Theoretical stability limit of diamond at ultrahigh pressure

Investigating off-Hugoniot states using multi-layer ring-up targets

Laser compression has long been used as a method to study solids at high pressure. This is commonly achieved by sandwiching a sample between two diamond anvils and using a ramped laser pulse to slowly compress the sample, while keeping it cool enough to stay below the melt curve. We demonstrate a different approach, using a multilayer ‘ring-up’ target whereby laser-ablation pressure compresses Pb up to 150 GPa while keeping it solid, over two times as high in pressure than where it would shock melt on the Hugoniot. We find that the efficiency of this approach compares favourably with the commonly used diamond sandwich technique and could be important for new facilities located at XFELs and synchrotrons which often have higher repetition rate, lower energy lasers which limits the achievable pressures that can be reached.

Transformation of diamond to fullerene-type onions at pressure 70 GPa and temperature 2400 K

We report the observation of a phase transition of diamond to denser than diamond carbon phase composed from 2 to 3 fullerene-type shells of onions. Raman spectra indicate the fullerene-type of the onions shells. The onions phase is a stable phase in a diamond instability zone of a phase diagram of carbon at pressure 70 GPa and temperature 2400 K. A mixture of diamond and Ni powders was heated by a laser beam under pressure in a diamond anvil cell. Both direct and catalytic diamond to onions transitions were observed during heating. The catalytic transformation includes the following steps. Melting of Ni during the laser heating at pressure 70 GPa, a 'diamond solution' (a transfer of carbon atoms from diamond) in liquid Ni and the formation of an equilibrium carbon phase from the supersaturated solution upon cooling. The catalytic process is a reverse one relative to the catalytic synthesis of diamond in a diamond stability zone at pressure around 6 GPa. The main result of our study is the presence of fullerene-type structures in the phase diagram of carbon in the region of diamond instability under high sub-Mbar pressure and wide range of temperatures.

Crystal structure prediction usingab initioevolutionary techniques: Principles and applications

We have developed an efficient and reliable methodology for crystal structure prediction, merging ab initio total-energy calculations and a specifically devised evolutionary algorithm. This method allows one to predict the most stable crystal structure and a number of low-energy metastable structures for a given compound at any P-T conditions without requiring any experimental input. Extremely high (nearly 100%) success rate has been observed in a few tens of tests done so far, including ionic, covalent, metallic, and molecular structures with up to 40 atoms in the unit cell. We have been able to resolve some important problems in high-pressure crystallography and report a number of new high-pressure crystal structures (stable phases: epsilon-oxygen, new phase of sulphur, new metastable phases of carbon, sulphur and nitrogen, stable and metastable phases of CaCO3). Physical reasons for the success of this methodology are discussed.

Carbon under extreme conditions: phase boundaries and electronic properties from first-principles theory

At high pressure and temperature, the phase diagram of elemental carbon is poorly known. We present predictions of diamond and BC8 melting lines and their phase boundary in the solid phase, as obtained from first-principles calculations. Maxima are found in both melting lines, with a triple point located at approximately 850 GPa and approximately 7,400 K. Our results show that hot, compressed diamond is a semiconductor that undergoes metalization upon melting. In contrast, in the stability range of BC8, an insulator to metal transition is likely to occur in the solid phase. Close to the diamond/liquid and BC8/liquid boundaries, molten carbon is a low-coordinated metal retaining some covalent character in its bonding up to extreme pressures. Our results provide constraints on the carbon equation of state, which is of critical importance for devising models of Neptune, Uranus, and white dwarf stars, as well as of extrasolar carbon-rich planets.

Carbon phase diagram from ab initio molecular dynamics

We compute the free energy of solid and liquid diamond from first-principles electronic structure theory using efficient thermodynamic integration techniques. Our calculated melting curve is in excellent agreement with the experimental estimate of the graphite-diamond-liquid triple point and is consistent with shock wave experiments. We predict the phase diagram of diamond at pressures and temperatures that are difficult to access experimentally. We confirm early speculations on the presence of a reentrant point in the diamond melting line but find no evidence for a first order liquid-liquid phase transition near the reentrant point.

Properties of diamond under hydrostatic pressures up to 140 GPa

Diamond is the archetypal covalent material. Each atom in an sp(3) configuration is bonded to four nearest neighbours. Because of its remarkable properties, diamond has been extensively studied. And yet our knowledge of the properties of diamond under very high pressure is still incomplete. Although diamond is known to be the preferred allotrope of carbon at high pressure, the possibility of producing under pressure high-density polymorphs of diamond, including metallic forms, has been discussed. Structural changes have already been reported in diamond under non-hydrostatic pressures around 150 GPa and large deformation. However, measurements of the properties of diamond under hydrostatic pressure have been limited to below 40 GPa. Here, we report accurate measurements of the volume and of the optical phonon frequency of diamond under hydrostatic pressure up to 140 GPa. We show that diamond is more compressible than currently expected. By combining the volume and the frequency pressure shifts, we deduce that diamond remains very stable under pressure: it is a Gruneisen solid up to at least 140 GPa, and the covalent bond is even slightly strengthened under pressure. Finally, the optical phonon frequency versus pressure is calibrated here to be used as a pressure gauge for diamond anvil cell studies in the multi-megabar range.