Nitridosilicate und Oxonitridosilicate: von keramischen Materialien zu struktureller und funktioneller Diversität

Ammonothermal Synthesis and Solid-State NMR Study of the Imidonitridosilicate Rb3Si6N5(NH)6

The imidonitridosilicate Rb3Si6N5(NH)6, being only the second representative of this compound class, was synthesized ammonothermally at 870 K and 230 MPa. Its crystal structure was solved from single-crystal X-ray diffraction data. The imidonitridosilicate crystallizes isotypically with the respective potassium compound in space group P4132 with the lattice parameter a=10.9422(4) Å forming a three-dimensional imidonitridosilicate tetrahedra network with voids for the rubidium ions. The structure model and the presence of the imide groups were verified by Fourier-Transform infrared (FTIR) and magic-angle spinning (MAS) NMR spectroscopy, using cross polarization 15N{1H} and 29Si{1H} MAS NMR experiments. Rb3Si6N5(NH)6 represents a possible intermediate during the ammonothermal synthesis of nitridosilicates. The characterization of such intermediates improves the understanding of the reaction pathway from ammonothermal solutions to nitrides. Thus, the ammonothermal synthesis is an alternative approach to the well-established high-temperature synthesis leading to the compound class of nitridosilicates.

Synthesis and Luminescence Properties of Amber Emitting La 7 Sr[Si 10 N 19 O 3 ] : Eu 2+ and Syntheses of the Substitutional Variants RE 8‐ x AE x [Si 10 N 20‐ x O 2+ x ] : Eu 2+ with RE =La, Ce; AE =Ca, Sr, Ba; 0≤ x ≤2

The oxonitridosilicate La₇Sr[Si₁₀N₁₉O₃] : Eu²⁺ and its substitutional variants RE_8‐xAE_x[Si₁₀N_20‐xO_2+x] : Eu²⁺ with RE=La, Ce; AE=Ca, Sr, Ba and 0≤x≤2 were synthesized starting from REN, SrN/Ca₃N₂/Ba₂N, SiO₂, amorphous Si₃N₄ and Eu₂O₃ as doping agent at 1600 °C in a radiofrequency furnace. The crystal structure of La₇Sr[Si₁₀N₁₉O₃] was solved and refined based on single‐crystal X‐ray diffraction data. La₇Sr[Si₁₀N₁₉O₃] crystallizes in the orthorhombic space group Pmn2₁ (no. 31). The crystal structures of the isotypic compounds RE_8‐xAE_x[Si₁₀N_20‐xO_2+x] were confirmed by Rietveld refinements based on powder X‐ray diffraction data using the single‐crystal data of La₇Sr[Si₁₀N₁₉O₃] as starting point. Crystal structure elucidation reveals a 3D network of vertex sharing SiN₄ and SiN₂(N_1/2‐x/4O_1/2+x/4)₂ (0≤x≤2) tetrahedra. When excited with UV to blue light, La₇Sr[Si₁₀N₁₉O₃] : Eu²⁺ shows amber luminescence with λ_em=612 nm and fwhm=84 nm/2194 cm⁻¹, which makes it interesting for application in amber phosphor‐converted light emitting diodes.

Nitridic Analogs of Micas AESi3 P4 N10 (NH)2 (AE=Mg, Mg0.94 Ca0.06 , Ca, Sr)

We present the first nitridic analogs of micas, namely AESi₃P₄N₁₀(NH)₂ (AE=Mg, Mg_0.94Ca_0.06, Ca, Sr), which were synthesized under high‐pressure high‐temperature conditions at 1400 °C and 8 GPa from the refractory nitrides P₃N₅ and Si₃N₄, the respective alkaline earth amides, implementing NH₄F as a mineralizer. The crystal structure was elucidated by single‐crystal diffraction with microfocused synchrotron radiation, energy‐dispersive X‐ray spectroscopic (EDX) mapping with atomic resolution, powder X‐ray diffraction, and solid‐state NMR. The structures consist of typical tetrahedra–octahedra–tetrahedra (T‐O‐T) layers with P occupying T and Si occupying O layers, realizing the rare motif of sixfold coordinated silicon atoms in nitrides. The presence of H, as an imide group forming the SiN₄(NH)₂ octahedra, is confirmed by SCXRD, MAS‐NMR, and IR spectroscopy. Eu²⁺‐doped samples show tunable narrow‐band emission from deep blue to cyan (451–492 nm).

