The ternary transition-metal nitridophosphates CdP₂N₄ and MnP₂N₄ have been synthesized under high-pressure high-temperature conditions (5–8 GPa, 1000–1300 °C) by using the multianvil technique. Cd and Mn azides can be used as the starting materials, however, with respect to safety considerations, it is much more advantageous to start from metal powders and phosphorus nitride imide, HPN₂. Both nitridophosphates crystallize in a structure closely related to the megacalsilite structure type. As a result of the known issues concerning superstructures with this type of structure, TEM investigations were performed on CdP₂N₄, which revealed that the megacalsilite superstructure is not equally pronounced in all crystallites. By adding NH₄Cl as mineralizer, single crystals were obtained that exhibit unequally pronounced superstructure reflections. Consequently, an averaged structural model was used and refined by the Rietveld method [P6₃22, a = 16.7197(3), c = 7.6428(2) Å, V = 1850.3(2) ų, R_p = 0.0671, wR_p = 0.0869 for CdP₂N₄ and P6₃22, a = 16.5543(2), c = 7.5058(2) Å, V = 1781.3(1) ų, R_p = 0.0526, wR_p = 0.0697 for MnP₂N₄]. The ³¹P NMR spectra exhibit four signal groups at (6.4, 4.8), 0.8, and –9.7 ppm with pronounced shoulders belonging to the same phase in an approximate area ratio of 4.8:1.1:2.0, thereby proving at least eight P sites.

Sn₆[P₁₂N₂₄] was synthesized from Sn and HPN₂ at 820 °C in an evacuated silica glass ampoule. According to powder X-ray diffraction investigations and Rietveld refinement [space group I3m (no. 217), a = 8.2882(2) Å, R_P = 0.03645, wR_P = 0.04613], Sn₆[P₁₂N₂₄] crystallizes with a sodalite-type structure with a reduced occupation factor of 3/4 for the Sn atom at Wyckoff site 8c and an empty Wyckoff site 2a at the center of the β-cages. The structural results are further corroborated by energy-dispersive X-ray spectroscopy (EDX) analyses, solid-state NMR spectroscopy, and theoretical investigations (DFT), including density of states (DOS), energy/volume, and electron localization (ELF) calculations. The ¹¹⁹Sn Mössbauer spectrum shows a single, quadrupole-split signal for Sn^II at an isomer shift of 3.05 mm/s. In addition to SiPN₃ with a defective wurtzite type of structure, the nitridophosphate sodalite Sn₆[P₁₂N₂₄] represents the second ternary nitridophosphate containing only p-block elements.

Li₁₄(PON₃)₂O was synthesized by reaction of phosphoric triamide PO(NH₂)₃ with LiNH₂ at 550 °C. It crystallizes in a trigonal structure [P (no. 147), a = 5.6880(5), c = 8.0855(8) Å, V = 226.55(5) ų, Z = 1] that can be described as a defect variant of the antifluorite structure type. The crystal structure was elucidated from X-ray powder diffraction data and corroborated by FTIR and solid-state NMR spectroscopy. Li₁₄(PON₃)₂O is composed of non-condensed PON₃^6– tetrahedra and O^2– ions that are surrounded by tetrahedrally coordinated Li⁺. This is the first example of an ortho-oxonitridophosphate.

Ca₂Mg[Li₄Si₂N₆] and Li₂Ca₂[Mg₂Si₂N₆] were synthesized in sealed tantalum ampules with Li as a fluxing agent. Both compounds crystallize in the monoclinic space group C2/m (no. 12). The crystal structures were solved and refined on the basis of single-crystal X-ray diffraction data [Z = 2; Ca₂Mg[Li₄Si₂N₆]: a = 5.9059(12), b = 9.817(2), c = 5.6109(11) Å, β = 94.90(3)°, R₁ = 0.015, wR₂ = 0.049; Li₂Ca₂[Mg₂Si₂N₆]: a = 5.5472(11), b = 9.844(2), c = 5.9978(12) Å, β = 97.13(3)°, R₁ = 0.024, wR₂ = 0.053]. Ca₂Mg[Li₄Si₂N₆] is isomorphic to Ca₃[Li₄Si₂N₆] and its crystal structure is homeotypic to that of Li₂Ca₂[Mg₂Si₂N₆]. Both structures are built up of edge-sharing [Si₂N₆]^10– tetrahedra (bow-tie units). In the nitridolithosilicate Ca₂Mg[Li₄Si₂N₆] the bow-tie units are connected via pairs of LiN₄ tetrahedra, whereas in the nitridomagnesosilicate Li₂Ca₂[Mg₂Si₂N₆] the nitridosilicate substructure is connected by chains of MgN₄ tetrahedra. Ca₂Mg[Li₄Si₂N₆] is only the second example of fourfold planar rectangular coordinated Mg²⁺ in a nitridosilicate. Li₂Ca₂[Mg₂Si₂N₆] is the first nitridosilicate with Li⁺ in threefold coordination. The crystal structures were confirmed by lattice-energy calculations (MAPLE), EDX measurements, and powder X-ray diffraction.

CuPN₂ was successfully synthesized from Cu₃N and P₃N₅ at 5 GPa and 1000 °C by employing the Walker-type multianvil technique. Its crystal structure was elucidated from powder X-ray diffraction data. CuPN₂ is isostructural to LiPN₂ and NaPN₂ [tetragonal I2d, no. 122, a = 4.5029(2) Å, c = 7.6157(2) Å, V = 154.42(1) ų, R_p = 1.303, wR_p = 1.741] with a structure that can be derived from both chalcopyrite and zincblende type. The electronic structure of CuPN₂ was investigated by means of DFT calculations. CuPN₂ is an indirect semiconductor with a bandgap of 1.67 eV.

The isotypic compounds La₃[SiN₄]F and La₃[SiN₃O]O were synthesized in a radio-frequency furnace at 1600 °C. The crystal structures [Pnma (no. 62), Z = 4; La₃(SiN₄)F: a = 9.970(3), b = 7.697(2), c = 6.897(2) Å, V = 529.3(3) ų; La₃(SiON₃)O: a = 9.950(2), b = 7.6160(15), c = 6.9080(14) Å, V = 523.48(18) ų] were elucidated from single-crystal X-ray diffraction data and corroborated by Rietveld refinement, lattice-energy calculations (Madelung part of lattice energy, MAPLE) and Raman/FTIR spectroscopy. Both compounds are homeotypic with Na₂Pr[GeO₄]OH forming a network of vertex-sharing FLa₆/OLa₆ octahedra, whose voids are filled with non-condensed SiN₄/SiN₃O tetrahedra. o-La₃[SiON₃]O is the orthorhombic polymorph of this compound, which probably represents the high-temperasture modification, whereas the tetragonal polymorph t-La₃[SiON₃]O represents the low-temperature modification. While the space group of the t-polymorph [I4/mcm (no. 140)] differs from the new La₃[SiN₄]F and o-La₃[SiN₃O]O, the crystal structure contains the same linking pattern.

Illumination sources based on phosphor-converted light emitting diode (pcLED) technology are nowadays of great relevance. In particular, illumination-grade pcLEDs are attracting increasing attention. Regarding this, the application of a single warm-white-emitting phosphor could be of great advantage. Herein, we report the synthesis of a novel nitridophosphate zeolite Ba₃P₅N₁₀Br:Eu²⁺. Upon excitation by near-UV light, natural-white-light luminescence was detected. The synthesis of Ba₃P₅N₁₀Br:Eu²⁺ was carried out using the multianvil technique. The crystal structure of Ba₃P₅N₁₀Br:Eu²⁺ was solved and refined by single-crystal X-ray diffraction analysis and confirmed by Rietveld refinement and FTIR spectroscopy. Furthermore, spectroscopic luminescence measurements were performed. Through the synthesis of Ba₃P₅N₁₀Br:Eu²⁺, we have shown the great potential of nitridophosphate zeolites to serve as high-performance luminescence materials.

