Gerhard Miehe

Synthesis of cubic silicon nitride

Silicon nitride (Si3N4) is used in a variety of important technological applications. The high fracture toughness, hardness and wear resistance of Si3N4-based ceramics are exploited in cutting tools and anti-friction bearings; in electronic applications, Si3N4 is used as an insulating, masking and passivating material. Two polymorphs of silicon nitride are known, both of hexagonal structure: α- and β-Si3N4. Here we report the synthesis of a third polymorph of silicon nitride, which has a cubic spinel structure. This new phase, c-Si3N4, is formed at pressures above 15 GPa and temperatures exceeding 2,000 K, yet persists metastably in air at ambient pressure to at least 700 K. First-principles calculations of the properties of this phase suggest that the hardness of c-Si3N4 should be comparable to that of the hardest known oxide (stishovite, a high-pressure phase of SiO2), and significantly greater than the hardness of the two hexagonal polymorphs.

Synthesis of a cubic Ge3N4 phase at high pressures and temperatures

The two known phases of germanium nitride (Ge3N4) have hexagonal and trigonal symmetries and consist of three-dimensional networks of corner-connected Ge–N tetrahedra. A new cubic spinel phase (space-group Fd3m, a0=8.3 A, Z=8, ρ=6.36 g/cm3) containing Ge–N octahedra and tetrahedra in a 2:1 ratio was synthesized from elemental germanium and molecular nitrogen starting materials in a laser-heated diamond-anvil cell above 14 GPa. This phase is isostructural to the recently discovered cubic spinel phase of Si3N4.

Material Properties and Structural Characterization of M3Si6O12N2:Eu2+ (M=Ba, Sr)—A Comprehensive Study on a Promising Green Phosphor for pc‐LEDs

The efficient green phosphor Ba(3)Si(6)O(12)N(2):Eu(2+) and its solid-solution series Ba(3-x)Sr(x)Si(6)O(12)N(2) (with x approximately = 0.4 and 1) were synthesized in a radio-frequency furnace under nitrogen atmosphere at temperatures up to 1425 degrees C. The crystal structure (Ba(3)Si(6)O(12)N(2), space group P3 (no. 147), a = 7.5218(1), c = 6.4684(1) A, wR2 = 0.048, Z = 1) has been solved and refined on the basis of both single-crystal and powder X-ray diffraction data. Ba(3)Si(6)O(12)N(2):Eu(2+) is a layer-like oxonitridosilicate and consists of vertex-sharing SiO(3)N-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 Ba(2+) sites are situated between the silicate layers. Luminescence investigations have shown that Ba(3)Si(6)O(12)N(2):Eu(2+) exhibits excellent luminescence properties (emission maximum at approximately 527 nm, full width at half maximum (FWHM) of approximately 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 Ba(3)Si(6)O(12)N(2) under these conditions. The band gap of Ba(3)Si(6)O(12)N(2):Eu(2+) 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.

Spinel‐Si3N4: Multi‐Anvil Press Synthesis and Structural Refinement

The third known polymorph of silicon nitride, which is cubic and was only recently discovered, has been prepared from two further, different precursors—Si2N2(NH) and a-Si3N4—in a high-pressure, high-temperature synthesis using multi-anvil presses. The synthesis and characterization of the products is described, which included a structural determination by Rietveld refinement of powder X-ray diffraction data. Spinel-type c-Si3N4 is significantly harder than the α and β phases and may possibly find applications as an ultrahard material.

NANOTUBES FORMED BY DETONATION OF C/N PRECURSORS

Individual carbon nanotubes, both unfilled and containing metal nanocrystals in a string-of-beads-like arrangement, as shown in the Figure, can be obtained reproducibly by detonative decomposition of a hydrogen-free C/N precursor, it is reported here. This method may challenge traditional techniques for the synthesis of carbon nanotubes, which are energy and hardware intensive.

The crystal structure of BaCuO2

Abstract The crystal structure of BaCuO2 has been redetermined by single crystal X-ray study on a crystal obtained as a parasitic phase during preparation of YBa2Cu3O7. The formula is Ba0.92Cu1.06O2.28 with a = 18.347(2) A in the cubic space group Im3m. Reflections numbering 10,288 were collected and the structure was refined to Rw = 0.031 on the basis of 496 unique reflections. The structure is composed of an ordered and a disordered part of copper-oxygen units. The results for the ordered part confirm earlier structure determinations. The disorder is discussed in detail.

Different zirconia-alumina nanopowders by modifications of chemical vapor synthesis

Abstract Ultra-fine, nanocrystalline zirconia/alumina (n-ZrO 2 /Al 2 O 3 ) powders are synthesized by chemical vapor synthesis (CVS) using a variable, modular gas flow reactor. Results for three different n-ZrO 2 /Al 2 O 3 powders are presented: single phase particles formed by doping zirconia with alumina, coated particles, and composite powder consisting of mixed particles of n-Al 2 O 3 and n-ZrO 2 .

Localisation of excess oxygen in the high-Tc 2223-phase Bi1.9Pb0.3Sr2.0Ca1.9Cu3.0O10+δ by neutron powder diffraction

Abstract Neutron and X-ray powder diffractograms of the title compound were refined by the Rietveld method. Within the bismuth layer an additional oxygen site with population parameter 0.12 was found which is interpreted as the excess oxygen. From the comparison of population parameters from neutron and X-ray data we conclude that Pb at least partially goes to the Ca position. The incommensurate modulation of the structure is reflected by the anisotropy of temperature factors.

Surfactant-free self-assembly route to hollow In2O3 microspheres

The self-assembly of In(2)O(3) nanoparticles into hollow microspheres is realized using in situ generation of gas microbubbles due to the melting and decomposition of ammonium nitrate in precipitates synthesized via hydrolytic routes.

Thermal treatment of YBa2Cu3−xAlxO6+δ single crystals in different atmospheres and neutron-diffraction study of excess oxygen pinned by the Al substituents

YBa2Cu3−xAlxO6+δ single crystals with different Al contents (x = 0.06, 0.14 and 0.19) were heat treated in different atmospheres. The oxidized as-grown crystals show a macroscopically tetragonal tweed-structure. The oxygen content δ in the Cu(1) layer of these oxidized samples is significantly lower than 1, which results from the Al coordination disturbing the ordering in the chain-oxygen sublattice. Neutron-diffraction data show that upon reduction at 1044 K the oxygen content δ within the Cu(1) layer remains at 0.25 due to pinning of excess oxygen by the aluminum at x = 0.19. Crystals annealed in reducing atmospheres at temperatures above 973 K develop orthorhombic long-range order after reoxidation at 673 K, probably due to short-range clustering of the AlOn defects. The obtained orthorhombicity of the reoxidized material decreases with decreasing temperature of the reduction treatment. No change in the structural state is obtained when the reducing treatment is performed below 950K; the reoxidized samples remain macroscopically tetragonal. Annealing the samples at 1073 K in oxygen restores the tweed structure. An evolution of the tweed structure with annealing time towards orthorhombic long-range order was not observed.