A brief overview on high-pressure synthesis of superhard and ultrahard materials is presented in this tutorial paper. Modern high-pressure chemistry represents a vast exciting area of research which can lead to new industrially important materials with exceptional mechanical properties. This field is only just beginning to realize its huge potential, and the image of "terra incognita" is not misused. We focus on three facets of this expanding research field by detailing: (i) the most promising chemical systems to explore (i.e. "where to search"); (ii) the various methodological strategies for exploring these systems (i.e. "how to explore"); (iii) the technological and conceptual tools to study the latter (i.e. "the research tools"). These three aspects that are crucial in this research are illustrated by examples of the recent results on high pressure -high temperature synthesis of novel super-and ultrahard phases (orthorhombic γ-B 28 , diamond-like BC 5 , rhombohedral B 13 N 2 and cubic ternary B-C-N phases). Finally, some perspectives of this research area are briefly reviewed. superhard-and ultrahard materials is crucial 1 ), has a dimensionality of pressure, and defined as a hardness named after the type of diamond pyramid used for indentation (so-called Vickers, Knoop or Berkovich hardness, typically used for superhard materials).Superhard and ultrahard materials can be defined as having Vickers microhardness (H V ) exceeding 40 GPa and 80 GPa respectively 2, 3 . In addition to high hardness, they usually possess other unique properties such as compressional strength, shear resistance, large bulk moduli, high melting temperatures and chemical inertness. This combination of properties makes these materials highly desirable for a number of industrial applications. Historically, the first high-pressure experiments designed to produce materials for industrial use were carried out during the second half of the 20th century with the laboratory synthesis of superhard materials, namely, diamond 4, 5 and cubic boron nitride (c-BN or zb-BN to denote its zinc-blende structure) 6, 7 . Nowadays, the chemical industries linked to these materials are flourishing all over the world with an annual production of 3 000 million carats (1 carat = 0.2 g). Industrial applications of bulk superhard materials to date have been dominated by superabrasives, such as stone and concrete sawing, cutting and grinding tools, polishing tools, petroleum exploration mining, high speed machining of various engineering materials, etc. Recent achievements in search for novel superhard materials indicate that synthesis of phases -other than carbon allotropes, which are of primary interest to this manual -with hardness exceeding that of various forms of diamond (Knoop hardness 56-115 GPa for different hkl index planes of natural single-crystals 8 and 120-145 GPa for nanocrystalline diamond 9 ) is very unlikely 10 . At the same time, the hardness and mechanical properties of diamond-based materials themselves can still be improved by microstructure ...

Orientation-dependent aloof-beam vibrational electron-energy-loss spectroscopy is carried out on uniaxial icosahedral B12P2 submicron crystals. We demonstrate that the high sensitivity of the signal to the crystal orientation allows for an unambiguous determination of the symmetry of normal-modes occurring at the Brillouin zone center of this anisotropic compound. The experimental results are assessed using first-principles quantum mechanical calculations (density functional theory) of the dielectric response of the specimen. The high spatial resolution inherent to this technique when implemented in the transmission electron microscope thus opens the door to nanoscale orientationdependent vibrational spectroscopy.

The p-V-T equation of state of magnesium metal has been measured up to 20 GPa and 1500 K using both multianvil and opposite anvil techniques combined with synchrotron X-ray diffraction. To fit the experimental data, the model of Anderson-Grüneisen has been used with fixed parameter δ T . The 300-K bulk modulus of B 0 = 32.5(1) GPa and its first pressure derivative, B 0 ' = 3.73(2), have been obtained by fitting available data up to 20 GPa to Murnaghan equation of state. Thermal expansion at ambient pressure has been described using second order polynomial with coefficients a = 25(2)×10 -6 K -1 and b = 9.4(4)×10 -9 K -2 . The parameter describing simultaneous pressure and temperature impact on thermal expansion coefficient (and, therefore, volume) is δ T = 1.5(5). The good agreement between fitted and experimental isobars has been achieved to relative volumes of 0.75. The Mg melting observed by X-ray diffraction and in situ electrical resistivity measurements confirms previous results and additionally confirms the p-T estimations in the vicinity of melting.

