Sr₃P₃N₇: Complementary Approach by Ammonothermal and High‐Pressure Syntheses

Abstract

Nitridophosphates exhibit an intriguing structural diversity with different structural motifs, for example, chains, layers or frameworks. In this contribution the novel nitridophosphate Sr₃P₃N₇ with unprecedented dreier double chains is presented. Crystalline powders were synthesized using the ammonothermal method, while single crystals were obtained by a high‐pressure multianvil technique. The crystal structure of Sr₃P₃N₇ was solved and refined from single‐crystal X‐ray diffraction and confirmed by powder X‐ray methods. Sr₃P₃N₇ crystallizes in monoclinic space group P2/c. Energy‐dispersive X‐ray and Fourier‐transformed infrared spectroscopy were conducted to confirm the chemical composition, as well as the absence of NH_x functionality. The optical band gap was estimated to be 4.4 eV using diffuse reflectance UV/Vis spectroscopy. Upon doping with Eu²⁺, Sr₃P₃N₇ shows a broad deep‐red to infrared emission (λ_em=681 nm, fwhm≈3402 cm⁻¹) with an internal quantum efficiency of 42 %.

Nitridophosphates: A complementary approach of medium‐ and high‐pressure methods with their respective advantages was used for the synthesis of Sr₃P₃N₇, indicating this strategy as promising for further investigation of novel nitridophosphates. Upon doping with Eu²⁺, Sr₃P₃N₇ exhibits luminescence in the deep‐red region.

Introduction

Due to the fact that the element combination P/N is isoelectronic to Si/O, nitridophosphates exhibit structural analogies to silicates and are built up from condensed or non‐condensed PN₄ tetrahedra. In recent years, numerous nitridophosphates have been synthesized with a high structural diversity with different degrees of condensation κ (ratio of tetrahedra centres to ligands) ranging from 1/4 to 4/7.1 Thereby, different structural motifs like non‐condensed tetrahedra, chains, layers or frameworks can occur.2, 3, 4, 5 Particularly due to their structural diversity, these compounds exhibit diverse, interesting physical properties, for example luminescence upon doping with Eu²⁺. So far, P/N‐network and ‐layer compounds such as MP₂N₄ (M=Ca, Sr, Ba), Ba₃P₅N₁₀ X (X=Cl, Br, I) or BaP₆N₁₀NH were used as host lattices, covering emission in almost the entire visible spectrum.6, 7, 8, 9 Especially, Ba₃P₅N₁₀Br, crystallizing in a zeolite‐like structure, exhibits intriguing luminescence properties and is discussed as a natural‐white‐light single emitter, demonstrating nicely the potential of nitridophosphate materials.7

But in contrast to silicates, only a few nitridophosphates could be synthesized under ambient pressure conditions (e.g., Ca₂PN₃, Li₁₀P₄N₁₀).3, 10, 11 This is due to the fact that the appropriate temperature range for crystallization of nitridophosphates is significantly beyond the decomposition temperature of the most important starting material P₃N₅, which can be circumvented by applying high‐pressure methods.

Among these, the multianvil approach is the most frequently employed method, following Le Chatelier's principle in order to compensate thermal decomposition. Over the years, different synthesis strategies using the multianvil approach were developed, including the azide route, high‐pressure metathesis or the synthesis with ammonium chloride as mineralizer.5, 6, 12 The most valuable advantage of this technique is that the enormous pressure allows for sufficient temperatures for the reconstructive cleavage of P−N bonds. Furthermore, in many cases these high temperatures enable the formation of single crystals, facilitating structure elucidation significantly. However, a drawback of such high‐pressure methods is the low sample volume, especially in the context of materials characterization and application.