The chemical and physical properties of phosphorus oxonitride (PON) closely resemble those of silica, to which it is isosteric. A new high-pressure phase of PON is reported herein. This polymorph, synthesized by using the multianvil technique, crystallizes in the coesite structure. This represents the first occurrence of this very dense network structure outside of SiO₂. Phase-pure coesite PON (coe-PON) can be synthesized in bulk at pressures above 15 GPa. This compound was thoroughly characterized by means of powder X-ray diffraction, DFT calculations, and FTIR and MAS NMR spectroscopy, as well as temperature-dependent diffraction. These results represent a major step towards the exploration of the phase diagram of PON at very high pressures and the possibly synthesis of a stishovite-type PON containing hexacoordinate phosphorus.

Developing a synthetic method to target an broad spectrum of unknown phases can lead to fascinating discoveries. The preparation of the first rare-earth-metal nitridophosphate LiNdP₄N₈ is reported. High-pressure solid-state metathesis between LiPN₂ and NdF₃ was employed to yield a highly crystalline product. The in situ formed LiF is believed to act both as the thermodynamic driving force and as a flux to aiding single-crystal formation in dimensions suitable for crystal structure analysis. Magnetic properties stemming from Nd³⁺ ions were measured by SQUID magnetometry. LiNdP₄N₈ serves as a model system for the exploration of rare-earth-metal nitridophosphates that may even be expanded to transition metals. High-pressure metathesis enables the systematic study of these uncharted regions of nitride-based materials with unprecedented properties.

Nitridophosphates MP₂N₄:Eu²⁺ (M=Ca, Sr, Ba) and BaSr₂P₆N₁₂:Eu²⁺ have been synthesized at elevated pressures and 1100–1300 °C starting from the corresponding azides and P₃N₅ with EuCl₂ as dopant. Addition of NH₄Cl as mineralizer allowed for the growth of single crystals. This led to the successful structure elucidation of a highly condensed nitridophosphate from single-crystal X-ray diffraction data (CaP₂N₄:Eu²⁺ (P6₃, no. 173), a=16.847(2), c=7.8592(16) Å, V=1931.7(6) ų, Z=24, 2033 observed reflections, 176 refined parameters, wR₂=0.096). Upon excitation by UV light, luminescence due to parity-allowed 4f⁶(⁷F)5d¹4f⁷(⁸S_7/2) transition was observed in the orange (CaP₂N₄:Eu²⁺, λ_max=575 nm), green (SrP₂N₄:Eu²⁺, λ_max=529 nm), and blue regions of the visible spectrum (BaSr₂P₆N₁₂:Eu²⁺ and BaP₂N₄:Eu²⁺, λ_max=450 and 460 nm, respectively). Thus, the emission wavelength decreases with increasing ionic radius of the alkaline-earth ions. The corresponding full width at half maximum values (2240–2460 cm⁻¹) are comparable to those of other known Eu²⁺-doped (oxo)nitrides emitting in the same region of the visible spectrum. Following recently described quaternary Ba₃P₅N₁₀Br:Eu²⁺, this investigation represents the first report on the luminescence of Eu²⁺-doped ternary nitridophosphates. Similarly to nitridosilicates and related oxonitrides, Eu²⁺-doped nitridophosphates may have the potential to be further developed into efficient light-emitting diode phosphors.

In this contribution, we report on novel functionalized triazines, which represent new precursors for C/N/(H) compounds or suitable building blocks for carbon-based functional networks. Our results provide insights into the structural properties of molecular carbon nitride materials and their design principles. Tris(1-propynyl)-1,3,5-triazine (C₃N₃(C₃H₃)₃) and tris(1-butynyl)-1,3,5-triazine (C₃N₃(C₄H₅)₃) were prepared by substitution reactions of cyanuric chloride (C₃N₃Cl₃) with prop-1-yne and but-1-yne. The crystal structure of tris(1-propynyl)-1,3,5-triazine was solved in the orthorhombic space group Pbcn (Z=4, a=1500.06 (14), b=991.48(10), c=754.42(6) pm, V=1122.03(18)×10⁶ pm³), whereas tris(1-butynyl)-1,3,5-triazine crystallized in the triclinic space group P−1 (Z=6, a=1068.36(12), b=1208.68(12), c=1599.38(16) pm, α=86.67(3), β=86.890(4), γ=86.890(4)°, V=1997.7(4)×10⁶ pm³). For both structures, planar triazine units and layerlike packing of the molecules were observed. Tris(1-propynyl)-1,3,5-triazine is built up from hydrogen-bonded zig-zag strands, whereas tris(1-butynyl)-1,3,5-triazine shows parallel layered arrangements. Both compounds were investigated by NMR spectroscopy, IR spectroscopy, and differential thermal analysis/thermogravimetric analysis, which provided insights into their structural, chemical, and thermal properties. In addition, tris(1-propynyl)-1,3,5-triazine was pyrolyzed and a new polymeric triazine-based compound containing mesitylene units was obtained. Its structural features and properties are discussed in detail.

Illumination sources based on phosphor-converted light emitting diode (pcLED) technology are nowadays of great relevance. In particular, illumination-grade pcLEDs are attracting increasing attention. Regarding this, the application of a single warm-white-emitting phosphor could be of great advantage. Herein, we report the synthesis of a novel nitridophosphate zeolite Ba₃P₅N₁₀Br:Eu²⁺. Upon excitation by near-UV light, natural-white-light luminescence was detected. The synthesis of Ba₃P₅N₁₀Br:Eu²⁺ was carried out using the multianvil technique. The crystal structure of Ba₃P₅N₁₀Br:Eu²⁺ was solved and refined by single-crystal X-ray diffraction analysis and confirmed by Rietveld refinement and FTIR spectroscopy. Furthermore, spectroscopic luminescence measurements were performed. Through the synthesis of Ba₃P₅N₁₀Br:Eu²⁺, we have shown the great potential of nitridophosphate zeolites to serve as high-performance luminescence materials.

Die chemischen Eigenschaften von Phosphoroxidnitrid (PON) ähneln denen des isosteren Siliciumdioxids. Hier berichten wir über eine neue Hochdruckphase von PON. Diese Modifikation kristallisiert in der Coesit-Struktur und wurde mittels Multianviltechnik hergestellt. Dies ist das erste Beispiel für das Auftreten dieser sehr dichten Netzwerkstruktur außerhalb von SiO₂. Coesit-PON (coe-PON) lässt sich phasenrein und in makroskopischen Mengen bei Drücken über 15 GPa synthetisieren. Wir konnten die Verbindung durch Röntgenpulverdiffraktometrie, DFT-Rechnungen, FTIR- und MAS-NMR-Spektroskopie sowie temperaturabhängige Röntgenbeugung gründlich charakterisieren. Diese Ergebnisse stellen einen fundamentalen Fortschritt bei der Erkundung des Phasendiagramms von PON bei sehr hohen Drücken dar und ebnen möglicherweise den Weg zur Synthese einer Stishovit-artigen Modifikation mit sechsfach koordiniertem Phosphor.

