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Sep 21, 2017 - ABSTRACT: Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here ...
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Luminescence Tuning, Thermal Quenching, and Electronic Structure of Narrow-Band Red-Emitting Nitride Phosphors Dianpeng Cui, Zhen Song, Zhiguo Xia, and Quanlin Liu* The Beijing Municipal Key Laboratory of New Energy Materials and Technologies, School of Materials Sciences and Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: Exploring high-performance narrow-band red-emitting phosphor is an important challenge for improving white light LEDs. Here, on the basis of three interesting nitride phosphors with similar vierer rings framework structure, two phosphor series, Eu2+-doped Sr(LiAl)1−xMg2xAl2N4 and Sr(LiAl3)1−y(Mg3Si)yN4 (x, y = 0−1), are successfully synthesized by a solid state reaction. They show narrowband red emission with tunable emission peaks from 614 to 658 nm and 607 to 663 nm. The varying luminescence behaviors with composition and structure are discussed based on centroid shift, crystal field splitting and Stokes shift. On the basis of experimental data, we construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/ trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors.

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INTRODUCTION Red phosphors, as a key component for general solid-state lighting, can improve color rendering index (CRI) and correlated color temperature (CCT).1−3 Besides, narrow-band red phosphors also balance optimal luminous efficacy and CRI, viz., enhancing luminous efficacy in comparison with conventional red phosphors under a comparable CRI. Moreover, redemitting narrow-band phosphor can provide a wide color gamut in the display area.4−7 Therefore, since the report of narrow-band red phosphor SrLiAl3N4:Eu2+,8 it has come into the public eye rapidly. SrLiAl3N4:Eu2+ could be synthesized by solid state reaction using different raw materials,9−11 with the emission peak centering at 650 nm. The width of emission peak is about half of that in CaAlSiN3. Hu et al.12 have reported the substitution of a portion of Sr with Ca. Besides the photoluminescence, its electronic structure has also been investigated.13,14 In addition, to protect luminescent materials from high-humidity atmosphere, Tsai et al.15 have improved its water resistance through coating with an organosilica layer. As is well-known, SrLiAl3N4 owns the vierer rings framework structure. These rings are comprised of four edge- and cornersharing [AlN4] and [LiN4] tetrahedra which are connected to each other.16 They arrange in an up−down sequence forming channels. In every second channel, it contains a strand of Sr sites which are centered in cuboidal [SrN8] polyhedra. The vierer rings framework contains only quaternary ammoniumtype N[4] atoms, building a rigid network and a maximum degree of condensation (i.e., atomic ratio (Li,Al):N) κ = 1 as well as that in AlN. This framework structure is favorable for narrow emission band and high thermal stability. The © 2017 American Chemical Society

compounds of vierer rings framework also include CaLiAl3N4, SrMg3SiN4, M[Mg2Al2N4] (M = Ca, Sr, Ba),17−19 etc. Their crystal structures are similar to SrLiAl3N4 and Eu2+-doped phosphors have a narrow-band red emission, especially SrMg2Al2N4 and SrMg3SiN4. The structure overview of the three hosts is shown in Figure 1. Because of the similar crystal structure and luminescence behavior, we expect that photoluminescence of Eu2+ can be tuned between SrLiAl3N4 and SrMg2Al2N4 /SrMg3SiN4 through the structural modification. One of the strategies of structural modification is the chemical unit cosubstitution.20 The number of substituted atoms and sum of oxidation states are the same between the two structural units. Based on this concept, SrMg2Al2N4 can be assumed as the substitution of structural unit [Mg−Mg] for [Li−Al] in SrLiAl3N4. Likewise, SrMg3SiN4 is the replacement of [LiAl3] in SrLiAl3N4 by [Mg3Si]. As reported in the articles,8,17,18 under the excitation of 440 nm, the phosphors show emission bands with peak position at 650 nm for SrLiAl3N4:Eu2+, 612 nm for SrMg2Al2N4:Eu2+, and 615 nm for SrMg3SiN4:Eu2+. Obviously, the emission peak position of the latter two phosphors is more typical for application, as compared with that in SrLiAl3N4:Eu2+. We therefore expect that the emission peak can be tuned between the positions in SrLiAl3N4:Eu2+ and the other two phosphors. More specifically, the blue-shift from 650 to 615/612 nm is expected to be realized through the structural evolution from SrLiAl3N4 to SrMg2Al2N4/SrMg3SiN4. In addition, the luminescent thermal Received: July 17, 2017 Published: September 21, 2017 11837

DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844

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Inorganic Chemistry

The relations between photoluminescence, composition and structure will be discussed. We construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes of divalent/trivalent lanthanide-doped endmember compounds, which account for the luminescent thermal quenching behavior of these phosphors.

