AlB4O6N, Al0.97Cr0.03B4O6N, and Al0.83Cr0.17B4O6N are the first representatives of the recently established structure-family of oxonitridoborates containing Al3+. These compounds are isotypic to CrB4O6N and are synthesized in a multi-anvil press under high-pressure/high-temperature conditions of 7.0 GPa/1350 degrees C. Structural refinement by single-crystal X-ray diffraction shows that they crystallize in the space group P6(3)mc (no. 186) with two formula units per cell. Detailed characterization including high-temperature X-ray powder diffraction (HT-XRD), electron probe microanalysis (EPMA), measurements of second harmonic generation (SHG), hardness, photoluminescence properties, vibrational spectroscopy, and band structure calculations reveal intriguing physicochemical properties that strongly resemble the famous material ruby.

The thermodynamics in spin-ice systems are governed by emergent magnetic monopole excitations and, until now, the creation of a pair of these topological defects was associated with one specific pair-creation energy. Here, we show that the electric dipole moments inherent to the magnetic monopoles lift the degeneracy of their creation process and lead to a splitting of the pair-creation energy. We consider this finding to extend the model of magnetic relaxation in spin-ice systems and show that an electric dipole interaction in the theoretically estimated order of magnitude leads to a splitting which can explain the controversially discussed discrepancies between the measured temperature dependence of the magnetic relaxation times and previous theory. By applying our extended model to experimental data of, various spin-ice systems, we show its universal applicability and determine a dependence of the electric dipole interaction on the system parameters, which is in accordance with the theoretical model of electric dipole formation.

We report the successful synthesis of up to millimeter-size single crystals of the pyrochlore rare earth element (RE) germanates Dy2Ge2O7 and Ho2Ge2O7. Crystals were grown from oxide starting materials with the addition of 0.2 to 0.9 wt % H2O in sealed AuPd capsules at 7.7 GPa and 1200-1400 degrees C with a Belt-type high-pressure apparatus. With the use of seed crystals, regular, octahedrally-shaped crystals up to a size of 0.9 mm edge-length could be recovered.

By using a high-pressure/high-temperature process (HP/HT), it is possible to synthesise new ultra-hard cutting materials called binderless nanocrystalline cubic boron nitride (BNNC). The starting material is a pyrolytically deposited hexagonal boron nitride (pBN), which is converted at temperatures of 1400 °C – 2200 °C and pressures of 10 - 20 GPa in a direct synthesis without any binding material. The average crystallite size of this material is 50 - 100 nm and is thus significantly smaller than conventional polycrystalline cubic boron nitride (PCBN) cutting materials. Compared to conventional PCBN cutting materials, this material has an increased hardness, hot hardness and better temperature resistance. This provides an excellent alternative to extend the process limits for the machining of hardened steels and superalloys. In this conference paper, the first technological results for groove turning in hardened steels with this new promising cutting material will be presented.

We report that the lattice constant of Dy2Ge2-xSixO7 (x = 0, 0.02, 0.08, 0.125) can be systematically reduced by substituting the nonmagnetic germanium ion in the cubic pyrochlore oxide with silicon. A multianvil high-pressure synthesis was performed up to 16 GPa and 1100 degrees C to obtain polycrystalline samples in a solid-state reaction. Measurements of magnetization, ac susceptibility, and heat capacity reveal the typical signatures of a spin-ice phase. From the temperature shift of the peaks, observed in the temperature-dependent heat capacity, we deduce an increase in the strength of the exchange interaction. In conclusion, the reduced lattice constant leads to a changed ratio of the competing exchange and dipolar interaction. This puts the new spin-ice compounds closer towards the phase boundary of a short-range spin-ice arrangement and antiferromagnetic long-range order consistent with an observed reduction in the energy scale of monopole excitations.

The direct synthesis of a binderless nanocrystalline cubic boron nitride (BNNC) from hexagonal boron nitride (hBN) has been developed by using a high-pressure / high-temperature process (HP/HT) with temperatures from 1400 °C - 2200 °C and pressures from 10 GPa - 20 GPa, the resulting BNNC compacts have a high hardness of more than 50 GPa as determined by Knoop-Indentation. A phase content of up to 97 % cBN could be determined by X-ray diffraction (XRD). The average crystallite size of the material is smaller than 100 nm and therefore significantly smaller than the crystallite size of conventional polycrystalline cubic boron nitride (PcBN) cutting materials. Compared to established PcBN cutting materials, the BNNC has an increased hardness and hot hardness and improved temperature stability. Utilization BNNC-based tools with defined cutting edge could provide an excellent alternative for the enhancement of the process limits for the machining of hardened steels and superalloys. This conference contribution will present the latest developments and challenges in the HP/HT direct synthesis of binderless nanocrystalline boron nitride (BNNC) as well as the investigation of its grinding characteristics for the application as cutting insert.

