computational chemistry, planetary science
The dramatic influence of high pressure (tens or hundreds of GPa) on reactivity is nowadays experimentally well established[i]. Crystal structure prediction approaches have become very effective in correctly anticipating experimental outcomes[ii]. Among the numerous applications, these techniques have been widely exploited to predict high-pressure reactions[iii],[iv],[v]. Composition-pressure phase diagrams can be built by comparing the free energy (which at T = 0 K reduces to enthalpy) of the most stable structures of elements and compounds at various pressures, in order to single out the thermodynamically stable compositions. The latter are defined as those compounds for which no exothermic decomposition reactions exist. In most cases, binary phase diagrams are targeted, for the high number of possible stoichiometries in a ternary phase diagram makes its ab initio exploration very computationally demanding.
The C-H-O ternary phase diagram at high pressure is of paramount interest for planetary science. H2O and CH4, not necessarily in their intact molecular forms, are among the major constituents of giant planets such as Neptune and Uranus, where pressure can reach values of hundreds of GPa [vi]. Moreover, large icy satellites (e.g. Ganymede, Callisto and Titan) [vii] and comets[viii] all contain water ice, mixed with a number of volatiles such as CH4 and CO2, experiencing pressures up to a few GPa.
In this contribution, the C-H-O phase diagram is explored up to 400 GPa by means of the powerful variable-composition evolutionary algorithm USPEX[ix] coupled with periodic density-functional calculations. Thorough, unbiased searches were performed sampling all possible C-H-O compositions, and a total of more than 125000 structures, generated by the evolutionary method, were relaxed to the closest minimum-enthalpy configuration.
Besides uncovering new stable polymorphs of high-pressure elements and known molecules, we predicted the formation of new compounds. A 2CH4:3H2 inclusion compound forms at low pressure and remains stable up to 215 GPa. So far, among the inclusion compounds, only gas hydrates were known to persist above 50 GPa[x]. Our 2CH4:3H2 co-crystal not only sets a new upper limit for the stability of inclusion compounds in general, but also introduces a qualitative shift of views, for it broadens the classes of inclusion compounds stable at very high pressures. Carbonic acid (H2CO3), highly unstable at ambient conditions, was predicted to form exothermically at mild pressure (about 1 GPa). This fact opens possibilities for new synthetic pathways and has also important implications for planetary science. On top of that, we show that carbonic acid displays a remarkable high-pressure behavior, for it polymerizes (44 GPa) and reacts with water to form orthocarbonic acid (314 GPa). This unexpected high-pressure chemistry is rationalized in this contribution by analyzing charge density and electron localization function distributions, and implications for general chemistry and planetary science are also discussed.
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