Critical Review—Narrow-Band Nitride Phosphors for Wide Color-Gamut White LED Backlighting

To achieve a brilliant image with high color saturation in white LED backlighting technology, innovative phosphor materials with narrow-band emission in the green and red spectral region are continuously pursed. Nitride phosphors are so far accepted as the most suitable phosphors for white LED backlights due to their high efficiency and excellent robustness. In this perspective, we will present an overview about the recent developments of state-of-the-art and newly-emerging nitride phosphors with a narrow emission band, and the emphasis would be placed on the relationships between the crystal structure and luminescence properties. Finally, some empirical rules for designing novel narrow-band phosphors for backlighting technologies would be summarized. © The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. [DOI: 10.1149/2.0051801jss] All rights reserved.

Nowadays, liquid crystal displays (LCDs) are ubiquitous in our daily lives, and their applications range from smartphones, tablets, computers, to large-screen TVs, and data projectors. As there is much concern on backlight units with larger color gamut, higher brightness, lower power consumption and mercury-free, white light-emitting diodes (LEDs) have been widely applied as backlighting components in modern LCD technologies to replace the traditional cold cathodefluorescence lamps (CCFLs). [1][2][3][4][5][6][7][8][9][10][11][12] Multi-chip white LEDs, which comprise red-, green-and blueemitting chips, exhibit excellent color performance and tuning abilities. However, the efficiencies of red, green, and blue LEDs vary overtime at different rates. In addition, separated driving circuits and complicated feedback-driven systems are required, which leads to a high cost. Moreover, the efficiency of green LEDs, known as "green gap", is much smaller than that of red and blue ones. [13][14][15] By contrast, phosphor-converted (pc) white LED backlights, which combine a single LED chip with phosphor materials, are mostly used due to their high efficiency, cost effectiveness and robustness. To achieve a large color gamut and to faithfully reproduce the natural colors, the emission peak of phosphors should match well with the RGB color filters of LCDs. Phosphors with narrow emission bands would loss less their luminescence after passing through color filters, and also promise high color purity. Therefore, narrow-band phosphors with appropriate emission maximum are required for high efficiency and wide color gamut backlights. 13,15 Quantum dots (QDs) materials have shown great potentials for next-generation displays due to their narrow emission band. [7][8][9][10][11][12] In comparison to traditional CCFL having a color gamut of ∼75% of the National Television Standard Committee (NTSC) standard, the Cdcontaining QDs-integrated white LED backlights promise a large color gamut of > 100% NTSC, but the application has been limited due to the usage restrictions of hazardous Cd compounds. Compared to Cdcontaining rivals, the non-toxic InP/ZnS QDs only yield a color gamut of 87% NTSC due to the quite broader emission band. 11 Moreover, it is difficult to maintain the initial optical properties of the QDs during device fabrication and long-term operation.
