Fluid CH4 and H2 trapped around metallic inclusions in HPHT synthetic diamond

Abstract

Diamonds grown by the high-pressure, high-temperature (HPHT) method commonly contain solid inclusions from the metallic solvent-catalyst they are grown in. Such inclusions give rise to their familiar magnetism. Upon recent analysis of 4 commercial HPHT synthetic diamonds from different sources, several of these solid inclusions were found to have a thin, fluid film or jacket of methane (CH4) and hydrogen (H2) trapped at the interface between the solid inclusion and surrounding diamond. Raman spectroscopy reveals peaks near 2918cm−1 and 4156cm−1 that correspond to the main vibrations of CH4 and H2, respectively. In some cases the peak position is shifted down by a few wavenumbers due to elevated pressure within the inclusions.The CH4 and H2 probably formed as the inclusion cooled, when dissolved hydrogen atoms were no longer soluble in the solidifying inclusion phases and were forced to exsolve as H2, as well as CH4 by combining with available carbon. Inclusions can provide an indirect means of studying the effective growth environment. These findings show that hydrogen is commonly present at the site of diamond growth in HPHT synthesis, even if it is not intentionally added among the starting materials. Hydrogen is known to influence diamond growth, based on doping experiments, suggesting that there could be unrealized benefits to gaining better control over hydrogen during routine HPHT diamond synthesis.