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EXPERIMENTAL SECTION

Synthesis. Three end-member phosphors, Eu2+- or Sm3+-doped SrLiAl3N4, SrMg2Al2N4, SrMg3SiN4, and intermediate components were synthesized via solid-state reaction in a high-purity nitrogen atmosphere. The raw materials, Sr3N2, Li3N, Mg3N2, EuN, and SmN were prepared through direct nitridation of metallic Sr (99.9%, Beijing Founde Star Science & Technology Co, Ltd.), Li (99.9%, Beijing Founde Star Science & Technology Co, Ltd.), Mg (99%, Lei Ming Mineral Processing Factory), Eu (99.99%, Beijing Founde Star Science & Technology Co, Ltd.) and Sm (99.99%, Beijing Founde Star Science & Technology Co, Ltd.), respectively. The as-prepared materials and AlN (≥97%, Tokuyama) or Si3N4 (α-phase >95%, UBE) were weighed stoichiometrically and ground thoroughly with BaF2 flux in an agate mortar in a N2-filled glovebox (Etelux, H2O < 1 ppm, O2 < 1 ppm). The obtained mixture was transferred into a molybdenum crucible. After firing the powder mixture at 1100 °C for 4 h in a graphite furnace the samples were prepared for measurements. Characterization. The X-ray diffraction (XRD) data were collected on a Rigaku TTR III diffraction (Cu Kα radiation, λ = 1.5418 Å) in the 2θ range from 10 to 90°. The Rietveld refinement was carried out using the Fullprof Suite package.24 The photoluminescence excitation (PLE) and photoluminescence (PL) spectra measurements were performed using a Hitachi F-4600 spectrophotometer equipped with a 150 W xenon lamp as the light source. The temperature-dependent luminescence properties were analyzed by Edinburgh FLS 920 fluorescence spectrophotometer with a 450 W xenon lamp as the excitation source and Hamamatsu R928P photomultiplier providing a low noise level.

Figure 1. Structure overview of the three hosts. The transitions from SrLiAl3N4 to SrMg2Al2N4/SrMg3SiN4 via the cosubstitution of [Li− Al] for [Mg−Mg] and [Li−Al3] for [Mg3−Si].

quenching is an important characteristic of phosphors, and it plays a crucial role in their application. The intermediate compositions in the solid solutions can yield the highest thermal quenching temperature, such as the case in SrxBa2−xSiO4:Eu2+ orthosilicate phosphors.21,22 Im and colleagues have reported a blue-emitting Na3−2xSc2(PO4)3:xEu2+ phosphor, which shows zero emission loss up to a temperature of 200 °C.23 Thus, herein, we study the luminescence tuning, luminescent thermal quenching and electronic structure of Eu2+-doped Sr(LiAl)1−xMg2xAl2N4:Eu2+ and Sr(LiAl3)1−y(Mg3Si)yN4:Eu2+ (x, y = 0−1) series phosphors.

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RESULTS Synthesis and Phase Identification. In order to realize the structural-unit cosubstitution between SrLiAl3N4 and the other two phases, first the three end-member compounds have

Figure 2. XRD patterns of (a) Sr(LiAl)1−xMg2xAl2N4:Eu2+ and (c) Sr(LiAl3)1−y(Mg3Si)yN4:Eu2+. Dependence of the lattice parameters and formula unit cell on (b) x and (d) y (x, y = 0−1). 11838

DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844

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Inorganic Chemistry been prepared separately and their phase identification from Rietveld refinement is presented in Figures S1 and S2. The results reveal that the three end-member phases have been obtained and there are merely some weak reflections of remaining AlN raw materials in the XRD of SrLiAl3N4 and SrMg2Al2N4. Considering the small amount of AlN and the broad band emission centered at 465 nm of AlN:Eu2+,25 it is believed that the secondary AlN phase has no impact on the narrow-band red emission of the three nitride phosphors. Therefore, we can continue to carry out structural modification experiments between SrLiAl3N4 and SrMg2Al2N4 /SrMg3SiN4. The crystallographic parameters and atomic coordinates of Rietveld refinement are listed in Tables S1 and S2. XRD data of structural modification samples are displayed in Figure 2a and c. It is seen that, with the composition change of solid solutions, the diffraction peaks shift regularly from SrLiAl3N4 to the other two phases. In addition, there exists no extra impurity phase. Thus, it can be concluded that a completely structural evolution from SrLiAl 3 N 4 to SrMg2Al2N4/SrMg3SiN4 has been achieved via structural-unit cosubstitution strategy. Furthermore, in Figure 2 the diffraction peaks shift to lower angles gradually with x or y values increasing, which is attributed to substitute larger ions for smaller ions. The effective ionic radius of Mg2+ (0.57 Å, CN = 4) is much larger than that of Al3+ (0.39 Å, CN = 4), a litter smaller than Li+ (0.59 Å, CN = 4), while Si4+ has a radius of 0.26 Å (CN = 4).26 As a result, when the structural unit [Mg− Mg] replaces [Li−Al] and [Mg3Si] substitutes for [LiAl3], the net effect is to expand the cell volume, which is confirmed by the plot of formula unit cell, i.e., unit cell volume divided by formula units per unit cell, against x or y in Figure 2b and d. However, it is very interesting to note that a significant structural transition happens at about x = 0.2 and y = 0.15, with the indication of the discontinuous change of lattice parameters. Before and after the structural transition, the lattice parameters all show a linear dependence on x or y, in agreement with Vegard’s law. Because the structural transition occurs at x and y values smaller than 0.5, it can be inferred that SrLiAl3N4 has a more rigid structure, in which a small amount of replacement leads to the collapse of the original structure. On the contrary, the structures of SrMg2Al2N4 and SrMg3SiN4 have more tolerance to accommodate the structural unit substitution. There is no doubt that the structural change will inevitably cause the variations of luminescence property. Controllable Photoluminescence Tuning. PLE spectra of the three phosphors show a broad-band absorption which can be efficiently excited by blue light chip (Figure 3). When excited by 460 nm blue light, the three phosphors all emit relatively narrow-band red luminescence. Specifically, the emission peak of SrLiAl3N4:Eu2+ is located at λem = 648 nm with fwhm (full-width at half-maximum) of ∼1092 cm−1 (∼50 nm), SrMg2Al2N4:Eu2+ at λem = 614 nm with fwhm of ∼1826 cm−1 (∼71 nm) and SrMg3SiN4:Eu2+ at λem = 607 nm with fwhm of ∼1404 cm−1 (∼52 nm). As depicted in Figures 4 and 5, luminescence property is affected by the structural modification. Especially, the emission band first undergoes a red-shift from 648 to 657 or 663 nm and then a blue-shift to 614 or 607 nm. Besides, in triclinic phase range, the emission spectra become wider with x/y increase. The excitation spectra have little change except an obvious blue-shift for Sr(LiAl3)1−y(Mg3Si)yN4:Eu2+ (y ≥ 0.15) samples. These phenomena will be discussed in detail in the following part.

Figure 3. PLE and PL spectra of three end-member phosphors.

Figure 4. PLE and PL spectra of the phosphors Sr(LiAl)1−xMg2xAl2N4:Eu2+ (x = 0−1).

Figure 5. PLE and PL spectra of the phosphors Sr(LiAl3)1−y(Mg3Si)yN4:Eu2+ (y = 0−1).

Thermal Quenching Property. Thermal stability is a critical parameter for application in solid-state lighting on account of its dramatic impact on the correlated color temperature, color rendering index, chromaticity point and luminous efficiency of a light source.27−32 With the temperature increasing, the luminescence intensities decrease gradually as displayed in Figure 6, in accordance with expectations. The SrLiAl3N4:Eu2+ end-member has the best luminescent thermal stability, which deteriorates gradually as x or y increase. However, for x = 1 and y = 1, the thermal stability of these two 11839

DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844

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Inorganic Chemistry

recent years, especially accompanying the development of the white LEDs. Dorenbos has conducted a systematic study of the energies of the lowest 4fn−15d1 states of Eu2+/Ce3+ ions.35 The centroid shift εc depends on the nephelauxetic effect, i.e., being related to covalency between the lanthanide ion and the anion ligands of host lattice.36 The crystal-field splitting ΔCFS is associated with coordination environments of RE ions. In summary, both centroid shift and crystal-field splitting control the excitation spectrum features. The location of emission band originated from the lowest-5d-to-4f is dependent on Stokes shift, usually being illuminated by using the configurational coordinate model, which is also dominated by the crystal structure of the host. Photoluminescence Excitation Spectra. As mentioned above, the lattice parameters and cell volume show a linear increasing trend before and after the structural transition. In general, the volume accommodated by Eu2+ ion becomes larger as the cell volume increases. We calculate coordination volumes of the three end-member phosphors, as listed in Table 1. The Table 1. Calculated coordination Volumes for [(Sr,Eu)N8] Polyhedra Figure 6. Temperature-dependent PL spectra of (a) Sr(LiAl)1−xMg2xAl2N4:Eu2+ and (b) Sr(LiAl3)1−y(Mg3Si)yN4:Eu2+ (x, y = 0, 0.1, 0.2, 0.3, 0.7 and 1).

SrLiAl3N4 SrMg2Al2N4 SrMg3SiN4

end-members recover better than some of the intermediate components, such as line 5 in Figure 6.

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experimental values (Å3)

reference values8,17,18 (Å3)

34.89, 34.91 35.32 37.28

34.00, 34.26 34.27 36.95

experimental and reference values are comparable. The coordination volumes for [(Sr,Eu)N 8 ] polyhedra in SrMg2Al2N4 and SrMg3SiN4 are larger than that in SrLiAl3N4. It is believed that the coordination space of Eu2+ keeps an increase trend for intermediate components with the cell volume increasing linearly. The larger coordination volume of Eu2+ usually makes the crystal-field splitting decrease, causing PLE blue-shift.21,37 We adopt the change of the lowest 5d absorption energy Efd to show PLE shift. The lowest 5d absorption band is represented as the projection position; namely, take the position of zero-phonon energy as the center, then project the distance between the position of zero-phonon energy and the emission wavelength onto the excitation spectrum, where the projection position is the lowest 5d absorption band. As shown in Table S4 and Figure 8a, the lowest 5d absorption energy Efd increases gradually, corresponding to PLE blue-shift, which is consistent with the above analysis. Photoluminescence Spectra. The emission band is dependent on the lowest 5d level and the Stokes shift. The Stokes shift is determined as 2-fold difference between the emission wavelength and the position of zero-phonon energy. As plotted in Figure 8b, the Stokes shift obviously increases with x or y values before the structural transition. Since the lowest 5d absorption band has a little increase, the red-shift of PL spectra can be primarily ascribed to the Stokes shift for S r (L iAl ) 1 − x M g 2 x A l 2 N 4 : E u 2 + ( x ≤ 0 . 1 5) a n d S r (LiAl3)1−y(Mg3Si)yN4:Eu2+ (y ≤ 0.1) as mentioned above. After the structural transition, the Stokes shift has a decrease tendency, which generates the blue-shifted emission band, as shown in Figure 4 and 5. Besides the luminescence peak position, the structural modification also has a significant influence on the fwhm of emission band. The fwhm values increase before the structural transition, and, then, they decrease after the structural

DISCUSSION Generally, the luminescent properties of RE ions involving 4fn4fn−15d (f−d) transition in such ions as Eu2+ or Ce3+, are highly sensitive to the host lattice because of strong interaction of 5delectrons with the neighboring anion ligands. This f−d transition is a parity-allowed electric dipole transition and, therefore, high quantum efficiencies of absorption and emission could be achieved. As shown in Figure 7, there exists a large

Figure 7. Relations between luminescence and ligand surrounding of rare-earth ions.

energy gap between the 4f ground state and the 5d excited state for a free ion, e.g., ∼4.216 eV (34,000 cm−1) for Eu2+ and ∼6.2 eV (50,000 cm−1) for Ce3+ ions.33,34 The gaps can substantially be decreased by a downward shift of the 5d centroid (εc) and crystal field splitting (ΔCFS), causing absorption from f−d transition in the region of violet or blue light. Therefore, Eu2+/ Ce3+-doped phosphors have attracted increasing attention in 11840

DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844

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Figure 8. Correlation of (a) the lowest 5d absorption energy Efd, (b) Stokes shift, (c) fwhm, and (d) spectral emission intensity versus x and y (x, y = 0−1). All data on these values are listed in Table S4.

