Static atomic displacements of Sn in disordered NiAs/Ni2In type HT-Ni1+δSn

The crystal structures of the intermediate solid solution HT (high temperature) Ni_1+δSn with δ=0.28, 0.52 and 0.61 (refined Ni contents) have been analyzed in detail by X-ray diffraction on single crystals. The previously reported basic atomic arrangement, i.e., a NiAs/Ni₂In structure type (P6₃/mmc, Ni(1) on 2a, 0 0 0, Ni(2) with an occupancy δ on 2d, and Sn on 2c, ), is confirmed. However, strong anisotropic atomic displacements occur for Sn within the a-b plane of the hexagonal unit cell, which require a Gram-Charlier expansion of the probability density function of Sn in order to obtain a good fit to the diffraction data. Direction, magnitude and the concentration dependence of the displacements can be interpreted in terms of the geometrical requirements of the different local atomic configurations in the planes z=±1/4, so that the displacements can be identified as static ones.     

Ce10Cl4Ga5 and Ln3ClGa4 (Ln = La,Ce): reduced halides or oxidized intermetallics?

The compounds Ce(10)Cl(4)Ga(5) and Ln(3)ClGa(4) (Ln = La, Ce) were synthesized from stoichiometric mixtures of Ln, LnCl(3), and Ga under Ar atmosphere in sealed Ta ampules at 910-1020 degrees C for 25-26 days. Ce(10)Cl(4)Ga(5) is isostructural to La(10)Cl(4)Ga(5) (space group I4/mcm, No. 140) with lattice constants a = 7.9546(11) A, c = 31.793(6) A. Ln(3)ClGa(4) represents a new structural type, also in the space group I4/mcm, with a = 8.1955(8) and 8.1123(11) A, c = 11.363(2) and 11.229(2) A, respectively, for Ln = La and Ce. Ce(10)Cl(4)Ga(5) features building blocks of Ga-centered Ce(6) trigonal prisms and distinctive two-dimensional intermetallic CuAl(2) and U(3)Si(2) type nets. Its electronic structure falls within the realm of reduced rare-earth halides. Ln(3)ClGa(4) also contains the intermetallic CuAl(2) type nets, but the interstitials are inverted: The building blocks are Cl-centered Ln(6) octahedra. Its electronic structure is characterized by strong peripheral Ln-Ga bonding stabilizing the Ln(6)Cl octahedron which normally would have its Ln-Ln antibonding orbitals filled with electrons from interstitials beyond chalcogen. Magnetic susceptibility and conductivity measurements confirm the metallic nature of all three compounds.     

Synthese,Kristallstruktur und festkörper‐NMR‐spektroskopische Untersuchung neuer Oxonitridosilicate der Mischkristallreihe Ba4−xCaxSi6N10O

The new oxonitridosilicates Ba_4?xCa_xSi₆N₁₀O have been synthesized by means of high‐temperature synthesis in a radio‐frequency furnace, starting from calcium, barium, silicon diimide and amorphous silicon dioxide. The maximum reaction temperature was at about 1450 °C. The solid solution series Ba_4?xCa_xSi₆N₁₀O with a phase width 1.81 ≤ x ≤ 2.95 was obtained. The crystal structure of Ba_1.8Ca_2.2Si₆N₁₀O was determined by X‐ray single‐crystal structure determination (P2₁3, no. 198), a = 1040.2(1) pm, Z = 4, wR2 = 0.082). It can be described as a highly condensed network of corner‐sharing SiN₄ and SiON₃ tetrahedra, the voids of which are occupied by the alkaline earth ions. The structure is isotypic with that of BaEu(Ba_0.5Eu_0.5)YbSi₆N₁₁. In the ²⁹Si solid‐state MAS‐NMR spectrum two isotropic resonances at ?50.0 and ?53.6 ppm were observed.     

High-Pressure High-Temperature Synthesis of Mixed Nitridosilicatephosphates and Luminescence of AESiP3N7: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 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.     

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‐Ca₂Si₅N₈ and Sr₂Si₅N₈. Powder X‐ray diffraction (XRD) and synchrotron single‐crystal XRD showed random vacancy distribution in Sc_0.2Ca_1.7Si₅N₈, and partial vacancy ordering in U_0.5xSr_2?0.75xSi₅N₈ with x≈1.05. The high chemical stability of U nitridosilicates makes them interesting candidates for immobilization of actinides.     

HP-Ca2Si5N8--a new high-pressure nitridosilicate: synthesis, structure, luminescence, and DFT calculations

HP-Ca(2)Si(5)N(8) was obtained by means of high-pressure high-temperature synthesis utilizing the multianvil technique (6 to 12 GPa, 900 to 1200 degrees C) starting from the ambient-pressure phase Ca(2)Si(5)N(8). HP-Ca(2)Si(5)N(8) 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)x10(6) pm(3), Z=8, R1=0.1191). The HP-Ca(2)Si(5)N(8) 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 SiN(4) tetrahedra are interconnected by further SiN(4) units. The Ca(2+) ions are situated between these layers with coordination numbers 6+1 and 7+1, respectively. HP-Ca(2)Si(5)N(8) as well as hypothetical orthorhombic o-Ca(2)Si(5)N(8) (isostructural to the ambient-pressure modifications of Sr(2)Si(5)N(8) and Ba(2)Si(5)N(8)) were studied as high-pressure phases of Ca(2)Si(5)N(8) up to 100 GPa by using density functional calculations. The transition pressure into HP-Ca(2)Si(5)N(8) was calculated to 1.7 GPa, whereas o-Ca(2)Si(5)N(8) will not be adopted as a high-pressure phase. Two different decomposition pathways of Ca(2)Si(5)N(8) (into Ca(3)N(2) and Si(3)N(4) or into CaSiN(2) and Si(3)N(4)) and their pressure dependence were examined. It was found that a pressure-induced decomposition of Ca(2)Si(5)N(8) into CaSiN(2) and Si(3)N(4) is preferred and that Ca(2)Si(5)N(8) is no longer thermodynamically stable under pressures exceeding 15 GPa. Luminescence investigations (excitation at 365 nm) of HP-Ca(2)Si(5)N(8):Eu(2+) reveal a broadband emission peaking at 627 nm (FWHM=97 nm), similar to the ambient-pressure phase Ca(2)Si(5)N(8):Eu(2+).     

Struktur,Verzwillingung und Eigenschaften von Ce4Br3C4

Structure, Twinning, and Properties of Ce₄Br₃C₄ The new compound Ce₄Br₃C₄ can be prepared from Ce metal, CeBr₃ and C (3 : 3 : 2) at 1020 °C. It crystallizes in P 1 with a = 422.7(1) pm, b = 1103.4(3) pm, c = 1126.8(2) pm, α = 77.15(3)°, β = 90.13(2)° and γ = 84.42(3)°. The crystals are characteristically twinned, the twin law being (1 0 0, ¹/₂ –1 0, 0 0 –1). The crystal structure contains puckered layers of edge sharing Ce₆C₂ octahedra. The mean C–C distance in the C₂ units is 133(5) pm. Ce₄Br₃C₄ has at room temperature a specific resistivity of 100 mΩ cm and an effective magnetic moment of 2.55(3) μ_B (Ce³⁺).