Crystal structures and luminescence properties of Li 6 M N 4

: The compounds Li 6 MoN 4 and Li 6 WN 4 were prepared from the reactants M ( M = Mo, W) and Li 3 N in a radi-ofrequency furnace at 1000 ° C. The lithonitridometallates crystallize in the tetragonal system with the lattice parameters: a = 6.6844(1), c = 4.9294(1) Å for Li 6 WN 4 based on single-crystal X-ray di ﬀ raction data and a = 6.6611(3), c = 4.9338(3) Å for Li 6 MoN 4 taken from powder X-ray di ﬀ raction data. Colorless to slightly reddish single-crystals of the tungsten compoundwere isolatedand thecrystalstructurewasre ﬁ ned in the space group P 4 2 / nmc (no. 137) with Z = 2 and the powder X-ray data of the molybdenum compound were analyzed by a Rietveld re ﬁ nement. Both structures belong to the Li 6 [ZnO 4 ] type published by Hoppe et al. in 1987 (Untenecker H., Hoppe R. Z. Anorg. Allg. Chem. 1987, 551 , 147 – 150) and could be doped with Ce 3 + for the ﬁ rst time. The investigated compounds show a reddish color impression upon UV to blue irradiation and exhibit a broad emission band with a maximum at λ max = 693 nm (fwhm 97 nm) for Li 6 MoN 4 and at λ max = 653 nm (fwhm 133 nm) for Li 6 WN 4 .


Introduction
In 1961, Juza et al. first reported the preparation of ternary nitrides of lithium with the transition metals Cr, Mo and W. They succeeded by reacting Li 3 N with the pure metals in high-temperature autoclaves, obtaining compounds with the composition Li 9 MN 5 (M = Cr, Mo, W) [1].Gudat et al. synthesized the compounds Li 6 MN 4 (M = Cr, Mo, W) and Li 15 Cr 2 N 9 for the first time in 1990 and described them as fluoriterelated superstructures [2].In the 1990s, lithium nitride was reacted with a larger selection of metals and a wide variety of lithonitridometallates were obtained [3][4][5][6].In 2005, Yuan et al. synthesized the compound Li 6 WN 4 by high-temperature synthesis with N 2 overpressure and analyzed it using powder X-ray diffraction data [7].
The field of nitride chemistry in connection with the element lithium has made great progress in recent decades.Not only because it had received little attention in the past compared to lithooxides, but also because the use of these compounds in batteries or as ion conductors has recently become increasingly important [5,8,9].In the corrosion of Crcontaining steels, the formation of lithium nitridometallates is also observed [10,11].It was found that the extremely reactive lithium nitride melts attack metal crucibles forming a passivating layer.Specifically, when tungsten crucibles are used, a layer with the composition Li 6 WN 4 is formed and thus prevents ongoing progressive attack [12,13].
Another area of application of nitride compounds is as phosphors in so-called phosphor-converted LEDs (pc-LEDs).
In this contribution, we report on the normal-pressure syntheses and crystal structures of Li 6 MN 4 (M = Mo, W).For Li 6 WN 4 , we present single-crystal data for the first time.The structure model of the title compounds is compared and discussed with previously published compounds that crystallize in the Li 6 [ZnO 4 ] structure type [28].In addition, both title compounds were doped with the activator ion Ce 3+ and their optical properties were investigated.