Figure 9. (a−c) Energy level schemes of the divalent (blue) and trivalent (red) lanthanides in the three hosts; the dashed line represents the exciton creation energy (Ex) in the host lattice. (d) Vacuum referred binding energies of electrons at the bottom of the conduction band (Ec) and the top of the valence band (Ev); blue horizontal bars denote the VRBE of electrons in the 5d excited state, red bars in the 4f ground state of Eu2+ and black solid symbols indicate the exciton energy.

Stokes shift, which is signed as the equilibrium distance between the bottoms of two parabolas in the configurational coordinate diagram. One parabola stands for the ground state and the other is the excited state. The less rigid structure can make the equilibrium distance increase, which results in a larger Stokes shift, and vice versa. Therefore, as shown in Figure 8b, the change of Stokes shift is a reflection of the less rigid structure of the intermediate components. Simultaneously, this

transition, as shown in Figure 8c, due to the size mismatch of substituted atoms. Generally, the intermediate compositions have wider fwhm due to the larger disorder degree. As is wellknown, the three end-member phosphors have a relatively narrow emission band due to the rigid crystal structure. Now the substituted structural units leading to fwhm broadening inevitably have an impact on the rigidity of the intermediate components. This can be demonstrated from the change in 11841

DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844

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Figure 10. (a−c) PLE and PL spectra of the three Sm-doped end-member phosphors. (d) Diffuse reflectance spectra of the three hosts and their corresponding optical band gaps at RT.

than the exciton creation energy Ex.41 Generally, the value Ex is derived at 10 K, otherwise it is 0.1−0.2 eV larger than the optical band gap at RT.42,43 To pin the 4f-zigzag curve in the scheme, the charge transfer (CT) energy of Sm3+ is measured via the CT-model.44 The PLE and PL spectra of Sm3+-doped end-member phosphors are shown in Figure 10, giving the CT energy which is listed in Table 2. With the data on the ΔEvf(n+1,7,2+) parameters,45 the

change is in line with the fwhm because both are influenced by the rigidity of structure. Thermal Quenching Property and HRBE/VRBE Schemes. Thermal quenching behavior is an important characteristic of phosphor, and it plays a key role in the application. Two mechanisms regarding thermal quenching of Ce3+/Eu2+ d−f emission have been proposed. One quenching process is caused by the thermal stimulated ionization process from the 5d to the conduction band.28,38 This thermal quenching temperature is mainly determined by the energy difference between 5d excitation level and the conduction band bottom. The other thermal quenching mechanism is usually ascribed to a large displacement between the ground and excited states in the configurational coordinate diagram.39 This thermal quenching temperature is generally related to structure rigidity.22,40 SrLiAl3N4:Eu2+ phosphor has a higher thermal quenching temperature than the other end-member phosphors, i.e., SrMg2Al2N4:Eu2+ and SrMg3SiN4:Eu2+, as shown in Figure 6. This phenomenon can be explained by thermal stimulated ionization of 5d electron to conduction band. With the data on the optical band gap, the charge transfer (CT) energies of Sm3+, the centroid shift, and the lowest 5d level, we can construct the host referred binding energy (HRBE) and vacuum referred binding energy (VRBE) schemes for the three end-member phosphors. The HRBE scheme presents the location of the 4fn and 4fn−15d levels of all trivalent and divalent rare earth ions relative to the top of the valence band. In the VRBE scheme, all level energies in HRBE scheme are related and converted to that of an electron at rest in vacuum at an infinite distance.35 In order to picture the HRBE scheme, the energy Evc between the top of the valence band Ev and the bottom of the conduction band Ec is derived from the measured optical band gap at room temperature (RT). Evc is assumed 8% larger

Table 2. Overview of the Experimental Values Obtained for the Band Gap, the CT Band of Sm3+, and the Lowest 5d Absorption Band of Eu2+ in Three End-Member Phosphors optical band gap (eV) Sm3+ CT (eV) Efd (eV)

