:

I = k[1 + β(x)θ /3 ]−1 x

where θ is an indication of electric multipolar character, and k and β are constants for a certain host under the same excitation conditions. There can be three different quenching mechanisms, depending on the θ value, namely, dipole−dipole (d−d), dipole−quadrupole (d−q), and quadrupole−quadrupole (q− q) for θ = 6, 8, and 10, respectively. By applying logarithm manipulation on both sides of eq 3, it is converted to ⎛I⎞ θ log⎜ ⎟ = − log(x) + c ⎝x⎠ 3

(5)

The value of θ can be obtained by the slope (−θ/3) of the fitting line in the plot of log(I/x) versus log(x). As is shown in the inset of Figure 3c, the fitted slope is −1.79, that is, θ is closest to 6, shedding light on the fact that the resonant d−d interaction31,32 is the crucial quenching mechanism for the MgIn2P4O14:Tm phosphors. 3.3.2. UV-Excited Luminescence Properties of MgIn2P4O14:Dy Phosphors. Figure 4 exhibits the excitation and emission spectrum of representative phosphor MgIn2P4O14:Dy0.04. Monitored at 573 nm, the excitation

(2)

Thus, the critical transfer distance can be calculated by the following formula: ⎛ 3V ⎞1/3 R c = 2⎜ ⎟ ⎝ 4πxcN ⎠

(4)

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Figure 5. (a, c, e) Vacuum ultraviolet (VUV) excitation and emission spectra of MgIn2P4O14:Tm0.04 and (b, d, f) MgIn2P4O14:Dy0.04 measured at various low temperatures within a temperature range of 27−298 K.

452 nm (6H15/2 → 4H11/2), and 474 nm (6H15/2 → 4F9/2).29,33,34 All of the peaks are attributed to the f−f radiative transitions of Dy3+ ions, among which the one at 347 nm has the highest intensity. Upon excitation at the UV (347 nm), three major peaks

spectrum exhibits a series of peaks in the range of 290−500 nm, namely, 294 nm (6H15/2 → 4L13/2), 322 nm (6H15/2 → 6P3/2), 336 nm (6H15/2 → 4F5/2), 347 nm (6H15/2 → 6P7/2), 362 nm (6H15/2 → 6P5/2), 385 nm (6H15/2 → 4I13/2), 424 nm (6H15/2 → 2G11/2), 12907


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Figure 6. (a) Comparison of both excitation and emission spectra of MgIn2P4O14:Tm with the excitation spectra of MgIn2P4O14:Dy. (b) CIE chromaticity diagram of phosphors MgIn2P4O14:Tm0.04 (λex = 355 nm) and MgIn2P4O14:Dy0.04 (λex = 347 nm).

at 481 nm (4F9/2 → 6H15/2), 573 nm (4F9/2 → 6H13/2), and 668 nm (4F9/2 → 6H11/2) can be observed in the emission spectrum, which correspond to the characteristic emission of trivalent Dy.29,33,34 In the inset of Figure 4, it is obvious that the quenching concentration for the MgIn2P4O14:Dyy phosphor is 0.04 as well, and the quenching mechanism is also ascribed to the d−d interaction. 3.3.3. VUV-Excited Luminescence Properties of MgIn2P4O14:Tm and MgIn2P4O14:Dy Phosphors. In order to investigate the luminescence behaviors dependence on temperature in the VUV region, VUV excitation and emission spectra of Tm3+/Dy3+-doped MgIn2P4O14 phosphors were measured at different temperatures. Figures 5a, 5c, and 5e correspond to the result of representative phosphor MgIn2P4O14:Tm0.04, and Figures 5b, 5d, and 5f are for representative phosphor MgIn2P4O14:Dy0.04. From Figures 5a and 5b, it can be seen that, besides the characteristic f−f transitions of doping ions, a strong broad excitation band ranging from 150 nm to 270 nm can be observed. Since the broad band does not belong to the characteristic transition of Tm3+ or Dy3+ ions, it should be classified as the intrinsic host absorption of MgIn2P4O14.35−40 Excited by the radiation of the characteristic wavelength of Tm3+/Dy3+, the VUV emission spectra of both MgIn2P4O14:Tm0.04 and MgIn2P4O14:Dy0.04 exhibit similar profile shapes to those obtained from the UV measurements (see Figures 5c and 5d). Nevertheless, when excited by 216 nm (approximately the maximum of the broad excitation band), wide instinct emission bands located in the visible region ranging from 330 to 600 nm appear in the emission spectra for both MgIn2P4O14:Tm0.04 and MgIn2P4O14:Dy0.04 phosphors (see Figures 5e and 5f). An obvious temperature quenching effect is observed for the instinct emission. The integrated intensity of the blue emission at 27 K is ∼3 times higher than that at 95 K. Furthermore, the peak position of luminescence is red-shifted by 0.03 eV and with a 0.06 eV broadening at the full width at half maximum (fwhm) at 95 K, in comparison to that at 27 K. These results indicate that the intense blue luminescence should be attributed to self-trapped exciton (STE) emission. It could be produced by the radiative recombination of an electron on a donor formed by oxygen vacancies with a hole on an acceptor

