Rules of formation of H–C–N–O compounds at high pressure and the fates of planetary ices

The solar system’s outer planets, and many of their moons, are dominated by matter from the H–C–N–O chemical space, based on solar system abundances of hydrogen and the planetary ices H2O, CH4, and NH3. In the planetary interiors, these ices will experience extreme pressure conditions, around 5 Mbar at the Neptune mantle–core boundary, and it is expected that they undergo phase transitions, decompose, and form entirely new compounds. While temperature will dictate the formation of compounds, ground-state density functional theory allows us to probe the chemical effects resulting from pressure alone. These structural developments in turn determine the planets’ interior structures, thermal evolution, and magnetic field generation, among others. Despite its importance, the H–C–N–O system has not been surveyed systematically to explore which compounds emerge at high-pressure conditions, and what governs their stability. Here, we report on and analyze an unbiased crystal structure search among H–C–N–O compounds between 1 and 5 Mbar. We demonstrate that simple chemical rules drive stability in this composition space, which explains why the simplest possible quaternary mixture HCNO—isoelectronic to diamond—emerges as a stable compound and discuss dominant decomposition products of planetary ice mixtures.

Crystal structure prediction coupled to electronic structure calculations has emerged as a powerful tool in computational materials science, in particular in the area of high-pressure science, where it can overcome the chemical imagination attuned to ambient conditions: The predictions—and subsequent experimental confirmations—of unusual compounds such as Na2He, H3S, or LaH10 attest to the predictive power of these approaches (1–6). The planetary ices H2O, CH4, and NH3 may dominate the interiors of icy planets, but under extreme conditions (7–10). High-pressure phases of these ices have been explored computationally, and some predictions of exotic phases of individual ices have also been confirmed by experiments (11–15). Arguably, computational predictions in this field are of crucial importance because of the challenges for laboratory experiments and the indirect nature of astronomical observations. However, with increasing number of constituents the structure searches become computationally much more demanding. Hence, while structure predictions for elemental and binary systems are routine there are far fewer extensive searches of ternary systems, and none for quaternary systems.The situation in the H–C–N–O quaternary system reflects this. A vast number of publications exist on the high-pressure evolution of the individual constituents H, C, N, and O (16–19). Binary systems have also been looked at in great detail (20–24). The ternary systems are much less investigated: While H–C–O, H–N–O, and C–N–O phase diagrams have been reported (25–27), the H–C–N ternary has not, for example. The PubChem database (28) lists just under 4.8 million molecular H–C–N compounds. Overall, it contains 44.6 million H–C–N–O compounds at ambient pressure, 78% of which are of true quaternary composition—this highlights both the complexity and the relevance of this quaternary system for organic chemistry. Back in the high-pressure area, some binary mixtures of planetary ices, for instance H2O–NH3 or N2–CH4 mixtures, which form a subset of the H–N–O and H–C–N ternaries, have been studied in simulations into the megabar pressure range (29–32). However, despite the overall importance of H–C–N–O both to planetary science and Earth-bound chemical and life sciences, to our knowledge there are no computational high-pressure studies of this system, or even subsets that include all four elements, for example binary or ternary molecular mixtures such as CO2–NH3 or NH3–H2O–CH4. Here, we explore the full H–C–N–O chemical space via crystal structure searches performed at 500 GPa. This resembles the pressure—if not the temperature—at Neptune’s core–mantle boundary and therefore helps illuminate potential pressure-induced chemical reactions in the deep interiors of the outer planets and giant icy exoplanets. It also helps us understand more generally what rules govern a familiar composition space at highly unfamiliar external conditions.