Die Erschließung eines großen Spektrums neuer Verbindungen durch eine neue Synthesemethode kann zu faszinierenden Entdeckungen führen. Hier berichten wir über die Synthese des ersten Seltenerdnitridophosphats, LiNdP₄N₈. Durch Festkörpermetathese zwischen LiPN₂ und NdF₃ wurde unter Hochdruck ein makrokristallines Produkt erhalten. Wir vermuten, dass in situ gebildetes LiF als thermodynamische Triebkraft und als Flux wirkt. Das Wachstum von für Kristallstrukturanalyse geeigneten Einkristallen wird durch den Flux unterstützt. Die von Nd³⁺ stammenden magnetischen Eigenschaften wurden mit SQUID-Magnetometrie gemessen. LiNdP₄N₈ fungiert als Modellsystem für die Erforschung von Seltenerdnitridophosphaten, die möglicherweise auch auf Übergangsmetalle erweitert werden kann. Unsere Hochdruckmetathese ermöglicht die systematische Untersuchung der unbekannten Gebiete innerhalb nitridbasierter Materialien, die interessante Eigenschaften aufweisen können.

CaMg₂P₆O₃N₁₀ has been synthesized starting from stoichiometric amounts of Ca(N₃)₂, Mg₃N₂, P₃N₅, and PON in a high-pressure/high-temperature reaction at 8 GPa and 1100 °C. Adding small amounts of NH₄Cl to the starting mixture afforded single crystals of CaMg₂P₆O₃N₁₀, which form transparent, colorless truncated octahedra. The crystal structure [space group I4₁/acd (no. 142), a = 12.494(1), c = 23.797(2) Å, Z = 16] was solved and refined by single-crystal X-ray diffraction analysis and confirmed by electron diffraction and transmission electron microscopy, including HRTEM image simulations. Rietveld refinement proved the phase purity of the product. FTIR analysis confirmed the absence N–H groups in the structure. Bond valence and lattice energy calculations (MAPLE) of the title compound are discussed. The crystal structure consists of polyhedral building units constructed from vertex-sharing P(O,N)₄ tetrahedra with condensed dreier and sechser rings.

Phosphorus nitride imide, PN(NH), is of great scientific importance because it is isosteric with silica (SiO₂). Accordingly, a varied structural diversity could be expected. However, only one polymorph of PN(NH) has been reported thus far. Herein, we report on the synthesis and structural investigation of the first high-pressure polymorph of phosphorus nitride imide, β-PN(NH); the compound has been synthesized using the multianvil technique. By adding catalytic amounts of NH₄Cl as a mineralizer, it became possible to grow single crystals of β-PN(NH), which allowed the first complete structural elucidation of a highly condensed phosphorus nitride from single-crystal X-ray diffraction data. The structure was confirmed by FTIR and ³¹P and ¹H solid-state NMR spectroscopy. We are confident that high-pressure/high-temperature reactions could lead to new polymorphs of PN(NH) containing five-fold- or even six-fold-coordinated phosphorus atoms and thus rivalling or even surpassing the structural variety of SiO₂.

Coordination numbers higher than usual are often associated with superior mechanical properties. In this contribution we report on the synthesis of the high-pressure polymorph of highly condensed phosphorus nitride imide P₄N₆(NH) representing a new framework topology. This is the first example of phosphorus in trigonal-bipyramidal coordination being observed in an inorganic network structure. We were able to obtain single crystals and bulk samples of the compound employing the multi-anvil technique. γ-P₄N₆(NH) has been thoroughly characterized using X-ray diffraction, solid-state NMR and FTIR spectroscopy. The synthesis of γ-P₄N₆(NH) gives new insights into the coordination chemistry of phosphorus at high pressures. The synthesis of further high-pressure phases with higher coordination numbers exhibiting intriguing physical properties seems within reach.

Phosphorus nitride imide, PN(NH), is of great scientific importance because it is isosteric with silica (SiO₂). Accordingly, a varied structural diversity could be expected. However, only one polymorph of PN(NH) has been reported thus far. Herein, we report on the synthesis and structural investigation of the first high-pressure polymorph of phosphorus nitride imide, β-PN(NH); the compound has been synthesized using the multianvil technique. By adding catalytic amounts of NH₄Cl as a mineralizer, it became possible to grow single crystals of β-PN(NH), which allowed the first complete structural elucidation of a highly condensed phosphorus nitride from single-crystal X-ray diffraction data. The structure was confirmed by FTIR and ³¹P and ¹H solid-state NMR spectroscopy. We are confident that high-pressure/high-temperature reactions could lead to new polymorphs of PN(NH) containing five-fold- or even six-fold-coordinated phosphorus atoms and thus rivalling or even surpassing the structural variety of SiO₂.

Coordination numbers higher than usual are often associated with superior mechanical properties. In this contribution we report on the synthesis of the high-pressure polymorph of highly condensed phosphorus nitride imide P₄N₆(NH) representing a new framework topology. This is the first example of phosphorus in trigonal-bipyramidal coordination being observed in an inorganic network structure. We were able to obtain single crystals and bulk samples of the compound employing the multi-anvil technique. γ-P₄N₆(NH) has been thoroughly characterized using X-ray diffraction, solid-state NMR and FTIR spectroscopy. The synthesis of γ-P₄N₆(NH) gives new insights into the coordination chemistry of phosphorus at high pressures. The synthesis of further high-pressure phases with higher coordination numbers exhibiting intriguing physical properties seems within reach.

Ca[LiAlN₂] was synthesized from LiAlH₄, LiN₃, calcium, and lithium metal as fluxing agent in welded-shut tantalum crucibles at 900 °C. The compound crystallizes in the form of transparent colorless platelets that undergo hydrolysis in air and under moisture. The crystal structure [P2₁/c (no. 14), a = 5.7587(12) Å, b = 6.8773(14) Å, c = 5.7960(12) Å, β = 90.28(3)°, Z = 4] was solved from single-crystal X-ray diffraction data and was confirmed with Rietveld refinement methods and lattice-energy calculations (Madelung part of lattice energy, MAPLE). Ca[LiAlN₂] forms layers of edge- and vertex-sharing AlN₄ tetrahedra isotypic with LiCaGaN₂. Li⁺ ions are positioned in tetrahedral voids within the [Al₂N₂N_4/2] layers resulting in a highly condensed structure of Al- and Li-centered polyhedra.

Until recently, melam, [C₃N₃(NH₂)₂]₂NH, has been regarded as a short-lived intermediate in the condensation process of melamine that is only detectable under special reaction conditions owing to its high reactivity. A new synthetic approach has allowed a closer look at the formation and condensation behavior of melam by using elevated ammonia pressure in autoclaves. Whereas the thermal treatment of dicyandiamide at 450 °C and 0.2 MPa ammonia yielded melam in large amounts, prolonged treatment under these conditions (9 days) led to the formation of a melam–melem adduct, thus enabling the first insight into the condensation process of melam into melem. The hydrothermal treatment of melam at 300 °C (24 h) yields melam hydrate, [C₃N₃(NH₂)₂]₂NH⋅2 H₂O (space group P2₁/c; a=676.84(2), b=1220.28(4), c=1394.24(4) pm; β=98.372(2)°; V=1139.28(6)×10⁶ pm³; Z=4), which crystallizes as a layered structure that is composed of almost-planar melam molecules, thereby forming ellipsoidal rosette-like motifs. The resulting voids are filled with four water molecules, thus forming a dense network of hydrogen bonds.

