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Single-phase BC4N can be formed at high pressure and temperature, starting with either a mixture of diamond and cubic boron nitride or graphite and hexagonal boron nitride. The samples exhibit extreme Vickers hardness (85±4GPa), reaching the hardness of single crystal diamond. Within a random solid solution model, the X-ray diffraction patterns of cubic BC x N solid solutions were calculated. The results indicate that the prepared B–C–N materials are continuous solid solutions with a zinc-blende structure. [Copyright &y& Elsevier]

Although it has been nearly 70 years since iron-based catalysts were used to synthesize diamond from graphite at high pressure and high temperature (HPHT), the graphite to diamond transformation mechanism in this process is still a mystery. It is found that the formation of a eutectic melt between carbon and an iron-based catalyst plays a critical role in the graphite to diamond transformation at HPHT. In this work, the eutectic temperature of Fe–Ni–C (diamond and graphite) was determined using the in situ high-pressure temperature measurement (HPTM) and the high-pressure differential thermal analysis (HPDTA) method up to 8 GPa in a large-volume cubic press. The experimental results of XRD, Raman, SEM and XPS indicate that the transformation of graphite to diamond occurs in the Fe–Ni–C eutectic melt: the C–C bonds in graphite are broken in the eutectic melt, and the free carbon atoms in the melt form sp3 bond states in the thermodynamically stable region of diamond. In this process, the role of iron-based catalysts is to reduce the environmental conditions, in which the C–C bonds in graphite are destroyed, and to make the carbon atoms that are separated from the sp2 bond state form the sp3 bond state in the diamond P–T phase stable region. [ABSTRACT FROM AUTHOR]

Pure diamond powder and mixed powder of diamond and sodium chloride were treated at high pressures (5 GPa and 10 GPa) and high temperatures (1000–2700 °C), and then the graphitization behavior of diamond powder under non-hydrostatic (pure diamond powder) and quasi-hydrostatic (mixed powder of diamond and sodium chloride) compression was quantitatively studied by analyzing the X-ray diffraction peaks' area of treated samples. It is found that the graphitization under non-hydrostatic compression is much more easily triggered by heating due to the heterogeneous stress distribution within the diamond grains. Under the quasi-hydrostatic compression of 5 GPa, the starting graphitization temperature is about 1600 °C, which is 300 °C higher than that under non-hydrostatic compression. Under a pressure of 10 GPa, graphitization could be observed after pure diamond powder was treated at 1400 °C, but no graphitization occurred in the mixed powder even after a high temperature treatment at 2300 °C. Optimizing the stress distribution among diamond grains by the mixing of grain-size can significantly reduce the graphitization at high temperature even under non-hydrostatic compression. [Display omitted] • Graphitization of diamond was quantitatively studied by x-ray diffraction peak areas. • The graphitization behavior of diamond is affected by stress state. • Higher temperature of graphitization is under quasi-hydrostatic compression of 5 GPa. • No graphitization occurs under quasi-hydrostatic compression of 10 GPa at 2300 °C. • The graphitization degree is greatly weakened by the mixing of grain-size. [ABSTRACT FROM AUTHOR]