P1-xTa8+xN13 (x = 0.1-0.15): A Phosphorus Tantalum Nitride Featuring Mixed-Valent Tantalum and P/Ta Disorder Visualized by Scanning Transmission Electron Microscopy.

We report on the synthesis, crystal, and electronic structure, as well as the magnetic, and electric properties of the phosphorus-containing tantalum nitride P1-xTa8+xN13 (x = 0.1-0.15). A high-pressure high-temperature reaction (8 GPa, 1400 °C) of Ta3N5 and P3N5 with NH4F as a mineralizing agent yields the compound in the form of black, rod-shaped crystals. Single-crystal X-ray structure elucidation (space group C2/m (no. 12), a = 16.202(3), b = 2.9155(4), c = 11.089(2) Å, ß = 126.698(7)°, Z = 2) shows a network of face- and edge-sharing Ta-centered polyhedra that contains small vacant channels and  PN6 octahedra strands. Atomic resolution transmission electron microscopy reveals an unusual P/Ta disorder. Mixed-valent tantalum atoms exhibit interatomic distances similar to those in metallic tantalum, however, the electrical resistivity is quite high in the order of 10­1 Ω cm. The density of states and the electron localization function indicate localized electrons in both covalent and ionic bonds between P/Ta and N atoms, combined with less localized electrons that do not contribute to interatomic bonds.

Structural Influence of Lone Pairs in GeP2 N4 , a Germanium(II) Nitridophosphate.

Owing to their widespread properties, nitridophosphates are of high interest in current research. Explorative high-pressure high-temperature investigations yielded various compounds with stoichiometry MP2 N4 (M=Be, Ca, Sr, Ba, Mn, Cd), which are discussed as ultra-hard or luminescent materials, when doped with Eu2+ . Herein, we report the first germanium nitridophosphate, GeP2 N4 , synthesized from Ge3 N4 and P3 N5 at 6 GPa and 800 °C. The structure was determined by single-crystal X-ray diffraction and further characterized by energy-dispersive X-ray spectroscopy, density functional theory calculations, IR and NMR spectroscopy. The highly condensed network of PN4 -tetrahedra shows a strong structural divergence to other MP2 N4 compounds, which is attributed to the stereochemical influence of the lone pair of Ge2+ . Thus, the formal exchange of alkaline earth cations with Ge2+ may open access to various compounds with literature-known stoichiometry, however, new structures and properties.

Discovery of Two Polymorphs of TiP4 N8 Synthesized from Binary Nitrides.

TiP4 N8 was obtained from the binary nitrides TiN and P3 N5 upon addition of NH4 F as a mineralizer at 8 GPa and 1400 °C. An intricate interplay of disorder and polymorphism was elucidated by in situ temperature-dependent single-crystal X-ray diffraction, STEM-HAADF, and the investigation of annealed samples. This revealed two polymorphs, which consist of dense networks of PN4 tetrahedra (degree of condensation κ=0.5) and either augmented triangular TiN7 prisms or triangular TiN6 prisms for α- and ß-TiP4 N8 , respectively. The structures of TiP4 N8 exhibit body-centered tetragonal (bct) framework topology. DFT calculations confirm the measured band gaps of α- and ß-TiP4 N8 (1.6-1.8 eV) and predict the thermochemistry of the polymorphs in agreement with the experiments.

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 AESi3 P4 N10 (NH)2 (AE=Mg, Mg0.94 Ca0.06 , Ca, Sr), which were synthesized under high-pressure high-temperature conditions at 1400 °C and 8 GPa from the refractory nitrides P3 N5 and Si3 N4 , the respective alkaline earth amides, implementing NH4 F 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).

High-Pressure Synthesis of Sc5 P12 N23 O3 and Ti5 P12 N24 O2 by Activation of the Binary Nitrides ScN and TiN with NH4 F.

Multinary transition metal nitrides and oxonitrides are a versatile and intriguing class of compounds. However, they have been investigated far less than pure oxides. The compounds Sc5 P12 N23 O3 and Ti5 P12 N24 O2 have now been synthesized from the binary nitrides ScN and TiN, respectively, by following a high-pressure high-temperature approach at 8 GPa and 1400 °C. NH4 F acts as a mineralizing agent that supports product formation and crystallization. The starting materials ScN and TiN are seemingly an uncommon choice because of their chemical inertness but, nevertheless, react under these conditions. Sc5 P12 N23 O3 and Ti5 P12 N24 O2 crystallize isotypically with Ti5 B12 O26 , consisting of solely vertex-sharing P(O/N)4 tetrahedra forming two independent interpenetrating diamond-like nets that host TM(O/N)6 (TM=Sc, Ti) octahedra. Ti5 P12 N24 O2 is a mixed-valence compound and shows ordering of Ti3+ and Ti4+ ions.

