Narrow-Band Red Emission in the Nitridolithoaluminate Sr4

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Narrow-Band Red Emission in the Nitridolithoaluminate Sr4[LiAl11N14]:Eu2+ Dominik Wilhelm, Dominik Baumann, Markus Seibald, Klaus Wurst, Gunter Heymann, and Hubert Huppertz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b04519 • Publication Date (Web): 26 Dec 2016 Downloaded from http://pubs.acs.org on December 30, 2016

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Chemistry of Materials

Narrow-Band Red Emission in the Nitridolithoaluminate Sr4[LiAl11N14]:Eu2+ Dominik Wilhelm†, Dominik Baumann‡, Markus Seibald‡, Klaus Wurst†, Gunter Heymann†, and Hubert Huppertz†* †

Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck, Innrain

80-82, A-6020 Innsbruck, Austria ‡

OSRAM GmbH, Corporate Innovation, Mittelstetter Weg 2, D-86830 Schwabmünchen, Germany Reprint requests to H. Huppertz. E-mail: [email protected] ABSTRACT: The new narrow-band red-emitting phosphor material Sr4[LiAl11N14]:Eu2+ was synthesized by solid-state reaction using a tungsten crucible with a cover plate in a tube furnace. When excited with blue light (460 nm), it exhibits red fluorescence with an emission maximum at 670 nm and a full width at half maximum of 1880 cm-1 (~85 nm). The crystal structure was solved and refined from single-crystal X-ray diffraction data. This new compound from the group of the nitridolithoaluminates crystallizes in the orthorhombic space group Pnnm (No. 58) with the unit-cell parameters a = 10.4291(7), b = 10.4309(7) c = 3.2349(2) Å. Sr4[LiAl11N14]:Eu2+ shows a pronounced tetragonal pseudosymmetry. It consists of a framework of disordered (Al/Li)N4 and AlN4 tetrahedra which are connected to each other by common corners and edges. Along [001], the tetrahedral network creates empty four membered ring channels as well as five membered ring channels in which the Sr2+ cations are located.

Introduction Because of the many advantages compared to other light sources such as longer lifetime, high energy efficiency, less energy consumption, and a small form factor, the usage of LEDs has increased rapidly during the last decade.1-4 The most common approach to generate white light is done by phosphor conversion. By using a green and a red phosphor which are on top of a highly efficient blue-emitting (In,Ga)N-LED chip, white light can be generated.5-9 Especially in general lighting, there are steep requirements on the light’s spectral properties. Due to the low human eye sensitivity to red photons, it is necessary to improve or to develop new narrow-band red-emitting phosphors, in order to maximize the overlap with the eyesensitivity curve. Besides the general demands of thermal and chemical stability, there are two important factors to obtain this spectral requirement: the maximum emission wavelength and the bandwidth (full width at half maximum (fwhm)) of the red phosphor.5,10,11 There are several nitridic substance classes for which new (narrow-banded) red-emitting phosphors have been reported. Nitridosilicates like Ba2Si5N8:Eu2+ (λem ~590-625 nm; fwhm ~2050-2600 cm-1),12 nitridoalumosilicates like CaAlSiN3:Eu2+ (λem ~610-660 nm;

fwhm ~2100-2500 cm-1),13

nitridomagnesosilicates

like

1 ACS Paragon Plus Environment

Sr[Mg3SiN4]:Eu2+

(λem ~615

nm;


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fwhm ~1170 cm-1)14 - which is until today the most narrow red-emitting Eu2+-doped phosphor in the literature - and nitridomagnesoaluminates like M[Mg2Al2N4]:Eu2+ (M = Ca, Sr, Ba) (λem ~607-666 nm; fwhm ~1815-2331 cm-1).15 In 2014, Pust et al. gained a lot of attention with the discovery of the nitridolithoaluminate Sr[LiAl3N4]:Eu2+ (λem ~654 nm; fwhm ~1180 cm-1) which was called a “next-generation LED-phosphor material” due to its outstanding narrow emission band in the deep-red spectral region and excellent thermal luminescence behavior (thermal quenching losses at 200 °C < 5% relative to the quantum efficiency at 20 °C).10 These properties and the fact that Ca[LiAl3N4]:Eu2+ is another narrow-band red-emitting phosphor (λem ~668 nm; fwhm =1333 cm-1)16 makes the class of nitridolithoaluminates a potential field for further research towards new red-emitting LED phosphors. During our research of new representatives of this class of substances, we discovered the compound Sr4[LiAl11N14]:Eu2+, whose synthesis, structural characterization, and luminescence properties are presented in this article. This compound has the potential for an application in phosphor-converted (pc) LEDs for general lighting.