Oxonitridophosphate Ba₆P₁₂N₁₇O₉Br₃ was synthesized by heating a multicomponent mixture of BaBr₂, BaS, phosphoryl triamide and thiophosphoryl triamide in an evacuated and sealed silica-glass ampoule to 750 °C. Ba₆P₁₂N₁₇O₉Br₃ was obtained as the main product as a nanocrystalline powder. The crystal structure was determined ab initio on the basis of electron diffraction data acquired from a single needle-shaped nanocrystal by automated diffraction tomography. Ba₆P₁₂N₁₇O₉Br₃ crystallizes in the hexagonal space group P6₃/m (no. 176) with unit cell parameters a = 14.654(19), c = 8.255(9) Å and Z = 2. Its structure includes triangular, column-shaped anions of _∞¹{(P₁₂N₁₇O₉)^9–}, which are built from vertex-sharing P(O,N)₄ tetrahedra with 3-rings and three-coordinate nitrogen atoms. The 1D anions are separated by Ba²⁺ and Br^– ions, which are arranged in channels parallel to the phosphate anions along [001]. The Ba²⁺ ions are eight- and nine-coordinated by Br^– and O/N atoms, respectively.

Biuretooxophosphates represent a link between carbon nitride and phosphorus (oxo)nitride precursor chemistry being closely related to cyanurates and trimetaphosphimates. The group of alkali biuretooxophosphates has been complemented by the synthesis of four salts M[PO₂(NH)₃(CO)₂]·xH₂O in which M = Li, K, Rb, and Cs (x = 1, 0, 0.5, 0, respectively). The structures were solved by single-crystal X-ray diffraction and compared with the corresponding ammonium and sodium salts. For all of the salts, the 1-phospha-2,4,6-s-triazine ring exhibits a nearly planar conformation with the phosphorus atom being slightly deflected. In the sequence Li to Cs, the crystal structures show a significant change in orientation leading from a parallel to a perpendicular arrangement of the rings, the latter being bridged by N–H···O bonds. The thermal behavior of the biuretooxophosphates was examined by means of temperature-dependent powder X-ray diffraction measurements and combined thermogravimetric analysis (TGA) and differential thermal analysis (DTA). Moreover, the FTIR and photoluminescence spectra of the salts are discussed.

The rare-earth melonates LnC₆N₇(NCN)₃·xH₂O (Ln = La, Ce, Pr, Nd, Sm, Eu, Tb; x = 8–12) have been synthesized by metathesis reactions in aqueous solution and characterized by single-crystal and powder XRD, FTIR spectroscopy, thermal analysis, and photoluminescence studies. Powder XRD patterns revealed isotypism of the La–Sm compounds. The structure of LaC₆N₇(NCN)₃·8H₂O has been solved and refined from single-crystal diffraction data and those of the remaining salts have been refined from powder XRD data by Rietveld refinement. In the crystal structures, the melonate entities are arranged in corrugated layers, which alternate with layers of crystal water molecules. The lanthanide ions are coordinated by two melonate and six water molecules. LnC₆N₇(NCN)₃·xH₂O (Ln = Eu, Tb; x = 9–12) have also been investigated by photoluminescence studies. Neither hydrated nor dehydrated europium melonate exhibits luminescence under UV excitation, whereas photoluminescence studies of terbium melonate showed green emission with a maximum at 545 nm due to the ⁵D₄⁷F₅ transition. Thermal analysis revealed rather low thermal stability of the rare-earth melonates, which is probably due to the tight binding of crystal water that results in hydrolytic decomposition at elevated temperatures.

Compounds M[PO₂(NHCONH₂)₂] with M = Na, K, and Rb, which are the salts of N,N′-bis(aminocarbonyl)phosphorodiamidic acid, were synthesized by ammonolysis of the corresponding alkali biuretooxophosphates M[PO₂(NH)₃(CO)₂] (M = Na, K, Rb) at 350 °C and 120–150 bar in an autoclave. The structures were solved by single-crystal X-ray diffraction and exhibit 3D frameworks in which N,N′-bis(aminocarbonyl)phosphorodiamidate ions connect the corresponding alkali ions. The thermal behavior was investigated by combined thermogravimetric (TG) and differential thermal analysis (DTA) as well as temperature-dependent powder X-ray diffraction to evaluate applicability of the alkali N,N′-bis(aminocarbonyl)phosphorodiamidates as precursors for the synthesis of CNP(O) networks. In this context, the concept of Le Chatelier's principle for the hindrance of premature decomposition by applying elevated pressure was examined.

With BaSi₄O₆N₂ it was possible to synthesize a new compound in the hexacelsian structure type that has not been observed for nitrido- or oxonitridosilicates before. BaSi₄O₆N₂ was obtained by employing the multi-anvil technique at pressures of 12–15 GPa and temperatures between 900 and 1200 °C starting from BaSi₂O₂N₂. The crystal structure of BaSi₄O₆N₂ was solved on the basis of X-ray powder diffraction data [P6/mmm, no. 191, a = 5.3512(2) Å, c = 7.5235(4) Å, V = 186.57 ų, Z = 1, R_wp = 0.065] and was refined by the Rietveld method employing difference Fourier analyses. BaSi₄O₆N₂ represents a nonbranched double-layer silicate with a high degree of condensation [κ = 0.5, i.e. the atomic ratio n(Si)/n(O,N)]. The layers of BaSi₄O₆N₂ are made up of vertex-sharing SiO₃N tetrahedra forming hexagonal double layers that exhibit Sechser-rings as fundamental building units (FBU). Unlike most known nitrido- and oxonitridosilicates, no N^[3] but only double bridging N^[2] atoms are found in the silicate substructure. All of the oxygen atoms (O^[2]) bridge two Si atoms as well. Along [001] the Ba²⁺ ions are located in the Sechser-ring channels. The cations are coordinated by twelve O and six N atoms and are situated between and within the double layers. In-situ high-temperature investigations at low pressure with synchrotron radiation indicated that decomposition reactions occurred. Attempts to dope BaSi₄O₆N₂ with Eu²⁺ are described; however, no luminescence was observed.

Self-assembly of melem C₆N₇(NH₂)₃ in hot aqueous solution leads to the formation of hydrogen-bonded, hexagonal rosettes of melem units surrounding infinite channels with a diameter of 8.9 Å. The channels are filled with strongly disordered water molecules, which are bound to the melem network through hydrogen bonds. Single-crystals of melem hydrate C₆N₇(NH₂)₃⋅xH₂O (x≈2.3) were obtained by hydrothermal treatment of melem at 200 °C and the crystal structure (R c, a=2879.0(4), c=664.01(13) pm, V=4766.4(13)×10⁶ pm³, Z=18) was elucidated by single-crystal X-ray diffraction. With respect to the structural similarity to the well-known adduct between melamine and cyanuric acid, the composition of the obtained product was further analyzed by solid-state NMR spectroscopy. Hydrolysis of melem to cyameluric acid during syntheses at elevated temperatures could thus be ruled out. DTA/TG studies revealed that, during heating of melem hydrate, water molecules can be removed from the channels of the structure to a large extent. The solvent-free framework is stable up to 430 °C without transforming into the denser structure of anhydrous melem. Dehydrated melem hydrate was further characterized by solid-state NMR spectroscopy, powder X-ray diffraction, and sorption measurements to investigate structural changes induced by the removal of water from the channels. During dehydration, the hexagonal, layered arrangement of melem units is maintained whereas the formation of additional hydrogen bonds between melem entities requires the stacking mode of hexagonal layers to be altered. It is assumed that layers are shifted perpendicular to the direction of the channels, thereby making them inaccessible for guest molecules.