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.

BaGe8As14: a semiconducting sodalite-type compound.

A new sodalite-type compound, namely BaGe8As14 was synthesized via solid-state reactions and structurally characterized with single crystal X-ray diffraction (space group I4[combining macron]3m). Vertex-sharing GeAs4-tetrahedra form ß-cages with additional Ge/As-mixed sites located slightly above or below the six-membered rings. The structure is similar to the borate mineral rhodizite. Barium atoms are disordered due to a slight shift off the centers of large ß-cages. This partially disordered structure together with a narrow bandgap of 0.43 eV in line with low resistivity (2 × 10-2Ω cm), and a high carrier concentration (1.6 × 1020 cm-3) at 300 K qualifies BaGe8As14 as a potential thermoelectric material.

Nitridophosphate-Based Ultra-Narrow-Band Blue-Emitters: Luminescence Properties of AEP8 N14 :Eu2+ (AE=Ca, Sr, Ba).

The nitridophosphates AEP8 N14 (AE=Ca, Sr, Ba) were synthesized at 4-5 GPa and 1050-1150 °C applying a 1000 t press with multianvil apparatus, following the azide route. The crystal structures of CaP8 N14 and SrP8 N14 are isotypic. The space group Cmcm was confirmed by powder X-ray diffraction. The structure of BaP8 N14 (space group Amm2) was elucidated by a combination of transmission electron microscopy and diffraction of microfocused synchrotron radiation. Phase purity was confirmed by Rietveld refinement. IR spectra are consistent with the structure models and the chemical compositions were confirmed by X-ray spectroscopy. Luminescence properties of Eu2+ -doped samples were investigated upon excitation with UV to blue light. CaP8 N14 (λem =470 nm; fwhm=1380 cm-1 ) and SrP8 N14 (λem =440 nm; fwhm=1350 cm-1 ) can be classified as the first ultra-narrow-band blue-emitting Eu2+ -doped nitridophosphates. BaP8 N14 shows a notably broader blue emission (λem =417/457 nm; fwhm=2075/3550 cm-1 ).

A Layered Tin Bismuth Selenide with Three Different Building Blocks that Account for an Extremely Large Lattice Parameter of 283†Å.

The layered compound Sn2.8(4) Bi20.2(4) Se27 exhibits an extraordinarily long-periodic 150R stacking sequence. The crystal structure contains three different building blocks, which form upon the addition of Sn to a Bi-rich bismuth selenide. Sn-doped Bi2 double ("2") layers similar to those in elemental bismuth, Sn0.3 Bi1.7 Se3 quintuple ("5") layers and Sn0.4 Bi2.6 Se4 septuple ("7") layers are arranged in a 7525757525|7525757525|7525757525 sequence, which corresponds to a structure with a=4.1819(4) and c=282.64(6) Šin space group R 3 ‾ m. The structure of a microcrystal was determined using microfocused synchrotron radiation and refined as a formally commensurately modulated structure in (3+1)D superspace (superspace group R 3 ‾ m(00γ)00), with a trivial basic structure that contains just one atom. The stacking sequence as well as the cation distribution are confirmed by aberration-corrected scanning transmission electron microscopy (STEM) in combination with chemical mapping by X-ray spectroscopy with atomic resolution. Stacking faults are not typical but have been observed occasionally.

BaP6 N10 NH:Eu2+ as a Case Study-An Imidonitridophosphate Showing Luminescence.

Barium imidonitridophosphate BaP6 N10 NH was synthesized at 5 GPa and 1000 °C with a high-pressure high-temperature approach using the multianvil technique. Ba(N3 )2 , P3 N5 and NH4 Cl were used as starting materials, applying a combination of azide and mineralizer routes. The structure elucidation of BaP6 N10 NH (P63 , a=7.5633(11), c=8.512(2) Å, Z=2) was performed by a combination of transmission electron microscopy and single-crystal diffraction with microfocused synchrotron radiation. Phase purity was verified by Rietveld refinement. 1 H and 31 P solid-state NMR and FTIR spectroscopy are consistent with the structure model. The chemical composition was confirmed by energy-dispersive X-ray spectroscopy and CHNS analyses. Eu2+ -doped samples of BaP6 N10 NH show blue emission upon excitation with UV to blue light (λem =460 nm, fwhm=2423 cm-1 ) representing unprecedented Eu2+ -luminescence of an imidonitride.

The Long-Periodic Loop-Branched Chain Structure of the Oxonitridophosphate La21 P40 O46 N57 , Elucidated by a Combination of TEM and Microfocused Synchrotron Radiation.