Crystal structure
The lithonitridotungstate crystallizes in the tetragonal crystal system in the space group P4 2 /nmc (no.137).For the crystal structure of Li 6 WN 4 , the lattice parameters a = 6.6844(1) and c = 4.9294(1) Å and a cell volume of V = 220.252(8)Å 3 were obtained from single-crystal X-ray diffraction data.The details of the structure refinement are given in Table 1 and the atomic coordinates, displacement parameters, interatomic distances and angles are given in Tables 2-5.
The tungsten and lithium atoms are tetrahedrally coordinated by nitrogen atoms (Figure 1).The W-N distances of 1.882(2) Å are in the range of the values known in the literature.For example, in the compound Ba 3 [WN 4 ] of Rabenau et al., in which the tungsten atom is also tetrahedrally coordinated by nitrogen atoms, the W-N distances have a mean value of 1.902 Å [2,29,30].Likewise, the Li-N distances with a range of 2.041(2)-2.227(3)Å (see Figure 1) agree with published values for other lithium nitridometallates of different transition Table : Wyckoff positions and atomic coordinates of Li  MoN  based on the Rietveld refinement of the powder data (the Li positions were not refined) (standard deviations in parentheses).
metals.E.g., Ma et al. give a Li-N distance of 2.252 Å for the compound LiSiON, in which lithium is tetrahedrally coordinated by three oxygen atoms and one nitrogen atom [2,23,31].
Regarding the coordination polyhedra, the structure is built up from a three-dimensional network of edge-and corner-linked LiN 4 tetrahedra.Along [001], channels are formed by an alternating arrangement of eight edge-linked LiN 4 tetrahedra of the Table : Interatomic distances and angles in Li  MN  (M = Mo, W) of the single-crystal structure refinement and the Rietveld refinements (standard deviations in parentheses).
Mean values are written in bold letters.Li1 site, with the Li2 site forming a square.The channels consist of alternating WN 4 tetrahedra and vacancies (see Figure 2).In the crystal structure of Li 6 WN 4 , the positions of the nitrogen atoms are close to a ccp (cubic closest packing) arrangement where seven out of eight tetrahedral voids are occupied by W and Li (see Figure 3).

MAPLE, CHARDI and BLBS calculations
Madelung Part of Lattice Energy (MAPLE) [32,33], bondlength/bond-strength (BLBS) [34,35] and Charge Distribution (CHARDI) [36,37] calculations were performed to prove the electrostatic consistency of the crystal structure model.The MAPLE sums of Li 6 WN 4 are listed in Table 6.The results of the CHARDI and BLBS calculations are shown in Table 7.
Comparing the MAPLE sum based on WO 3 , Li 3 N and Si 3 N 4 with the calculated MAPLE sum of Li 6 WN 4 and SiO 2 , it was found that the values differ by only 0.26 %.The results of the BLBS method show the expected charges for the atoms, which are in good agreement with the oxidation states according to an electro-neutral sum formula.The result obtained using the BLBS method is supported by the CHARDI calculations, which is also in good agreement.
The validity of the structural model was confirmed for the powder phases by Rietveld refinement (a phase-pure sample obtained via the described synthesis pathway for the tungsten variant, and a high-quality powder sample with a minor amount of an unknown side phase for the molybdenum variant), as shown in Figures 4 and 5.The results of the Rietveld analysis for the W and Mo phases are shown in Table 8.

