High‐Pressure High‐Temperature Synthesis of Mixed Nitridosilicatephosphates and Luminescence of AESiP₃N₇:Eu²⁺ (AE=Sr, Ba)

Abstract

Tetrahedra‐based nitrides with network structures have emerged as versatile materials with a broad spectrum of properties and applications. Both nitridosilicates and nitridophosphates are well‐known examples of such nitrides that upon doping with Eu²⁺ exhibit intriguing luminescence properties, which makes them attractive for applications. Nitridosilicates and nitridophosphates show manifold structural variability; however, no mixed nitridosilicatephosphates except SiPN₃ and SiP2N4NH have been described so far. The compounds AESiP₃N₇ (AE=Sr, Ba) were synthesized by a high‐pressure high‐temperature approach using the multianvil technique (8 GPa, 1400–1700 °C) starting from the respective alkaline earth azides and the binary nitrides P₃N₅ and Si₃N₄. The latter were activated by NH₄F, probably acting as a mineralizing agent. SrSiP₃N₇ and BaSiP₃N₇ were obtained as single crystals. They crystallized in the barylite‐1O (M=Sr) and barylite‐2O structure types (M=Ba), respectively, with P and Si being occupationally disordered. Cation disorder was further supported by solid‐state NMR spectroscopy and energy‐dispersive X‐ray spectroscopy (EDX) mapping of BaSiP₃N₇ with atomic resolution. Upon doping with Eu²⁺, both compounds showed blue emission under UV excitation.

The first nitridosilicatephosphates are synthesized using high‐pressure high‐temperature reactions. The incorporation of P and Si as well as their occupational disorder in the compounds AESiP₃N₇ (AE=Sr, Ba) is proven by solid‐state NMR spectroscopy, STEM‐EDX mapping, and single‐crystal diffraction. Eu²⁺‐doped samples show blue luminescence under irradiation with UV light and therefore expand the range of possible host lattices for luminescent materials.

Introduction

Emerging environmental consciousness has pushed the development of solid‐state lighting solutions forward. The invention of efficient InGaN‐based blue LEDs (light‐emitting diodes) enabled the development of pc‐LEDs (phosphor‐converted) with remarkable properties in terms of color temperature, color rendition, and efficacy. Significant improvements in the aforementioned properties were possible due to nitride compounds such as M ₂Si₅N₈:Eu²⁺ (M=Sr, Ba), MSi₂O₂N₂:Eu²⁺ (M=Ca, Sr, Ba), SrLiAl₃N₄:Eu²⁺, and MAlSiN₃:Eu²⁺.[ ¹ , ² , ³ , ⁴ ]

Materials properties concerning solid‐state lighting can be tuned by dopant concentration to a limited extent, affecting Stokes shifts in emission spectra, or by a variation of the size of coordination polyhedra by substitution such as introducing Sr on Ba sites. Completely shifted emission properties, however, can only be achieved by a fundamental alteration of the host lattice.[ ⁵ , ⁶ ]

The main goal of this work was to expand the compositional and structural diversity of tetrahedra‐based luminescent materials. Thus, discovery of the title compounds SrSiP₃N₇ and BaSiP₃N₇ opens up the novel compound class of mixed nitridosilicatephosphates, which can now be further explored as innovative host lattices. While nitridosilicates have been investigated thoroughly and nitridophosphates show similar promising structures and properties, only two compounds that contain both SiN_x (x=4, 6) and PN₄ units have been reported so far, that is, SiPN₃ and SiP₂N₄NH.[ ⁷ , ⁸ , ⁹ ] The crystal structure of SiPN₃ corresponds to a defect wurtzite‐type arrangement with mixed occupation of Si and P at the tetrahedral sites. The crystal structure of SiP₂N₄NH is related to sillimanite‐type Al₂SiO₅. It is built up from edge‐sharing SiN₆ octahedra interconnected by all‐side vertex‐sharing PN₄ tetrahedra. A possible explanation for the challenges concerning syntheses that are involved in the preparation of mixed nitridosilicatephosphates could be the chemical inertness of Si₃N₄, while P₃N₅ already decomposes at temperatures above 850 °C if no external pressure is applied. As shown in previous publications, according to Le Chatelier's principle, the decomposition of P₃N₅ under the formation of N₂ is suppressed by external pressure. ^[10]

