Crystal Structure

The crystal structure of SALON (Sr[Li 2 Al 2 O 2 N 2 ]:Eu2+) was solved and refined based on single-crystal X-ray diffraction data (for detailed information on the synthesis and structure see Supplementary Table 1 to Supplementary Table 5)37,38,39. SALON crystallizes in a hitherto unknown structure type in the tetragonal space group P4 2 /m (no. 84) with the unit-cell parameters a equals 7.959(2) Å and c equals 3.184(1) Å. The structure is an ordered variant of the UCr 4 C 4 -structure type with Sr on the corresponding uranium site, an ordering of aluminium and lithium on the corresponding chromium site and an ordering of nitrogen and oxygen on the corresponding carbide site. Based on the data quality and the different scattering factors, the location and occupation of the Sr, Li and Al sites could be unambiguously determined. Two kinds of tetrahedra form a highly condensed network hosting the strontium cations in one of the resulting channels (Fig. 1a). In the first type of tetrahedra (T1, grey tetrahedra), aluminium is coordinated by three nitrogen and one oxygen atom forming a [AlON 3 ]8- unit. In the second type (T2, orange tetrahedra), lithium is coordinated by one nitrogen and three oxygen atoms forming a [LiO 3 N]8- unit. Each oxygen or nitrogen atom acts as a fourfold bridging atom leading to a sphalerite-like degree of condensation κ, which is equivalent to the atomic ratio of (Li,Al)/(O,N) equals 1. This is unusual for anionic 3D tetrahedra networks (an illustration of this network can be found in Supplementary Fig. 1).

Fig. 1 Structural overview of SALON. Red spheres represent strontium, blue spheres oxygen, and green spheres nitrogen atoms. The [LiO 3 N]8- tetrahedra are shown in orange, the [AlON 3 ]8− tetrahedra in grey, and the red polyhedron represents the cube-like coordination of Sr2+. a Viewing of a 2 × 2 × 2 supercell of Sr[Li 2 Al 2 O 2 N 2 ]:Eu2+ along [001̅]. b Perspective viewing of the eight-fold coordination of Sr2+ by O and N Full size image

The tetrahedra form a network of vierer rings40 arranged in three types of channels along the [001] axis. These channels are connected via common tetrahedra. Two of the channels are empty and solely built up from T1- or T2-tetrahedra. The third type of channel, which hosts the strontium cations, is built up from alternating T1- and T2-tetrahedra. As the strontium cations are located at a centre of inversion, an unprecedented coordination incorporating four nitrogen (Sr–N: 2.760(5) Å) and four oxygen (Sr–O: 2.659(4) Å) atoms is obtained, resulting in a highly symmetrical cube-like coordination (Fig. 1b). The Eu2+ activator partly replaces Sr2+, as the ionic radii of Eu2+ (1.39 Å) and Sr2+ (1.40 Å) are nearly identical41. The structure model was confirmed by BLBS42 and CHARDI43 calculations (see Supplementary Table 6 and 7), which are all in good agreement with the single-crystal X-ray diffraction data.

Due to the limitations of X-ray diffraction, the ordering of the nitrogen and oxygen atoms could not be determined with absolute certainty. However, the obtained residuals and figures of merit were much better using this approach. With a ratio of O/N equals 1/1, this model is in good agreement with the elemental analysis data (see next chapter). Additionally, one has to keep in mind that the absence of the described ordering would lead to a variety of different local coordination environments for the europium cations, which would in turn cause a definitely broader emission band. To confirm the above mentioned ordering MAPLE (Madelung part of lattice energy) calculations have been performed. The results of these calculations are in good agreement with the other data and the oxygen/nitrogen-ordering is therefore a reasonable assumption. For the future, an in-depth analysis of the oxygen/nitrogen ordering via neutron diffraction could be employed.

To ensure that the crystal used for the structure determination is representative for the whole sample, a powder diffraction pattern of a bulk sample was recorded. A Rietveld refinement of the PXRD data revealed that 93 wt% of the bulk material was indeed the afore described phase Sr[Li 2 Al 2 O 2 N 2 ], with impurities of SrO (7 wt%) and AlN (not quantifiable). For further details see Supplementary Note 1, Supplementary Table 8, and Supplementary Fig. 2.

