Liquid Structure of Shock-Compressed Hydrocarbons at Megabar Pressures

Abstract

We present results for the ionic structure in hydrocarbons (polystyrene, polyethylene) that were shock compressed to pressures of up to 190 GPa, inducing rapid melting of the samples. The structure of the resulting liquid is then probed using in situ diffraction by an X-ray free electron laser beam, demonstrating the capability to obtain reliable diffraction data in a single shot, even for low-Z samples without long range order. The data agrees well with ab initio simulations, validating the ability of such approaches to model mixed samples in states where complex interparticle bonds remain, and showing that simpler models are not necessarily valid. While the results clearly exclude the possibility of complete carbon-hydrogen demixing at the conditions probed, they also, in contrast to previous predictions, indicate that diffraction is not always a sufficient diagnostic for this phenomenon.

Ab initio dielectric response function of diamond and other relevant high pressure phases of carbon

“…KG, BSE, and TDDFT results of this work. 4 show the shift to higher energies as well as a broadening with increase of pressure.…”

“…Such conditions can be recreated in the laboratory using compression methods like laser driven shock waves to such an effect that nano-diamonds in a high pressure fluid of carbo-hydrates can indeed be observed [2,3]. Meanwhile computer simulations need to provide significant information prior to the design of such experiments and are essential for the evaluation of the measurements [3,4].…”

High Pressure Hydrocarbons Revisited: From van der Waals Compounds to Diamond

“…Dynamic compression experiments on hydrocarbons are used to probe their properties at combined high-pressure and -temperature conditions [33][34][35][36][37][38][39][40], both to investigate their potential decomposition, but also to better understand a critical ablator material used in inertial confinement fusion experiments. The thermal equation of state of hydrocarbons has therefore been a subject of several ab initio molecular dynamics studies [41][42][43][44][45][46].…”

Demonstration of X-ray Thomson scattering as diagnostics for miscibility in warm dense matter

“…In case of demixing into significantly non-pure constituents, e.g. C 2 H + CH 2 , the inferred demixing would be larger due to a non-linear scaling of W R for this case 45 .…”

“…Here, we perform a linear interpolation between an expected value of W R = 2.70 for completely mixed CH (no species separation) and W R = 0.38 for fully demixed CH (100% species separation and the isolated carbon being in a diamond state) to obtain the corresponding values for the degree of species separation inside the sample. This procedure omits the first shock, however, DFT-MD simulations predict the Rayleigh scattering to be of very similar magnitude or slightly larger as for the double-shock conditions for mixed CH 45 . Moreover, the linear interpolation assumes demixing into pure constituents and no splitting into carbon-rich and hydrogen-rich phases in the regions where phase separation occurs.…”