Experimental Section. Synthesis. Sr4[LiAl11N14]:Eu2+ was synthesized by solid-state reaction in a tube furnace. Sr3N2 (6.722 g, 0.023 mol, Materion, 99.5%), SrH2 (6.214 g, 0.069 mol, Materion, 99.5%), LiAlH4 (4.483 g, 0.118 mol, Sigma Aldrich, >97%), Li3N (0.242 g, 0.007 mol, Materion, 99.5%), AlN (10.796 g, 0.263 mol, Tokuyama, >97%), and EuF3 (0.115 g, 0.557 mmol, Materion, 99.6%) were used as starting materials. Due to the tendency of evaporation for Li-based starting materials in an open system, the initial weight is not stoichiometric regarding the investigated compound (Li excess) but determined empirically yielding a product with only AlN as an impurity phase. AlN has no negative influence on the luminescence data presented or on the structure refinement based on single-crystal data. The starting materials were mixed by ball-milling, filled into a tungsten crucible, and capped with a cover plate. These procedures were performed in a nitrogen-filled glove box. The crucible was placed in a tube furnace with an alumina tube and heated to 1400 °C with a rate of 250 °C/h, kept at that temperature for 15 minutes and then cooled down to room temperature using the same rate. The reaction was carried out in a constant flow of forming gas (N2:H2 = 92.5:7.5). After grinding, Sr4[LiAl11N14]:Eu2+ is obtained as a pink powder. Subsequent handling of the samples was carried out under air.

Single-Crystal X-Ray Diffraction. Single crystals were collected under a polarization microscope by separating the crystals out of dry paraffin oil and gluing them on glass wires. Resulting data of Sr4[LiAl11N14]:Eu2+ was collected using a Bruker D8 Quest diffractometer with monochromatic MoKα1 radiation (λ =0.71073 Å) and an Incoatec Microfocus diffraction source. A multi-scan absorption correction (SADABS-2014 17) was applied to the intensity data sets. Graphical representations of the structure were produced with the program DIAMOND.18 Powder X-Ray Diffraction. Powder X-ray diffraction data was collected on a PANalytical Empyrean diffractometer using CuKα1 radiation (λ =1.54056 Å) and reflection mode. A Rietveld refinement was performed using the program suite TOPAS 4.219. Therefore, powder X-ray diffraction data were collected using a Stoe stadi P diffractometer with Ge (111)monochromatized MoKα1 (λ =0.71073 Å). Hardware parameter and reflection shape refinement were carried out by a LaB6 standard. Luminescence. In order to determine the luminescence properties of the title compound, a HORIBA Fluoromax spectrophotometer was used. The emission spectrum was measured in the wavelength range between 550 and 800 nm using an excitation wavelength of 460 nm and an integration time of 0.2 seconds per step. The excitation spectrum was measured in the range between 250 and 600 nm by monitoring the emission intensity at 670 nm. Both, excitation and emission spectra were measured with a step size of 1 nm.

Results and Discussion Single-Crystal Structure Analysis. The final crystal structure of Sr4LiAl11N14:Eu2+ was solved and refined in the orthorhombic space group Pnnm (No. 58) with a = 10.4291(7) Å, b = 10.4309(7) Å and c = 3.2349(2) Å. Because of the very


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similar lengths of the a- and b-axes, the compound showed a pronounced tetragonal pseudosymmetry. When first trying to solve and refine the structure, no reasonable solution within the tetragonal system could be found. Changing to the monoclinic system assuming a pseudomeroedral twin with the matrix [010 100 001ത ] led to a first reasonable solution. In this case, the ADDSYM routine of PLATON suggested the orthorhombic space group Pnnm and the setting of this solution was finally transformed to this orthorhombic space group. The crystallographic data of Sr4[LiAl11N14]:Eu2+ are listed in Table 1. Due to the insignificant scattering power of 0.4 mol % Eu, it is not included in the structural refinement. Furthermore, one of the Wyckoff positions 4g (Li1/Al1) was refined with a pronounced statistical disorder of Li+ and Al3+ cations in the ratio of 1:3, respectively. The atomic coordinates and displacement parameters are presented in Table 2. A comparison of the powder diffraction pattern with the theoretical powder pattern based on the single-crystal diffraction data is shown in Figure 1.20-22 It clearly shows that the single crystal is representative for the bulk sample investigated.