The first crystalline phosphorus oxonitride imide H₃P₈O₈N₉ (=P₈O₈N₆(NH)₃) has been synthesized under high-pressure and high-temperature conditions. To this end, a new, highly reactive phosphorus oxonitride imide precursor compound was prepared and treated at 12 GPa and 750 °C by using a multianvil assembly. H₃P₈O₈N₉ was obtained as a colorless, microcrystalline solid. The crystal structure of H₃P₈O₈N₉ was solved ab initio by powder X-ray diffraction analysis, applying the charge-flipping algorithm, and refined by the Rietveld method (C2/c (no. 15), a=1352.11(7), b=479.83(3), c=1820.42(9) pm, β=96.955(4)°, Z=4). H₃P₈O₈N₉ exhibits a highly condensed (κ=0.47), 3D, but interrupted network that is composed of all-side vertex-sharing (Q⁴) and only threefold-linking (Q³) P(O,N)₄ tetrahedra in a Q⁴/Q³ ratio of 3:1. The structure, which includes 4-ring assemblies as the smallest ring size, can be subdivided into alternating open-branched zweier double layers {oB,}[²P₃(O,N)₇] and layers containing pairwise-linked Q³ tetrahedra parallel (001). Information on the hydrogen atoms in H₃P₈O₈N₉ was obtained by 1D ¹H MAS, 2D homo- and heteronuclear (together with ³¹P) correlation NMR spectroscopy, and a ¹H spin-diffusion experiment with a hard-pulse sequence designed for selective excitation of a single peak. Two hydrogen sites with a multiplicity ratio of 2:1 were identified and thus the formula of H₃P₈O₈N₉ was unambiguously determined. The protons were assigned to Wyckoff positions 8f and 4e, the latter located within the Q³ tetrahedra layers.

New nitridosilicates Ca₃Sm₃[Si₉N₁₇] and Ca₃Yb₃[Si₉N₁₇] were synthesized from the reactions of the pure metals (calcium and samarium/ytterbium) with silicon diimide “Si(NH)₂” in a radio-frequency (rf) furnace at temperatures of up to 1650 °C. These isotypic compounds crystallize in the cubic space group P3m (no. 215) with lattice parameters a=739.50(3) pm; V=0.4044(1) nm³; Z=1; wR₂=0.029 (240 diffraction data, 26 parameters) for Ca₃Sm₃[Si₉N₁₇] and a=730.20(2) pm; V=0.3893(1) nm³; wR₂=0.039 (387 diffraction data, 27 parameters) for Ca₃Yb₃[Si₉N₁₇]. The new structure type of Ca₃RE₃[Si₉N₁₇] (RE=Sm, Yb) consists of two independent infinite networks, each of which have an expanded sphalerite (ZnS) topology in which the positions of the Zn and S atoms are replaced by voluminous [N^[4](SiN₃)₄] units and [Si₅N₁₆] supertetrahedra, respectively, thereby displaying twofold interpenetration. As well, a structural description of Ca₃Yb₃[Si₉N₁₇], its thermal stability, and magnetic properties, as well as UV/Vis, IR, and Raman spectra, are presented.

Owing to a parity allowed 4f⁶(⁷F)5d¹4f⁷(⁸S_7/2) transition, powders of the nominal composition Sr_0.25Ba_0.75Si₂O₂N₂:Eu²⁺ (2 mol % Eu²⁺) show surprising intense blue emission (λ_em=472 nm) when excited by UV to blue radiation. Similarly to other phases in the system Sr_1−xBa_xSi₂O₂N₂:Eu²⁺, the described compound is a promising phosphor material for pc-LED applications as well. The FWHM of the emission band is 37 nm, representing the smallest value found for blue emitting (oxo)nitridosilicates so far. A combination of electron and X-ray diffraction methods was used to determine the crystal structure of Sr_0.25Ba_0.75Si₂O₂N₂:Eu²⁺. HRTEM images reveal the intergrowth of nanodomains with SrSi₂O₂N₂ and BaSi₂O₂N₂-type structures, which leads to pronounced diffuse scattering. Taking into account the intergrowth, the structure of the BaSi₂O₂N₂-type domains was refined on single-crystal diffraction data. In contrast to coplanar metal atom layers which are located between layers of condensed SiON₃-tetrahedra in pure BaSi₂O₂N₂, in Sr_0.25Ba_0.75Si₂O₂N₂:Eu²⁺ corrugated metal atom layers occur. HRTEM image simulations indicate cation ordering in the final structure model, which, in combination with the corrugated metal atom layers, explains the unexpected and excellent luminescence properties.

Pyrolysis of prominent precursor compounds for the synthesis of carbon nitride type materials (e.g., melamine, thiourea) have been studied in detail. Molecular adducts containing monoprotonated melamium C₆N₁₁H₁₀⁺ and melaminium HC₃N₃(NH₂)₃⁺ ions, respectively, have been identified as intermediates. The adduct C₆N₁₁H₁₀Cl⋅0.5NH₄Cl was obtained by the reaction of melamine C₃N₃(NH₂)₃ with NH₄Cl at 450 °C. During the pyrolysis of thiourea, guanidinium thiocyanate was initially formed and subsequently the melamium thiocyanate melamine adduct C₆N₁₁H₁₀SCN⋅2C₃N₃(NH₂)₃ was isolated at 300 °C. A second melaminium thiocyanate melamine adduct with the formula HC₃N₃(NH₂)₃SCN⋅2C₃N₃(NH₂)₃ represents an intermediary reaction product that is best accessible at low pressures. The crystal structures of the compounds were solved by single-crystal XRD. Unequivocal proton localization at the C₆N₁₁H₁₀⁺ ion was established. A typical intramolecular and interannular hydrogen bridge and other characteristic hydrogen-bonding motifs were identified. Additionally, the adducts were investigated by solid-state NMR spectroscopy. Our study provides detailed insight into the thermal condensation of thiourea by identifying and characterizing key intermediates involved in the condensation process leading to carbon nitride type materials. Furthermore, factors promoting the formation of melamium adduct phases over melem are discussed.

The quaternary alkaline earth nitridosilicate nitride Li₂Sr₄[Si₂N₅]N has been synthesized at 900 °C using liquid lithium as fluxing agent in weld-shut tantalum ampoules. The structure of Li₂Sr₄[Si₂N₅]N {space group Im2 [no. 119, a = 751.46(11), c = 1508.9(3) pm, Z = 4, R1 = 0.049, wR2 = 0.135, 499 data, 41 parameters]} is built up from vertex sharing [SiN₄] tetrahedra forming layers which can be derived from the apophyllite structure type. According to the condensation degree of κ = 2:5 discrete N^3– ions which are coordinated by five Sr²⁺ and one Li⁺ site occur besides the silicate layers in the crystal structure. The electronic structure and chemical bonding in Li₂Sr₄[Si₂N₅]N has been analyzed by Madelung calculations (MAPLE) and DFT (VASP) calculations.