SrLiAl3N4

SrMg2Al2N4

SrMg3SiN4

4.90 3.90 2.13

3.90 3.46 2.29

4.10 3.54 2.28

4f-zigzag curve for the divalent lanthanides is pinned. In addition, we need U(6,A), which is known as the Coulomb repulsion energy, to pinpoint the 4f-zigzag curve for the trivalent lanthanides. U(6,A) is defined as the energy difference between the ground state energy of Eu2+ and Eu3+. Because the values for U(6,CaAlSiN3) and U(6,M2Si5N8) are about 6 eV,46,47 as a reference, the value for U(6,A) has been estimated to be 6 eV in this work. Combining the 4f-zigzag curve for the divalent lanthanides and the known data on the ΔEvf(n,6,3+) parameters,45 the 4f-zigzag curve for the trivalent lanthanides can be pinned. To place the lowest 5d-state location in the scheme, the energy difference Efd between the 4f ground state and the lowest 5d excited state is essential to know. Efd is acquired from the luminescence spectroscopy. After Efd(7,2+,A) is obtained in 11842

DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844

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Inorganic Chemistry this work, Efd(1,3+,A) can be calculated according to the red shift model.33,48,49 Efd(7, 2 + , A) = Efd(7, 2 + , free) − D(2 + , A)

(1)

D(2 + , A) = 0.64D(3 + , A) − 0.233eV

(2)

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With the data on the ΔEvd (n+1,7,2+) and ΔEvd (n,1,3+) parameters,44 the lowest 5d-level locations for divalent and trivalent lanthanides are placed in the scheme. At this point, all data to construct the HRBE of the divalent and trivalent lanthanides have been given. The schemes for the three end-member phosphors are shown in Figure 9. The lowest 5d-level locations for Eu2+ in SrMg2Al2N4 and SrMg3SiN4 are much closer to the conduction band than that in SrLiAl3N4, as seen in Figure 9d. This can lead to autoionization of Eu2+ and luminescence quenching.50−52 As a consequence, the luminescent thermal stability is lower than that of SrLiAl3N4:Eu2+ phosphor, as exhibited in Figure 6. In addition, there is a relationship between the thermal stability and structural rigidity. Namely, the less rigid structure has a lower luminescence thermal stability and vice versa. As a result, the thermal stability of intermediate components should be not as good as that of the end-member phosphors because the structural rigidity of two terminals is better. Accordingly, it reconfirms that the luminescence thermal stability of SrMg2Al2N4:Eu2+ and SrMg3SiN4:Eu2+ phosphor is lower compared with SrLiAl3N4:Eu2+ phosphor, because the thermal stability of these two end-member phosphors is worse than some of the intermediate components close to the end-member SrLiAl3N4:Eu2+ phosphor, such as line 2, 3, 4 in Figure 6.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Zhiguo Xia: 0000-0002-9670-3223 Quanlin Liu: 0000-0003-3533-7140 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51602019 and 51472028) and Fundamental Research Funds for the Central Universities (FRF-TP-15-012A1).

