European Microscopy Congress 2016: Proceedings || Graphite-to-diamond (13C) direct transition in a diamond anvil high-pressure cell

摘要： As the hardest material in nature, diamond is of great importance and interest for scientific studies. However, formation of a diamond is complicated process and requires extreme conditions. Bundy and Kasper (1967) for the first time synthesized a new form of carbon—hexagonal diamond – under conditions of static pressure exceeding about 13 GPa and temperature greater than about 1000°C [1]. At room temperature the crystal structure of graphite is stable up to pressure 15 GPa and loses some of the graphite features at higher pressure, forming metastable graphitic or amorphous phases [2]. Transition of polycrystalline graphite to diamond occurs after hydrostatic pressure treatment near 70 GPa [3]. The development of solid-state phase transitions, including those at the stage of nucleation and development of a new phase practically always is connected with the relaxation of elastic stress [4], and in case of graphite-diamond transformation the latter can play main role. The goal of the present work is the formation of diamond from graphite in direct phase transition in a diamond anvil high-pressure cell, where the relaxation of elastic stress can be realized by means of plastic deformation of the sample. The experiment was performed at room temperature without a catalyst.13С was subjected to the shear deformation under pressure of 25 GPa. The structure studies of the obtained material were made by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). In order to prevent the confusion of the diamond obtained in the experiment with one of the diamond anvils we used graphite composed of 13C carbon isotope atoms as a precursor. The diamond anvils consisted of conventional 12C diamond. Before TEM examination of each sample a Raman spectroscopy was used to verify that it contains only 13 C (diamond) and no 12C. TEM and EELS were carried out using JEOL JEM-2010 high-resolution transmission electron microscope. TEM analysis has shown that the samples obtained in the series of our experiments contain several phases of carbon simultaneously. After the high pressure treatment in shear diamond anvil cell (SDAC) there were observed some fragments of the sample, which contained both hexagonal and rhombohedral graphite (significant amounts of the last one), and also diamond and lonsdaleite. Fig. 1 shows the fragment, where the rhombohedral graphite presents. Fig. 2a shows the diamond structure fragment with {111}-planes composing 70o. Interplanar distances are 0.206 nm. Fig. 2b shows the EELS-spectrum which can be unambiguously attributed to a diamond. Thus, it was shown that 13С-graphite directly transforms into 13С-diamond (at least particularly) without a catalyst at room temperature after treatment in SDAC under pressure of 25 GPa.