Zur Kenntnis von LiSi2N3: Synthese und Verfeinerung der Kristallstruktur

On LiSi₂N₃ – Synthesis and Crystal Structure Refinement Single-crystalline LiSi₂N₃ was obtained by the reaction of lithium and silicon diimide under N₂ atmosphere in a radio-frequency furnace at 1300 °C. LiSi₂N₃ is isotypic with Li₂SiO₃ and it crystallizes as an ordered variant of wurtzite. A single-crystal X-ray diffraction analysis (Cmc2₁, a = 922.15(9), b = 529.64(8), c = 477.98(5) pm, Z = 4, R1 = 0.0173, wR2 = 0.0382) confirms and improves the structural data which previously had been obtained from powder diffraction data.     

Synthesis and Crystal Structure of the Nitridosilicates Ca5[Si2N6] and Ca7[NbSi2N9]

Ca₅[Si₂N₆] and Ca₇[NbSi₂N₉] were obtained by reaction of Ca₃N₂, Ca₂N and Si₃N₄ (with addition of niobium powder in case of Ca₇[NbSi₂N₉]) in closed tantalum ampoules at temperatures at 1060 °C and 1000 °C, respectively. Ca₅[Si₂N₆] is monoclinic C2/c with a = 983.6(2) pm, b = 605.2(1) pm, c = 1275.7(3), β = 100.20(3)° and Z = 4 crystallising homotypically to Ba₅[Si₂N₆]. The crystal structure contains pairs of edgesharing SiN₄ tetrahedra forming isolated nitridosilicate anions of [Si₂N₆]^10?. Ca₇[NbSi₂N₉] is monoclinic P2₁/m with a = 605.1(1), b = 994.6(2), c = 899.7(2), β = 92.10(1)°, Z = 2 and crystallises in an hitherto unknown structure type. Ca₇[NbSi₂N₉] contains isolated anions [NbSi₂N₉]^14? which are composed of two edgesharing SiN₄ tetrahedra and an edge‐sharing NbN₅ pyramid. So far, such a pseudotrisilicate unit has not been observed in the family of silicates.     

Synthesis,Crystal and Electronic Structure of the Nitridoaluminosilicate Ca5[Si2Al2N8]

Ca₅[Si₂Al₂N₈] was synthesized from elementary aluminum and silicon with phase‐pure tricalcium dinitride at 1280 K under dry argon in a sealed niobium ampoule. Ca₃N₂ was freshly prepared from distilled calcium metal in a dry nitrogen atmosphere. The compound crystallizes in form of transparent yellow distorted octahedra. In air and under moisture Ca₅[Si₂Al₂N₈] undergoes hydrolysis. The structure was determined from a single crystal to be orthorhombic (space group Pbcn – no. 60, a = 925.5, b = 614.0 and c = 1557.8 pm). The nitridoaluminate and ‐silicate substructures are separated into planes of edge and corner‐shared aluminate tetrahedra, which are linked by edge‐sharing double tetrahedral pillars of the silicate. The structure was confirmed by electrostatic and quantum mechanical analysis.     

Ce16Si15O6N32—An Oxonitridosilicate with Silicon Octahedrally Coordinated by Nitrogen

A rarity in solid‐state chemistry is octahedrally coordinated silicon. Ce₁₆Si₁₅O₆N₃₂ is the first nitridosilicate with this structural motif. Most oxosilicates containing octahedrally coordinated silicon are high‐pressure phases. In contrast to that Ce₁₆Si₁₅O₆N₃₂ has been synthesized under ambient pressure. An SiN₆ octahedron that is surrounded by two parallel rings formed from six Si(O,N)₄ tetrahedra is depicted.     

Neue Vertreter des Er6[Si11N20]O‐Strukturtyps Hochtemperatur‐Synthesen und Kristallstrukturen von Ln(6+x/3)[Si(11–y)AlyN(20+x–y)]O(1–x+y) mit Ln = Nd,Er, Yb,Dy und 0 ≤ x ≤ 3, 0 ≤ y ≤ 3