On the other hand, a blue LED covered with a combination of rareearth-doped green and red narrow-band phosphors have been proposed to systematically achieve improvements in color gamut. [2][3][4]6, 16 Ito et al. attempted the application of narrow-band SrGa 2 S 4 :Eu 2+ (green) z E-mail: rjxie@xmu.edu.cn; Xie.Rong-Jun@nims.go.jp and CaS:Eu 2+ (red) phosphor sheets in white LED backlights, and obtained a large color gamut of 90% NTSC, but limitations such as strong thermal quenching and high sensitivity toward moisture hindered the practical application of sulfide phosphors to some extent. 4 Over the past decade, rare-earth-doped nitride or oxynitride phosphors have been vindicated to be excellent down-conversion luminescent materials in white LEDs due to their superiority over others. The narrow-band nitride phosphors are also accepted as the most suitable phosphors for white LED backlights due to their high efficiency and excellent robustness. For example, Xie et al. has developed a wide color gamut of 92% NTSC by pumping β-Sialon:Eu 2+ (green) and CaAlSiN 3 :Eu 2+ (red) nitride phosphors with a blue LED chip. 2 Fukuda et al. has succeeded in using Sr 3 Si 13 Al 3 O 2 N 21 :Eu 2+ (green) and CaAlSiN 3 :Eu 2+ (red) phosphors to fabricate a white LED backlight with a color gamut of 94.2%. 6 Yoshimura et al. has achieved a superwide color gamut of >100% by using narrow-band γ-AlON:Mn 2+ ,Mg 2+ (green) and K 2 SiF 6 :Mn 4+ (red) phosphors. 16 To meet the requirements of backlighting technologies, new types of narrow-band nitride phosphors with enhanced or specialized properties are continuously pursued by chemists or materials scientists. Recently, Pust et al. reported the discovery of a highly promising narrow-band red phosphor, SrLiAl 3 N 4 :Eu 2+ , with an emission peak position at 650 nm and a full width at half-maximum (FWHM) of 50 nm, 17 while the chemically similar CaLiAl 3 N 4 :Eu 2+ narrowband red-emitting phosphor has a deep-red emission peak at 668 nm with the FWHM of 60 nm. 18 Another narrow-band red phosphor, SrMg 3 SiN 4 :Eu 2+ , has an ideal peak position at 615 nm and a narrow bandwidth of only 43 nm. 19 Takeda et al. have discovered a narrow-band green phosphor Ba 2 LiSi 7 AlN 12 :Eu 2+ peaking at approximately 515 nm with FWHM of 61 nm via the single particle diagnosis approach. 20 Strobel et al. have reported a narrowband green phosphor Ba[Li 2 (Al 2 Si 2 )N 6 ]:Eu 2+ peaking at 532 nm with FWHM of 57 nm by using the single crystal X-ray diffraction method. 21 Therefore, the focus in this regard is on some commonly used and newly-emerging narrow-band nitride phosphors and to highlight the important structural principles to realize the narrow emission bands. Firstly, state-of-the-art and newly-discovered narrow-band green and red nitride phosphors with narrow emission bands have been demonstrated. Then, the emphasis would be placed on the relationships between the crystal structure and luminescence properties. Finally, some empirical rules for designing novel narrow-band phosphors for white LED backlighting technologies would be presented.

Narrow-Band Green Phosphors
β-SiAlON:Eu 2+ .-β-SiAlON is derived from β-Si 3 N 4 (hexagonal system, space group P63/m) by partial substitution of Si by Al and N by O, and its chemical formula can be written as Si 6-z Al z O z N 8-z , with z typically ranging from 0 to 4.2. The crystal structure shown in Fig. 1a tells that the densely packed, corner-sharing (Si,Al)(N,O) 4 tetrahedra form a three-dimensional network with a one-dimensional channel running along the c-axis. 22 Brgoch. 24 β-SiAlON can be readily prepared by a variety of methods, such as solid-state reaction, 28 gas pressure sintering, 29 and combustion synthesis. 30 17), which yields an ideal emission peak of 535 nm and a narrow bandwidth of 55 nm. Later, systematical investigations have been conducted to clarify the influences of z value and Eu 2+ concentration on phase purity and photoluminescence properties. 31,32 Fig. 3a shows the XRD patterns of β-SiAlON with varying z values. Single β-SiAlON phase can be obtained in samples with z = 0.1 and 0.5. Higher z values of 1.0, 1.5, and 2.0 lead to impurity phase formation, and the amount of secondary phases increases with z value. Fig. 3b shows the photoluminescence spectra for samples with small z (<0.25) values, which exhibit fine structure with several sharp peaks. 33 The excitation band covers a broad range of 250-500 nm, allowing excitation by a variety of commercial LED chips. The intense narrow-band emission of Eu 2+ corresponds to the allowed 4f 6 5d→4f 7 transition of Eu 2+ . As z value decreases, the emission maximum shifts to short wavelengths (i.e., blueshift). Meanwhile, the emission bandwidth decreases from 55 to 45 nm. Fig. 3c depicts the peak emission wavelength as a function of z value and Eu 2+ concentration. It is found that the emission peak can be tuned in a wide range of 528-550 nm via composition tailoring. With increasing z value and Eu 2+ concentration, the emission band shifts toward the long wavelength side (i.e., redshift), and higher z (>1) causes red-shifted emission (λ em = 545-550 nm) along with broadened bandwidth (>63 nm). Therefore, limits on z < 0.5 is established for practical applications. Moreover, the oxygen content is proven to have a great impact on the photoluminescence properties. β-SiAlON:Eu 2+ also shows excellent thermal quenching resistance. For example, the emission intensity of the sample z = 1.0 at 150 • C maintains 84-87% of that measured at room temperature, as shown in Fig. 3d. The thermal quenching of samples becomes smaller with z ranging from 0.1 to 1.5, and the sample of z = 1.5 exhibits the best thermal property. Recently, Wang 23 et al. have reported that the predicted barrier of thermal ionization for β-SiAlON:Eu 2+ is relatively high (∼0.56-0.68 eV), which is in good accordance with its experimentally measured excellent thermal stability.