NH₄Cl has been successfully employed as a mineralizer facilitating crystal growth of nitridophosphates. HCl formed in situ most likely leads to reversible P−N bond formation and cleavage.[ ¹¹ , ¹² ] After nitridosilicatephosphates proved to be not accessible with the help of NH₄Cl, changing the mineralizing agent to NH₄F afforded the title compounds AESiP₃N₇ (AE=Sr, Ba). This may be explained by the fact that HF cannot only reversibly cleave and form P−N bonds, but also Si−N bonds. The surface of Si₃N₄ features SiNH₂ groups that can be attacked by F⁻ in a nucleophilic substitution. ^[13]

Results and Discussion

The nitridosilicatephosphates AESiP₃N₇ (AE=Sr, Ba) were synthesized by high‐pressure high‐temperature (HP/HT) reactions at 8 GPa and 1400 °C (Ba) and 1700 °C (Sr), respectively, using a modified Walker‐type multianvil apparatus. ^[14] The synthesis of SrSiP₃N₇ at temperatures below 1700 °C resulted in samples with significant amounts of unknown side phases. Reactions followed the so‐called azide route using P₃N₅, Si₃N_4, and the respective metal azide as starting materials with additional NH₄F (≈5 wt %) as a mineralizing agent [Eq. 1]. To investigate luminescence properties, samples with the addition of approximately 1 mol % of EuF₃ (concerning AE ²⁺) to the starting mixture were prepared.

$${\displaystyle 3AE\left(\mathrm{N}{}_3\right){}_2+3\mathrm{P}{}_3\mathrm{N}{}_5+\mathrm{Si}{}_3\mathrm{N}{}_4\underset{}{\overset{\mathrm{N}{\mathrm{H}}_4\mathrm{F}}{\to }}3AE\mathrm{SiP}{}_3\mathrm{N}{}_7+8\mathrm{N}{}_2}$$ (1)

The title compounds were obtained as colorless powders (Eu²⁺‐doped samples of SrSiP₃N₇ show a yellow tint) and showed no sensitivity to air or moisture. More detailed information on the HP/HT synthesis is given in the Supporting Information.

The crystal structures were elucidated by single‐crystal X‐ray diffraction (SCXRD) using direct methods and least‐squares refinement. SrSiP₃N₇ crystallizes in space group Pmn2₁ (no. 31) with Z=2. BaSiP₃N₇ crystallizes in space group Pnma (no. 62) with Z=4); details are given in Tables 1 and S2–7. In addition, Rietveld refinements indicate the presence of BaSiP₃N₇ crystallizing in space group Pmn2₁ (no. 31) with Z=2 as a side phase. Both compounds are isotypic to the two polymorphs of barylite BaBeSi₂O₇. SrSiP₃N₇ corresponds to the barylite‐1O polymorph, whereas BaSiP₃N₇ features the structure of barylite‐2O. The structures of barylite‐1O and barylite‐2O represent the maximum degree of order (MDO) polytypes of their polytype family. Both structures consist of a network of all‐vertex‐sharing PN₄ and (Si_0.5P_0.5)N4 tetrahedra and elongated square pyramid (J₈) AEN₉ (AE=Sr, Ba) polyhedra (Figure S1). ^[15]

Table 1.

Crystallographic data of the single‐crystal structure refinements of AESiP₃N₇ (AE=Sr, Ba). Standard deviations are given in parentheses.

Formula	SrSiP3N7	BaSiP3N7
molar mass [g mol−1]	306.69	356.41
crystal system	orthorhombic	orthorhombic
space group	Pmn21 (no. 31)	Pnma (no. 62)
lattice parameters [Å]	a=11.979(2)	a=9.9048(3)
	b=4.9040(10)	b=12.1858(3)
	c=4.6870(9)	c=4.73580(10)
cell volume [Å3]	275.34(10)	571.60(3)
formula units/unit cell	2	4
density [g cm−3]	3.699	4.142
μ [mm−1]	10.807	7.927
temperature [K]	296(2)	297(2)
absorption correction	semiempirical
radiation	Mo‐Kα (λ=0.71073 Å)
F(000)	292	656
θ range [°]	3.4≤θ≤44.09	3.34≤θ≤38.44
total no. of reflections	9890	10 523
Independent reflections [I≥2σ(I)/all]	1253/1379	959/1128
R σ, R int	0.0393, 0.0952	0.0234, 0.0481
refined parameters	60	58
goodness of fit	1.100	1.044
R‐values [I≥2σ(I)]	R1=0.0293 wR2=0.0731	R1=0.0261 wR2=0.0576
R‐values (all data)	R1=0.0343 wR2=0.0750	R1=0.0340 wR2=0.0599
Δρ max, Δρ min [e Å−3]	2.18, −1.49	0.81, −1.53