Therefore, a promising alternative for nitridophosphate synthesis is the ammonothermal approach as a medium pressure method. Herein, the reduced pressure, compared to the multianvil technique, is compensated by the usage of a highly reactive supercritical ammonia atmosphere. In 1997 Jacobs and co‐workers used this approach to synthesize K₃P₆N₁₁.13 In the meantime, the ammonothermal method enabled synthesis of nitridophosphates with isolated tetrahedra units, chains, layers and frameworks.14, 15 Even nitridophosphates like SrP₈N₁₄ or Li₁₈P₆N₁₆, which were so far only accessible by the high‐pressure multianvil approach, could be synthesized under ammonothermal conditions in high yield, simplifying the investigation of their physical properties significantly. Although, the ammonothermal method was already used for synthesis of numerous nitride materials,16, 17, 18, 19, 20, 21, 22 the crystal growth of ternary or quaternary nitrides during ammonothermal synthesis, except for a few examples like Mg₂PN₃ or MTaN₂ (M=Na, K, Rb, Cs), is still challenging.15, 23

In this contribution, we present a combination of medium‐ and high‐pressure methods with their respective advantages for the investigation of unknown nitridophosphates. We report on Sr₃P₃N₇ as a case study for a complementary approach of ammonothermal and multianvil syntheses. Sr₃P₃N₇ extends the class of ternary alkaline earth nitridophosphates by the degree of condensation of κ=3/7. Structure elucidation was based on single‐crystal X‐ray diffraction data, enabled by high‐pressure synthesis, while ammonothermal methods facilitated bulk synthesis for further analysis, as well as studies on the luminescence properties of Sr₃P₃N₇:Eu²⁺.

Results and Discussion

Synthesis

The title compound initially has been observed in heterogeneous products obtained by ammonothermal as well as high‐pressure methods, targeting new nitridophosphates with a low degree of condensation. While high‐pressure high‐temperature synthesis leads to suitable single crystals for structure elucidation, ammonothermal synthesis provided the highest yield of bulk samples, expressing the complementary approach. The synthesis of single crystals was started from stoichiometric amounts of Sr(N₃)₂ and P₃N₅ according to Equation (1) together with small amounts of EuCl₂. The starting mixture was treated by high‐pressure high‐temperature reaction at 5 GPa and 1270 K. These reaction conditions were achieved by using a hydraulic 1000 t press and a modified Walker‐type multianvil apparatus.24, 25

$${\displaystyle 3\mathrm{Sr}\left(\mathrm{N}{}_3\right){}_2+\mathrm{P}{}_3\mathrm{N}{}_5\to \mathrm{Sr}{}_3\mathrm{P}{}_3\mathrm{N}{}_7+8\mathrm{N}{}_2}$$ (1)

Analogously, translucent orange Sr₃P₃N₇:Eu²⁺ crystals of up to 40 μm in length were isolated (see Scanning electron microscopy section). After optimizing the synthesis based on the results of single‐crystal X‐ray diffraction and energy dispersive X‐ray spectroscopy, bulk samples of Sr₃P₃N₇ were prepared under ammonothermal conditions in custom‐built high‐temperature autoclaves using SrH₂, P₃N₅ and the ammonobasic mineralizer NaN₃. NaN₃ decomposes at elevated temperatures and forms NaNH₂ in situ, which increases the solubility of the other starting materials by formation of intermediate species (e.g., Na₂Sr₃(NH₂)₈, Na₁₀[P₄(NH)₆]N₄(NH₂)₆(NH₃)_0.5 or P₃N₃(NH₂)₆) which have already been observed under ammonothermal conditions at temperatures around 670 K.26, 27, 28 Subsequent heating to 1070 K, reaching a maximum pressure of 140 MPa in the autoclave, resulted in synthesis of Sr₃P₃N₇ as a crystalline white powder, which hydrolyses slowly on exposure to moist air. The product was therefore washed with dry ethanol to remove residual mineralizer and intermediate species. Analogous syntheses with red phosphorus instead of P₃N₅ also resulted in the desired product. Doping with Eu²⁺ (nominal concentration of 2 mole% with respect to Sr) in the form of Eu(NH₂)₂ resulted in deep‐red luminescence of Sr₃P₃N₇:Eu²⁺ when irradiated with UV light (see Luminescence section).