More recently, first-principles calculations have been utilized to understand the structure-composition-property relationships in β-SiAlON:Eu 2+ . Wang 23 et al. demonstrated that z value (Si/Al ratios or oxygen concentrations) could have a significant effect on the local environment of Eu 2+ in β-SiAlON. As shown in Fig. 4, increasing z leads to an elongation in the Eu-N bond length and the enhanced distortion of EuN 9 polyhedron. The magnitude of the crystal field splitting is related to the activator-anion local geometry, which can be described by the average bond length (l av ) and distortion index (D) of the activator-anion polyhedral. 35,36 Shorter l av and higher D typically lead to a larger splitting. Both l av and D increase with increasing z at constant Eu 2+ concentration (y value). However, the percentage increase in D is 3.32%, 5 times larger than the increase in l av (0.63%). Therefore, the effect of increasing D dominates over the increase in l av , resulting in a larger crystal field splitting with increasing z, thereby causing a red-shifted emission. Fig. 5a shows a schematic of the different transitions in broadand narrow-band phosphors. In both cases, the main emission is the result of the transition of an electron from the lowest 5d band into the empty 4f top band, i.e., a 4f 6 5d 1 → 4f 7 transition. When there are multiple Eu 2+ 4f levels within 0.1 eV from the highest band, overlapping emissions result in a broad bandwidth. Conversely, a large energy splitting, E S > 0.1 eV between the two highest 4f bands results in a narrow-band emission. Therefore, E S > 0.1 eV was proposed as a threshold for narrow-band emission. 27 The E S values were calculated with z ranging from 0.125 to 0.75, as shown in Fig. 5b. We may observe that E S at low z values are greater than 0.1 eV, implying a narrow emission band. On the other hand, E S at a large z = 0.75 is less than 0.1 eV, indicating broadening in emission spectra. The calculated results suggest that the synthesis should be directed toward achieving lower z value or oxygen content in β-SiAlON in order to achieve narrower emission bands and to minimize the red-shift, which are in line with previous experimental observations. 31,34 In order to understand the distribution of Al and O in β-SiAlON structure, Cozzan 37 et al. utilized high-resolution NMR techniques in combination with DFT calculations to unravel the exquisite details of local compositional variation in the crystal structure. As shown in Fig. 6, 27 Al NMR spectral features are assigned to different AlO q N 4-q (0 ≤ q ≤ 4) species, and these results suggest unambiguously a definite preference for AlON 3 formation and a decrease in the AlN 4 and AlO 2 N 2 /AlO 3 N populations. This is in stark contrast to the stochastic  38 It shows a face-centered cubic and nonstoichiometric spinel structure (space group Fd3m). In the three-dimensional rigid network, Al atoms occupy both tetrahedral (8a) and octahedral (16d) sites which are coordinated by four and six anions, respectively, and the O/N atoms occupy the anion sites of a spinel lattice (32e), as depicted in Fig. 7. In addition, γ-AlON contains vacancies as regular part of the crystal, which are mainly distributed in the octahedral site. 39,40 When doped with Mn 2+ or rare earths, γ-AlON can be developed into interesting luminescent materials. Xie 41 et al. reported that this phosphor produced an interesting green emission band at 512 nm and a FWHM of 32 nm upon 445 nm excitation, which shows a shorter and narrower emission spectrum than β-SiAlON:Eu 2+ . This indicates that γ-AlON:Mn 2+ could promise a higher color purity than β-SiAlON:Eu 2+ . However, γ-AlON:Mn 2+ has a quite low absorption efficiency (only 21% under 450 nm excitation) due to the spinforbidden transition of 3d 5 electrons in Mn 2+ .
