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.