Silicate sind eine der wichtigsten Verbindungsklassen auf diesem Planeten, und mehr als 1000 Vertreter wurden im Mineralreich bereits identifiziert. Zusätzlich wurden auch mehrere hundert künstliche Silicate synthetisiert. Der Ersatz von Sauerstoff durch Stickstoff führt zur strukturell vielseitigen Klasse der Nitridosilicate. Siliciumnitrid, eines der wichtigsten nichtoxidischen keramischen Materialien, ist die binäre Stammverbindung der Nitridosilicate, und sie symbolisiert die inhärenten Materialeigenschaften dieser Verbindungen. Allerdings ist eine breite systematische Erforschung der Nitridosilicate erst in den letzten Dekaden erfolgt. Mittlerweile haben diese und verwandte Verbindungen ein bemerkenswertes Ausmaß an industrieller Anwendung erreicht. Dieser Aufsatz illustriert jüngste Fortschritte bei der Synthese, der Aufklärung von Struktur-Eigenschafts-Beziehungen und den Anwendungen von Nitridosilicaten, Oxonitridosilicaten und den verwandten SiAlONen.

Silicates are one of the most important classes of compounds on this planet, and more than 1000 silicates have been identified in the mineral kingdom. Additionally, several hundreds of artificial silicates have been synthesized. The substitution of oxygen by nitrogen leads to the structurally diverse and manifold class of nitridosilicates. Silicon nitride, one of the most important non-oxidic ceramic materials, is the binary parent compound of nitridosilicates, and it symbolizes the inherent material properties of these refractory compounds. However, prior to the last decades, a broad systematic investigation of nitridosilicates had not been accomplished. In the meantime, these and related compounds have reached a remarkable level of industrial application. This review illustrates recent progress in synthesis and structure–property relationships and also applications of nitridosilicates, oxonitridosilicates, and related SiAlONs.

Poly(triazine imide) with intercalation of lithium and chloride ions (PTI/Li⁺Cl⁻) was synthesized by temperature-induced condensation of dicyandiamide in a eutectic mixture of lithium chloride and potassium chloride as solvent. By using this ionothermal approach the well-known problem of insufficient crystallinity of carbon nitride (CN) condensation products could be overcome. The structural characterization of PTI/Li⁺Cl⁻ resulted from a complementary approach using spectroscopic methods as well as different diffraction techniques. Due to the high crystallinity of PTI/Li⁺Cl⁻ a structure solution from both powder X-ray and electron diffraction patterns using direct methods was possible; this yielded a triazine-based structure model, in contrast to the proposed fully condensed heptazine-based structure that has been reported recently. Further information from solid-state NMR and FTIR spectroscopy as well as high-resolution TEM investigations was used for Rietveld refinement with a goodness-of-fit (χ²) of 5.035 and wRp=0.05937. PTI/Li⁺Cl⁻ (P6₃cm (no. 185); a=846.82(10), c=675.02(9) pm) is a 2D network composed of essentially planar layers made up from imide-bridged triazine units. Voids in these layers are stacked upon each other forming channels running parallel to [001], filled with Li⁺ and Cl⁻ ions. The presence of salt ions in the nanocrystallites as well as the existence of sp²-hybridized carbon and nitrogen atoms typical of graphitic structures was confirmed by electron energy-loss spectroscopy (EELS) measurements. Solid-state NMR spectroscopy investigations using ¹⁵N-labeled PTI/Li⁺Cl⁻ proved the absence of heptazine building blocks and NH₂ groups and corroborated the highly condensed, triazine-based structure model.

The oxonitridophosphate SrP₃N₅O has been synthesized by heating a multicomponent reactant mixture that consisted of phosphoryl triamide OP(NH₂)₃, thiophosphoryl triamide SP(NH₂)₃, SrS, and NH₄Cl enclosed in evacuated and sealed silica-glass ampoules up to 750 °C. The compound was obtained as nanocrystalline powder with needle-shaped crystallites. The crystal structure was solved ab initio on the basis of electron diffraction data by means of automated electron diffraction tomography (ADT) and verified by Rietveld refinement with X-ray powder diffraction data. SrP₃N₅O crystallizes in the orthorhombic space group Pnma (no. 62) with unit-cell data of a=18.331(2), b=8.086(1), c=13.851(1) Å and Z=16. The compound is a highly condensed layer phosphate with a degree of condensation κ=1/2. The corrugated layers {(P₃N₅O)²⁻} consist of linked, triangular columns built up from P(O,N)₄ tetrahedra with 3-rings and triply binding nitrogen atoms. The Sr²⁺ ions are located between the layers and exhibit six-, eight-, and ninefold coordination. FTIR and solid-state NMR spectra of SrP₃N₅O are discussed as well.

The quaternary lithium alkaline earth nitridosilicatesLi₂MSi₂N₄ with M = Ca, Sr were synthesized at 900 °C by employment of liquid lithium as fluxing agent in weld shut tantalum ampoules. The title compounds crystallize in space group Pa [Li₂CaSi₂N₄: a = 1056.9(12) pm, Z = 12, R₁ = 0.0439, wR2 = 0.0839, 412 data, 42 parameters; Li₂SrSi₂N₄: a = 1071.37(12) pm, Z = 12, R₁ = 0.0476, wR2 = 0.1227, 528 data, 42 parameters]. The structure is built up from vertex sharing [SiN₄] tetrahedra forming a three-dimensional network comprising the alkaline earth and lithium ions in the voids. The network is the first example of a nitridosilicate adopting topology of Net 39 according to O'Keeffe. The two crystallographically independent M²⁺ sites are highly symmetrically coordinated by six nitrogen atoms. Lattice energy calculations (MAPLE) and EDX measurements confirmed the electrostatic bonding interactions and the chemical compositions. ⁷Li and ²⁹Si solid-state MAS NMR spectra of Li₂SrSi₂N₄ are reported. Electron energy-loss spectroscopy (EELS) elucidates further information upon the lithium incorporation.

The quaternary lithium rare-earth nitridosilicates Li₅Ln₅Si₄N₁₂ with Ln = La, Ce were synthesized at moderate temperatures below 900 °C in closed tantalum ampoules employing liquid lithium as a flux. Thereby, nitridosilicate substructures with a low degree of condensation were obtained. The air and moisture sensitive title compounds are the first representatives of quaternary lithium rare-earth nitridosilicates and crystallize in space group Pb2 [Li₅La₅Si₄N₁₂: a = 1104.28(16), c = 557.30(11) pm, Z = 2, R₁ = 0.0498, 873 data, 56 parameters; Li₅Ce₅Si₄N₁₂: a = 1097.78(16), c = 551.43(11) pm, Z = 2, R₁ = 0.0334, 748 data, 56 parameters]. The nitridosilicate substructure consists of infinite nonbranched zweier single-chains of vertex sharing SiN₄ tetrahedra running parallel to [001]. The chains exhibit a stretching factor f_S = 0.981 for La and 0.977 for Ce, respectively. The two crystallographically independent Li⁺ sites are each coordinated by four nitrogen atoms. Lattice energy calculations (MAPLE) and EDX measurements confirmed the electrostatic bonding interactions and the chemical compositions. For the La containing compound ⁷Li solid-state MAS NMR investigations are reported.