Luminescence
The two title compounds could be successfully doped with the activator ion Ce 3+ and the luminescence properties of Li 6 WN 4 :Ce 3+ single crystals and Li 6 MoN 4 :Ce 3+ single particles were investigated.The compound Li 6 WN 4 :Ce 3+ with a nominal activator concentration of 1 mol% Ce 3+ can be excited with UV-near to blue light.The single crystals, also taken for the structure determination, excited at λ = 448 nm show an emission maximum at λ max = 693 nm with a full width at half maximum (fwhm) of 97 nm (0.269 eV, 2166 cm −1 ).In comparison, single    particles taken from a powder sample of the isotypic compound Li 6 MoN 4 :Ce 3+ with a nominal activator concentration of 1 mol% Ce 3+ show an emission maximum at λ max = 653 nm with a full width at half maximum (fwhm) of 133 nm (0.385 eV, 3108 cm −1 ).In the CIE-xy color space, this corresponds to x = 0.707(1) and y = 0.293(1) for Li 6 WN 4 :Ce 3+ and x = 0.598(1) and y = 0.400(1) for Li 6 MoN 4 :Ce 3+ (see Figure 6).The single crystals of the compound Li 6 WN 4 :Ce 3+ have a slightly reddish body color in daylight and are stable for several months under atmospheric conditions.In the compounds Li 6 MN 4 :Ce 3+ (M = Mo, W), two possible positions occur that can be occupied by the activator ion Ce 3+ , although a precise assignment is not possible (see Figure 7).Comparing the distances in the MN 4 tetrahedra of the two Rietveld refinements (Table 5) and the ionic radii of tungsten (0.56 Å) [42] and molybdenum (0.55 Å) [42], the molybdenumcentered tetrahedra have a larger distance between the nitrogen and the metal atoms than the tungsten-centered tetrahedra (the ionic radii applied in each case are those of the oxidation states 6+ and the coordination numbers 4).This difference leads to a smaller splitting of the d orbitals and thus results in a shift of the bands (see Figure 6 left).
For the Ce 3+ dopant, an octahedral or even larger coordination is more likely than the tetrahedral coordination (see Figure 7) owing to the comparatively large ionic radius of cerium (r = 1.15 Å) [42].Two different possible positions of the octahedrally coordinated activator ion Eu 2+ were described for the compound Li 2 CaSi 2 N 4 :Eu 2+ [43].Therein, the octahedral coordination polyhedra are occupied by Ca and N atoms, with Ca-N distances ranging from 2.489(3) to 2.509(3) Å for Ca1 and 2.586(3) Å for Ca2.Compared to the distances between the center of the octahedron to its nitrogen atoms in Li 6 MN 4 :Ce 3+ (M = Mo, W) (Mo: Ø center-N = 2.41(2) Å and W: Ø center-N = 2.42(2) Å), the distances found in Li 2 CaSi 2 N 4 :Eu 2+ are slightly larger.Considering that Eu 2+ (r = 1.31Å) has a slightly larger ionic radius than Ce 3+ (r = 1.15 Å), the occupation of an octahedrally coordinated center in the structure of the title compounds by Ce 3+ would also be possible [42].
A spectrum unfolding was performed for the emission spectrum of the single-crystal Li 6 WN 4 :Ce 3+ (see Figure 8).This unfolding shows two bands, of which one band has a maximum at λ maxP1 = 1.799 eV (14,507 cm −1 ) with a full width at half maximum (fwhm) of 0.218 eV (1757 cm −1 ) and the second band shows a maximum at λ maxP2 = 1.718 eV (13,855 cm −1 ) with a full width at half maximum (fwhm) of 0.301 eV (2432 cm −1 ).This double band is a result of spinorbit coupling of the two sublevels 2 F 5/2 and 2 F 7/2 of the 4f 1 ground state configuration of Ce 3+ [44].
In order to exclude defect luminescence and host structure emission, several experiments were carried out.For this purpose, the compounds Li 6 MN 4 (M = Mo, W) were prepared without the activator ion Ce 3+ , for which no luminescence could be detected.Likewise, the possibility of    falsification of the emissions due to impurities in the crucibles was eliminated.

Conclusions
The two lithonitridometallates described here were prepared in a solid-state reaction.They crystallize in the Li 6 [ZnO 4 ] structure type described by Hoppe et al. [28].The quality of the crystals of the tungsten compound could be improved by adjusting the reactor of the radiofrequency furnace.
Compared to the synthesis routes of Gudat et al. [2] and Yuan et al. [7], a gas flow was used in a radiofrequency furnace and the reaction was thus carried out at normal pressure.The plausibility of the structure model was checked by singlecrystal data, Rietveld refinement, MAPLE, CHARDI and BLBS calculations.The results of the structure models agree with those of other variants known in the literature.Doping of the compounds with Ce 3+ afforded single crystals of Li 6 WN 4 :Ce 3+ , which show an emission maximum at λ max = 693 nm with a full width at half maximum (fwhm) of 97 nm upon excitation with UV near to blue light.Single particles taken from a powder sample of the isotypic compound Li 6 MoN 4 :Ce 3+ show an emission maximum at λ max = 653 nm with a full width at half maximum (fwhm) of 133 nm.The two phosphors are stable to humidity and atmospheric conditions.
In several experiments in which attempts were made to synthesize the Cr phase isotypic to the title compound, the products showed an emission with a maximum at lower wavelengths than the two reported compounds.This observation is consistent with the hypothesis of shifting the emission maximum to lower wavelengths using a smaller central atom (W, Mo vs. Cr).Due to the aggressiveness of the lithium melt during the syntheses, we were not able to produce the pure Cr phase, despite the use of different crucible materials (tungsten, tantalum, corundum) and crucible inlays (molybdenum, titanium).However, there is reason to assume that the observed emission originates from the compound Li 6 CrN 4 :Ce 3+ .