New Representatives of the Er₆[Si₁₁N₂₀]O Structure Type. High‐Temperature Synthesis and Single‐Crystal Structure Refinement of Ln_(6+x/3)[Si_(11–y)Al_yN_(20+x–y)]O_(1–x+y) with Ln = Nd, Er, Yb, Dy and 0 ≤ x ≤ 3, 0 ≤ y ≤ 3 According to the general formula Ln_(6+x/3)[Si_(11–y)Al_yN_(20+x–y)]O_(1–x+y) (0 ≤ x ≤ 3, 0 ≤ y ≤ 3) four nitridosilicates, namely Er₆[Si₁₁N₂₀]O, Yb_6.081[Si₁₁N_20.234]O_0.757, Dy_0.33Sm₆[Si₁₁N₂₀]N, and Nd₇[Si₈Al₃N₂₀]O were synthesized in a radiofrequency furnace at temperatures between 1300 and 1650 °C. The homeotypic crystal structures of all four compounds were determined by single‐crystal X‐ray diffraction. The nitridosilicates are trigonal with the following lattice constants: Er₆[Si₁₁N₂₀]O: a = 978.8(4) pm, c = 1058.8(3) pm; Yb_6.081[Si₁₁N_20.243]O_0.757: a = 974.9(1) pm, c = 1055.7(2) pm; Dy_0.33Sm₆[Si₁₁N₂₀]N: a = 989.8(1) pm, c = 1078.7(1) pm; Nd₇[Si₈Al₃N₂₀]O: a = 1004.25(9) pm, c = 1095.03(12) pm. The crystal structures were solved and refined in the space group P31c with Z = 2. The compounds contain three‐dimensional networks built up by corner sharing SiN₄ and AlN₄ tetrahedra, respectively. The Ln³⁺ and the “isolated” O^2– ions are situated in the voids of the structures. According to Ln_(6+x/3)[Si_(11–y)Al_yN_(20+x–y)]O_(1–x+y) an extension of the Er₆[Si₁₁N₂₀]O structure type has been found.     

Synthese und Kristallstruktur von BaEu(Ba0,5Eu0,5)YbSi6N11

Synthesis and Crystal Structure of BaEu(Ba_0.5Eu_0.5)YbSi₆N₁₁ Pure BaEu(Ba_0.5Eu_0.5)YbSi₆N₁₁ was obtained by stoichiometric reaction of silicon diimide with metallic barium, europium, and ytterbium. The reaction was carried out under nitrogen atmosphere in a high-frequency furnace at 1650 °C. BaEu(Ba_0.5Eu_0.5)YbSi₆N₁₁ is cubic (space group P2₁3, a = 1043.64(5) pm, Z = 4, R1 = 0.0406, wR2 = 0.0988). The compound contains alkaline earth and rare earth ions in a three-dimensional network structure of corner-sharing SiN₄ tetrahedra.     

Preparation via the polymer route and photoluminescence properties of CeSi3N5

CeSi₃N₅ has been prepared via the polymer route, a one-pot and liquid-phase synthesis, based on a reaction between SiCl₄ and liquid NH₃ in which Ce[N(Si(CH₃)₃)₂]₃ was suspended. The sample was characterized by powder X-ray diffraction analysis, diffuse reflection and photoluminescence spectroscopies. CeSi₃N₅ exhibits a strong absorption peaking at 365 nm. Furthermore, it has a broad emission band at 468 nm due to 4f⁰5d¹ → 4f¹ transition of Ce³⁺. Obviously, CeSi₃N₅ appears to be a good candidate for use as a blue phosphor, which can be pumped by near-UV-LEDs.     

Li4Ca3Si2N6 and Li4Sr3Si2N6 – Quaternary Lithium Nitridosilicates with Isolated [Si2N6]10– Ions

The isotypic nitridosilicates Li₄Ca₃Si₂N₆ and Li₄Sr₃Si₂N₆ were synthesized by reaction of strontium or calcium with Si(NH)₂ and additional excess of Li₃N in weld shut tantalum ampoules. The crystal structure, which has been solved by single‐crystal X‐ray diffraction (Li₄Sr₃Si₂N₆: C2/m, Z = 2, a = 6.1268(12), b = 9.6866(19), c = 6.2200(12) Å, β = 90.24(3)°, wR₂ = 0.0903) is made up from isolated [Si₂N₆]^10– ions and is isotypic to Li₄Sr₃Ge₂N₆. The bonding angels and distances within the edge‐sharing [Si₂N₆]^10– double‐tetrahedra are strongly dependent on the lewis acidity of the counterions. This finding is discussed in relation to the compounds Ca₅Si₂N₆ and Ba₅Si₂N₆, which also exhibit isolated [Si₂N₆]^10– ions.     

Analytical evidence of amorphous microdomains within nitridosilicate and nitridoaluminosilicate single crystals

Single crystals of new nitridosilicates and nitridoaluminosilicates with excellent R values in X-ray investigations were analysed quantitatively using 30 to 60 μm single-spot LA-ICP-MS. Significant discrepancies between expected and measured chemical composition could not be explained by the crystallographic data. High spatial resolution analysis using electron probe microanalysis (EPMA, 10 μm) leads to the discovery of inhomogeneities in the crystalline material. The application of standard single-spot LA-ICP-MS with a spatial resolution of 30 to 60 μm is not suitable for the analysis of these crystals as the existing inhomogeneities dominate and alter the determined concentrations. However, owing to the better detection capabilities, a scanning LA-ICP-MS procedure enables a more representative analysis of single crystals of Ca₅Si₂Al₂N₈ than single-spot LA-ICP-MS as a result of a larger sampling volume. It is highly likely that these impurities consist of amorphous, vitreous phases as powder diffraction X-ray data indicates the existence of a significant fraction of an X-ray amorphous material besides crystalline silicates. These microdomains contain less aluminium, silicon and calcium or are nearly free of aluminium, which explains the detected discrepancies in the chemical composition.