Later, it is found that the green luminescence band of γ-AlON:Mn 2+ can be significantly enhanced by the addition of Mg 2+ ion into γ-AlON:Mn 2+ . The photoluminescence properties of Mn 2+ -, and Mn 2+ -Mg 2+ codoped γ-AlON are shown in Fig. 8. The excitation spectra of both samples consist of five peaks at 340, 358, 381, 424, and 445 nm, and the emission spectra show a single band centered at 512 and 520 nm for Mn 2+ -and Mn 2+ -Mg 2+ doped samples, which correspond to the characteristic transitionof Mn 2+ 4 T 1 ( 4 G) → 6 A 1 .  The Mn 2+ -Mg 2+ co-doped sample yields 1.7 times of the emission intensity of the sample containing only Mn 2+ , and the emission band is red-shifted with the FWHM increasing from 32 to 44 nm (see Fig. 8a). Xie 41  ions at the tetrahedral Al 3+ sites and finally leads to the luminescence enhancement. Fig. 8b presents the temperature dependent luminescence intensity of γ-AlON:Mn 2+ -Mg 2+ . The emission intensity measured at 150 • C maintains 88% of that at room temperature, which is comparable to that of SiAlON-based phosphors. Fig. 8c tells that γ-AlON:Mn 2+ -Mg 2+ has a sharper emission spectrum on the longwavelength side than the β-SiAlON:Eu 2+ phosphor. Thanks to the narrower and blue-shifted emission spectrum, white LED backlighting devices using γ-AlON:Mn 2+ -Mg 2+ as green phosphor achieve a color gamut larger than 100% of the NTSC, whereas the color gamut is only 95.7% relative to NTSC using β-SiAlON:Eu 2+ as green phosphor (see Fig. 8d). 16 X-ray absorption fine structure (XAFS) analyses are commonly used to reveal the electronic state and coordination structure of specific elements. Takeda 22,43 et al. studied the valence and coordination structure of Mn in the γ-AlON:Mn-Mg green phosphor by XAFS technique, as shown in Fig. 9. The absorption edge of Mn K-edge XANES spectrum for Mn,Mg-codoped γ-AlON is located at 6542.5 eV, in between those of MnO and MnCO 3 . From comparisons with the XANES spectra of various manganese oxides, the valence of the Mn ions is proven to be divalent. Further, the EXAFS analyses of Mn K-edge reveal that Mn 2+ ions are predominantly placed at tetrahedral Al sites (see Fig. 9b). To determine the Mg sites in AlON structure, total energy with different Mg occupation sites is calculated to confirm that Mg occupation at tetrahedral sites is more favorable. 44 That is to say, although Al atoms occupy both tetradedral and octahedral sites, Mn and Mg substitute the Al atoms only in the tetrahedral sites. This is also in line with the luminescent properties because the emission of Mn 2+ strictly depends on the crystallographic site where it occupies, and Mn 2+ emits green light when it resides in a tetrahedron and red light in an octahedron.  Another strategy to improve the low absorption of Mn 2+ is to codope a sensitizer that transfers its energy to Mn 2+ . Eu 2+ is reported to be an efficient sensitizer for Mn 2+ . Liu 45 et al. prepared the Eu 2+ -and Mn 2+ -co-doped γ-AlON phosphors and investigated their photoluminescence properties. An appreciable overlap between the excitation spectrum of Mn 2+ and the emission spectrum of Eu 2+ is observed, and the energy transfer from Eu 2+ to Mn 2+ definitely occurs, leading to a 9 times improvement in luminescent intensity of Mn 2+ , as shown in Fig. 10. Moreover, the external quantum efficiency (EQE) is increased from 7% to 49% with the Eu 2+ → Mn 2+ energy transfer in γ-AlON, making γ-AlON:Mn 2+ -Eu 2+ to be a suitable green phosphor excited by UV LEDs. Ba 2 LiSi 7 AlN 12 :Eu 2+ .-Recently, "single-particle-diagnosis approach" is proposed by Xie's group, which is an effective way to search for new phosphor hosts without fabrication of highquality powder or single-crystal samples. 46 Till now, several new phosphor systems have been discovered by this method, such as, blue-emitting BaSi 4 Al 3 N 9 :Eu 2+ , 46  phosphors. Among them, Ba 2 LiSi 7 AlN 12 :Eu 2+ , whose single particle is selected from the powder mixtures formed in the Ba 3 N 2 -Li 3 N-Si 3 N 4 -AlN system, possesses interesting emission peaking at ∼515 nm with bandwidth as small as 61 nm.