Experimental section 4.1 Synthesis
Because the syntheses involve reactants that are unstable or sensitive to hydrolysis in air, the preparations were carried out in a glovebox filled with inert gas (Ar 5.0, Messer Austria GmbH).In an agate mortar, the starting materials M (M: W -Plansee, 99.9 %; Mo -Plansee, 99.9 %) and Li 3 N (Alfa Aesar, 99.4 %) were mixed in a stoichiometric ratio of 1:4 together with 1 mol% Ce (Sigma Aldrich, 99.9 %) based on the metal contents.The ratio 1:4 has been found to be the best from empirical studies, although traces of corrosion on the tungsten crucible and sublimation of reactant and subsequent deposition on the cooler reaction apparatus were always observed during the syntheses.The reactants were then poured into a tungsten crucible and transferred to the radiofrequency furnace system (type TrueHeat HF 5010; Hüttinger Elektronik, Freiburg, Germany) under an inert gas atmosphere.To prevent a reaction of the lithium nitride melt with the tungsten crucible during the production of the Mo variant, the reactants were packed in a capsule made of Mo foil (Alfa Aesar, 99.95 %, 0.025 mm thick) and then placed in the tungsten crucible.During the entire annealing process, forming gas (forming gas H10 5.0, Messner Austria GmbH), which was regulated to 10 mL min −1 , flowed into the reaction apparatus to ensure a reducing reaction atmosphere.The annealing process consisted of a heating ramp of 200 K h −1 to the final temperature of 1000 °C, a holding time of 12 h at this temperature, a cooling ramp of 0.15 K min −1 to 600 °C and finally quenching to room temperature.The products obtained, which had a slightly reddish body color, could be further investigated and characterized under atmospheric conditions and showed no decomposition due to humidity or atmospheric conditions for several days.

Single-crystal X-ray diffraction
Single-crystals were isolated from the obtained products using a needle selected under a polarization microscope and analyzed with the Bruker D8 Quest single-crystal diffractometer (Bruker, Billerica, USA) equipped with a PHOTON III C14 detector and an Incoatec Microfocus diffraction source (Incoatec, Geesthacht, Germany) with MoKα radiation (λ = 0.71073 Å).The programs SAINT [45] and SADABS [46] were used for data processing and multi-scan absorption correction.OLEX2.solve 1.5 [47] using Direct Methods and SHELXL [48] using the least-squares method, both implemented in the program OLEX2 1.5 [49], were used for structure solution and parameter refinement, respectively.The structural data were standardized using STRUCTURE TIDY [50] implemented in PLATON (version 170613) [51].
Further details of the crystal structure investigation on the singlecrystal of Li 6 WN 4 can be obtained from the Cambridge Crystallographic Data Center at www.ccdc.cam.ac.uk/structures/ using the deposition number 2282797 (Li 6 WN 4 ).

Powder X-ray diffraction
Powder X-ray diffraction (PXRD) was used for phase analysis, using a STOE Stadi P powder diffractometer (STOE & Cie., Darmstadt, Germany) with Ge(111)-monochromatized MoKα 1 (λ = 0.7093 Å) radiation and a Mythen 1K detector in modified Debye-Scherrer geometry over a 2θ range of 2-50°.The experimental powder pattern was compared with the calculated powder pattern derived from single-crystal data of Li 6 WN 4 on the basis of Rietveld analysis.Secondary-phase identification was performed with the ICDD PDF-2 database and the TOPAS 4.2 program [52] was used for Rietveld refinements.

Luminescence
The emission spectra of the compounds Li 6 MN 4 :Ce 3+ (M = Mo, W) were recorded with a setup consisting of a blue laser diode (λ = 448 nm, THORLABS, Ostfildern, Germany) in combination with a CCD-based spectrometer (AVA AvaSpec 2048, AVANTES, Apeldoorn, The Netherlands).A tungsten-halogen calibration lamp (HL3-PLUS CAL, Ocean Insight, Ostfildern, Germany) was used to calibrate the spectral radiance of the system for the first time.AVA AVASOFT software (version 7) was used for data preparation.