Structure Elucidation of Complex Endotaxially Intergrown Lanthanum Barium Oxonitridosilicate Oxides by Combination of Microfocused Synchrotron Radiation and Transmission Electron Microscopy

Single‐crystalline domains in intergrown microcrystalline material of the new compounds Ba_22.5+xLa_55−x[Si₁₂₉N_240−xO_x]O₃:Ce³⁺ and Ba_25.5+xLa_77−x[Si₁₇₀N_312−xO_9+x]O₄:Ce³⁺ were identified by transmission electron microscopy (TEM). Precise diffraction data from these domains were collected with microfocused synchrotron radiation so that crystal structure elucidation of the complex disordered networks became possible. They are composed of two different interconnected slabs of which one is similar in both compounds, which explains their notorious intergrowth. The distribution of Ba and La is indicated by the analysis of bond‐valence sums and by comparison with isostructural Sr_28.5+xLa_75−x[Si₁₇₀N_312−xO_9+x]O₄. Ce³⁺ doping leads to yellow luminescence. This is a showcase that highlights the discovery and accurate characterization of new compounds relevant for luminescence applications from heterogeneous microcrystalline samples by exploiting the capability of the combination of TEM and diffraction using the latest focusing techniques for synchrotron radiation.

The Reduced Nitridogermanates(III) Ca6 [Ge2 N6 ] and Sr6 [Ge2 N6 ] with Ge-Ge Bonds

The first nitridogermanates(III) Ca₆[Ge₂N₆] and Sr₆[Ge₂N₆] were synthesized from sodium flux and structurally characterized by powder and single crystal X‐ray diffraction, respectively. They crystallize isostructurally to each other and homeotypic to Ca₆[Cr₂N₆]H in space group R3‾ . They feature unprecedented, mutually isolated, ethane‐like [Ge^III₂N₆]¹²⁻ anions in a staggered conformation. The compounds are semiconductors according to resistivity measurements and electronic structure calculations, yielding band gaps of 1.1 eV for Ca₆[Ge₂N₆] and 0.2 eV for Sr₆[Ge₂N₆].

Na3 SO4 H-The First Representative of the Material Class of Sulfate Hydrides

The first representative of a novel class of mixed-anionic compounds, the sulfate hydride Na3 SO4 H, and the corresponding deuteride Na3 SO4 D were obtained from the solid-state reaction of NaH or NaD with dry Na2 SO4 . Precise reaction control is required, because too harsh conditions lead to the reduction of sulfate to sulfide. A combined X-ray and neutron diffraction study revealed that the compound crystallizes in the tetragonal space group P4/nmm with the lattice parameters a=7.0034(2) Å and c=4.8569(2) Å. The sole presence of hydride and absence of hydroxide ions is proven by vibrational spectroscopy and comparison with spectra predicted from quantum chemical calculations. 1 H and 23 Na MAS NMR spectra are consistent with the structure of Na3 SO4 H: a single 1 H peak at 2.9 ppm is observed, while two peaks at 15.0 and 6.2 ppm for the inequivalent 23 Na sites are observed. Elemental analysis and quantum chemical calculations further support these results.

High-Pressure High-Temperature Synthesis of Mixed Nitridosilicatephosphates and Luminescence of AESiP3 N7 :Eu2+ (AE=Sr, Ba)

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 AESiP₃ N₇ (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 P₃ N₅ and Si₃ N₄ . The latter were activated by NH₄ F, probably acting as a mineralizing agent. SrSiP₃ N₇ and BaSiP₃ N₇ 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 BaSiP₃ N₇ with atomic resolution. Upon doping with Eu²⁺ , both compounds showed blue emission under UV excitation.