The efficient green phosphor Ba₃Si₆O₁₂N₂:Eu²⁺ and its solid-solution series Ba_3−xSr_xSi₆O₁₂N₂ (with x≈0.4 and 1) were synthesized in a radio-frequency furnace under nitrogen atmosphere at temperatures up to 1425 °C. The crystal structure (Ba₃Si₆O₁₂N₂, space group P (no. 147), a=7.5218(1), c=6.4684(1) Å, wR2=0.048, Z=1) has been solved and refined on the basis of both single-crystal and powder X-ray diffraction data. Ba₃Si₆O₁₂N₂:Eu²⁺ is a layer-like oxonitridosilicate and consists of vertex-sharing SiO₃N-tetrahedra forming 6er- and 4er-rings as fundamental building units (FBU). The nitrogen atoms are connected to three silicon atoms (N3), while the oxygen atoms are either terminally bound (O1) or bridge two silicon atoms (O2) (numbers in superscripted square brackets after atoms indicate the coordination number of the atom in question). Two crystallographically independent Ba²⁺ sites are situated between the silicate layers. Luminescence investigations have shown that Ba₃Si₆O₁₂N₂:Eu²⁺ exhibits excellent luminescence properties (emission maximum at ≈527 nm, full width at half maximum (FWHM) of ≈65 nm, low thermal quenching), which provides potential for industrial application in phosphor-converted light-emitting diodes (pc-LEDs). In-situ high-pressure and high-temperature investigations with synchrotron X-ray diffraction indicate decomposition of Ba₃Si₆O₁₂N₂ under these conditions. The band gap of Ba₃Si₆O₁₂N₂:Eu²⁺ was measured to be 7.05±0.25 eV by means of X-ray emission spectroscopy (XES) and X-ray absorption near edge spectroscopy (XANES). This agrees well with calculated band gap of 6.93 eV using the mBJ-GGA potential. Bonding to the Ba atoms is highly ionic with only the 4p_3/2 orbitals participating in covalent bonds. The valence band consists primarily of N and O p states and the conduction band contains primarily Ba d and f states with a small contribution from the N and O p states.

BeP₂N₄ was synthesized in a multi-anvil apparatus starting from Be₃N₂ and P₃N₅ at 5 GPa and 1500 °C. The compound crystallizes in the phenakite structure type (space group R, no. 148) with a=1269.45(2) pm, c=834.86(2) pm, V=1165.13(4)×10⁶ pm³ and Z=18. As isostructural and isovalence-electronic α-Si₃N₄ transforms into β-Si₃N₄ at high pressure and temperature, we studied the phase transition of BeP₂N₄ into the spinel structure type by using density functional theory calculations. The predicted transition pressure of 24 GPa is within the reach of today’s state of the art high-pressure experimental setups. Calculations of inverse spinel-type BeP₂N₄ revealed this polymorph to be always higher in enthalpy than either phenakite-type or spinel-type BeP₂N₄. The predicted bulk modulus of spinel-type BeP₂N₄ is in the range of corundum and γ-Si₃N₄ and about 40 GPa higher than that of phenakite-type BeP₂N₄. This finding implies an increase in hardness in analogy to that occurring for the β- to γ-Si₃N₄ transition. In hypothetical spinel-type BeP₂N₄ the coordination number of phosphorus is increased from 4 to 6. So far only coordination numbers up to 5 have been experimentally realized (γ-P₃N₅), though a sixfold coordination for P has been predicted for hypothetic δ-P₃N₅. We believe, our findings provide a strong incentive for further high-pressure experiments in the quest for novel hard materials with yet unprecedented structural motives.

Amorphous silicon bis(carbodiimide) “Si(CN₂)₂” has been identified as a reactive precursor for the synthesis of nitridosilicates that is specifically useful in the temperature region below 1000 °C. In this context the applicability and reactivity of amorphous “Si(CN₂)₂” towards Li₃N in comparison to silicon diimide Si(NH)₂ has been studied using high-temperature in situ powder diffraction and DSC measurements. During the current investigation single crystals of Li₂SiN₂ have been obtained and the crystal structure of this Li⁺ ion conductor has been determined and refined: [Pbca, no. 61, a = 9.907(2), b = 9.907(2), c = 15.014(3) Å, Z = 32, R₁ = 0.038, 1460 data, 142 parameters]. In the solid, Li₂SiN₂ consists of two interpenetrating cristobalite type nets which are made up from hetero-adamantane-like [Si₄N₆]N_4/2 groups. The eight crystallographically independent Li⁺ sites are ordered at room temp. and exhibit coordination numbers 3 to 5. The ⁷Li and ²⁹Si solid-state MAS NMR spectra of Li₂SiN₂ are reported. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The cover picture shows the [SiN₂]^2– framework of the lithium ion conductor Li₂SiN₂. The hitherto unknown crystal structure reveals possible Li⁺ pathways (red channels) for lithium ion conductivity. Li₂SiN₂ was synthesized from “Si(CN₂)₂”, highlighted as a promising precursor for nitridosilicates in arc-welded (top left) tantalum crucibles. The structure was solved from single crystals, and the ⁷Li MAS solid-state NMR spectrum (bottom right) is presented. Details are discussed in the article by W. Schnick et al. on p. 1579 ff.

A new precursor approach leading to NPO-zeolite analogous nitridosilicates with cavities containing carbodiimide ions is presented. The reaction of amorphous “Si(CN₂)₂” and barium in liquid sodium afforded Ba₆Si₆N₁₀O₂(CN₂) as yellow crystals. The structure is a rare example of the NPO-zeolite framework type and the first nitridosilicate incorporating carbodiimide ions. The partially ordered integration of carbodiimide moieties in the channels leads to the formation of a superstructure (P, no. 174, a = 16.255(2), c = 5.4690(11) Å, Z = 3, R₁ = 0.0299, 2139 data, 100 parameters) and merohedral twinning. A comprehensive structure solution is presented, taking all possible ordering variants and twin laws into account.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

(Sr_1–xCa_x)_{(11+16y–25z)/2}(Si_1–yAl_y)₁₆(N_1–zO_z)₂₅ (x ≈ 0.24, y ≈ 0.18, z ≈ 0.19) was obtained by high-temperature synthesis (1650 °C – 1700 °C) using CaSiAlN₃, Si₃N₄, AlN, SiO₂, and Sr₂N as starting materials. The crystal structure [Imm2 (no. 44), a = 20.6973(6), b = 10.7292(4), c = 4.8881(2) Å, Z = 2, R1 = 0.0326, wR2 = 0.0735] was determined from single-crystal X-ray data using dual space methods and confirmed for the bulk material by Rietveld refinement on X-ray powder diffraction data. The structural results are corroborated by lattice energy calculations (MAPLE). The compound is the first representative of a novel silicate framework. The anionic part of the structure is built up from highly-condensed dreier ring layers extending parallel (100), which are interconnected by common N and O atoms. In the resulting voids of this framework, there are three different cation sites, which are coordinated by six, eight, and twelve nitrogen and oxygen atoms, respectively. The largest one is fully occupied by Sr atoms, whereas the other two sites host Sr and Ca atoms or vacancies, respectively.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

The synthesis of potassium melonate, K₃[C₆N₇(NCN)₃], by reaction of a potassium thiocyanate melt with the polymer melon [C₆N₇(NH)(NH₂)]_n is an established, though poorly understood, reaction. We have modified the original approach by using salt melts containing Na⁺ ions and/or cyanate ions to yield the respective melonate salts. These melonates, however, are not the final reaction products. We have identified them to decompose in cyanate melts to formtricyanomelaminates at higher temperatures and prolonged reaction times. This is the first selective decomposition reaction leading from heptazines to triazines. The progress of the reactions was studied by using thermal analysis, thus allowing the exact determination of reaction temperatures and weight losses. With the data at hand we are now able to gain better insight into the formation and properties of alkali melonates while establishing new synthetic routes to these compounds. We were able to isolate crystals of anhydrous potassium melonate directly from a thiocyanate melt. The structure of this compound was solved by single-crystal X-ray-diffraction. The new reaction conditions involving cyanates on the one hand avoid the release of CS₂ and are no longer highly corrosive to most metallic reaction vessels and on the other hand these reagents provide new, cheap, and convenient access to melonates and tricyanomelaminates. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

