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International journal of ceramic engineering & science 2(2), 76 - 82 (2020). doi:10.1002/ces2.10036, Transparent nanoceramics, synthesized at extreme conditions of high pressure and temperature, are new classes of materials highly attractive for photonic applications, such as optical windows, which require additional increased hardness and toughness. In this study, mechanical properties of transparent polycrystalline nanoceramics consisting of triclinic Al$_2$SiO$_5$ kyanite (~91.4 vol%) and trigonal Al$_2$O$_3$ corundum (~8.6 vol%) fabricated at high pressure (10 GPa) and temperature (1200‐1400°C) were investigated. It is already known that the optical transparency of kyanite‐based nanoceramics increases with decreasing average grain size. The present study shows that the hardness of these ceramics increases with decreasing grain sizes down to ~70 nm according to the Hall‐Petch strengthening. This grain size seems to mark a transition range where an inverse Hall‐Petch effect is indicated due to signs of a moderate hardness decrease at a smaller grain size of ~35 nm. The observed hardness‐grain size relation can fairly be described by an existing composite model, which considers the crystals to be harder than the noncrystalline grain boundaries. Within the range of average grain sizes examined, the kyanite habit changes from more equant to more columnar. This behavior is associated with the observed strong crack deflection by the columnar kyanite grains with aspect (length to diameter) ratios ranging from ~2 to 10 and may positively affect the fracture toughness., Published by Wiley, Hoboken, NJ

A-site columnar-ordered double perovskites, LnMn(Ga0.5Ti0.5)2O6 (Ln = Sm, Gd), were successfully synthesized under high pressure and high temperature (6 GPa, 1375 K). From the synchrotron powder X-ray diffraction patterns, all of the diffraction peaks can be indexed by the P42/nmc space group with lattice parameters a, c ≈ 2ap (ap: primitive cubic perovskite lattice) and no ordering of the B-site cations. Rietveld analysis of the synchrotron powder X-ray diffraction patterns and Curie–Weiss fitting of their magnetizations reveal that the ionic formulae of these perovskites are Ln3+Mn2+(Ga3+0.5Ti4+0.5)2O2–6. SmMn(Ga0.5Ti0.5)2O6 shows canted-antiferromagnetic behavior, whereas GdMn(Ga0.5Ti0.5)2O6 exhibits two different magnetic states at low temperature depending on the applied magnetic field and shows an unusual magnetization curve. These magnetic behaviors originate by decreasing the antiferromagnetic interaction by substituting Ga3+(d10) for Ti4+(d0) and by decreasing the ferromagnetic interaction between columnar-ordered Ln3+ and Mn2+.

The last remaining marcasite-type RuN2 was successfully synthesized by direct chemical reaction between ruthenium and molecular nitrogen above the pressure of 32 GPa. For the first time, we found that Ru 4d is weakly hybridized with N 2p in the structure by using transmission electron microscopy equipped with electron-energy-loss spectroscopy. Our finding give important knowledge about the platinum-group pernitride with respect to the chemical bonding between platinum-group element and nitrogen.

This paper reports a procedure to combine the focused ion beam micro-sampling method with conventional Ar-milling to prepare high-quality site-specific transmission electron microscopy cross-section samples. The advantage is to enable chemical and structural evaluations of oxygen dissolved in a molten iron sample to be made after quenching and recovery from high-pressure experiments in a laser-heated diamond anvil cell. The evaluations were performed by using electron energy-loss spectroscopy and high-resolution transmission electron microscopy. The high signal to noise ratios of electron energy-loss spectroscopy core-loss spectra from the transmission electron microscopy thin foil, re-thinned down to 40 nm in thickness by conventional Argon ion milling, provided us with oxygen quantitative analyses of the quenched molten iron phase. In addition, we could obtain lattice-fringe images using high-resolution transmission electron microscopy. The electron energy-loss spectroscopy analysis of oxygen in Fe(0.94)O has been carried out with a relative accuracy of 2%, using an analytical procedure proposed for foils thinner than 80 nm. Oxygen K-edge energy-loss near-edge structure also allows us to identify the specific phase that results from quenching and its electronic structure by the technique of fingerprinting of the spectrum with reference spectra in the Fe-O system.

Marcasite-type RuN2 is synthesized by direct reaction between ruthenium metal and molecular nitrogen above a minimum pressure of 32 GPa (laser-heated diamond-anvil cell

RhN2 is prepared by high temperature/high-pressure reaction of Rh and N2 (IR laser-heated diamond anvil cell, 43.2 GPa, ≈3000 K).