As shown in Fig. 11, the green-emitting micro-crystal is found to be a new phosphor out of the many luminescent particles, and then its crystal structure is resolved via the super-resolution single-crystal X-ray diffraction. The analysis results show that the new phosphor has an orthorhombic unit cell of a = 14.0993(2) Å, b = 4.89670(10) Å, c = 8.07190(10) Å, and Z = 2 with the Pnnm space group (no.58). The crystal structure of Ba 2 LiSi 7 AlN 12 :Eu 2+ is depicted in Figs. 11c-11d. As can be seen, Si and Al randomly locates at the tetrahedral site (blue tetrahedron) whereas Li occupies the independent red tetrahedral site (red tetrahedron). Vertex-sharing (Si,Al)N 4 tetrahedra form a corrugated layer (marked A) along the c-axis direction, and edge-sharing (Si,Al)N 4 tetrahedra and edge-sharing LiN 4 tetrahedra alternately align along the b direction and form a pillar (marked B). The corrugated layer (A) and pillar (B) form a large one-dimensional channel along the b direction. Ba occupies the one-dimensional channel in a zigzag manner along the b direction. There is only one crystallographic site for Ba, which is coordinated by 11 N atoms, and the BaN 11 polyhedra are linked by face-sharing manner. The activator Eu is also coordinated by 11 N atoms, and the distance between Eu and N ranges from 2.86 Å to 3.24 Å with an average distance of 3.12 Å, as shown in Table I.
The photoluminescence spectra of Ba 2 LiSi 7 AlN 12 :Eu 2+ single particle are shown in Fig. 12a. The excitation spectrum covers from 350 to 450 nm while the emission spectrum has a peak at approximately 515 nm with a bandwidth of 61 nm. It is expected that the powder sample will have a narrower bandwidth since the higher energy range of the emission spectrum would be absorbed by other Ba 2 LiSi 7 AlN 12 :Eu 2+ particles. 46 The temperature dependence of the emission spectra, peak intensity, and integrated intensity is displayed in Fig. 12b. At 200 and 300 • C, the peak intensities are 84 and 76% of the intensity observed at room temperature, whereas the integrated intensities are 95 and 91% of that at room temperature, respectively. The rigid condensed structure is one reason for the high thermal stability of this phosphor. Given the high internal quantum efficiency of 79% upon excitation at 405, this compound is characterized as a promising candidate for the backlighting displays.   3 3.077(4) Eu-N 4 3.238(9) Eu-N 3 3.077(4) Eu-N 4 3.238(9) Eu-N 3 3.199 (10) lizing single-crystal X-ray diffraction technique, the compound is found to crystallize in tetragonal space group P4/ncc (no. 130) (Z = 4, a = 7.8282(4), c = 9.9557(5) Å). As shown in Figs. 13a-13b, the crystal structure is built up from corner-and edge-sharing (Al/Si)N 4 tetrahedra with a statistical distribution of Al 3+ and Si 4+ . The tetrahedra form two vierer ring layers (a vierer ring comprising four tetrahedral centers), which are opposed to each other in [100] direction and are connected by common corners. Two types of vierer ring channels are built running along [001]. Smaller vierer ring channels of (Al/Si)N 4 tetrahedra