Although SALON was found to be an oxonitride compound with an oxygen to nitrogen ratio of 1:1, it exhibits some of the structural motives typical for nitride compounds. This is very unusual as oxygen rich compounds usually do not exhibit the same connectivity as purely nitride based compounds. To the best of our knowledge, this is the first time that a cube-like coordination, similar to the one found in many nitride based red phosphors, could be realized in an oxonitride with an oxygen/nitrogen ratio as high as 1:1.

Elemental analysis

Analytical scanning transmission electron microscopy (STEM) in combination with energy-dispersive X-ray spectrometry (EDX) confirmed the composition of SALON. The STEM micrograph (Supplementary Fig. 3) shows two phosphor particles, both with diameters of a few hundred nanometres (100 nm at their thinnest, 500 nm at their thickest point), embedded in a SrO-matrix. This matrix is the result of the sample preparation process which was carried out on a larger SALON particle and not inherent to the sample. EDX element maps, obtained by plotting the lateral distribution of the EDX signal intensity of the respective X-ray lines within the spectrum, indicate that the crystals are rich in nitrogen, aluminium, and strontium, and also contain oxygen (see Fig. 2a). The surrounding matrix is rich in strontium and oxygen. A quantification of the EDX spectra acquired from the phosphor particles resulted in an element ratio of Sr:Al:O:N approximately equal to 1:1.8:2.1:1.7. Within the accuracy of the measurement, this result is very close to Sr:Al:O:N equals 1:2:2:2, thus in agreement with the X-ray diffraction data.

Fig. 2 Scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM/EDX) and time of flight secondary ion mass spectrometry (ToF-SIMS) analyses of SALON. a Qualitative false-colour representation of the lateral distribution of the elements Sr, Al, O, and N. b Qualitative false-colour representation of the lateral distribution of the elements Li and Cu for a Sr[Li 2 Al 2 O 2 N 2 ]:Eu2+ phosphor particle that has been placed into a pocket of a copper transmission electron microscopy grid with 50 µm mesh width. c Time of flight mass spectrometry depth profile of the Li+ secondary ions signal intensities in Sr[LiAl 3 N 4 ] and Sr[Li 2 Al 2 O 2 N 2 ], divided by the respective signal intensities of the Sr+ secondary ions. Below the particle surface, the normalized signal intensities approach a linear level (further details are provided in the Method section) Full size image

Since the EDX setup did not allow for the detection of lithium, time-of-flight secondary ion mass spectroscopy (ToF-SIMS) was performed to verify the lithium content of the phosphor. As Fig. 2b shows, the ToF-SIMS analysis unambiguously detected Li with homogeneous distribution in the phosphor particle. In order to gain quantitative information concerning the composition, ToF-SIMS depth profiles were acquired from a phosphor particle, and also from a reference with known composition, namely Sr[LiAl 3 N 4 ]. Within these depth profiles, the signal intensities of the Li+ secondary ions, divided by the signal intensities of the Sr+ secondary ions, approached a linear level in the phosphor bulk below the surface. As can be seen in Fig. 2c, the intensity of the Li+ signal in SALON is higher than that of the Sr[LiAl 3 N 4 ] reference by a factor of 2.4, which is close to 2. The remaining 0.4 difference can be attributed to matrix effects, which cannot be accounted for as there is no established reference available, due to the fact that SALON is the only known representative of this compound class. Hence, by combination of the EDX and ToF-SIMS results, taking electro-neutrality into account, the only possible composition of SALON was determined to be Sr[Li 2 Al 2 O 2 N 2 ], which confirmed the results obtained from single-crystal X-ray diffraction.

