Shock compression of liquid carbon monoxide and methane to 90 GPa (900 kbar)

Chemistry-mediated Ostwald ripening in carbon-rich C/O systems at extreme conditions

There is significant interest in establishing a capability for tailored synthesis of next-generation carbon-based nanomaterials due to their broad range of applications and high degree of tunability. High pressure (e.g., shockwave-driven) synthesis holds promise as an effective discovery method, but experimental challenges preclude elucidating the processes governing nanocarbon production from carbon-rich precursors that could otherwise guide efforts through the prohibitively expansive design space. Here we report findings from large scale atomistically-resolved simulations of carbon condensation from C/O mixtures subjected to extreme pressures and temperatures, made possible by machine-learned reactive interatomic potentials. We find that liquid nanocarbon formation follows classical growth kinetics driven by Ostwald ripening (i.e., growth of large clusters at the expense of shrinking small ones) and obeys dynamical scaling in a process mediated by carbon chemistry in the surrounding reactive fluid. The results provide direct insight into carbon condensation in a representative system and pave the way for its exploration in higher complexity organic materials. They also suggest that simulations using machine-learned interatomic potentials could eventually be employed as in-silico design tools for new nanomaterials.

Modelling the growth of carbon nanoclusters in shock experiments is computationally demanding. Here the authors employ a machine-learned reactive interatomic model to perform large-scale simulations of nanocarbon formation from prototypical shocked C/O-containing precursor.

Investigating 3,4-bis(3-nitrofurazan-4-yl)furoxan detonation with a rapidly tuned density functional tight binding model

We describe a machine learning approach to rapidly tune density functional tight binding models for the description of detonation chemistry in organic molecular materials. Resulting models enable simulations on the several 10s of ps scales characteristic to these processes, with "quantum-accuracy." We use this approach to investigate early shock chemistry in 3,4-bis(3-nitrofurazan-4-yl)furoxan, a hydrogen-free energetic material known to form onion-like nanocarbon particulates following detonation. We find that the ensuing chemistry is significantly characterized by the formation of large C_xN_yO_z species, which are likely precursors to the experimentally observed carbon condensates. Beyond utility as a means of investigating detonation chemistry, the present approach can be used to generate quantum-based reference data for the development of full machine-learned interatomic potentials capable of simulation on even greater time and length scales, i.e., for applications where characteristic time scales exceed the reach of methods including Kohn-Sham density functional theory, which are commonly used for reference data generation.

Ultrafast shock synthesis of nanocarbon from a liquid precursor

Carbon nanoallotropes are important nanomaterials with unusual properties and promising applications. High pressure synthesis has the potential to open new avenues for controlling and designing their physical and chemical characteristics for a broad range of uses but it remains little understood due to persistent conceptual and experimental challenges, in addition to fundamental physics and chemistry questions that are still unresolved after many decades. Here we demonstrate sub-nanosecond nanocarbon synthesis through the application of laser-induced shock-waves to a prototypical organic carbon-rich liquid precursor-liquid carbon monoxide. Overlapping large-scale molecular dynamics simulations capture the atomistic details of the nanoparticles' formation and evolution in a reactive environment and identify classical evaporation-condensation as the mechanism governing their growth on these time scales.

Nanocarbon condensation in detonation

We analyze the definition of the Gibbs free energy of a nanoparticle in a reactive fluid environment, and propose an approach for predicting the size of carbon nanoparticles produced by the detonation of carbon-rich explosives that regards their condensation as a nucleation process and takes into account absolute entropy effects of the cluster population. The results are consistent with experimental observations and indicate that such entropy considerations are important for determining chemical equilibrium states in energetic materials that contain an excess of carbon. The analysis may be useful for other applications that deal with the nucleation of nanoparticles under reactive conditions.

Dense Hydrocarbon Structures at Megabar Pressures

The structure, bonding, and other properties of phases in the carbon-hydrogen system over a range of conditions are of considerable importance to a broad range of scientific problems. However, the phase diagram of the C-H system at high pressures and temperatures is still not known. To search for new low-energy hydrocarbon structures, we carried out systematic structure prediction calculations for the C-H system from 100 to 300 GPa. We confirmed several previously predicted structures but found additional compositions that adopt more stable structures. In particular, a C₂H₄ structure is found that has an indirect band gap, and phonon calculations confirm that it is dynamically stable over a broad pressure range. We also identify more carbon-rich structures that are energetically favorable. The results are important for understanding carbon-hydrogen interactions in high-pressure experiments, dense astrophysical environments and the deep carbon cycle in planetary interiors.