are connected to each other by common corners in an up-down sequence, which are centered by LiN 4 tetrahedra. The LiN 4 tetrahedra are linked to each other by common edges forming bow-tie units of Li 2 N 6 . These units are connected to form a tetragonal Li 4 N 12 -bisphenoid (see Fig. 13c) inside the smaller vierer ring channels. Larger vierer ring channels consist of equally aligned (Al/Si)N 4 tetrahedra, which are exclusively centered by Ba 2+ . Ba 2+ is eight-fold coordinated by N 3− . The Ba-N bond lengths are in the order of 2.93(2)-3.10(9) Å (see Fig. 13d and Table II). More interestingly, TOPOS analysis of the (Al/Si) network reveals the first representative of an only theoretically calculated tetragonal Fischer-Koch sphere packing (symbol: whj) that has not been observed yet. In this framework, the tetrahedral centers of the network build tetrahedra on their own (see Fig. 13e). It is assumed that the doped Eu 2+ occupy the eight-fold coordinated atom position of Ba 2+ . The UV/vis reflectance spectroscopy is investigated to determine the bandgap of Ba[Li 2 (Al 2 Si 2 )N 6 ], as shown in Fig. 14a. A broad absorption band around 280 nm is observed, and thus the bandgap is estimated to be ∼4.6 eV. This value is consistent with the calculated value of 4.82 ± 0.20 eV, 49 and a large bandgap is the prerequisite for efficient emissions from the Eu 2+ dopant. The detected absorption band from blue to yellow region is attributed to the 4f 7 to 4f 7−n 5d n absorptions of Eu 2+ . The photoluminescence spectra of Ba[Li 2 (Al 2 Si 2 )N 6 ]:Eu 2+ are depicted in Fig. 14b. The excitation spectrum shows absorption in the blue spectral range with a maximum at 395 nm while the emission shows luminescence in the green spectral range peaking at 532 nm with FWHM ∼57 nm. Moreover, the sample shows a relatively low thermal quenching and the emission intensity at 200 • C keeps ∼70% of that at room temperature. 3 N 4 ]:Eu 2+ is reported as a new narrow-band red phosphor, which exhibits a highly efficient red emission peaking at ∼650 nm with a FWHM of ∼50 nm, together with low thermal quenching (>95% relative to the quantum efficiency at 200 • C). 17 Later, facile synthesis methods have been investigated. 50,51 The crystal structure of Sr[LiAl 3 N 4 ]:Eu 2+ is solved and refined from single-crystal and powder X-ray diffraction (XRD) data, revealing a triclinic variant of the UCr 4 C 4 structure type (space group P1   53 There are two crystallographic Sr sites, each coordinated by eight N atoms (Sr-N: 2.69-2.91 Å) in a highly symmetric cuboidlike environment (see Table III   accordance with the calculated value of (4.56 ± 0.25) eV. 54 To further understand the origin of luminescent features, low-temperature measurements at 6 K are performed, as shown in Fig. 16b. Well-resolved vibronic structure of the emission transitions along with the zero phonon line energy U 0 at 15,797 cm −1 are observed. The low number of vibronic structures in the high-energy tail of the emission band indicates a weak electron-phonon coupling of Eu 2+ in Sr[LiAl 3 N 4 ]. Moreover, the Stokes shift of the luminescence is calculated to be as small as 956 cm −1 . Smaller Stokes shifts and thus narrower emission bands observed for Eu 2+ phosphors are commonly associated with a high symmetric dopant-site geometry. 