consisting of either indium or indium−oxygen vacancy pairs. Analogously, the characteristic emission peaks of Tm3+/Dy3+ show a slightly slower decline (compared to the instinct bands) in intensities with the temperature increasing. 3.4. Photoluminescence Properties of Tm3+/Dy3+ Codoped MgIn2P4O14 Phosphors. 3.4.1. Possible Energy Transfer of MgIn2P4O14:TmDy Phosphors. Comparison of both excitation and emission spectra of MgIn2P4O14:Tm with the excitation spectra of MgIn2P4O14:Dy is shown in Figure 6a. First, the distinct overlap between the excitation spectra of singledoped phosphor MgIn2P4O14:Tm (3H6 → 1D2) and the excitation spectra of single-doped phosphor MgIn2P4O14:Dy (6H15/2 → 6P7/2, 5/2) indicates that the characteristic emission of Tm3+ and Dy3+ could be excited simultaneously by identical radiation. Second, the excitation spectrum of MgIn2P4O14:Dy (6H15/2 → 4H11/2) and the emission spectrum of MgIn2P4O14:Tm (1D2 → 3F4) have a significant overlapped region, implying that effective energy transfer (ET) process from Tm3+ to Dy3+ ions could be expected in the Dy3+/Tm3+ co-doped MgIn2P4O14 phosphors. Moreover, the connection line between the chromaticity coordinates points of Tm3+/Dy3+ single-doped MgIn2P4O14 phosphors passes exactly through the white light area, according to Figure 6b. Therefore, the MgIn2P4O14:TmDy single-component phosphors are expected to generate white light under the appropriate Tm and Dy concentrations. The characteristic f−f transitions and the possible ET process of Tm3+ and Dy3+ ions in MgIn2P4O14:TmDy are illustrated in the schematic energy level diagram (see Figure 7). 3.4.2. Energy Transfer Mechanism of MgIn2P4O14:TmDy phosphors. In order to investigate the ET process in Tm3+/Dy3+ co-doped MgIn 2P4O14, a selection of typical phosphors MgIn2P4O14:Tm0.04Dyy were synthesized. Figure 8 gives the emission spectra of these phosphors MgIn2P4O14:Tm0.04Dyy (y = 0.005, 0.01, 0.03, 0.04, 0.05) under the excitation of 351 nm radiation. It can be seen that the intensity of Tm3+ emission declines fast while the Dy3+ emission increases gradually with the Dy3+ doping concentration increasing (see the inset of Figure 8). Based on the foregoing analysis, it is plausible to conclude that the resonant multipolar interaction, rather than exchange interaction, is the corresponding ET mechanism.41 The 12908


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Figure 7. Schematic energy level diagram illustrating characteristic excitation and emission of Tm3+ and Dy3+, and energy transfer process from Tm3+ to Dy3+ (NR denotes nonradiative transition).