Crystal structure

The crystal structure of Sr₃P₃N₇ was solved and refined from single‐crystal X‐ray diffraction data in monoclinic space group P2/c (no. 13). Details on the structure determination are summarized in Table 1. Atomic coordinates and anisotropic displacement parameters are given in Table S1 and S2 in the Supporting Information. Selected interatomic distances and bond angles are summarized in the Supporting Information (Table S3). With κ=n(P)/n(N)=3/7, Sr₃P₃N₇ has a hitherto unknown degree of condensation of an alkaline earth nitridophosphate and is composed of infinite PN₄‐tetrahedra dreier double chains. A degree of condensation of κ=3/7 is already known for rare earth nitridophosphates (RE ₂P₃N₇ with RE=La, Ce, Pr, Nd, Sm, Eu, Ho, Yb), however, there is no structural correlation between these compounds.29 Apart from Ca₂PN₃, Sr₃P₃N₇ is the only known alkaline earth nitridophosphate with a chain structure, while Mg₂PN₃ crystallizes strictly speaking in a wurtzite‐type superstructure and can be interpreted as a double nitride.3, 30 The chains in Sr₃P₃N₇ show a periodicity of P=3 and a stretching factor of f _c=0.90 (see Figure 1) and are built up from dreier‐rings according to Liebau.31, 32 Two of these dreier‐rings, which are structurally related to each other by a rotating mirror axis (2/m), are connected through two common corners forming an additional vierer‐ring. The resulting [P₆N₁₆]‐units were already found in Li₁₈P₆N₁₆ as non‐condensed [P₆N₁₆]¹⁸⁻ anions.14, 33 In the title compound these subunits are connected by two common corners on each side, forming infinite chains. The P−N distances vary from 1.575 to 1.683 Å. As expected, the shortest distances belong to the terminal P(2)−N(2) atoms. The corresponding N‐P‐N bond angles are between 103.3(5) and 115.0(4)°. Angles as well as distances are in good agreement with values observed in other nitridophosphates described in literature.3, 4, 13, 29

Table 1.

Crystallographic data of Sr₃P₃N₇ (synthesized by the multianvil approach) obtained from single‐crystal X‐ray diffraction.

Formula	Sr3P3N7
crystal system	monoclinic
space group	P2/c (no. 13)
lattice parameters [Å,°]	a=6.882(8)
	b=7.416(9)
	c=7.036(8)
	β=104.96(3)
cell volume [Å3]	346.9(7)
formula units / cell	2
Density [g cm−3]	4.345
crystal size [mm]	0.02×0.02×0.03
μ [mm−1]	23.617
T [K]	298(2)
Diffractometer	Bruker D8 Quest
radiation (λ [Å])	MoKα (0.71073)
F (000)	416
θ range [°]	2.7–30.5
total no. of reflections	3621
no. of independent reflections	1064
observed reflections (F 2>2σ(F 2))	728
R int; R σ	0.0858; 0.0970
structure solution	SHELXT
structure refinement	SHELXL‐97
refined parameters	61
goodness of fit (χ 2)	1.044
R1 (all data); R1 (F 2>2σ(F 2))	0.100; 0.059
wR2 (all data); wR2 (F 2>2σ(F 2))	0.131; 0.117
Δρ max; Δρ min [e Å−3]	1.892, −1.790

Figure 1.

Infinite dreier double chain (a) and dreier‐ring subunit (b). Thermal ellipsoids are depicted at 90 % probability.

The infinite chains are oriented along [0 0 1] and are stacked congruently in a and b (see Figure 2). The crystal structure contains two crystallographically different Sr sites. The Sr1 site is located between two chains stacked along [1 0 0] and is coordinated by 10 N atoms with distances between 2.740(9) and 3.068(10) Å. In contrast, the second Sr site (Sr2) is located between two chains stacked along [0 1 0] and is coordinated by 9 N atoms with distances ranging from 2.504(10) to 3.283(10) Å. The coordination polyhedra of both sites are illustrated in Figure S1 in the Supporting Information. All Sr−N distances are in good agreement with values from other Sr (oxo)nitridophosphates known from literature (e.g., SrP₂N₄, SrP₃N₅O, Sr₃P₆O₆N₈).34, 35, 36

Figure 2.