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Table 1. Crystallographic data of Sr4LiAl11N14:Eu2+

Empirical formula

Sr4LiAl11N14

Molar mass, g·mol-1

850.31

Crystal system

Orthorhombic

Space group

Pnnm (No. 58)

Single crystal data Single crystal diffractometer

Bruker D8 Quest Kappa

Radiation

MoKα1 (λ = 0.71073 Å)

a, Å

10.4291(7)

b, Å

10.4309(7)

c, Å V, Å

3.2349(2) 3

351.9(2)

Formula units per cell Calculated density, g·cm

1 -3

4.01

T, K µ, mm

193(2) -1

15.80

F(000)

396

Crystal size, mm3

0.076 × 0.054 × 0.038

2θ range, deg

5.5 - 70.0

Range in hkl

-16 ≤ h ≤ 16, -9 ≤ k ≤ 16, -5 ≤ l ≤ 5

Total no. of reflections

7155

Independent reflections

884

Reflections with I ≥ 2σ(I)

873

Data/parameters

884/48

Absorption correction Goodness-of-fit on

multi-scan (SADABS)

Fi2

1.123

Final R indices [I ≥ 2σ(I)]

R1 = 0.0213 wR2 = 0.0513

Final R indices (all data)

R1 = 0.0220 wR2 = 0.0517

Largest diff. peak and hole, e Å

-3

1.26/-1.23


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Figure 1. Top: experimental powder pattern of Sr4LiAl11N14:Eu2+. Bottom: theoretical powder pattern based on the singlecrystal diffraction data. Reflections marked with asterisks are caused by the side phase AlN.

Table 2. Atomic coordinates and equivalent isotropic displacement parameters (Å²) of Sr4LiAl11N14:Eu2+ including site occupancies. Standard deviations in parentheses.

WyckoffAtom Position

x

y

z

Ueq

SOF

Sr1

4g

0.35249(4)

0.36915(4)

0

0.00770(7)

Al1

4g

0.1104(2)

0.1432(2)

0

0.0116(4)

0.75

Li1

4g

0.1104(2)

0.1432(2)

0

0.0116(4)

0.25

Al2

4g

0.1591(2)

0.5933(2)

0

0.0034(2)

1

Al3

4g

0.5794(2)

0.1551(2)

0

0.0069(2)

1

N1

4g

0.0127(3)

0.6981(3)

0

0.0065(4)

1

N2

4g

0.6762(3)

0.0022(3)

0

0.0078(5)

1

N3

4g

0.6933(3)

0.3024(3)

0

0.0083(3)

1

N4

2a

0

0

0

0.0092(6)