DFT Calculations

The DFT (density functional theory) electronic band-structure calculations identify Sr[Li 2 Al 2 O 2 N 2 ] as an insulating material with highly ionic bonding character. Sr[Li 2 Al 2 O 2 N 2 ] has a calculated band gap of 4.9 to 5.3 eV (depending on the mBJ (modified Becke-Johnson) parametrisation), which is larger than the band gap of Sr[LiAl 3 N 4 ] (4.14 eV) due to the stronger ionic character in the oxonitride material44. The band gap has also been estimated by means of diffuse reflectance spectroscopy and analysis via the Tauc-method45, resulting in a value of ~4.4 eV (for details see Supplementary Fig. 4). This value is well within the expected scope, as optical band gaps are usually slightly smaller than the band gaps calculated via DFT. The calculated large band gap is in good agreement with the very low thermal quenching (see next chapter). Figure 3 shows the total density-of-states (DOS) and atom-resolved partial density-of-states curves (pDOS). The latter gives the contributions of the constituent elements to the total DOS. The valence bands originate from the oxygen and nitrogen 2p-orbitals and are very narrow, which is typical for salt-like ionic compounds. Empty states of strontium and aluminium form the bottom of the conduction band, while the empty states of Li are at slightly higher energies.

Fig. 3 Projected total density of states (DOS) obtained from Linear combination of atomic orbitals and atom-resolved partial electronic density-of-states (pDOS) of SALON. The energy zero is taken at the Fermi level Full size image

Integrations of the partial atom-resolved pDOS from the projected LCAO (linear combination of atomic orbitals) base yield atom charges that reflect the expected ionic distribution as well as the charges obtained from Bader´s atom-in-molecules (AIM) approach (further details are provided in the Method section and Supplementary Table 9)46. The calculated bulk-modulus of SALON is 107 GPa and thus comparable with that of silicon (100 GPa).

Luminescence and LED efficacy data of SALON

Eu2+-doped samples of SALON exhibit intense red luminescence when excited with UV to green light. Figure 4a shows the excitation spectrum (grey curve) and the emission spectrum (red curve) of a SALON bulk sample. The emission of the single-crystal used for the structure determination has also been measured and is very similar to the emission of the powder (see Supplementary Note 2 and Supplementary Fig. 5). Therefore, the luminescence properties of the bulk sample can clearly be associated with SALON. This also rules out the possibility that SrO, which is present as a side phase in the bulk sample of SALON and can also exhibit a red emission when doped with Eu2+, contributes to the observed emission spectra47. The excitation spectrum’s maximum is located at roughly 450 nm, which is in the region of common wavelengths for primary blue LEDs. Additionally, the broad absorption band can be assigned to parity-allowed 4f7(8S 7/2 ) → 4f6(7F)5d1 transitions within the Eu2+ activator. SALON exhibits a narrow-band emission (FWHM equals 48 nm, 1286 cm−1, 0.1594 eV) in the red spectral region (λ max equals 614 nm, perfectly meeting the requirement defined for red conversion phosphors in the solid state lighting R&D plan 2016). In comparison to Sr[LiAl 3 N 4 ]:Eu2+ (λ max approximately equals 654 nm; FWHM approximately equals 50 nm, 1180 cm−1, 0.1463 eV, purple curve22), a very good red phosphor which has received significant attention, the main advantage of SALON lies in the position of its emission maximum, which is located at higher energies than the emission maximum of SLA and thereby significantly increases the emission’s overlap with the human eye sensitivity curve (dotted black curve in Fig. 4a). The optimized spectral position of the SALON phosphor leads to an LER increase by a factor of roughly 3.5 compared to Sr[LiAl 3 N 4 ]:Eu2+ (266 lm/W opt for SALON vs. 77 lm/W opt for Sr[LiAl 3 N 4 ]:Eu2+, both at roughly 300 K)22 regarding full-conversion red-emitting pc-LEDs built up from a primary blue LED and one single red-emitting phosphor. The relative blue shift of SALON’s emission maximum is caused by a less pronounced nephelauxetic effect and a lower crystal field splitting due to the presence of oxygen atoms in the Eu2+-coordination sphere (Fig. 1b), whereas only nitrogen atoms are present in the case of Sr[LiAl 3 N 4 ]:Eu2+. In addition to the well suited spectral position of SALON for red components in high-performance white LEDs, its small FWHM of 48 nm makes a huge step on the way towards the goal of reducing the FWHM of red phosphors to strongly increase the LED’s overall efficacy18.