55 Owing to similar local electronic structure and comparable crystal-field splitting effects for both cuboidal coordination Sr sites, inhomogeneous line broadening of the emission band, resulting from different activator environments, is reduced to a minimum. This clarifies why, with two sites which can be occupied by the activator Eu 2+ ion, such a narrow emission band is still observed. Fig. 16c shows the temperature dependence of the relative integrated photoluminescence intensity of Sr[LiAl 3 N 4 ]:Eu 2+ . At 500 K, the integrated intensity drops by only 5%, demonstrating excel-lent thermal stability. The spectral emission changes at elevated temperature are shown in Fig. 16d. Notably, the emission band only shifts toward shorter wavelength side by ∼1 nm and broadens by 374 cm −1 from 303 to 465 K. The Eu 2+ 5d conduction band separation in Sr[LiAl 3 N 4 ]:Eu 2+ is calculated to be as large as ∼0.28 eV, thus avoiding photoionization and keeping outstanding thermal stability at elevated temperature. 56 Moreover, Sr[LiAl 3 N 4 ]:Eu 2+ has been reported to be moisture-sensitive. Coating technology is a potential avenue for improving stability property that is needed for application in the LED industry. Liu et al recently improved the unstable Sr[LiAl 3 N 4 ]:Eu 2+ by coating it with organosilica surfactants. 57,58 The coated samples showed excellent moisture resistance while retaining an external quantum efficiency (EQE) of 70% of their initial EQE after aging for 5 days under harsh conditions. These results indicate that Sr[LiAl 3 N 4 ]:Eu 2+ is an outstanding red-emitting candidate for backlighting applications with high color purity.  of only ∼43 nm. 19 This compound crystallizes in the Na[Li 3 SiO 4 ] structure type (see Fig. 17a) and exhibits specific structural features facilitating narrow-band red emission.
The strands of SiN 4 and MgN 4 tetrahedra are connected to each other by common corners in an up-down sequence forming vierer rings (a vierer ring comprising four tetrahedral centers) with one-half of them centered by Sr 2+ , forming a rigid and condensed framework. Sr 2+ occupies only one crystallographic site coordinated by eight N atoms (Sr-N: 2.65-3.29 Å) in a distorted cuboidal surrounding (see Table IV). Nevertheless, density of states of the nonequivalent N sites shows strong uniformity, 56 which is beneficial to reduce inhomogeneous line broadening. There are two possible coordination configurations for distorted SrN 8 , resulting in enantiomeric patterns in the unit cell. The "SrN 8 cubes" are connected via face sharing with ones from the same chirality, forming strands of "SrN 8 cubes" along [001]. There are two enantiomeric strands of "cubes" per unit cell, linked by a SiN 4 and MgN 4 strand (see Fig. 17c). All SiN 4 and MgN 4 tetrahedra have two common edges in trans position with the enantiomeric strands of "SrN 8 cubes". As a consequence, the enantiomeric strands of "SrN 8 cubes" are shifted one-half of the edge length against each other (see Fig. 17d). The UV/Vis spectrum of Sr[Mg 3 SiN 4 ] bulk sample is investigated to determine the bandgap (see Fig. 18a). From the absorption edge in the UV region, the bandgap of Sr[Mg 3 SiN 4 ] is derived to be ∼3.9 eV, which is in good accordance with the calculated valueof (3.28 ± 0.20) eV. 56 In Fig. 18b are presented the excitation and emission spectra of Sr[Mg 3 SiN 4 ]:Eu 2+ . The excitation spectrum shows a broadband with a maximum at 450 nm while the emission spectrum exhibits a narrow band peaking at 615 nm with the FWHM of only ∼43 nm. The Stokes shift is determined to be ∼772 cm −1 , even smaller than that of Sr[LiAl 3 N 4 ] (∼956 cm −1 ). The smaller Stokes shift and narrower emission band are ascribed to the only one crystallographic Sr site, the high symmetry of dopant site geometry, as well as the comparable ionic radii of Sr 2+ (1.26 Å) and Eu 2+ (1.25 Å).
