Transparent polycrystalline nanoceramics consisting of triclinic Al2SiO5 kyanite (91.4 vol%) and Al2O3 corundum (8.6 vol%) were fabricated at 10 GPa and 1200‐1400°C. These materials were obtained by direct conversion from Al2O3‐SiO2 glasses fabricated using the aerodynamic levitation technique. The material obtained at 10 GPa and 1200°C shows the highest optical transparency with a real in‐line transmission value of 78% at a wavelength of 645 nm and a sample‐thickness of 0.8 mm. This sample shows equigranular texture with an average grain size of 34 ± 13 nm. The optical transparency increases with decreasing mean grain size of the constituent phases. The relationship between real in‐line transmission and grain size is well explained by a grain‐boundary scattering model based on a classical theory.

The phase stability of γ‐P3N5 and the possible formation of new phosphorus nitrides were investigated via high‐pressure in situ Raman spectroscopy, X‐ray diffraction measurements, and first‐principles calculations up to approximately 80 GPa. In this study, γ‐P3N5 was synthesized via the direct nitridation of black phosphorus at a pressure approximately above 12 GPa. The Raman spectrum, bulk modulus (K0 = 130.27(43) GPa), and compression behaviors (order of axial compressibility: βc > βa > βb) were experimentally measured for the first time. These experimental results were in good agreement with those of first‐principles calculations. Our high‐pressure in situ measurements and first‐principles calculations revealed that γ‐P3N5 persisted up to 80 GPa at room temperature. The compression of γ‐P3N5 proceeded with the folding of the layer consisting of the corner‐ and edge‐sharing PN4 and PN5. The present findings indicate that the P–N bonding with a low coordination number (PN4 and PN5) is preferable and stabilized for phosphorus nitride over a wide pressure range. However, laser heating between 67 and 70 GPa in the presence of nitrogen resulted in the formation of new PxNy, which included the possibility of a new high‐pressure P3N5 phase. The Raman scattering measurements along with the decompression demonstrated that the local structure of the newly synthesized PxNy metastably persisted at atmospheric pressure. The present experimental and theoretical studies on phosphorus nitrides offer new insights into the high‐pressure behaviors of covalent compounds consisting of highly coordinated polyhedra.

We report the synthesis of alumina/stishovite nano‐nano composite ceramics through a pressure‐induced dissociation in Al2SiO5 at a pressure of 15.6 GPa and temperatures of 1300°C‐1900°C. Stishovite is a high‐pressure polymorph of silica and the hardest known oxide at ambient conditions. The grain size of the composites increases with synthesis temperature from ~15 to ~750 nm. The composite is harder than alumina and the hardness increases with reducing grain size down to ~80 nm following a Hall–Petch relation. The maximum hardness with grain size of 81 nm is 23 ± 1 GPa. A softening with reducing grain size was observed below this grain size down to ~15 nm, which is known as inverse Hall–Petch behavior. The grain size dependence of the hardness might be explained by a composite model with a softer grain‐boundary phase.

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 Al2SiO5 kyanite (~91.4 vol%) and trigonal Al2O3 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.