Figure 9. Energy transfer efficiencies from Tm3+ to Dy3+ in phosphor MgIn2P4O14:Tm0.04Dyy.

multipolar interaction consists of three different types, namely dipole−dipole (d-d), dipole−quadrupole (d-q), and quadrupole−quadrupole (q-q) interaction, similar to that of the concentration quenching mechanism. Hence, further analysis was implemented to define the type of reaction by applying the Dexter’s energy transfer theory and Reisfeld’s approximation via the following formula:42,43 IS0 ∝ C θ /3 IS (6)

3.4.3. Luminescence Decay Properties of MgIn2P4O14:TmDy Phosphors. Luminescence decay time measurements were performed to further analyze the ET efficiency. Figure 10 shows typical luminescence decay curve of 1D2 → 3F4 emission of the Tm3+ at 458 nm for the MgIn2P4O14:Tm0.04Dyy phosphors under the excitation of 351 nm radiation. Double exponential decay mode44 can fit well with the decay curve of Tm3+: ⎛ t ⎞ ⎛ t⎞ I(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠

where IS0 and IS represent the emission intensity of Tm3+ ions in the absence and presence of Dy3+ ions, respectively, C is the sum of doping ions concentration, and θ is an indicator of the ET mechanism type (θ = 6, 8, and 10 correspond to dipole−dipole (d-d), dipole−quadrupole (d-q), and quadrupole−quadrupole (q-q) interaction, respectively). Figure 9 exhibits the linear fitted line by assigning different values to θ. Obviously, when θ = 6, it finds the best match of the linear relationship, which demonstrates that the ET mechanism from Tm3+ to Dy3+ in the MgIn2P4O14:Tm0.04Dyy phosphors is mainly attributed to the type of multipolar d-d interaction.

(7)

where t is time, I(t) is the luminescence intensity at time t, A1 and A2 are weighting parameters, and τ1 and τ1 are decay lifetime components. The average decay life (τav) can be calculated using the following equation: τav =

A1τ12 + A 2 τ2 2 A1τ1 + A 2 τ2

(8)

Figure 8. Photoluminescence emission spectra of MgIn2P4O14:Tm0.04Dyy phosphors with different Dy3+ doping concentrations under the excitation of 351 nm. Inset shows dependence of emission intensities peaking at 458 and 573 nm on Dy3+ concentration. 12909


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sion Internationale de L’Eclairage (CIE) chromaticity diagram of a set of MgIn2P4O14:TmxDyy phosphors under the excitation of 351 nm. By varying the doping concentration of Tm3+ and Dy3+ ions, the emitting color can change from blue to yellow, passing through the white light region, which is consistent with our previous predictions based on Figure 6b. The phosphor components and their corresponding CIE chromaticity coordinates in Figure 11 are listed in Table 2. It is clear that the Table 2. CIE Chromaticity Coordinates and Correlated Color Temperature (CCT) of Tm3+/Dy3+-Doped Phosphors Calculated by the PL Spectra under 351 nm Excitation

3+

Figure 10. Photoluminescence decay curves for Tm ions in MgIn2P4O14:Tm0.04Dyy phosphors (λex = 351 nm, λem = 458 nm).

The value of τ a v of the Tm 3+ emission for the MgIn2P4O14:Tm0.04Dyy phosphors are calculated to be 28.0, 27.9, 25.9, 22.6, 21.0, and 19.7 μs, corresponding to y = 0, 0.005, 0.01, 0.03, 0.04, and 0.05, respectively. Obviously, the increase of the Dy3+ doping concentration results in a downtrend in the decay time of Tm3+ emission. The ET efficiency ηET can be estimated, in terms of decay time, by using the following equation:45−47 τ ηET = 1 − s τs0 (9)

label

components

(x, y)

CCT (K)

1 2 3 4 5 6 7 8 9 10

Mg(In0.96Tm0.04Dy0.000)2P4O14 Mg(In0.955Tm0.04Dy0.005)2P4O14 Mg(In0.95Tm0.04Dy0.01)2P4O14 Mg(In0.93Tm0.04Dy0.03)2P4O14 Mg(In0.92Tm0.04Dy0.04)2P4O14 Mg(In0.91Tm0.04Dy0.05)2P4O14 Mg(In0.95Tm0.03Dy0.02)2P4O14 Mg(In0.95Tm0.02Dy0.03)2P4O14 Mg(In0.95Tm0.01Dy0.04)2P4O14 Mg(In0.96Tm0.00Dy0.04)2P4O14

