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Two boron-rich chalcogenides B12S and B12Se isostructural to {\alpha}-rhombohedral boron were synthesized by chemical reaction of the elements at high-pressure - high-temperature conditions. The crystal structures and stoichiometries of both compounds were confirmed by Rietveld refinement and elemental analysis. The experimental Raman spectra of B12S and B12Se were investigated for the first time. All observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed.

The effect of doping of cubic boron nitride with beryllium, silicon, sulfur and magnesium on the lattice parameters, electrical conductivity and EPR spectra has been studied. It is established that the degree of doping increases significantly in the case of crystallization of cubic boron nitride from BN solutions in supercritical ammonia at 3.9-4.2 GPa and 1100{\deg}C in comparison with the conventional synthesis from melts of the Mg-B-N system at 4.2 GPa and 1400{\deg}C. Doping with silicon and beryllium results in semiconductor properties of cubic boron nitride.

Melting of orthorhombic boron silicide B6Si has been studied at pressures up to 8 GPa using in situ electrical resistivity measurements and quenching. It has been found that in the 2.6-7.7 GPa range B6Si melts congruently, and the melting curve exhibits negative slope of -31(2) K/GPa that points to a higher density of the melt as compared to the solid phase. At very high temperatures B6Si melt appears to be unstable and undergoes disproportionation into silicon and boron-rich silicides. The onset temperature of disproportionation strongly depends on pressure, and the corresponding low-temperature boundary exhibits negative slope of -92(3) K/GPa which is indicative of significant volume decrease in the course of B6Si melt decomposition.

New boron-rich sulfide B6S and selenide B6Se have been discovered from high pressure - high temperature synthesis combined with ab initio evolutionary crystal structure prediction, and studied by synchrotron X-ray diffraction and Raman spectroscopy at ambient conditions. As it follows from Rietveld refinement of powder X-ray diffraction data, both chalcogenides have orthorhombic symmetry and belongs to Pmna space group. All experimentally observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed. Prediction of mechanical properties (hardness and elastic moduli) of new boron-rich chalcogenides have been made using ab initio routines, and both compounds were found to be members of a family of hard phases (Hv ~ 31 GPa).

Microstructure of boron subphosphide B12P2 produced by self-propagated high-temperature synthesis has been studied by high-resolution transmission electron microscopy. Two systems of twins have been found i.e. conventional twins on the (0003)h plane and nanotwins resulting from duplication of the rhombohedral unit cell of B12P2 along one of the basic vectors.

Melting of rhombohedral boron subnitride B13N2 has been studied in situ at pressures to 8 GPa using synchrotron X-ray diffraction and electrical resistivity measurements. It has been found that above 2.6 GPa B13N2 melts incongruently, and the melting curve exhibits positive slope of 31(3) K/GPa that points to a lower density of the melt as compared to the solid phase.

The electrical conductivity of melts of boron and its carbide (B4C), nitride (BN), and phosphide (BP) has been studied at pressures to 7.7 GPa and temperatures to 3500 K. It has been shown that these melts are good conductors with specific electrical conductivity values comparable with that of iron melt at ambient pressure.

Melting of boron subphosphide (B12P2) to 26 GPa has been studied by in situ synchrotron X-ray powder diffraction in a laser-heated diamond anvil cell, and by quenching and electrical resistance measurements in a toroid-type high-pressure apparatus. B12P2 melts congruently, and the melting curve has a positive slope of 23(6) K/GPa. No solid-state phase transition was observed up to the melting in the whole pressure range under study.

Melting of cubic boron phosphide, BP has been studied at pressures to 9 GPa using synchrotron X-ray diffraction and electrical resistivity measurements. It has been found that above 2.6 GPa BP melts congruently, and the melting curve exhibits negative slope -60(7) K/GPa, which is indicative of a higher density of the melt as compared to the solid phase., Comment: 3 pages, 1 figure

Two new methods to produce nanopowders of B12P2 boron subphosphide by self-propagating high-temperature synthesis have been proposed. Bulk polycrystalline B12P2 with microhardness of HV = 35(3) GPa and stability in air up to 1300 K has been prepared by sintering these powders at 5.2 GPa and 2500 K., Comment: 6 pages, 4 figures

A new method of producing boron phosphide (BP) submicron powders by self-propagating high-temperature reaction between boron phosphate and magnesium in the presence of an inert diluent (sodium chloride) has been proposed. Bulk polycrystalline BP with microhardness of HV = 28(2) GPa has been prepared by sintering the above powders at 7.7 GPa and 2600 K., Comment: 5 pages, 3 figures

The melting of silicon carbide has been studied at pressures 5-8 GPa and temperatures up to 3300 K. It has been found that SiC melts congruently, and its melting curve has negative slope of -44 +/- 4 K/GPa., Comment: 5 pages, 2 figures

The pressure dependence of melting temperatures for boron carbide and B4C-carbon eutectic has been studied up to 8 GPa, and it was found that in both cases the melting curves exhibit negative slope (-13\pm6 K/GPa), that is indicative of higher density of the melt as compared to the solid phase., Comment: 5 pages, 2 figures

In the present work we have proposed the method that allows one to easily estimate hardness and bulk modulus of known or hypothetical solid phases from the data on Gibbs energy of atomization of the elements and corresponding covalent radii. It has been shown that hardness and bulk moduli of compounds strongly correlate with their thermodynamic and structural properties. The proposed method may be used for a large number of compounds with various types of chemical bonding and structures; moreover, the temperature dependence of hardness may be calculated, that has been performed for diamond and cubic boron nitride. The correctness of this approach has been shown for the recently synthesized superhard diamond-like BC5. It has been predicted that the hypothetical forms of B2O3, diamond-like boron, BCx and COx, which could be synthesized at high pressures and temperatures, should have extreme hardness.