The main difference between the two polymorphs concerns the arrangement of tetrahedra. While in SrSiP₃N₇ all tetrahedra vertices point in the same direction, those in BaSiP₃N₇ alternate, which results in a doubled unit cell with 2b (SrSiP₃N₇)=a (BaSiP₃N₇) (Figure 1). Although tetrahedra orientation differs in both compounds, the environment of AE atoms is strikingly similar. The tetrahedra connection patterns show that both compounds consist of dreier, vierer, and sechser rings that, apart from slight distortions, are arranged and distributed in the same manner (Figure 2), leading to the same topology point symbol {3².4³.5.6⁴}{3⁴.4⁵.5⁴.6²}. ^[16] Both compounds exhibit one tetrahedrally coordinated site shared by Si and P, while the other site is solely occupied by P. Further details of the crystal structure investigations may be obtained from the joint CCDC/FIZ Karlsruhe online deposition service by quoting the deposition numbers CSD‐2050660 and 2050661. In the case of SrSiP₃N₇, potential ordering of Si and P was considered by symmetry reduction and refinement of the structure against SCXRD data in the subgroups P2₁, Pn, and Pm of space group Pmn2₁. However, no indications of complete ordering were found. In all structure models, the volumes of the resulting four symmetrically independent tetrahedra were compared as (P,Si)−N bond lengths were not sufficiently meaningful for discrimination. ^[17] This investigation led to two different kinds of tetrahedra.

Figure 1.

Crystal structures of SrSiP₃N₇ (top) and BaSiP₃N₇ (bottom) both along [001]. For SrSiP₃N₇ PN₄ tetrahedra (red) and (Si_0.5P_0.5)N₄ (orange) all vertices point in the same direction (behind the plane of projection). For BaSiP₃N₇ the orientation of tetrahedra vertices alternates.

Figure 2.

Tetrahedra connection patterns of SrSiP₃N₇ (left) and BaSiP₃N₇ (right). Patterns of both compounds show dreier (red), vierer (lilac), and sechser rings (green).

Structure models in subgroups P2₁, Pn, and Pm reveal two tetrahedra exhibiting a volume of 2.21–2.24 ų that coincides with the volume of PN₄ tetrahedra from known nitridophosphates in literature and corresponds to one site in the final structure model in Pmn2₁. The other two tetrahedra have a volume of 2.37–2.40 ų, which lies between the volumes of PN₄ (2.13–2.28 ų) and SiN₄ tetrahedra (2.52–2.76 ų) (Tables S10 and 11).[ ⁸ , ⁹ , ¹⁸ , ¹⁹ , ²⁰ , ²¹ , ²² , ²³ , ²⁴ , ²⁵ , ²⁶ , ²⁷ , ²⁸ ] BVS (bond valence sum) calculations performed on all structure models revealed two tetrahedral sites fully occupied by P and two tetrahedral sites occupied by Si and P in a 1:1 ratio (Tables S13–15). ^[29]

In the case of BaSiP₃N₇, the ordering of tetrahedra was considered by symmetry reduction and refinement of the structure against SCXRD data in subgroups Pna2₁, P2₁ ma, and P2₁/c of space group Pnma. Only subgroups retaining the extinction condition of the a glide planes present in Pnma were taken into account because electron diffraction parallel to [001] showed no violation of the extinction conditions. Additional electron diffraction patterns parallel to [100] showed no violation of the extinction conditions of the n glide, too, further supporting the structure model in space group Pnma (a comparison of experimental diffraction patterns with simulated ones based on the structure model in space group Pnma is given in Figure S5).

The comparison of resulting tetrahedral volumes showed the same features as for SrSiP₃N₇ (Table S12). BVS calculations performed for the different structure models again suggested two sites completely occupied by P and two sites shared by Si and P (Tables S16–18). The simple approach of comparing tetrahedra volume as a tool for assigning Si and P, which lack scattering contrast, to the respective sites was indeed confirmed by scanning transmission electron microscope energy‐dispersive X‐ray spectroscopy (STEM‐EDX) mapping with atomic resolution for BaSiP₃N₇. These data support the model in space group Pnma, showing two sites with mixed Si/P occupation (Figure 3, enlarged version see Figure S6). This result is also corroborated by ³¹P solid‐state magic angle spinning (MAS)‐NMR spectra, which show a broad signal [full width at half‐maximum (fwhm)=19.7 ppm] that is consistent with a disordered model (Figure 4, Figure S4). Line broadening in the NMR spectrum is probably due to disorder in the second coordination sphere of P atoms. In contrast, ordered nitrides like BP₃N₆ or Li₁₂P₃N₉ show very narrow signals in their ³¹P NMR spectra.[ ⁸ , ³⁰ ]

Figure 3.