(0.174, 0.093) (0.228, 0.170) (0.267, 0.230) (0.311, 0.303) (0.319, 0.316) (0.329, 0.331) (0.299, 0.291) (0.342, 0.346) (0.362, 0.375) (0.380, 0.401)

6859 6237 5663 8021 5108 4525 4169

coordinates of the phosphors labeled “4”, “5”, “6”, and “8” are all close to standard white light coordinate (0.333, 0.333). In addition, based on the analysis on the type of light white according to the correlated color temperature (CCT), the color of the Tm3+/Dy3+ co-doped MgIn2P4O14 phosphors can be tuned to warm white by increasing Dy3+ ions with CCT < 5000 K. To calculate the CCT, the most widely used equation proposed by McCamy was applied:49

where τs and τs0 are the decay times of Tm3+ emission in the presence and absence of Dy3+ ions, respectively. The inset of Figure 10 shows that the ET process becomes more efficient with the increase of Dy3+ doping concentration, which is consistent with the literature results.8,41,48 3.4.4. Color-Tunable Luminescent Properties of MgIn2P4O14:TmDy Phosphors. Figure 11 depicts the Commis-

CCT = 449.0n3 + 3525.0n2 + 6823.3n + 5520.22

(10)

where n=

x − xe ye − y

xe = 0.3320 ye = 0.1858

(11) 3+

As the concentration of Dy increases, the colors of MgIn2P4O14:TmxDyy phosphors excited by 351 nm change from the cold white region (point 4) to the warm white region (point 10). The most attractive phosphor is MgIn2P4O14:Tm0.01Dy0.04 with a chromaticity coordinate of (0.362, 0.375). It presents neutral white light emitting at CCT = 4525 K, which can cater to the requirement of wLEDs. 3.5. Thermal Quenching Behavior of Tm3+/Dy3+ Codoped MgIn2P4O14 Phosphors. As is known to all, heat generation (at ∼200 °C) during LED operation usually leads to emission losses, which can be described as a thermal quenching (TQ). Notably, thermal quenching (TQ) of luminescence is one of the significant indexes to be considered for LED phosphors, because it has a considerable influence on the light output during service. The temperature dependence of the luminescence property for the representative MgIn 2 P 4 O 14 :Tm0.02 Dy0.03 phosphor excited by 351 nm UV radiation was investigated. Temperature-dependent emission spectra of MgIn2P4O14:Tm0.02Dy0.03 phosphor measured from 25 °C to 300 °C is illustrated in Figure 12a, with dependence of integral

Figure 11. CIE chromaticity diagram of a series of phosphors MgIn2P4O14:TmxDyy under the excitation of 351 nm (x = 0−0.05, y = 0−0.04). 12910


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Figure 12. (a) Photoluminescence emission spectra of MgIn2P4O14:Tm0.02Dy0.03 phosphor measured at various temperatures from 25 °C to 300 °C under the excitation of 351 nm radiation. (b) Dependence of integral emission intensity on test temperatures.

yields of MgIn2P4O14:Dy0.04, MgIn2P4O14:Tm0.01Dy0.04, and MgIn2P4O14:Tm0.02Dy0.03 phosphors under the excitation of 351 nm ultraviolet radiation are measured by monitoring within the luminescence range of 400−690 nm, which are 70.50%, 53.24% and 52.31%, respectively. Compared to the absolute quantum yields of the Tm3+/Dy3+ co-doped MgIn2P4O14 phosphors, it can be seen that the absolute quantum yield of single-doped MgIn2P4O14:Dy0.04 is much higher than that of the others. Considering that the concentration quenching effect of both Tm3+ and Dy3+ single-doped MgIn2P4O14 phosphors occur at a doping content of 0.04, it can be reasonably presumed that the increase of the overall doping concentration in the co-doped phosphors leads to an enhancement of the cross-relaxation effect between the luminescence centers. Therefore, more absorbed photons are consumed in the resonant process and the emitted photons are reduced correspondingly.

