Diamond formation in double-shocked epoxy to 150 GPa

Abstract

We present measurements of diamond formation in doubly shocked Stycast 1266 epoxy (comprising C, H, Cl, N, and O) using in situ x-ray diffraction. Epoxy samples were reshocked against a LiF window to pressures between 80 and 148 GPa in experiments at the Omega Laser Facility. The pressure and temperature conditions were diagnosed in situ using velocimetry and optical pyrometry, respectively. X-ray diffraction patterns of the compressed epoxy are consistent with cubic diamond (Fd[Formula: see text]m), indicating that diamond can precipitate not only from twice-shocked CH polystyrene [Kraus et al. Nat. Astron. 1, 606 (2017)] at these conditions but also from twice-shocked CH polymers with the addition of oxygen, nitrogen, and chlorine. These results, in combination with previous works on CH, [Formula: see text], [Formula: see text], and methane hydrate, support that diamond often, but not always, forms from CH-based compounds at extreme pressures and temperatures, indicating that the chemical composition, thermodynamic compression path, and kinetics play an important role.

Thermodynamics of diamond formation from hydrocarbon mixtures in planets

“…Our phase boundaries in Fig. 3 g put largely scattered experimental measurements 11 , 12 , 15 – 17 into context, and provide a mechanistic understanding of the thermodynamics involved. They also help gauge the accuracy of the experimental determination of diamond formation conditions, extrapolate between different experiments, and guide future efforts to validate these boundaries.…”

“…The shaded area below the bright blue line indicates the depletion zone, where diamond formation is thermodynamically favorable regardless of the carbon ratio. Experimental observation of diamonds (D) is indicated by diamond symbols and dashed/dotted rectangular regions, while no diamond observation (no D) is marked by cross symbols: laser-heated DAC data for methane 11 , 12 , for methane hydrate (MH) 14 , and shock-compression experiments for polystyrene (-C 8 H 8 -) 16 , polyethylene (-C 2 H 4 -) 17 , and epoxy 15 . The symbols are colored according to the N C /( N C + N H ) ratio of the starting materials, using the same color scheme for the phase boundaries.…”

“…The hollow diamond symbols in Fig. 3 g show the diamond formation conditions from starting materials of more complex compositions: Marshall et al 15 used epoxy (C:H:Cl:N:O ≈ 27:38:1:1:5) and Kadobayashi et al 14 used methane hydrate. We find little agreement between Kadobayashi et al 14 and our phase boundaries, if we compare solely in terms of χ C including all atomic species, although the agreement is improved if based on the χ C of methane alone, and indeed CH 4 may be an intermediate product in the experiment 14 .…”

“…In methane hydrates, Kadobayashi et al reported diamond formation in a DAC between 13 and 45 GPa above 1600 K but not at lower temperatures 14 . Laser shock-compression experiments found diamond formation in epoxy (C,H,Cl,N,O) 15 and polystyrene (-C 8 H 8 -) 16 , but none in polyethylene (-C 2 H 4 -) 17 .…”

Mixture of hydrogen and methane under planetary interior conditions

“…[21][22][23][24][25][26] Note that more complex mixtures containing hydrogen, carbon, nitrogen, and oxygen were also studied. [27][28][29] In particular, evidence for diamond formation under conditions typical for the deep interior of Uranus has been found in laser-driven shock compression experiments on plastics like polystyrene. 28,[30][31][32] Molecular dynamics (MD) simulations combined with density functional theory (DFT) were used to calculate the equation of state (EOS) and analyze the dissociation, polymerization into carbon chains, and chemical bonding properties of methane.…”