Phase diagram of carbon and the factors limiting the quantity and size of natural diamonds

Abstract

Phase diagrams of carbon, and those focusing on the graphite-to-diamond transitional conditions in particular, are of great interest for fundamental and applied research. The present study introduces a number of experiments carried out to convert graphite under high-pressure conditions, showing a formation of stable phase of fullerene-type onions cross-linked by sp-bonds in the 55-115 GPa pressure range instead of diamonds formation (even at temperature 2000-3000 K) and the already formed diamonds turn into carbon onions. Our results refute the widespread idea that diamonds can form at any pressure from 2.2 to 1000 GPa. The phase diagram built within this study allows us not only to explain the existing numerous experimental data on the formation of diamond from graphite, but also to make assumptions about the conditions of its growth in Earth's crust.

Pressure-Induced Transformation of Graphite and Diamond to Onions

“…The existence of denser phases should lead to a loss of stability in diamond [9,10], which was also observed in [1][2][3]. The loss of stability of diamond can be initiated by a critical shear stress at room temperature.…”

“…On the basis of experimental data, we have recently proposed a new phase diagram of carbon with a region of diamond instability in the 55-115 GPa pressure range [1,2]. In this range, the data on shock compression of single-crystal graphite [3] indicate formation of a phase denser (by 2%) than diamond.…”

The Effect of Shear Deformation on C-N Structure under Pressure up to 80 GPa

“…According to recent experimental and computer modeling studies, fullerene-type onions cross-linked by sp 3 bonds are the equilibrium phase of the carbon phase diagram at 55-115 GPa [1,2]. This discovery allows us to re-evaluate the phase diagram of C-N compounds from the perspective of obtaining new nanocluster-based materials.…”

The Effect of Severe Plastic Deformations on Phase Transitions and Structure of Solids

“…1120) To date, experiments with a shear are carried out up to 130 GPa. 21) To avoid displacement and tilting of diamond anvils under load, and to increase the pressure at which it is possible to investigate phase transitions in samples under conditions of non-hydrostatic loading and large shear deformation, the following changes have been made to the SDAC design described in Refs. 911,19,20).…”

“…The above changes made to the cell design, allowed to increase the pressure in the sample during the shear to 150 GPa. 21) In the present paper, the structure of different forms of carbon (C 60 , graphite, diamond, and nanotubes), as well as silicon and zirconium after pressure treatment under conditions of severe plastic deformations in a high-pressure cell with a shear, was studied by TEM and Raman spectroscopy. It is established that in the same conditions of processing by pressure and deformation the listed materials behave differently, that in C 60 and Zr transformation-deformation bands are formed, containing highpressure phases.…”