Crystal structure of Sr₃P₃N₇ viewed along [0 0 1] (a) and [0 1 0] (b). PN₄ tetrahedra and N atoms are depicted in blue, Sr atoms in grey. Thermal ellipsoids are depicted at 90 % probability.

To confirm the structure model obtained from single‐crystal X‐ray data, a PXRD measurement and subsequent Rietveld refinement were conducted (Figure 3). The refined crystallographic data, as well as Wyckoff positions, are summarized in Table S4 and S5 in the Supporting Information. As described above (Synthesis section), residual mineralizer (NaNH₂) can be removed by washing with dry ethanol. However, the sample partially decomposes during the washing process, resulting in the formation of an amorphous side‐phase. Due to this fact, the unwashed sample was used for Rietveld refinement. A comparison of washed and unwashed samples is illustrated in Figure S2 in the Supporting Information.

Figure 3.

Rietveld refinement of PXRD measurements of ammonothermally synthesized Sr₃P₃N₇ with experimental data (black line), calculated data (red line), difference profile (blue line) and reflection positions (Sr₃P₃N₇: black bars, NaNH₂: green bars).

Scanning electron microscopy

Energy dispersive X‐ray (EDX) spectroscopy was used for determination of the chemical composition. The determined atomic ratios are in good agreement with the expected chemical formulas (see Table S5 in the Supporting Information). Traces of europium can be attributed to doping of the sample and oxygen impurities to surface hydrolysis of the products. Furthermore, a scanning electron micrograph of the product was collected and is illustrated in Figure 4.

Figure 4.

SEM image of a Sr₃P₃N₇ single crystal obtained from high‐pressure synthesis.

Fourier Transform Infrared spectroscopy

A FTIR spectrum was recorded in order to prove the absence of any NH_x functionality in Sr₃P₃N₇. The spectrum (see Figure S3 in the Supporting Information) shows no significant absorption bands around 3000 cm⁻¹, indicating the absence of N−H groups in accordance to the crystal structure obtained from single‐crystal X‐ray diffraction, since the presence of imide or amide groups would lead to strong absorption bands.4 Weak signals could be attributed to partial surface hydrolysis of the products. The absorption bands between 500 and 1300 cm⁻¹ can be attributed to symmetric and asymmetric stretching modes of the P‐N‐framework and are characteristic for nitridophosphates.

UV/Vis spectroscopy

Diffuse reflectance UV/Vis spectroscopy was conducted to estimate the optical band gap of the undoped sample. Therefore, the Kubelka–Munk function F(R)=(1−R)²/2R was used to convert the measured diffuse reflectance spectrum to a pseudo‐absorption spectrum.37 A Tauc plot (see Figure 5) was then used to estimate the optical band gap by plotting (F(R)⋅hν)^1/n versus hν (with n=1/2, assuming direct transition) and drawing of a tangent at the inflection point.38 The diffuse reflectance spectrum shows an absorption band around 250 nm (see Figure S4 in the Supporting Information). The determined band gap is approximately 4.4 eV.

Figure 5.

Tauc plot (black line) of ammonothermally synthesized Sr₃P₃N₇. Red line as a tangent at the inflection point.

A diffuse reflectance spectrum of the Eu²⁺ doped sample is illustrated in Figure S5. It shows an additional broad absorption band between 400 and 550 nm, which can be attributed to dopant absorption and is in good agreement with the corresponding excitation spectrum (see Luminescence section).