Atomic‐resolution STEM‐EDX of BaSiP₃N₇ along [001]. STEM high‐angle annular dark‐field (HAADF) image (top) with structure overlay (Ba: cyan, P: red, Si: yellow). The inset shows the corresponding area for EDX maps (bottom) with a combined color map, Ba map (cyan), Si map (yellow), and P map (red).

Figure 4.

³¹P NMR spectrum showing one broad signal for BaSiP₃N₇ instead of two signals for the different crystallographic sites, most likely due to the occupational disorder.

In both structures, the connectivity of the tetrahedra via their vertices can explain the presence of different tetrahedral volumes. The smaller tetrahedra have three vertices occupied by twofold bridging nitrogen atoms N ^[2] and one vertex occupied by a threefold bridging nitrogen atom N ^[3] . The larger tetrahedra, in contrast, feature two vertices occupied by N ^[2] and two vertices occupied by N ^[3] . Chemical analysis by EDX agrees with the sum formulas. Due to ambiguous O contents (as indicated by EDX measurements) either surface hydrolysis or slight compositional variations cannot completely be ruled out so that a phase width according to AESi_1+xP_3−xN_7−xO_x (AE=Sr, Ba) with x<1 could also be considered (Table S8) even though some analyses show no O.

Upon doping with Eu²⁺, both compounds emit blue light under UV excitation. Luminescence spectra show emission maxima of λ _max=430 nm for SrSiP₃N₇:Eu²⁺ and λ _max=424 nm for BaSiP₃N₇:Eu²⁺ upon excitation with λ _exc=400 nm. The emission curves were extrapolated to give an estimate of the fwhm, which amount to 45 nm (2404 cm⁻¹) for SrSiP₃N₇:Eu²⁺ and 53 nm (2731 cm⁻¹) for BaSiP₃N₇:Eu²⁺ (Figure 5). The corresponding Stokes shifts are 38 nm (2254 cm⁻¹) for SrSiP₃N₇:Eu²⁺ and 32 nm (1925 cm⁻¹) for BaSiP₃N₇:Eu²⁺. The presence of a single narrow emission band for both phosphors can be explained by the emission properties of Eu²⁺and the presence of a single crystallographic site for the alkaline earth ions suitable for doping with Eu²⁺ with AE−N distances ranging from 2.696(3)–3.270(3) Å (SrSiP₃N₇) to 2.872(3)–3.230(3) Å (BaSiP₃N₇). The similarity of emission properties in terms of fwhm are most likely to be explained by the P‐Si_0.5P_0.5 „cages“ around the AE position, which are very similar. Thus, only minute deviations are caused by different AE cation sizes even though both structures differ with respect to their space groups, unit cell volumes, and tetrahedra orientations.

Figure 5.

Emission spectra of (a) SrSiP₃N₇:Eu²⁺ (blue) and (b) BaSiP₃N₇:Eu²⁺ (black); measured data in solid lines and extrapolation in dotted lines; respective excitation spectra (red) (insets: micrographs of luminescent particles).

Conclusions

High‐pressure high‐temperature synthesis with the addition of NH₄F is a suitable approach to the synthesis of mixed nitridosilicatephosphates. The compounds AESiP₃N₇ (AE=Sr, Ba) adopt the structure types of the two polymorphs of the mineral barylite. This structure type has not been observed for nitride compounds so far. Although silicon and phosphorus exhibit little contrast in X‐ray diffraction, the comparison of polyhedra volumes led to structure models with an occupationally disordered site that also persists if potential ordering is considered by symmetry reduction. The disordered model for BaSiP₃N₇ is further supported by solid‐state NMR spectroscopy. Scanning transmission electron microscopy energy‐dispersive X‐ray spectroscopy (STEM‐EDX) mapping with atomic resolution enables to directly observe said disorder, which is additionally in accordance with systematic absences observed in electron diffraction patterns. Therefore, nitridosilicatephosphates have the potential to significantly diversify the structural chemistry of nitrides. Their suitability as host lattices for rare‐earth activator ions seems especially intriguing considering the emission properties of other compounds with multiple tetrahedra centers like CaAlSiN₃:Eu²⁺, Sr[Li₂Al₂O₂N₂]:Eu²⁺, or Sr[LiAl₃N₄]:Eu²⁺.[ ² , ³¹ , ³² ]

Conflict of interest

The authors declare no conflict of interest.

Supporting information

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Supplementary

L. Eisenburger, O. Oeckler, W. Schnick, Chem. Eur. J. 2021, 27, 4461.