Experimental measurement of speeds of sound in dense supercritical carbon monoxide and development of a high-pressure, high-temperature equation of state

We report the adiabatic sound speeds for supercritical fluid carbon monoxide along two isotherms, from 0.17 to 2.13 GPa at 297 K and from 0.31 to 3.2 GPa at 600 K. The carbon monoxide was confined in a resistively heated diamond-anvil cell, and the sound speed measurements were conducted in situ using a recently reported variant of the photoacoustic light scattering effect. The measured sound speeds were then used to parametrize a single site dipolar exponential-6 intermolecular potential for carbon monoxide. PρT thermodynamic states, sound speeds, and shock Hugoniots were calculated using the newly parametrized intermolecular potential and compared to previously reported experimental results. Additionally, we generated an analytical equation of state for carbon monoxide by fitting to a grid of calculated PρT states over a range of 0.1-10 GPa and 150-2000 K. A 2% mean variation was found between computed high-pressure solid-phase densities and measured data-a surprising result for a spherical interaction potential. We further computed a rotationally dependent fluid to β-solid phase boundary; results signal the relative magnitude of short-range rotational disorder under conditions that span existing phase boundary measurements.

Dissociation of methane under high pressure

Methane is an extremely important energy source with a great abundance in nature and plays a significant role in planetary physics, being one of the major constituents of giant planets Uranus and Neptune. The stable crystal forms of methane under extreme conditions are of great fundamental interest. Using the ab initio evolutionary algorithm for crystal structure prediction, we found three novel insulating molecular structures with P2(1)2(1)2(1), Pnma, and Cmcm space groups. Remarkably, under high pressure, methane becomes unstable and dissociates into ethane (C(2)H(6)) at 95 GPa, butane (C(4)H(10)) at 158 GPa, and further, carbon (diamond) and hydrogen above 287 GPa at zero temperature. We have computed the pressure-temperature phase diagram, which sheds light into the seemingly conflicting observations of the unusually low formation pressure of diamond at high temperature and the failure of experimental observation of dissociation at room temperature. Our results support the idea of diamond formation in the interiors of giant planets such as Neptune.

Brillouin scattering study of liquid methane under high pressures and high temperatures

Brillouin scattering measurements were performed on liquid methane using diamond anvil cell along five isotherms and at the pressures up to solidification points. Sound velocity, refractive index, and adiabatic bulk modulus of liquid methane as function of pressure were determined with the measurements from the platelet and backscattering geometries. The maximum pressure and temperature reached up to 5.12 GPa and 539 K. The sound velocity, refractive index, and adiabatic bulk modulus increased with pressure along each isotherm. The equation of state of liquid methane was determined from the present Brillouin results.

Probing hydrogen-rich molecular systems at high pressures and temperatures

Information on the behavior of hydrogen-containing simple molecular systems at high pressures and temperatures is central to many problems in chemistry and allied sciences. The combination of increasing pressure and temperature causes a redistribution of the electronic density, and the resulting alterations in interatomic interactions give rise to changes in macroscopic properties. Experimental studies of hydrogen-rich systems under extreme conditions have advanced considerably in recent years as a result of improvements in diamond anvil cell technology in combination with resistive- and laser-heating techniques. These experimental methods are combined with spectroscopic and diffraction techniques, and the results interpreted with the aid of molecular simulations. In this tutorial review we discuss recent findings in hydrogen, water, and other selected hydrogen-rich systems including compounds formed from these materials. The results are examined in terms of the evolution of effective potentials and interatomic vibrational coupling, molecular dissociation and recombination, ionization and polymerization, quantum effects, and order-disorder phenomena.