Luminescence

Luminescence properties of Sr₃P₃N₇:Eu²⁺ were measured from crystalline powder samples. Sr₃P₃N₇:Eu²⁺ shows deep‐red luminescence when irradiated with UV to blue light (see Figure S6 in the Supporting Information). The emission spectrum (λ_exc=450 nm) and the excitation spectrum are illustrated in Figure 6. The excitation spectrum has two maxima at around 405 and 465 nm, while the emission spectrum (2 mole% Eu with respect to Sr) shows one broad band in the deep‐red to infrared region (λ_em=681 nm) with a full width at half maximum (fwhm) of 162 nm/3402 cm⁻¹. The broad emission band probably results from the different coordination of the cations (see Crystal structure section). In order to capture the complete emission spectrum, a second spectrometer was used for the measurement (λ_exc=410 nm) of the emission in the IR region beyond 800 nm. This leads to the higher background level above 800 nm. The internal quantum efficiency (IQE) was determined with an excitation wavelength of 438 nm to be 42 % in the range up to 800 nm. These luminescent properties make Sr₃P₃N₇:Eu²⁺ interesting, as such materials may be applied in horticultural lighting, as they can convert green light into the red to IR spectral range with increased photosynthesis quantum efficiency. A further possible application field is, for example, hyper‐ or multispectral imaging, with diverse applications in for example, agriculture, automated driving or molecular biology.39, 40

Figure 6.

Excitation (red line) and emission (black line) spectra of ammonothermally synthesized Sr₃P₃N₇:Eu²⁺.

Conclusions

In this contribution we present a complementary approach of ammonothermal and multianvil techniques as a powerful combination for a simplified and much faster access to a detailed structural and physical analysis of nitridophosphates. Exploitation of the advantages of each method led to the discovery of the new nitridophosphate Sr₃P₃N₇. Thereby, the best crystallization conditions were realized via a high‐pressure approach using the azide route, allowing higher temperatures during syntheses compared to the ammonothermal approach. The structure model was solved and refined from single‐crystal X‐ray diffraction data. Sr₃P₃N₇ is composed of unprecedented dreier double chains made up of PN₄ tetrahedra with a degree of condensation κ=3/7, which is hitherto unknown for alkaline earth nitridophosphates. Due to larger sample volumes, studies on the physical properties of the title compound were carried out with bulk samples obtained from ammonothermal synthesis. The band gap (4.4 eV) was determined using diffuse reflectance spectroscopy. Upon doping with Eu²⁺, Sr₃P₃N₇ exhibits a broad emission band in the deep‐red to infrared region (λ_em=681 nm, fwhm≈3402 cm⁻¹) with an internal quantum efficiency of 42 %, making it practically interesting for commercial applications (e.g., horticultural lighting). For this purpose, stoichiometry and doping concentration optimization as major parts of an industrial process development will further improve the luminescence efficiency.

Experimental Section

All manipulations were conducted in argon‐filled gloveboxes (Unilab, MBraun, Garching, O₂<1 ppm, H₂O<1 ppm) under exclusion from oxygen and moisture because of the air sensitivity of starting materials and products. A vacuum line (≤0.1 Pa) with argon and ammonia (both: Air Liquide, 99.999 %) supply was used for filling of the autoclaves with NH₃. The gases were further purified by gas cartridges (Micro torr FT400‐902 (for Ar) and MC400‐702FV (for NH₃), SAES Pure Gas Inc., San Luis Obispo, CA, USA), providing a purity level of <1 ppbV H₂O, O₂ and CO₂.

Synthesis of Eu(NH₂)₂: Eu(NH₂)₂ was synthesized ammonothermally starting from Eu metal (99.99 %, smart elements). The experiment was carried out analogously to the synthesis described in literature.41

Synthesis of P₃N₅: P₃N₅ was synthesized following Stock et al.42 by ammonolysis of P₄S₁₀ (Sigma Aldrich, 99 %) at 1125 K for 4 h (heating rate: 5 K min⁻¹). Before reaction, the silica tube was saturated with NH₃ for 4 h. After cooling down to room temperature (5 K min⁻¹), the received product was washed in multiple steps with ethanol, water and acetone and dried under vacuum. Powder X‐ray diffraction was conducted to confirm phase purity.