A new structure type of nitridosilicates with an interrupted framework has been identified for M₇Si₆N₁₅ with M=La, Ce, and Pr. The materials have been synthesized in a radio-frequency furnace at temperatures between 1550–1625 °C, starting from the respective metals, metal nitrides, and silicon diimide. The crystal structure of Ce₇Si₆N₁₅ has been determined by using single-crystal X-ray diffraction. Besides ordered crystals 1 with a complicated triclinic superstructure and multiple twinning (P, no. 2; a=13.009(3), b=25.483(5), c=25.508(10) Å; α=117.35(3), β=99.59(3), γ=99.63(3)°; V=7114(2) ų; Z=18; R1=0.0411), disordered crystals 2 with identical composition exhibiting a trigonal average structure (R, no. 148) have also been observed (a=43.420(6), c=6.506(2) Å; V=10 623(3) ų; Z=27; R1=0.0309). Pr₇Si₆N₁₅ (3) and La₇Si₆N₁₅ (4) are isostructural with 1 as evidenced by twinned single-crystal data for 3 (P, no. 2; a=12.966(3), b=25.449(10), c=25.459(10) Å; α=117.28(3), β=99.70(4), γ=99.60(4)°; V=7068(4) ų; Z=18; R1=0.0526) and powder diffraction data for 4 (P, no. 2; a=13.109(9), b=25.606(18), c=25.609(18) Å; V=7223(12) ų; Z=18; R_P=0.0194; R_F=0.0936). The crystal structure of M₇Si₆N₁₅ (M=La, Ce, Pr) is built up exclusively of corner-sharing tetrahedrons that appear as Q²-, Q³-, and Q⁴-type tetrahedrons forming different ring sizes within a less condensed three-dimensional network. Among the characteristic structural motifs are saw-blade-shaped 12-rings and finite chains consisting of four corner-sharing SiN₄ tetrahedrons. High-resolution transmission electron micrographs indicate both ordered and disordered crystallites. In the diffraction patterns of disordered rhombohedral crystals, diffuse maxima appear in reciprocal space at those positions in which sharp superstructure reflections are found in the case of the respective ordered crystallites. Magnetic susceptibility measurements of Ce₇Si₆N₁₅ show paramagnetic behavior with an experimental magnetic moment of 2.29 μ_B per Ce, thereby corroborating the existence of Ce³⁺.

By studying the thermal condensation of melamine, we have identified three solid molecular adducts consisting of melamine C₃N₃(NH₂)₃ and melem C₆N₇(NH₂)₃ in differing molar ratios. We solved the crystal structure of 2 C₃N₃(NH₂)₃⋅C₆N₇(NH₂)₃ (1; C2/c; a=21.526(4), b=12.595(3), c=6.8483(14) Å; β=94.80(3)°; Z=4; V=1850.2(7) ų), C₃N₃(NH₂)₃⋅C₆N₇(NH₂)₃ (2; Pcca; a=7.3280(2), b=7.4842(2), c=24.9167(8) Å; Z=4; V=1366.54(7) ų), and C₃N₃(NH₂)₃⋅3 C₆N₇(NH₂)₃ (3; C2/c; a=14.370(3), b=25.809(5), c=8.1560(16) Å; β=94.62(3)°; Z=4; V=3015.0(10) ų) by using single-crystal XRD. All syntheses were carried out in sealed glass ampoules starting from melamine. By variation of the reaction conditions in terms of temperature, pressure, and the presence of ammonia-binding metals (europium) we gained a detailed insight into the occurrence of the three adduct phases during the thermal condensation process of melamine leading to melem. A rational bulk synthesis allowed us to realize adduct phases as well as phase separation into melamine and melem under equilibrium conditions. A solid-state NMR spectroscopic investigation of adduct 1 was conducted.

HP-Ca₂Si₅N₈ was obtained by means of high-pressure high-temperature synthesis utilizing the multianvil technique (6 to 12 GPa, 900 to 1200 °C) starting from the ambient-pressure phase Ca₂Si₅N₈. HP-Ca₂Si₅N₈ crystallizes in the orthorhombic crystal system (Pbca (no. 61), a=1058.4(2), b=965.2(2), c=1366.3(3) pm, V=1395.7(7)×10⁶ pm³, Z=8, R1=0.1191). The HP-Ca₂Si₅N₈ structure is built up by a three-dimensional, highly condensed nitridosilicate framework with N^[2] as well as N^[3] bridging. Corrugated layers of corner-sharing SiN₄ tetrahedra are interconnected by further SiN₄ units. The Ca²⁺ ions are situated between these layers with coordination numbers 6+1 and 7+1, respectively. HP-Ca₂Si₅N₈ as well as hypothetical orthorhombic o-Ca₂Si₅N₈ (isostructural to the ambient-pressure modifications of Sr₂Si₅N₈ and Ba₂Si₅N₈) were studied as high-pressure phases of Ca₂Si₅N₈ up to 100 GPa by using density functional calculations. The transition pressure into HP-Ca₂Si₅N₈ was calculated to 1.7 GPa, whereas o-Ca₂Si₅N₈ will not be adopted as a high-pressure phase. Two different decomposition pathways of Ca₂Si₅N₈ (into Ca₃N₂ and Si₃N₄ or into CaSiN₂ and Si₃N₄) and their pressure dependence were examined. It was found that a pressure-induced decomposition of Ca₂Si₅N₈ into CaSiN₂ and Si₃N₄ is preferred and that Ca₂Si₅N₈ is no longer thermodynamically stable under pressures exceeding 15 GPa. Luminescence investigations (excitation at 365 nm) of HP-Ca₂Si₅N₈:Eu²⁺ reveal a broadband emission peaking at 627 nm (FWHM=97 nm), similar to the ambient-pressure phase Ca₂Si₅N₈:Eu²⁺.

We present two organometallic precursor approaches leading to the hitherto-unknown dioxo monocarbodiimides (Ln₂O₂CN₂) of the late lanthanides Ho, Er, and Yb as well as yttrium. One involves insertion of CO₂, and the other one is a straightforward route using a molecular single-source precursor. To this end the reactivity of the activated amido lanthanide compound [(Cp₂ErNH₂)₂] towards carbon dioxide absorption under supercritical conditions was studied. Selective insertion of CO₂ into the amido complex yielded the single-source precursor [Er₂(O₂CN₂H₄)Cp₄], which was characterized by vibrational spectroscopy and thermal and elemental analyses. Ammonolysis of this amorphous compound at 700 °C affords Er₂O₂CN₂. To gain deeper insight into the structural characteristics of the amorphous precursor, a similar molecular carbamato complex was synthesized and fully characterized. X-ray structure analysis of the dimeric complex [Cp₄Ho₂{μ-η¹:η²-OC(OtBu)NH}] shows an unusual bonding mode of the tert-butylcarbamate ligand, which acts as both a bridging and side-on chelating group. Ammonolysis of this compound also yielded dioxo monocarbodiimides, and therefore the crystalline carbamato complex turned out to be an alternative precursor for the straightforward synthesis of Ln₂O₂CN₂. Analogously, the dioxo monocarbodiimides of Y, Ho, Er, and Yb were synthesized by this route. The crystal structures were determined from X-ray powder diffraction data and refined by the Rietveld method (Ln=Ho, Er). Further spectroscopic characterization and elemental analysis evidenced the existence of phase-pure products. The dioxo monocarbodiimides of holmium and erbium crystallize in the trigonal space group Pm1. According to X-ray powder diffraction, they adopt the Ln₂O₂CN₂ (Ln=Ce–Gd) structure type.