A graphite relative of composition BC2N has been subjected to high-pressure (7.7 GPa) and high-temperature (2000-2400 °C) conditions to explore the possibility for the formation of a cubic phase via direct transformation. Several cubic phases with a diamond-like structure were confirmed in the products above 2150 °C by the powder X-ray diffraction patterns. The deconvolution of the broad and partly split diffraction lines suggested that the products obtained at 2150-2300 °C consisted of cBN, "diamond" (containing minor amount of B and N), and a cubic B-C-N substance. At 2400 °C, however, the cubic products tended to segregate into two major phases assigned as cBN and "diamond". This tendency was strongly supported by the microstructural and elemental observation of the products using a highresolution scanning electron microscope and Auger electron spectroscopy. The present study concludes that not a cubic B-C-N compound but a mixture of cBN and diamond exists as the thermodynamically stable phases in the ternary system under the conditions employed.

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Diamond has been found to grow from copper, zinc, and germanium when temperatures and pressures in excess of those usually used for growth via conventional catalysts are used. Around their melting temperatures these metals are inert with respect to graphite. However, under the conditions used in this study, namely temperatures of 1600 °C and pressures of 6 GPa, they exhibit catalytic action. The conventional catalysts, which were first discovered by General Electric, act as catalysts immediately after melting in the presence of graphite, and this distinguishes them from the catalysts used in this study which should therefore be placed in a different category. A new model of diamond growth is proposed in order to explain the behavior of these new catalysts.

Fine structures appearing on the phase transition from h (hexagonal) to c (cubic) boron nitride under high pressure (7.7 GPa) and high temperature (1800–2150 °C) are examined by high-resolution transmission electron microscopy. A prominent contraction of the interplanar spacing between sp2 sheets from 3.33 to 3.10 Å in so-called ‘‘compressed h-BN’’ is attributable to a monoclinic lattice distortion of the residual h-BN, which originates from the difference in the compressibility as well as the thermal expansion between adjoining h- and c-BN grains. The parameters of the monoclinic unit cell are am=4.33, bm=2.50, cm=3.1–3.3 Å, and β=92–95°. Thin plates of h-BN are often folded and the folding also causes the monoclinic structure. The sheet sequence of r (rhombohedral)-BN locally appears when the strong volume shrinkage occurs due to the formation of a c-BN grain. Nanoscale twins appear in resulting c-BN grains, as long as they are small, and w (wurzite)-BN is sometimes included in them.