The inorganic chemistry of the Na-Si system at high pressure is fascinating, with a large number of interesting compounds accessible in the industrial pressure scale, below 10 GPa. Especially, Na4Si4 is stable in this whole pressure range, and thus plays an important role for understanding the thermodynamics and kinetics underlying materials synthesis at high pressures and high temperatures. In the present work, the melting curve of the Zintl compound Na4Si4 made of Na + and Si4 4tetrahedral cluster ions is studied at high pressures up to 5 GPa, by using in situ electrical measurements. During melting, the insulating Na4Si4 solid transforms into an ionic conductive liquid that can be probed through the conductance of the whole high-pressure cell, i.e. the system constituted of the sample, the heater and the high-pressure assembly. Na4Si4 melts congruently in the studied pressure range and its melting point increases with pressure with a positive slope dTm/dp of 20(4) K/GPa.

High-pressure synthesis (which refers to pressure synthesis in the range of 1 to several GPa) adds a promising additional dimension for exploration of compounds that are inaccessible to traditional chemical methods and can lead to new industrially outstanding materials. It is nowadays a vast exciting field of industrial and academic research opening up new frontiers. In this context, an emerging and important methodology for the rapid exploration of composition-pressure-temperature-time space is the in situ method by synchrotron X-ray diffraction. This review introduces the latest advances of high-pressure devices that are adapted to X-ray diffraction in synchrotrons. It focuses particularly on the “large volume” presses (able to compress the volume above several mm3 to pressure higher than several GPa) designed for in situ exploration and that are suitable for discovering and scaling the stable or metastable compounds under “traditional” industrial pressure range (3–8 GPa). We illustrated the power of such methodology by (i) two classical examples of “reference” superhard high-pressure materials, diamond and cubic boron nitride c-BN; and (ii) recent successful in situ high-pressure syntheses of light-element compounds that allowed expanding the domain of possible application high-pressure materials toward solar optoelectronic and infra-red photonics. Finally, in the last section, we summarize some perspectives regarding the current challenges and future directions in which the field of in situ high-pressure synthesis in industrial pressure scale may have great breakthroughs in the next years.

Ice-templating, also known as directional freezing or freeze-casting, features the tunability of microstructure, the wide applicability of functional nanomaterials, and the fabrication of multiscale well-controlled biomimetic materials. Recently, integrating ice-templating with other materials' processing technologies (such as, spraying, spinning, filtration, and hydrothermal), it has been investigated to tailor pore morphology of scaffolds for emerging applications. Such integration endows materials with various structures (cellular, dendritic, and lamellar) and dimensions (0D, 1D, 2D, and 3D), which opens up a new avenue for improving material properties and developing new materials. Herein, this review probes into the relationship of integrative ice frozen assembly with structure and describes the fundamental principles and synthesis strategies for preparing multi-scale materials with complex biomimetic structures via ice-templating. Focusing on ice crystal nucleation and growth, it summarizes the performance of ice-templating in constructing pore geometries. Additionally, the review analyzes in depth the correlation between microstructure and macromorphology of final scaffolds, highlighting the application of integrative ice frozen assembly in electrochemical energy storage and conversion, and prospects for future research directions for this field.

Low-temperature heat capacities (Cp) of nanostructured rock salt (rs-ZnO) and wurtzite (w-ZnO) polymorphs of zinc oxide were measured in the 2–315 K temperature range. No significant influence of nanostructuring on Cp of w-ZnO has been observed. The measured Cp of rock salt ZnO is lower than that of wurtzite ZnO below 100 K and is higher above this temperature. Using available thermodynamic data, we established that the equilibrium pressure between nanocrystalline w-ZnO and rs-ZnO is close to 4.6 GPa at 300 K (half as much as the onset pressure of direct phase transformation) and slightly changes with temperature up to 1000 K.

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