Synthesis of Sr(N₃)₂: Sr(N₃)₂ was synthesized starting from SrCO₃ (Sigma Aldrich, 99.995 %) and HN₃ formed in situ (using aqueous NaN₃ (Acros Organics, 99 %) and a cation exchanger (Amberlyst 15)) following the syntheses of Suhrmann and Karau.43, 44 HN₃ was slowly added to an aqueous suspension of SrCO₃ until the liquid turned clear. After filtration, the solution was evaporated under reduced pressure (50 mbar, 40 °C), recrystallized from acetone and dried under vacuum. FTIR and PXRD measurements were conducted to confirm phase purity. Caution: Since HN₃ solutions are potentially explosive and the vapour is highly poisonous, special care issues are necessary.

Ammonothermal synthesis: Sr₃P₃N₇ was synthesized under ammonothermal conditions starting from 0.5 mmol P₃N₅ (81.5 mg), 1.5 mmol SrH₂ (134.4 mg, Materion, 99.5 %) and 5 mmol NaN₃ (325.0 mg, Sigma–Aldrich, 99.5 %) as ammonobasic mineralizer. For the synthesis of Eu²⁺‐doped product 0.03 mmol Eu(NH₂)₂ (5.5 mg) was added to the reaction mixture. The starting materials were ground using an agate mortar and transferred into a Ta‐liner, which protects the reaction mixture against autoclave impurities. The liner was placed in a high‐temperature autoclave constructed of a nickel based super‐alloy (Haynes® 282®, max. 1100 K, 170 MPa, 10 mL). The autoclave is sealed with a lid through flange joints using a silver coated Inconel® 718 ring (GFD seals). An Inconel® 718 high‐pressure tube connects the lid with a hand valve (SITEC), which is equipped with a pressure transmitter (HBM P2VA1/5000 bar) and a bursting disc (SITEC). After evacuation, the sealed autoclave was cooled to 198 K using an ethanol/liquid nitrogen mixture and filled with NH₃ (≈3.7 mL). A mass flow meter (d‐6320‐DR, Bronkhorst, Ruurlo, Netherlands) was used for detection of the amount of inserted ammonia. The filled autoclave was heated in two steps (heated to 670 K within 2 h, held at this temperature for 16 h, heated to 1070 K within 3 h and held at this temperature for 96 h) to 1070 K reaching a maximum pressure of 140 MPa. After cooling to room temperature and removal of NH₃, the reaction products were separated under argon, washed with ethanol and dried under vacuum. While the undoped product exhibits a white color, the Eu²⁺‐doped sample is orange.

High‐pressure synthesis: Single crystals of Sr₃P₃N₇:Eu²⁺ were synthesized starting from stoichiometric amounts of Sr(N₃)₂ and P₃N₅ as well as small amounts of EuCl₂ (Strem Chemicals, 99.9 %) as dopant using a modified Walker‐type multianvil press.24, 25 The reactants were mixed and grounded in an agate mortar and packed in a capsule of hexagonal boron nitride (Henze, Kempten). After sealing with a BN‐cap the sample was placed in the middle of a MgO octahedron (doped with 5 % Cr₂O₃, edge length 18 mm, Ceramic Substrates & Components Ltd, Isle of Wight) using two MgO spacers (Cesima Ceramics, Wust‐Fischbach). To ensure heating of the sample, the octahedron was further equipped with two graphite furnaces (Schunk Kohlenstofftechnik GmbH, Zolling), a ZrO₂ tube (Cesima Ceramics, Wust‐Fischbach) for thermal insulation and two Mo plates for electrical contact between the graphite furnaces and the anvils of the multianvil press. The assembled octahedron was placed between eight WC cubes (doped with 7 % Co, Hawedia, Marklkofen, Germany) with truncated edges (edge length 11 mm), which were separated with pyrophyllite gaskets (Ceramic Substrates & Components, Isle of Wight, UK). Further details on the experimental setup and the multianvil apparatus are given in the literature.25 The sample was slowly compressed to 5 GPa and subsequently heated to 1270 K within 30 min. After 30 min at 1270 K the sample was allowed to cool down to room temperature within 30 minutes and slowly decompressed. The crystalline orange product was isolated and stored under argon.