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REFERENCES

(1) Xia, Z. G.; Liu, Q. L. Progress in discovery and structural design of color conversion phosphors for LEDs. Prog. Mater. Sci. 2016, 84, 59−117. (2) Lin, C. C.; Meijerink, A.; Liu, R. S. Critical red components for next-generation white LEDs. J. Phys. Chem. Lett. 2016, 7, 495−503. (3) Li, J. H.; Yan, J.; Wen, D. W.; Khan, W. U.; Shi, J. X.; Wu, M. M.; Su, Q.; Tanner, P. A. Advanced red phosphors for white light-emitting diodes. J. Mater. Chem. C 2016, 4, 8611−8623. (4) Huang, X. Y. Solid-state lighting: Red phosphor converts white LEDs. Nat. Photonics 2014, 8, 748−749. (5) Wang, L.; Xie, R. J.; Li, Y. Q.; Wang, X. J.; Ma, C. G.; Luo, D.; Takeda, T.; Tsai, Y. T.; Liu, R. S.; Hirosaki, N. Ca1−xLixAl1−xSi1+xN3:Eu2+ solid solutions as broadband, color-tunable and thermally robust red phosphors for superior color rendition white light-emitting diodes. Light: Sci. Appl. 2016, 5, e16155. (6) Yoshimura, K.; Fukunaga, H.; Izumi, M.; Masuda, M.; Uemura, T.; Takahashi, K.; Xie, R. J.; Hirosaki, N. White LEDs using the sharp β-sialon: Eu phosphor and Mn-doped red phosphor for wide-color gamut display applications. J. Soc. Inf. Disp. 2016, 24, 449−453. (7) Zhang, X. J.; Wang, H. C.; Tang, A. C.; Lin, S. Y.; Tong, H. C.; Chen, C. Y.; Lee, Y. C.; Tsai, T. L.; Liu, R. S. Robust and stable narrow-band green emitter: an option for advanced wide-color-gamut backlight display. Chem. Mater. 2016, 28, 8493−8497. (8) Pust, P.; Weiler, V.; Hecht, C.; Tücks, A.; Wochnik, A. S.; Henß, A. K.; Wiechert, D.; Scheu, C.; Schmidt, P. J.; Schnick, W. Narrowband red-emitting Sr[LiAl3N4]:Eu2+ as a next-generation LEDphosphor material. Nat. Mater. 2014, 13, 891−896. (9) Kim, S. W.; Hasegawa, T.; Hasegawa, S.; Yamanashi, R.; Nakagawa, H.; Toda, K.; Ishigaki, T.; Uematsu, K.; Sato, M. Improved synthesis of SrLiAl3N4:Eu2+ phosphor using complex nitride raw material. RSC Adv. 2016, 6, 61906−61908. (10) Zhang, X. J.; Tsai, Y. T.; Wu, S. M.; Lin, Y. C.; Lee, J. F.; Sheu, H. S.; Cheng, B. M.; Liu, R. S. Facile atmospheric pressure synthesis of high thermal stability and narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor for high color rendering index white light-emitting diodes. ACS Appl. Mater. Interfaces 2016, 8, 19612−19617. (11) Cui, D. P.; Xiang, Q. C.; Song, Z.; Xia, Z. G.; Liu, Q. L. The synthesis of narrow-band red-emitting SrLiAl3N4:Eu2+ phosphor and improvement of its luminescence properties. J. Mater. Chem. C 2016, 4, 7332−7338. (12) Hu, W. W.; Ji, W. W.; Khan, S. A.; Hao, L. Y.; Xu, X.; Yin, L. J.; Agathopoulos, S. Preparation of Sr1−xCaxLiAl3N4:Eu2+ solid solutions and their photoluminescence properties. J. Am. Ceram. Soc. 2016, 99, 3273−3279.

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CONCLUSIONS In this paper, two phosphor series, Eu 2+ -doped Sr(LiAl)1−xMg2xAl2N4 and Sr(LiAl3)1−y(Mg3Si)yN4 (x, y = 0− 1), are successfully synthesized by a solid state reaction, using the chemical-unit cosubstitution with [Mg−Mg] substituting for [Li−Al] and [Mg3Si] for [LiAl3]. The composition change induces structural transitions from SrLiAl3N4 phase to SrMg2Al2N4/SrMg3SiN4 occurring at about x = 0.2 or y = 0.15, respectively. With the structural transition, the PL spectra are first red-shifted from 648 to 657/663 nm and then blueshifted to 614/607 nm. Further analysis shows that the PL spectra shift, fwhm, and Stokes shift are correlated and dependent on the composition and structural change. These series phosphors exhibit different luminescent thermal quenching behaviors, and SrLiAl3N4 has the highest thermal quenching temperature. We construct the host referred binding energy and vacuum referred binding energy schemes of divalent/ trivalent lanthanide-doped end-member compounds, and further give thermal quenching mechanism of these series phosphors. We think these results are beneficial to deepen the recognition of these narrow-band red-emitting nitride phosphors, and helpful for exploring new high-performance phosphors.

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experimental values extracted from the luminescence spectroscopy of some mentioned compounds for Table S1−S4; the transformation relations between SrMg2Al2N4 and SrMg3SiN4 for Figure S3 (PDF)

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b01816. Rietveld refinements of two series phosphors for Figures S1 and S2; main parameters of refinement and 11843

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Article

Inorganic Chemistry

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DOI: 10.1021/acs.inorgchem.7b01816 Inorg. Chem. 2017, 56, 11837−11844