Fig. 4 Photoluminescence properties of SALON. a Normalized excitation (grey, for the emission at λ max equals 614 nm) and emission spectrum (red, excited with λ exc equals 460 nm) of SALON in comparison to Sr[LiAl 3 N 4 ]:Eu2+ 22, reference (purple, data taken from literature) and the human-eye sensitivity curve (black dotted). b Comparison of two warm-white pc-LEDs (CCT equals 2700 K, CIE coordinates (SALON based LED): x/y equals 0.463/0.415; CIE coordinates (SLA based LED): x/y approximately equal 0.455/0.405, spectra normalized to integral intensity) with high colour rendering index (CRI greater than 90, R9 greater than 40). Purple curve taken from literature (Sr[LiAl 3 N 4 ]:Eu2+ as red phosphor), the red curve represents a mid-power pc-LED using the SALON phosphor as red component. c Relative luminous efficacy of radiation (LER) values for different warm-white pc-LED solutions (data normalized to the state of the art). Solutions marked with an asterisk are taken from the literature22. The solid red bar represents the relative LER value for the SALON containing phosphor solution with R9 greater than 40. d Relative photoluminescence intensity of SALON measured from 298 K to 500 K Full size image

To ensure that SALON does not suffer from pronounced thermal quenching, the temperature dependence of the integrated photoluminescence intensity was investigated for a powder sample from 298 K to 500 K (see Fig. 4d and Supplementary Fig. 6). Low-temperature luminescence investigations showed a gradual reduction of the FWHM yielding 41 nm at 15 K. Interestingly, the FWHM reduction mainly occurs on the high energy side of the emission spectrum (see Supplementary Fig. 7). Even at typical LED operating temperatures of 420 K, the integrated light output of SALON drops by only 4% compared to the initial intensity at room temperature (298 K), while compounds, which suffer from considerable thermal quenching like SrMg 3 SiN 4 :Eu2+ 23, have already lost over 90% of their maximum intensity at this temperature (420 K)48. SALONs outstanding thermal quenching behaviour surpasses the performance of most other phosphors (e.g. CaAlSiN 3 :Eu2+, Ba[Li 2 (Al 2 Si 2 )N 6 ]:Eu2+)26,49 and is a valuable quality for application in pc-LEDs. A simultaneous shift in the CIE (Commission Internationale de l’Éclairage) colour coordinates, x/y equals 0.654/0.346 to x/y equals 0.625/0.374 (Δx/Δy equals 0.029/0.028), and LER-values of SALON was also recorded in the range from 298 to 473 K. This is a rather small colour shift and comparable to the shift found in Sr[LiAl 3 N 4 ]:Eu2+, from x/y equals 0.693/0.306 to x/y equals 0.668/0.330 (Δx/Δy equals 0.025/0.024) between 303 K and 465 K. Finally, the quantum efficiency (number of converted photons emitted/number of photons absorbed in %) of SALON was determined to be greater than or equal to 80%. This value should be comparable to the literature value for the “internal quantum efficiency” of Sr[LiAl 3 N 4 ]:Eu2+ of 76%22.

The luminescence decay time of SALON has been determined by a Time-Correlated Single Photon Counting (TCSPC) experiment (see Supplementary Fig. 8). The relaxation constant τ of 790 ns is in good agreement to those determined for other red-emitting Eu2+-doped phosphors34,50.

Although SALON powder in this early state without any protective coating is not stable against direct exposure to water, it was possible to construct a prototype pc-LED, where SALON was shown to be stable enough to be utilized in silicone under ambient conditions up to at least 250 °C. As demonstrated for SLA, even highly sensitive nitride materials can be stabilized enough to be used in commercial pc-LEDs51. A similar process will be necessary for this material in the future.