Single‐crystal X‐ray diffraction: Single crystals of Sr₃P₃N₇:Eu²⁺ were placed and sealed in glass capillaries (Hilgenberg GmbH) in argon atmosphere for single‐crystal XRD measurements. The data were collected using a Bruker D8 Quest diffractometer with Mo_Kα radiation (λ=0.71073 Å). The measured data were indexed and integrated with the software package APEX3.45, 46 APEX3 was also used for semi‐empirical absorption corrections (SADABS) and the determination of the space group.46, 47, 48 The crystal structure was solved using the SHELXT algorithm and refined by full‐matrix least‐squares methods using WinGX with implemented SHELXL.49, 50, 51

CCDC 1975990 contains the supplementary crystallographic data for this paper. These data are provided free of charge by The Cambridge Crystallographic Data Centrethrough the CCDC/FIZ Karlsruhe deposition service.

Powder X‐ray diffraction: For powder XRD measurements, the ground product was filled and sealed in a glass capillary (d=0.3 mm, Hilgenberg GmbH). A Stoe STADI P diffractometer with Cu${}_{\mathrm{K}\alpha {}_1}$ (λ=1.5406 Å) radiation, Ge(1 1 1) monochromator and Mythen 1 K detector in modified Debye–Scherrer geometry was used for the measurements. TOPAS was used for Rietveld refinement of the data.52

Scanning electron microscopy: A scanning electron microscope (Dualbeam Helios Nanolab G3 UC (FEI), equipped with an EDX detector (X‐Max 80 SDD, Oxford instruments)) was used for imaging of the crystals and for EDX measurements. Thereto, the crystallites were placed on adhesive carbon pads. A high‐vacuum sputter coater (BAL‐TEC MED 020, Bal Tec A) was used for coating of the samples with a conductive carbon film.

Fourier Transform Infrared (FTIR) spectroscopy: IR spectra (range between 400 and 4000 cm⁻¹) were collected using a FTIR‐IFS 66 v/S spectrometer (Bruker). The samples were mixed with KBr (Acros Organics, 99 %) and pressed into pellets under argon. OPUS was used for evaluation of the measurements.53

UV/Vis spectroscopy: The optical band gap was estimated using UV/Vis spectroscopy. For this purpose, diffuse reflectance measurements of the samples at room temperature were performed using a Jasco V‐650 UV/VIS spectrophotometer equipped with Czerny–Turner mount, photomultiplier tube detector and deuterium (190–350 nm)/ halogen (330–900 nm) lamps as light sources.

Luminescence: A microcrystalline powder of Sr₃P₃N₇:Eu²⁺ was used to determine luminescence properties. The measurement was conducted on a PTFE sample holder using an in‐house built system based on a 5.3’’ integration sphere and a spectrofluorimeter equipped with a 150 W Xe lamp, two 500 mm Czerny–Turner monochromators, 1800 1 mm⁻¹ lattices, and 250/500 nm lamps, with a spectral range from 230 to 820 nm (λ_exc=450 nm). Additional data from a spectrometer sensitive in the 600–1100 nm wavelength range (Avantes) (λ_exc=410 nm) were used to obtain the complete emission band by merging data of both measurements. A comparison of integrated emission intensities and absorption at excitation wavelength (λ_exc=438 nm) of the sample with reference materials (BaSO₄, Merck for white standard DIN 5033 commercial (Sr,Ca)AlSiN₃:Eu²⁺, Mitsubishi Chemical, and Y₃Al₅O₁₂:Ce³⁺, Philips) were conducted to determine the internal quantum efficiency (IQE).

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

M. Mallmann, S. Wendl, P. Strobel, P. J. Schmidt, W. Schnick, Chem. Eur. J. 2020, 26, 6257.