To investigate the thermal behavior of Sr[Mg 3 SiN 4 ]:Eu 2+ , lowtemperature luminescence measurements down to 6 K are carried out, as depicted in Fig. 18c. It is observed that the emission intensity increases significantly by cooling, and the temperature stability of Sr[Mg 3 SiN 4 ]:Eu 2+ leaves room for improvement. Fig. 18d 56 In other words, thermal photoionization, 59,60 due to the low bandgap, seems most likely to be responsible for the significant thermal quenching of Sr[Mg 3 SiN 4 ]:Eu 2+ . Modulation of the bandgap through cation substitution represents an effective means to improve the thermal stability. 3 N 4 ]:Eu 2+ is also reported to be an intriguing new narrow-band red-emitting phosphor material, which exhibits an emission maximum at 668 nm with a FWHM of only ∼60 nm. 18 The compound is isotypical to Na[Li 3 SiO 4 ] and forms a highly condensed framework of AlN 4 and LiN 4 tetrahedra.
UV/Vis reflectance spectra of Eu 2+ -doped and undoped samples of Li 2 Ca 2 [Mg 2 Si 2 N 6 ] are measured to determine the bandgap, as shown in Fig. 22a. The strong absorption band in the blue to yellow region with two maxima at ∼410 and ∼460 nm is attributed to the 4f 7 → 4f 7−n 5d n absorptions in Eu 2+ . The absorption band around 240-280 nm is ascribed to the host lattice absorption, and therefore the optical bandgap is estimated to be ∼4.6 eV. The luminescence spectra

Summary and Outlook
The structural parameters and luminescent properties of the narrow-band phosphors reviewed in this article are summarized in Table VI. From the viewpoint of materials design, some underlying requirements for designing novel narrow-band phosphors have been analyzed as follows: Firstly, a rigid and ordering network structure is very important to reduce lattice-phonon energies, and thus to inhibit nonradiative relaxation. For example, compared with ordered variants of the UCr 4 C 4structure type such as Ca[LiAl 3 65 Secondly, only one single crystallographic site and almost the same size for the activator is another important precondition for reduction of inhomogenenous line broadening caused by different crystal fields. For example, (Ba,Sr) 2 Si 5 N 8 :Eu 2+ shows two crystallographic sites that can be occupied by Eu 2+ . As a result of the larger chemical differences of the representative sites, the emission maxima are distinct and thus lead to a relatively broad composite emission. 66,67 In fact, all the compounds reviewed in this article except Sr[LiAl 3 N 4 ]:Eu 2+ have only one crystallographic site for the activator in the rigid network. As for Sr[LiAl 3 N 4 ]:Eu 2+ , both Sr sites are coordinated in almost identical spheres. Owing to comparable crystal-field splitting effects for both sites, inhomogeneous line broadening of the emission band, resulting from different activator environments, is reduced to a minimum.
Thirdly, a high symmetry around the activator site with a high coordination number, producing the same distance between the activator and ligand, is favorable for restricting inhomogeneous emission broadening. In other words, asymmetric dopant site geometry and smaller coordination numbers are commonly associated with pronounced structural relaxation, larger Stokes shifts and, thus, broader  Last but not least, the long distance between activators and a large distance between the lowest excited state of activator and the bottom of the conduction band can effectively avoid photoionization effects at elevated temperature. For example, β-SiAlON:Eu 2+ exhibits a large gap between the 5d levels and the conduction band of the host, indicating a large barrier toward thermal ionization (>0.5 eV) and, hence, excellent thermal quenching stability. 27 By contrast, the lowest Eu 2+ 5d state and the conduction band of Sr[Mg 3 SiN 4 ]:Eu 2+ is found to be only 0.13 eV, thus an inferior thermal stability. 56 Finally, it should also be pointed out that these rules are mainly suitable to rare-earth ions with 4f-5d transitions, e.g., Eu 2+ , but not activators having spin-or parity-forbidden electron transitions, such as Mn 2+ or Mn 4+ .