Figure 1 :
Figure 1: Coordination polyhedra of the tungsten site and the two lithium sites in Li 6 WN 4 with the interatomic distances in Å (standard deviations in parentheses).Nitrogen atoms are drawn as red spheres.

Figure 2 :
Figure 2: Crystal structure of Li 6 WN 4 in the viewing directions [001] (top left), [100] (top middle) and [010] (bottom left).The unit cell is illustrated with black edges.Nitrogen atoms are shown in red and tungsten atoms in turquoise.The differently colored LiN 4 tetrahedra (Li1: green and Li2: orange) represent the two different Li positions.The squarish channels are marked in yellow.

Figure 3 :
Figure 3: Left: edge-linked Li1 tetrahedra (green) and a W tetrahedron (turquoise) linked by corners to the Li1 channel.Middle: strands of edge-linked Li2 tetrahedra (orange) which are linked by corners.Right: the resulting distorted ccp of nitrogen atoms with the seven out of eight occupied tetrahedra voids.The distorted ccp arrangement (black lines) of the N atoms (red spheres) is drawn in all three representations for better understanding.
, Difference abs. in kJ mol −  Difference in % . Bold values just to emphasize the values.

Figure 4 :
Figure 4: Rietveld fit of the measured powder diffraction data (black), the calculated pattern based on single-crystal data of Li 6 WN 4 (red), and the difference plot (blue).Tick marks illustrate the theoretical reflection positions of Li 6 WN 4 (green).

Figure 5 :
Figure 5: Rietveld fit of the measured powder diffraction data (black), the calculated pattern (developed on Li 6 WN 4 single-crystal data by substituting W for Mo (red)), and the difference plot (blue).Tick marks illustrate the theoretical reflection positions of Li 6 MoN 4 (green).The asterisks denote reflections belonging to an unknown side phase.

Figure 6 :
Figure 6: Left: the blue line represents the luminescence spectrum of single particles taken from a powder sample of Li 6 MoN 4 :Ce 3+ recorded at an excitation wavelength of 448 nm.The red line represents the luminescence spectrum of a Li 6 WN 4 :Ce 3+ crystal recorded at an excitation wavelength of 448 nm.The three gray dashed lines signal detector artefacts that occur at wavelengths 688, 703 and 733 nm.Right: color points of the emission spectra of Li 6 MoN 4 :Ce 3+ particles and Li 6 WN 4 :Ce 3+ crystals in the CIE-xy color space.

Figure 7 :
Figure 7: Left: the two possible coordination environments of the activator ion Ce 3+ in the tungsten variant including the distances to the N 3− anions (red spheres) (distances taken from the powder data for both compounds).Right: the two possible coordination environments of the activator ion Ce 3+ in the molybdenum variant including the distances to the N 3− anions (red spheres).

Figure 8 :
Figure 8: Left: the black line represents the luminescence spectrum of a Li 6 WN 4 :Ce 3+ crystal recorded at an excitation wavelength of 448 nm.The green dashed line represents an ideal Gaussian fit of the experimental emission band.The bimodal Gaussian fit is represented by the blue and red curves.Right: optical appearance of the 1 mol% Ce 3+ -doped compound Li 6 WN 4 under UV light (365 nm) at the bottom of a tungsten crucible and black particles due to corrosion of the tungsten crucible by lithium.

Table  :
Single-crystal data and structure refinement of tetragonal Li  WN  .

Table  :
Wyckoff positions, atomic coordinates, and equivalent isotropic displacement parameters U eq (/Å  ) of Li  WN  based on single-crystal data (standard deviations in parentheses).

Table  :
Comparison between the calculated MAPLE values for Li  WN  and SiO  and the corresponding theoretical starting materials WO  , Li  N and Si  N  according to the following equation:

Table  :
Crystallographic data for Rietveld refinement of Li  MN  (M = Mo, W).