Borate Hydrides as a New Material Class: Structure, Computational Studies, and Spectroscopic Investigations on Sr5 (BO3 )3 H and Sr5 (11 BO3 )3 D

The unprecedented borate hydride Sr5 (BO3 )3 H and deuteride Sr5 (11 BO3 )3 D crystallizing in an apatite-related structure are reported. Despite the presence of hydride anions, the compound decomposes only slowly in air. Doped with Eu2+ , it shows broad-band orange-red emission under violet excitation owing to the 4f6 5d-4f7 transition of Eu2+ . The observed 1 H NMR chemical shift is in good agreement with previously reported 1 H chemical shifts of ionic metal hydrides as well as with quantum chemical calculations and very different from 1 H chemical shifts usually found for hydroxide ions in similar materials. FTIR and Raman spectroscopy of different samples containing 1 H, 2 H, nat B, and 11 B combined with calculations unambiguously prove the absence of hydroxide ions and the sole incorporation of hydride ions into the borate. The orange-red emission obtained by doping with Eu2+ shows that the new compound class might be a promising host material for optical applications.

Targeting Vacancies in Nitridosilicates: Aliovalent Substitution of M2+ (M=Ca, Sr) by Sc3+ and U3

Based on the known linking options of their fundamental building unit, that is the SiN₄ tetrahedron, nitridosilicates belong to the inorganic compound classes with the greatest structural variability. Although facilitating the discovery of novel Si-N networks, this variability represents a challenge when targeting non-stoichometric compounds. Meeting this challenge, a strategy for targeted creation of vacancies in highly condensed nitridosilicates by exchanging divalent M²⁺ for trivalent M³⁺ using the ion exchange approach is reported. As proof of concept, the first Sc and U nitridosilicates were prepared from α-Ca₂ Si₅ N₈ and Sr₂ Si₅ N₈ . Powder X-ray diffraction (XRD) and synchrotron single-crystal XRD showed random vacancy distribution in Sc_0.2 Ca_1.7 Si₅ N₈ , and partial vacancy ordering in U_0.5x Sr_2-0.75x Si₅ N₈ with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.

LiSr2SiO4H, an Air-Stable Hydride as Host for Eu(II) Luminescence

LiSr₂SiO₄H is synthesized by solid-state reaction of LiH and α-Sr₂SiO₄. It crystallizes in space group P2₁/ m ( a = 658.63(4) pm, b = 542.36(3) pm, c = 695.01(4) pm, β = 112.5637(9)°) as proven by X-ray and neutron diffraction, is isotypic to LiSr₂SiO₄F, and exhibits isolated SiO₄ tetrahedra. Hydride anions are located in Li₂Sr₄ octahedra, which share faces to form columns, with H-H distances of 271.18(2) pm. NMR, IR, and Raman spectroscopy, density measurements, elemental analysis, and theoretical calculations confirm these results. Despite its hydridic nature, it is stable in air up to 550 K. When doped with europium, it emits bright yellow-green light with an intensity maximum at 560 nm for LiSr_1.98Eu_0.02SiO₄H. Even after treatment in water for several hours, the solid shows luminescence. The broad emission peak is attributed to the allowed 4f⁶5d → 4f⁷ transition of divalent europium. LiSr₂SiO₄H is the first silicate hydride, a class of compounds that might have potential as host for luminescent materials.

Alkali Lithosilicates: Renaissance of a Reputable Substance Class with Surprising Luminescence Properties

A hitherto unknown synthetic access to alkali lithosilicates, a substance class first described by Hoppe in the 1980s, is reported. With the synthesis and characterization of NaK₇ [Li₃ SiO₄ ]₈ , a new representative has been discovered, expanding the family of known alkali lithosilicates. Astonishingly, NaK₇ [Li₃ SiO₄ ]₈ and the already established alkali lithosilicates Na[Li₃ SiO₄ ] as well as K[Li₃ SiO₄ ] display unforeseen luminescence properties, when doped with Eu²⁺ . Na[Li₃ SiO₄ ]:Eu²⁺ exhibits an ultra-narrow blue, K[Li₃ SiO₄ ]:Eu²⁺ a broadband, and NaK₇ [Li₃ SiO₄ ]₈ :Eu²⁺ a yellow-green double emission upon excitation with near-UV to blue light. Consequently, all of the investigated substances of this class of compounds are highly interesting phosphors for application in phosphor converted LEDs.

