Formation of diamond in the system of Ag2CO3 and graphite at high pressureand high temperatures

Diamonds were reproductively formed from graphite in the presence of Ag2CO3 and Ag2O at 7.7 GPa and 1500-2000 degrees C for 0.5-27 h. The complete con version from graphite to diamond was confirmed in the samples obtained at t emperatures above 1800 degrees C for 0.5-2 h, but no diamond was detected i n the system of Ag and graphite under these conditions. To make clear the m echanism of diamond formation from graphite-Ag2CO3 system, decomposition te mperatures of Ag2CO3 and Ag2O were investigated at 7.7 GPa and it was found that they were 1000 and 1200 degrees C, respectively. Then, the decomposit ion products of Ag, CO2 and O-2 were present instead of the carbonate in th e diamond-forming condition. Since O-2 should react with graphite to form C O, in the HP-HT condition, our experimental results strongly suggest that f luid CO, has a strong catalytic action for the formation of diamond from gr aphite. (C) 2000 Elsevier Science B.V. All rights reserved.