High-pressure high-temperature behavior of polymer derived amorphous B-C-N

Dense diamond-like BCN compounds are of interest due to their extreme hardness and predicted exceptional thermal and chemical stability superior to diamond and c-BN. Here, we report on high-pressure high-temperature (HP-HT) behavior of amorphous BC$_{2}$N and BC$_{4}$N -- potential precursors for HP-HT synthesis of diamond-like BCN. Prepared via hydroboration reaction of piperazine borane and pyridine borane, respectively,\footnote{R. Riedel, J.Bill, and G. Passing, Adv. Mater. 3 (1991) 551.} amorphous BC$_{2}$N and BC$_{4}$N are characterized by well-mixed B-N, C-C and C-N bonds, confirmed by XPS analysis. These BCN compositions were subjected to pressures between 5-24 GPa and temperatures up to 2000$^{\circ}$C using multi anvil press, toroid press and laser-heated diamond anvil cell (LH-DAC). In- and ex-situ X-ray diffraction reveals decomposition of BC$_{4}$N to graphite and h-BN between 5 to 12 GPa above 700$^{\circ}$C, in contrast to BC$_{2}$N which remains amorphous up to 1600$^{\circ}$C. Examination of the recovered LH-DAC samples using HR-TEM, EELS and EDS, indicates a tendency of BC$_{2}$N to transform into a mixture of c-BN (micron size) and nanocrystalline diamond between 20-24 GPa and 1500-2000$^{\circ}$C.