Sr[BeSi2 N4 ]:Eu2+ /Ce3+ and Eu[BeSi2 N4 ]: Nontypical Luminescence in Highly Condensed Nitridoberyllosilicates

M[BeSi₂ N₄ ] (M=Sr,Eu), crystallizing in the hexagonal space group P6‾ 2c, was synthesized from Sr(NH₂ )₂ , Be₃ N₂ , and "Si(NH)₂ " in W crucibles under a N₂ atmosphere in a radio-frequency furnace. The crystal structure was solved from powder X-ray diffraction data by the charge-flipping method (Sr[BeSi₂ N₄ ]: a=4.86082(2), c=9.42264(4) Å, Z=2; Eu[BeSi₂ N₄ ]: a=4.85848(1), c=9.41615(4) Å). M[BeSi₂ N₄ ] contains a highly condensed rigid network of trigonal planar [BeN₃ ] units that are connected to Si₂ N₇ double tetrahedra by common vertices. M[BeSi₂ N₄ ] (M=Sr,Eu) are the first examples of nitridoberyllosilicates and are isotypic to the oxoberyllate Sr[Be₃ O₄ ]. Eu²⁺ -doped Sr[BeSi₂ N₄ ] and Eu[BeSi₂ N₄ ] show orange-trapped exciton emission (λ_em =605 nm, fwhm ≈126 nm), whereas Ce³⁺ -doped samples of Sr[BeSi₂ N₄ ] show nontypical yellowish-orange luminescence. Sr[BeSi₂ N₄ ] has a large band gap of ≈4.4 eV and shows high chemical and thermal stability. Eu²⁺ -doped beryllates with regular 4f⁶ 5d¹ →4f⁷ -emission could be interesting for future application in LEDs upon doping with Eu²⁺ or Ce³⁺ due to large band gaps, rigid networks, and chemical and thermal stability.

Supramolecular Organization in Confined Nanospaces

Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

Ammonothermal Synthesis of Nitrides: Recent Developments and Future Perspectives

Nitrides represent an intriguing class of functional materials with a broad range of application fields. Within the past decade, the ammonothermal method became increasingly attractive for the synthesis and crystal growth of nitride materials. The ammonothermal approach proved to be eminently suitable for the growth of bulk III-nitride semiconductors like GaN, and furthermore provided access to numerous ternary and multinary nitrides and oxonitrides with promising optical and electronic properties. In this minireview, we will shed light on the latest research findings covering the synthesis of nitrides by this method. An overview of synthesis strategies for binary, ternary, and multinary nitrides and oxonitrides, as well as their properties and potential applications will be given. The recent development of autoclave technologies for syntheses at high temperatures and pressures, in situ methods for investigations of crystallization processes, and solubility measurements by ultrasonic velocity experiments is briefly reviewed as well. In conclusion, challenges and future perspectives regarding the synthesis and crystal growth of novel nitrides, as well as the advancement of autoclave techniques are discussed.

"114"-Type Nitrides LnAl(Si4-x Alx )N7 Oδ with Unusual [AlN6 ] Octahedral Coordination

Aluminum-nitrogen six-fold octahedral coordination, [AlN₆ ], is unusual and has only been seen in the high-pressure rocksalt-type aluminum nitride or some complex compounds. Herein we report novel nitrides LnAl(Si_4-x Al_x )N₇ O_δ (Ln=La, Sm), the first inorganic compounds with [AlN₆ ] coordination prepared via non-high-pressure synthesis. Structure refinements of neutron powder diffraction and single-crystal X-ray diffraction data show that these compounds crystallize in the hexagonal Swedenborgite structure type with P6₃ mc symmetry where Ln and Al atoms locate in anticuboctahedral and octahedral interstitials, respectively, between the triangular and Kagomé layers of [SiN₄ ] tetrahedra. Solid-state NMR data of high-purity La-114 powders confirm the unusual [AlN₆ ] coordination. These compounds are the first examples of the "33-114" sub-type in the "114" family. The additional site for over-stoichiometric oxygen in the structure of 114-type compounds was also identified.