Metastable processes, diamond synthesis

We have developed solvothermal synthesis as an important method in research of metastable structures. In the benzene-thermal synthesis of nanocrystalline GaN at 280°C through the metathesis reaction of GaClj and U3N, the ultrahigh pressure rocksalt type GaN metastable phase, which was previously prepared at 37 GPa, was obtained at ambient condition [5]. Diamond crystallites were prepared from catalytic reduction of CCI4 by metallic sodium in an autoclave at 700°C (Fig.l) [6]. In our recent studies, diamond was also prepared via the solvothermal process. In the solvothermal catalytic metathesis reaction of carbides of transition metals and CX4 (X = F, Cl, Br) at 600-700°C, Raman spectrum of the prepared sample shows a sharp peak at 1330 cm" (Fig. 1), indicating existence of diamond. In another process, multiwalled carbon nanotubes were synthesized at 350°C by the solvothermal catalytic reaction of CgCle with metallic potassium (Fig. 2) [7]. 

Diamond is metastable under normal conditions and only becomes the more stable form of carbon at pressures above 16 kbar. The synthesis of diamonds from graphite therefore requires high pressures and, to increase the rate of reaction, high temperatures. The processes used are either diffusion-controlled (so-called catalytic process) or diffusion-less. 

Asmussin, J., and D. K. Reinhard. 2002. Diamond Films Handbook. New York Marcel Dekker. Detailed information on the science, processes, and application of low-pressure diamond deposition, which has been developed over the past two decades to synthesize a form of diamond that is metastable with respect to graphite, as distinguished from the high-pressure synthesis of diamond that is the stable form of carbon. Available online on CRCnetBASE. 

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