Shock compression of deuterium near 100 GPa pressures

The shock-compression curve (Hugoniot) of D2 near 100 GPa pressures (1 Mbar) has been contro-versial because the two published measurements have limiting compressions of fourfold and sixfold. Our purpose is to examine published experimental results to decide which, if either, is probably correct. The published Hugoniot data of low-Z diatomic molecules have a universal behavior. The deuterium data of Knudson et al. (fourfold limiting compression) have this universal behavior, which suggests that Knudson et al. are correct and shows that deuterium behaves as other low-Z elements at high tem-peratures. In D2, H2, N2, CO, and O2, dissociation completes and average kinetic energy dominates average potential energy above approximately 60 GPa. Below approximately 30 GPa, D2, H2, N2, CO, and O2 are diatomic. D2 dissociation is accompanied by a temperature-driven nonmetal-metal transition at approximately 50 GPa.

Reflected shock experiments on the equation-of-state properties of liquid deuterium at 100-600 GPa (1-6 mbar)

New laser-driven shock experiments have been used to study the equation-of-state (EOS) properties of liquid deuterium. Reflected shocks are utilized to increase the shock pressure and to enhance the sensitivity to differences in compressibility. The results of these experiments differ substantially from the predictions of the Sesame EOS. EOS models showing large dissociation effects with much greater compressibility (up to a factor of 2) agree with the data. By use of independent techniques, this experiment offers the first confirmation of an earlier observation of enhanced compressibility in liquid deuterium.

Effects of high pressure on molecules

Recent high-pressure studies reveal a wealth of new information about the behavior of molecular materials subjected to pressures well into the multimegabar range (several hundred gigapascal), corresponding to compressions in excess of an order of magnitude. Under such conditions, bonding patterns established for molecular systems near ambient conditions change dramatically, causing profound effects on numerous physical and chemical properties and leading to the formation of new classes of materials. Representative systems are examined to illustrate key phenomena, including the evolution of structure and bonding with compression; pressure-induced phase transitions and chemical reactions; pressure-tuning of vibrational dynamics, quantum effects, and excited electronic states; and novel states of electronic and magnetic order. Examples are taken from simple elemental molecules (e.g. homonuclear diatomics), simple heteronuclear species, hydrogen-bonded systems (including H2O), simple molecular mixtures, and selected larger, more complex molecules. There are many implications that span the sciences.

Dissociation of CH4 at high pressures and temperatures: diamond formation in giant planet interiors?

Experiments using laser-heated diamond anvil cells show that methane (CH4) breaks down to form diamond at pressures between 10 and 50 gigapascals and temperatures of about 2000 to 3000 kelvin. Infrared absorption and Raman spectroscopy, along with x-ray diffraction, indicate the presence of polymeric hydrocarbons in addition to the diamond, which is in agreement with theoretical predictions. Dissociation of CH4 at high pressures and temperatures can influence the energy budgets of planets containing substantial amounts of CH4, water, and ammonia, such as Uranus and Neptune.

Dissociation of methane into hydrocarbons at extreme (planetary) pressure and temperature

Constant-pressure, first-principles molecular dynamic simulations were used to investigate the behavior of methane at high pressure and temperature. Contrary to the current interpretation of shock-wave experiments, the simulations suggest that, below 100 gigapascals, methane dissociates into a mixture of hydrocarbons, and it separates into hydrogen and carbon only above 300 gigapascals. The simulation conditions (100 to 300 gigapascals; 4000 to 5000 kelvin) were chosen to follow the isentrope in the middle ice layers of Neptune and Uranus. Implications on the physics of these planets are discussed.

The Nature of the Interior of Uranus Based on Studies of Planetary Ices at High Dynamic Pressure

Data from the Voyager II spacecraft showed that Uranus has a large magnetic field with geometry similar to an offset tilted dipole. To interpret the origin of the magnetic field, measurements were made of electrical conductivity and equation-of-state data of the planetary "ices" ammonia, methane, and "synthetic Uranus" at shock pressures and temperatures up to 75 gigapascals and 5000 K. These pressures and temperatures correspond to conditions at the depths at which the surface magnetic field is generated. Above 40 gigapascals the conductivities of synthetic Uranus, water, and ammonia plateau at about 20(ohm-cm)(-1), providing an upper limit for the electrical conductivity used in kinematic or dynamo calculations. The nature of materials at the extreme conditions in the interior is discussed.