1


1


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The crystal structure of Sr4[LiAl11N14]:Eu2+ is shown in Figure 2. It consists of a three-dimensional network of (Al,Li)N4 and AlN4 tetrahedra which are connected via common corners and edges to form four and five membered ring channels along [001] (see Figure 2A). The five membered rings are made by corner sharing of three AlN4 and two (Al,Li)N4 tetrahedra (see Figure 2B), whereas the four membered rings are built up by four AlN4 tetrahedra. Regarding the AlN4 tetrahedra solely, eight membered rings can be obtained, which can also be seen in Figure 2A. Inside of the five membered ring channels the Sr site is located possessing a cube-like coordination by eight nitrogen atoms forming endless, face sharing strands. Two neighboring strands of these SrN8 polyhedra share common edges (see Figure 2C). Figure 2D shows that there are chains of alternating AlN4 and (Al,Li)N4 tetrahedra, which are connected via common edges. These chains are further on connected via a chain of corner sharing AlN4 tetrahedra. Additionally, the (Al,Li)N4 tetrahedra are connected to themselves via a bridging nitrogen atom N[2] being displayed in the Figures 2E and 2B. This structure motif can also be obtained within the crystal structure of Ca18.75Li10.5[Al39N55]:Eu2+.23 Besides the bridging nitrogen atoms N[2], there are also three- (N[3]) and four-fold (N[4]) connecting nitrogen atoms, which can be clearly seen in Figure 2D. This connectivity results in a highly condensed framework structure, which is promising for hosting dopants for narrow-band emitting phosphors. The interatomic distances of Al-N [1.878(3)-1.942(3) Å] are in good agreement with other nitridolithoaluminates such as Ca[LiAl3N4]:Eu2+ [Al-N: 1.8273(2)-1.9511(3) Å].16 The values of Ca[LiAl3N4]:Eu2+ are used here, because they were determined by single-crystal X-ray diffraction, unlike those for Sr[LiAl3N4]:Eu2+. As already mentioned above, a statistical disorder on the Wyckoff position 4g was found (Li1/Al1), occupied with Li+ and Al3+ cations in a ratio of 1:3. Interatomic distances of this disordered (Al,Li)-site [Al,Li-N: 1.886(2)-2.095(3) Å] are within the range of the Al-N distances in Ca[LiAl3N4]:Eu2+ [Al-N: 1.8273(2)-1.9511(3) Å] and its Li-N distances [Li-N: 1.9680(2)-2.2788(3) Å].16 The Sr site is located inside the five membered ring channels possessing a cube-like coordination by eight N atoms with distances in the range from 2.617(1) to 2.928(2) Å. These are in good agreement with the distances found in Sr[LiAl3N4]:Eu2+ [2.672(12)– 2.974(12) Å].10 The Sr-Sr distances [3.235(2) Å] tally well to those in Sr[LiAl3N4]:Eu2+ [3.266(3) Å].10 Having a look on the intra-tetrahedral bond angels of the (Al,Li)N4 tetrahedra in Sr4[LiAl11N14]:Eu2+ [92.4(2)-114.9(2)°], a distortion of these tetrahedra can be observed. The angles within the AlN4 tetrahedra range from 103.0(1) to 115.9(2)°. Comparing the crystal structures of the three nitridolithoaluminates Sr4[LiAl11N14]:Eu2+, Sr[LiAl3N4]:Eu2+, and Ca[LiAl3N4]:Eu2+, networks of LiN4 and AlN4 tetrahedra can be observed forming channels, in which the cations are located. In all three compounds, the cations are eightfold coordinated by nitrogen atoms leading to face-sharing cuboidal AEN8 polyhedra. Regarding these points, there are several properties which are quite similar in these three nitridolithoaluminates. However, Sr4[LiAl11N14]:Eu2+ is the only compound in which the alkaline earth metal cation is located in five membered ring channels whereas the cations in Ca[LiAl3N4]:Eu2+ and Sr[LiAl3N4]:Eu2+ are located in four membered ring channels. Unlike Ca[LiAl3N4]:Eu2+ and Sr[LiAl3N4]:Eu2+, which are structurally related to UCr4C4, Sr4[LiAll11N14]:Eu2+ shows a structural relation to K2Zn6O7. K2Zn6O7 crystallizes in the tetragonal space group P42nm. In detail, the potassium cations of K2Zn6O7 are substituted by strontium cations, the zinc cations by aluminum/lithium, and the oxygen atoms are replaced by nitrogen atoms. The structure of K2Zn6O7 is built up of eight membered rings composed of ZnO4 tetrahedra in analogy to the AlN4 tetrahedra in Sr4[LiAl11N14]:Eu2+. In Sr4[LiAl11N14]:Eu2+, all Al/Li cations are coordinated by four nitrogen atoms forming (Al,Li)N4 tetrahedra, whereas merely two-thirds of the zinc cations are four-fold coordinated by oxygen atoms leading to ZnO4 tetrahedra in K2Zn6O7. The residual third of the zinc cations is three-fold coordinated by oxygen atoms in a trigonal planar geometry (ZnO3 groups). The displacement of this atom site in Sr4[LiAl11N14]:Eu2+ is the reason for the symmetry reduction from P42nm to Pnnm. Due to this and the different space group, Sr4[LiAl11N14]:Eu2+ is homeotypic to the oxozincate K2Zn6O7. An additional commonality consists in the eight-fold coordination of the potassium cations by oxygen atoms leading to cuboid-like KO8 polyhedra. This is in analogy to the crystal structure of Sr4[LiAl11N14]:Eu2+, also exhibiting strands of cuboidal SrN8 polyhedra sharing common edges.10,16,24,25


