High-Pressure, High-Temperature Phase Equilibria with Superhard Boron-Rich Compounds of B–C–N–O and B–C–Si Systems by In Situ X-ray Diffraction and CALPHAD Methodology

Boron-rich compounds within the B–C–N–O and B–C–Si systems exhibit exceptional functional properties, making them highly attractive for industrial applications such as those requiring superhardness, nuclear technologies, or thermoelectricity. High-pressure, high-temperature (HPHT) conditions allow obtaining the ingots of ceramics with best physical properties, as well as to explore advanced materials by means of in situ crystallography, high-pressure chemistry, and nanoscience. This review summarizes recent experimental and theoretical advances on high-pressure, high-temperature (HPHT) phase equilibria up to 20 GPa and 3000 K, focusing on boron carbide (B₄C), boron suboxide (B₆O), boron subnitride (B₁₃N₂), boron silicides, and some of their solid solutions. Emphasis is placed on in situ X-ray diffraction (XRD), density functional theory (DFT) calculations, and CALPHAD thermodynamic modeling. Despite recent progress, significant methodological challenges remain, requiring enhanced experimental accuracy and refined theoretical approaches. Future work should address these gaps to fully leverage the potential of these superhard materials.