emission intensity on test temperatures presented in Figure 12b. MgIn2P4O14:Tm0.02Dy0.03 phosphor exhibits high stability in luminescence intensity as the temperature (T) increases from 25 °C to 175 °C. When T is increased to 200 °C, the luminescence intensity remains as high as 91% of its initial value at room temperature. Moreover, no shift in the peak position occurs over the entire temperature range from 25 °C to 300 °C. It suggests that the phosphor has a good thermal stability, with regard to the color of luminescence. As is shown in Figure 12b, the loss of the overall integral intensity is 200 °C in this case), the effect of temperature on NRTP is greatly improved, compared to that of ETE. Thus, the compensation mentioned above cannot be achieved anymore and the overall luminescence intensity declines fast. Overall, a novel phosphate host with rigid bonding networks and containing disordering cationic occupation is beneficial to energy transfer and initiates the generation of excellent TQ behavior. 3.6. Quantum Efficiencies of Representative Tm3+/ Dy3+-Doped MgIn2P4O14 Phosphors. The absolute quantum

4. CONCLUSION Based on an ab initio method, the crystal structure of a novel magnesium indium pyrophosphate MgIn2P4O14 was determined in space group C2/c (No. 15) with a unit cell of dimensions of a = 9.8733(3) Å, b = 8.7595(2) Å, c = 12.5931(3) Å, â = 108.546(2)°, V = 1032.56(5) Å3, and Z = 4. The framework of MgIn2P4O14 can be regarded as built of anionic (PO4) and cationic (MO6) layers arranged alternatively. Each cationic layer consists of an infinite parallel wavy chain MnO4n+2, extending parallel to the [101] direction. A series of Tm3+/Dy3+ single- and co-doped MgIn2P4O14 phosphors were synthesized via conventional high-temperature solid-state reaction. Tunable emitted light colors from blue to white to yellow can be obtained by varying the Tm3+/Dy3+ doping contents in a single-component MgIn2P4O14:TmDy phosphor. In Tm/Dy co-doped MgIn2P4O14 phosphors, ET process from Tm3+ to Dy3+ can be explained by the dipole−dipole interaction mechanism. Under VUV excitation, except for the characteristic f−f peaks of the doping trivalent ions, broad instinct excitation (150−270 nm) and emission (330−600 nm) bands with high thermal quenching (TQ) features were observed far below room temperature. The investigation on the TQ property for the MgIn2P4O14:Tm0.02Dy0.03 phosphor demonstrates excellent thermal stability, given that the overall integral intensity at 150 12911


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Inorganic Chemistry °C can remain 99.5% of its initial value at room temperature. Integrating present results, the novel phosphate host MgIn2P4O14 with rigid bonding networks and disordering cationic occupation favoring the energy transfer of Tm/Dy activators initiates the generation of single-component whiteemitting phosphors with excellent TQ behavior.

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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.7b01670. Projection of MgIn2P4O14 along the c-axis (Figure S1); IR spectrum of MgIn2P4O14 (Figure S2); TGA/DTA curves of MgIn2P4O14 (Figure S3); X-ray diffraction patterns of MgIn2P4O14 host and representative Tm/Dy-doped MgIn2P4O14 phosphors (Figure S4); nominal and measured compositions of MgIn2P4O14 and Tm3+/Dy3+ single- and co-doped MgIn2P4O14 series determined via ICP-AES (Table S1); atomic parameters of MgIn2P4O14 (Table S2); selected interatomic distances (Å) and angles (deg) of MgIn2P4O14 (Table S3); bond valence analysis of MgIn2P4O14 (Table S4) (PDF) Accession Codes

CCDC 1532384 contains 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 data_ [email protected], 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 Author

*E-mail: [email protected]. ORCID

Ge-Mei Cai: 0000-0001-9046-6397 Author Contributions

All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial supports by grants from the National Natural Science Foundation of China (Nos. 51772330 and 51472273), National Key Research and Development Plan (No. 2016YFB0701301), the Major State Basic Research Development Program of China (No. 2014CB6644002), BSRF, and the Project of Innovationdriven Plan in Central South University (No. 2015CX004) are gratefully acknowledged. Authors thank Prof. Z.G. Xia for his valuable suggestion of Beijing Municipal Key Lab for Advanced Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China.

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