The title perhydridopolysilathianes are prepared by transsilylation and condensation of HSiCl 3 ·2Py with a five-fold stoichiometric excess of gaseous Tms 2 S and Py (Schlenk vessel at reduced pressure, 55 °C, 4 d) and subsequent calcination (200 °C, vacuum, 2 h).

Polysiloxane-analogous Si–S compounds have so far not been reported. Transsilylation reactions of the solid pyridine adduct of HSiCl 3 with gaseous Me 3 Si–S–SiMe 3 yielded perhydridopolysilathianes. 29 Si NMR, FTIR, and Raman spectroscopy; elemental and thermal analysis; XRD; SEM; and gas-adsorption measurements indicated that the product consists of D-, T-, and Q-units with a composition of ([H 2 SiS] 1.2 [HSiS 1.5 ] 2.6 [SiS 2 ] 1.0 ) n . Formation of the latter is caused by pyridine-catalyzed dismutation reactions. The polymer is mesoporous with a surface area of 187 m 2  g –1 ; furthermore, it is amorphous and insoluble in organic solvents.

(Amorphous-)SiC/TiC composites for resistive tubular heaters in HP/HT experiments were obtained via a polymer-precursor process. A slurry consisting of a commercial SiC-precursor polymer (allylhydridopolycarbosilane, AHPCS) and TiC powder as conductive filler was applied to the inner walls of zirconia insulation tubes, using a centrifugation-casting method. Resistive coatings with homogeneous thickness of similar to 200 mu m were obtained. The heaters were tested in octahedral multi-anvil assemblies at similar to 10 GPa with simultaneous recording of heating voltage and current. Up to a maximum temperature of similar to 1800 degrees C they showed temperature vs. power characteristics reproducible from batch to batch, with resistance decreasing from 0.08 to 0.02 Omega during heating. Microstructural characterization using SEM/EDX was carried out on the recovered SiC/TiC composite material, as well as on pristine resistive heaters directly after coating and curing to 230 degrees C, and after additional pyrolysis at 900 degrees C in argon. In all cases, a stable composite microstructure of an interpenetrating network of TiC particles with either silicon carbide polymer precursor or an amorphous SiC phase were found. The composites were characterized by XRD and thermogravimetry. Further improvement of coating procedure and materials combination (precursor/filler/insulator substrate) may result in advanced coatings, operational well beyond 2000 degrees C.

Among the microstructure defects in hexagonal graphitic boron nitride, the basal plane corrugations are of high relevance for the sp 2 to sp 3 phase transition under high pressures (HP) and high temperatures (HT). A microstructure model is described, which is capable of quantifying the amplitude of the basal plane corrugations on the basis of the anisotropic X-ray diffraction line broadening. It is illustrated that this model correctly reproduces the specific shape of the diffraction lines from corrugated basal planes, i.e., the characteristic splitting of the 00 l peaks. The results from XRD are verified by direct observation in the transmission electron microscope with high resolution. Subsequent HP/HT experiments were performed in order to highlight the difference in the phase transition kinetics between hexagonal boron nitride samples with different amount of basal plane corrugations. The effect of these microstructure defects on the conversion rate and on the obtained synthesis product is discussed.

The high pressure spinel-type γ-Si3N4 was prepared by using shock wave synthesis from amorphous precursors at the peak shock pressures around 34GPa. Due to the high liability of the precursor to oxidation, it is nearly impossible to obtain oxygen-free high-pressure material. The overall oxygen content varies between 7 and 12wt%. However, the X-ray and neutron diffraction experiments confirmed that the spinel-like phase dominates the samples. The neutron diffraction revealed that up to 5wt% of oxygen can be accommodated in the crystal structure of γ-Si3N4, where it replaces nitrogen. Additional oxygen seemed to increase the amount of an amorphous phase in the samples. The local chemical analysis in a transmission electron microscope proved that the amorphous phase contains more oxygen than the crystalline phase, thus the excess of oxygen was verified to be contained in the amorphous phase.