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A

B

C

D

E

Figure 2. Crystal structure of Sr4[LiAl11N14]:Eu2+, with (Al,Li)N4 tetrahedra (red), AlN4 tetrahedra (blue), Sr cations (yellow) and SrN8 polyhedra (green): (A): viewing direction along [001]; (B) arrangement of the tetrahedra to form a five membered ring channel; (C): Coordination of the face-sharing SrN8 polyhedra; (D): chains of edge-sharing (Al,Li)N4 and AlN4 tetrahedra connected via a row of corner-sharing AlN4 tetrahedra (E): arrangement of two (Al,Li)N4 tetrahedra. Rietveld-Refinement. The Rietveld refinement led to a phase composition of 89.3 w% Sr4[LiAl11N14]:Eu2+ and 10.7 w% AlN. The lattice parameters are comparable to those derived from single-crystal X-ray diffraction:



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a = 10.4764(2) Å, b = 10.3938(2) Å and c = 3.2356(2) Å. An Rexp of 0.0069 shows that the diffraction pattern can be reconstructed quite well using the Rietveld method in combination with the single-crystal structure model for Sr4[LiAl11N14]:Eu2+. Further information for the Rietveld refinement are shown in the supporting information. Bond-Valence Sum Calculations. Bond-Valence Sum Calculations (BVS) were performed in order to validate the disordered (Al,Li)-site and to check the reliability of the presented structure. The results of the calculations are shown in Table 3. Due to the fractional occupation of the (Al,Li) site, the values for nitrogen were calculated separately. Values for AlN bonds are marked with one asterisk, values for Li-N bonds are marked with two asterisks. 26,27 Table 3. Bond-Valence Sum Calculations for Sr4LiAl11N14:Eu2+.

Atom Sr1 Al1 Li1 Al2 Al3 N1* N1** N2* N2** N3* N3** N4* N4** BVS 1.89 2.62 1.61 3.01 2.79 3.02 2.85 2.75 2.71 3.15 2.61 2.95 2.36 The values for Li1 and Al1 differ from the expected values for Li and Al which confirms the existence of the fractional occupancy for this site in Sr4LiAl11N14:Eu2+. The remaining values are in the expected range. Luminescence. Luminescence measurements were performed on powder samples. Using a nominal doping level of 0.4%, Sr4[LiAl11N14]:Eu2+ shows a red luminescence with an emission maximum at λmax = 670 nm, a band width of 1880 cm-1 (~85 nm), and CIE color coordinates of u’ = 0.535, v’ = 0.519 (x = 0.698, y = 0.301), as shown in Figure 3. Figure 4 shows the excitation and the emission spectra of Sr4[LiAl11N14]:Eu2+ (0.4%). The excitation spectrum shows two maxima at ~312 and ~467 nm. The luminescence properties make Sr4[LiAl11N14]:Eu2+ another example of a narrow-band red-emitting nitride material like Sr[LiAl3N4]:Eu2+ (λem = 650 nm, fwhm ~1180 cm-1),10 Ca[LiAl3N4]:Eu2+ (λem = 668 nm, fwhm ~1333 cm-1),16 Sr[Mg3SiN4]:Eu2+ (λem = 615 nm, fwhm ~1170 cm-1),14 and (Ca, Sr, Ba)[Mg2Al2N4]:Eu2+ (λem = 607 – 666 nm, fwhm ~1815-2331 cm-1).15 However, there are significant differences in their optical properties. In order to understand the difference in luminescent properties between Sr4[LiAl11N14]:Eu2+ and Sr[LiAl3N4]:Eu2+, it is necessary to take a close look at the coordination around the Sr sites, which likely host the activator ions. We found that the mean Sr-N-distance in Sr4[LiAl11N14]:Eu2+ (2.781 Å) is significantly shorter than in Sr[LiAl3N4]:Eu2+ (Sr1: 2.797 Å; Sr2: 2.803 Å, overall: 2.800 Å). In addition, the shortest Sr-N-distance is only 2.617 Å for Sr4[LiAl11N14]:Eu2+, whereas it is 2.672 Å in Sr[LiAl3N4]:Eu2+. Both these effects lead to a stronger interaction between ligand and activator, which results in a red shift of the emission signal for Sr4[LiAl11N14]:Eu2+. At the same time, an increased full width at half maximum is visible in Sr4[LiAl11N14]:Eu2+ compared to Sr[LiAl3N4]:Eu2+. This might be explained by a stronger distortion of the cube-like coordination polyhedron around the activator site. This distortion can be measured with the standard deviation of the Sr-N distances. In Sr4[LiAl11N14]:Eu2+ this value (0.111 Å) is larger than in Sr[LiAl3N4]:Eu2+ (Sr1: 0.100 Å; Sr2: 0.090 Å, overall: 0.096 Å), meaning a higher degree of distortion in Sr4[LiAl11N14]:Eu2+. In contrast to Sr[LiAl3N4]:Eu2+, but similarly to M[Mg2Al2N4]:Eu2+ (M = Ca, Sr, Ba), there is structural disorder in the aluminate substructure of Sr4[LiAl11N14]:Eu2+. This leads to a variation in the second coordination sphere around the activator ions, and therefore to a different charge density distribution in the ligands. Consequently, there are multiple slightly different, but crystallographically indistinguishable charge distributions around the activator ions, resulting in line broadening.