The lanthanum oxonitridophosphate La21 P40 O46 N57 was synthesized by high-pressure metathesis from partially hydrolysed LiPN2 and LaCl3 at 750-950 °C and 7-9 GPa. The combination of transmission electron microscopy (TEM) and diffraction using microfocused synchrotron radiation revealed a monoclinic crystal structure (space group P21 /n, a=14.042(4), b=7.084(3), c=41.404(10) Å, ß=97.73(3)° and Z=2), which is characterized by loop-branched 21 member single chains of P(O,N)4 tetrahedra that extend along [2 0 1]. These chains are related to the loop-branched dreier single chains with dreier-ring loops in stillwellite (CeBSiO5 ). In La21 P40 O46 N57 , these chains are characterized by a complex long-periodic conformation and exhibit disorder that involves La/N and P split positions. This is an extraordinarily long periodicity with respect to branched single chains of tetrahedra. La21 P40 O46 N57 constitutes the first rare-earth oxonitridophosphate exhibiting a chain structure. Single-crystal data are consistent with electron and powder X-ray diffraction.

Cationic Pb2 Dumbbells Stabilized in the Highly Covalent Lead Nitridosilicate Pb2 Si5 N8.

Due to the weak oxidative force of N2 , nitrides are only typically formed with the less electronegative metals. Meeting this challenge, we here present Pb2 Si5 N8 , the first nitridosilicate containing highly electron-affine cations of a metal from the right side of the Zintl border. By using advanced synchrotron X-ray diffraction, the crystal structure was determined from a tiny single crystal of 1×3×3 µm3 in size, revealing a significantly different bonding situation compared to all other nitridosilicates known so far. Indeed, DFT calculations confirm distinct amounts of covalency not only between Pb and N but also between formal Pb2+ cations. Thus, unprecedented cationic Pb2 dumbbells with a stretching vibration at 117 cm-1 were found in Pb2 Si5 N8 , the first representative of a crystallographically elucidated lead nitride, stabilized by high amounts of covalency.

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 Sc0.2 Ca1.7 Si5 N8 , and partial vacancy ordering in U0.5x Sr2-0.75x Si5 N8 with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.

RE4Ba2[Si12O2N16C3]:Eu2+ ( RE = Lu, Y): Green-Yellow Emitting Oxonitridocarbidosilicates with a Highly Condensed Network Structure Unraveled through Synchrotron Microdiffraction.

The oxonitridocarbidosilicates RE4Ba2[Si12O2N16C3]:Eu2+ ( RE = Lu, Y) were synthesized by carbothermal reactions starting from RE2O3, graphite, Ba2Si5N8, Si(NH)2, and Eu2O3. The crystal structure of Lu4Ba2[Si12O2N16C3]:Eu2+ was elucidated on a submicron-sized single crystal by a combination of transmission electron microscopy and microfocused synchrotron radiation. The compound crystallizes in trigonal space group P3 (no. 143) with a = 16.297(4) Å, c = 6.001(2) Å, and Z = 3 ( R1 = 0.0332, wR2 = 0.0834, GoF = 1.034). According to Rietveld refinements on powder X-ray diffraction data, Y4Ba2[Si12O2N16C3]:Eu2+ is isotypic with a = 16.41190(6) Å and c = 6.03909(3) Å. The crystal structures are built up of vertex-sharing SiC(O/N)3 tetrahedra forming star-shaped units [C[4](Si(O/N)3)4] with carbon atoms in fourfold bridging positions. Energy-dispersive X-ray spectroscopy and CHNS analysis correspond to the sum formula, lattice energy, and charge distribution calculations support the assignment of O/N/C atoms. When excited with UV to blue light, Eu2+-doped samples show green luminescence for RE = Lu (λem ≈ 538 nm, full width at half-maximum (fwhm) ≈ 3600 cm-1) and yellow emission in the case of RE = Y (λem ≈ 556 nm, fwhm ≈ 4085 cm-1).

Nucleophilicity of Alkyl Zirconocene and Titanocene Precatalysts, and Kinetics of Activation by Carbenium Ions and by B(C6 F5 )3.

Kinetics of activation of methyl and benzyl metallocene precatalysts by benzhydrylium ions, tritylium ions, and triarylborane B(C6 F5 )3 were measured spectrophotometrically. The rate constants correlate linearly with the electrophilicity parameter E of the benzhydrylium and tritylium ions employed, allowing us to determine the σ-nucleophilicities of the metal-carbon bond of several zirconocenes and titanocenes. Bridging, substitution, metal, and ligand effects on the rates of metal-alkyl bond cleavage (M=Zr, Ti) were studied and structure-reactivity correlations were used to predict the kinetics of generation of metallocenium ions pairs, which are active catalysts in polymerization reactions and are highly electrophilic Lewis acids in frustrated Lewis pair catalysis.