An example for kinetic control of a solid-state phase transformation, in which the system evolves via the path with the lowest activation barrier rather than ending in the thermodynamically most favorable state, has been demonstrated. As a case study, the phase transitions of indium sesquioxide (In2O3) have been guided by theoretical calculations and followed in situ under high-pressure high-temperature conditions in multi-anvil assemblies. The corundum-type rh-In2O3 has been synthesized from stable bixbyite-type c-In2O3 in two steps: first generating orthorhombic Rh2O3-II-type o′-In2O3 which is thermodynamically stable at 8.5GPa/850°C and, thereafter, exploiting the preferred kinetics in the subsequent transformation to the rh-In2O3 during decompression. This synthesis strategy of rh-In2O3 was confirmed ex situ in a toroid-type high-pressure apparatus at 8GPa and 1100°C. The pressure–temperature phase diagrams have been constructed and the stability fields of In2O3 polymorphs and the crystallographic relationship between them have been discussed.

In order to clarify the difference between the deformation-induced ε-martensite (ε 1) and the pressure-induced ε-iron (ε 2), high-pressure quasi-hydrostatic experiments were performed on a low-carbon, high-alloy metastable austenitic steel. In situ synchrotron X-ray diffraction measurements as well as post-mortem investigations of the microstructure by electron backscatter diffraction were carried out to study the microstructural transformations. Three processes were observed during compression experiments: first, the formation of deformation-induced hexagonal ε 1-martensite, as well as small nuclei of deformation-induced bcc α′-martensite (α 1′) within the fcc γ-matrix due to non-hydrostaticity in the experiments; second, the onset of the phase transformation from the metastable fcc γ-austenite into the hexagonal pressure-induced ε 2-iron phase occurred at around 6 GPa; third, during decompression, the hexagonal pressure-induced ε 2-iron transformed partially into bcc α′-martensite (α 2′). Completely different characteristics with regard to habitus as well as to orientation relationships were observed between the pressure-induced phases (ε 2-iron phase and α 2′-martensite) and the deformation-induced martensites (ε 1- and α 1′-martensite).

Dense diamond-like BCN compounds are of interest due to their extreme hardness and predicted excellent thermal and chemical stability, which are superior to those of diamond and c-BN. Here, we report on the high-pressure high-temperature (HP-HT) behavior of amorphous BC2N and BC4N -as potential precursors for HP-HT synthesis of diamond-like BCN. Prepared via hydroboration reaction of piperazine borane and pyridine borane, respectively, amorphous BC2N and BC4N are characterized by well-mixed B-N, C-C and C-N bonds, confirmed by XPS analysis. These BCN compositions were subjected to pressures between 5-12 GPa and temperatures up to 1700 degrees C using multi-anvil apparatus and toroid-type press. In-and ex-situ X-ray diffraction reveals the decomposition of BC4N to graphite and h-BN between 5 and 12 GPa above 500 degrees C, in contrast to BC2N which remains amorphous up to 1600 degrees C.

The electronic properties of β-Si3N4 and β-sialons (β-Si6−z Al z O z N8−z ) solid solutions were characterized using a combination of X-ray emission spectroscopy (XES), X-ray absorption spectroscopy (XAS), and density functional theory (DFT). The electronic structure measurements reveal a single bonding environment for both the Si and Al atoms, which corresponds to a specific nonrandom structural arrangement of the Al–O solute atoms into nanotube-like clusters or channels, running parallel to the c-axis of the β-Si3N4 host structure. Compared to an arrangement of alternating Si–N and Al–O slabs (“Dupree model”), lower total energy and overall better agreement to the experimentally observed electronic features confirm this “Al–O nanotube” model for β-sialon originally proposed by Okatov to be closer to the true chemical topology of the β-Si6−z Al z O z N8−z solid solution series. The β-sialons are shown to be wide band gap semiconductors with the band gap reduction arising from the O p-states moving toward the Fermi level. This band gap reduction provides the ability for direct band gap transitions, which is very important for practical applications. In contrast to the previous observations, both measurement and theory indicate a linear dependence of band gap energy with composition z. The experimental (theoretical) electronic band gaps of β-Si6−z Al z O z N8−z with z = 0.0, 2.0, and 4.0 as determined by XAS/XES (DFT) are 7.2 ± 0.2 (5.88), 6.2 ± 0.2 (3.45), and 5.0 ± 0.2 (2.39) eV, respectively. The considerable discrepancy between experimental and theoretical values is attributable to the shortcomings of DFT, which often underestimates the electronic band gap energy.

Manganese-doped bixbyite-type c-In2O3 decomposes into corundum-type rh-In2O3 and cubic MnO at 8 GPa and 950 °C.