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Figure 3. Color coordinates (CIE u’ v’ chromaticity diagram) of Sr4[LiAl11N14]:Eu2+ (a), compared to Sr[LiAl3N4]:Eu2+ (b) and Ca[LiAl3N4]:Eu2+ (c). The inset shows a magnified view.10,16



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Figure 4. Excitation (blue, λem= 670 nm) and emission spectra (red, λexc= 460 nm) of Sr4[LiAl11N14]:Eu2+. Sr4[LiAl11N14]:Eu2+ shows a red luminescence with an emission maximum at λmax = 670 nm and a band width of 1880 cm-1 (~85nm).

Conclusion. With Sr4[LiAl11N14]:Eu2+, a new representative from the class of nitridolithoaluminates is reported. To the best of our knowledge there are until now only two other luminescent nitridolithoaluminate compounds described in literature. Regarding Sr4[LiAl11N14]:Eu2+ and the two other luminescent compounds within this system namely Ca[LiAl3N4]:Eu2+ and Sr[LiAl3N4]:Eu2+, one comes to the conclusion that nitridolithoaluminates are a very suitable substance class as host lattices for Eu2+-doped phosphors. The highly symmetric cube-like surrounding of the Sr site, which has already been found in structurally related narrow-band red emitting phosphors,10,14-16 might be a driving force towards narrow emission. In the title compound, this leads to an intense red luminescence with an emission maximum at 670 nm with a band width of 1880 cm-1 (~ 85 nm) upon excitation with blue light. With Sr4[LiAl11N14]:Eu2+, we found a new type of a nitridolithoaluminate with a rigid, highly condensed [LiAl11N14]8- network. Unlike other narrow-band red emitting phosphors like Sr[Mg3SiN4]:Eu2+, Ca[LiAl3N4]:Eu2+, and Sr[LiAl3N4]:Eu2+ which are strongly related to the UCr4C4 structure type, a structural relation to K2Zn6O7 is present for Sr4[LiAl11N14]:Eu2+. Although this type of crystal structure has been observed for the first time, it surprisingly conserves structural features like a cube-like activator coordination. Therefore, the search for new compounds in this material system seems to be a valuable strategy for obtaining for new phosphor materials. Due to the structural and luminescence properties, Sr4[LiAl11N14]:Eu2+ could be a possible material for phosphor-converted LEDs using a blueemitting (In,Ga)N-LED chip.

Acknowledgements. The authors like to thank Daniel Schildhammer for the support regarding Rietveld refinement.


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Supporting Information Available: Anisotropic Displacement parameters of Sr4[LiAl11N14]:Eu2+ resulting from the singlecrystal diffraction data, crystallographic data of the Rietveld refinement and atomic coordinates and site occupancies of the Rietveld refinement supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org.

References.

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Narrow-Band Red Emission in the Nitridolithoaluminate Sr4

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