Structures And Dynamic Behaviors Of Several Planetary Compounds From First-principles Calculations

The structure and dynamic evolution of planetary matter is one of the most funda-mental problems in the process of understanding the universe.According to the current planetary models,the atmospheres of giant planets such as Uranus and Neptune are composed of hydrogen and helium,while their cores are small and mainly composed of rocks.Buried between the rocky core and the atmosphere,there is a thick hot ice layer,mainly composed of 56%water,36%methane and 8%ammonia.Helium,on the other hand,is the most inert element in nature,and it is generally considered to be unreactive with other substances under atmospheric pressure.While along the plane-tary isentrope,the pressure and temperature conditions of such hot ice layer increases from 20 GPa and 2000 K up to 600 GPa and 7000 K.Under such extreme conditions,the reactivity of helium may be greatly improved.Therefore,it is very meaningful to study whether helium in the planetary atmosphere will react with water,ammonia and methane in the hot ice layer under the extreme internal conditions of Uranus and Nep-tune,and if so,what kind of structure and state of matter it can form.In addition,it is important to study the structure and thermodynamic evolution of the rock composition in the more inner rock core for revealing the early causes of giant planets.Based on the machine learning accelerated crystal structure search method de-veloped by our group and first principles calculations,we systematically searched the binary phase diagrams of helium water,helium ammonia,and silicon oxygen systems,and then we studied the stability and dynamic evolution of these compounds at high temperature by molecular dynamics simulations and quasi harmonic approximation cal-culations.The following innovative research results we have been obtained:1.Helium-water compounds.We found that helium and water can form stable compounds in a large pressure range.Surprisingly,we also found that these compounds have special melting behaviors by increasing temperature under high pressure,and can form two kinds of superionic states between solid phase and liquid phase.In the first superionic state,both hydrogen and oxygen atoms vibrate near their equilibrium posi-tions,showing solid behavior,while helium can diffuse freely like a liquid.In the sec-ond superionic state,only oxygen behaves as a solid,while helium and hydrogen atoms can diffuse freely.Although the mass of helium atom is larger than that of hydrogen atom,the helium atoms in these superionic states have larger diffusion coefficient and lower”melting”temperature than that of hydrogen atom,because the helium oxygen interaction is weaker than that of hydrogen oxygen interaction.The insertion of helium atoms substantially decreases the pressure at which superionic states may be formed,compared to those in pure ice.2.Helium-ammonia compounds.We found that helium and ammonia can form three different stoichiometries of He-NH₃compounds,including eight stable crystal structures,in pressures range of 0-500 GPa.In these structures,He NH₃and He₂NH₃are similar to perovskite structure,and all He(NH₃)₂compounds are host guest struc-ture.Under high pressure and high temperature,helium-ammonia compounds can not only form superion state similar to helium water compound,but also form a plastic state.In this plastic state,the hydrogen atom can rotate around the nitrogen atom,but it does not diffuse outward.Moreover,the behavior of helium atoms in helium ammo-nia compounds is quite different from that in helium water compounds.As mentioned above,a large range of helium diffusion states appear in the phase diagram of helium water compounds,which are almost absent in the phase diagrams of helium-ammonia compounds.We found that the hydrogen bond density in helium-ammonia is higher than that of helium-water compounds.Because of lattice structure,hydrogen bond in helium-ammonia compounds forms a cage structure to bind helium atoms?in helium water compounds,water atoms form a hexagonal channel,and helium atoms can be diffused freely in the channel.3.Silicon-oxygen compounds.We found that silica will form a new crystal silica with R?3 symmetry at around 645-890 GPa.In this structure,silicon atoms have three different coordination numbers of 6,8,and 9,resulting in an average coordination num-ber of 8 for silicon atoms.This is a kind of mixed coordination stable phase that people have been looking for a long time but never found in silica system and it just fills the gap between the density,electronic band gap and coordination number between the 6-fold pyrite type and the 9-fold Fe₂P type silica.This R?3 silica is more stable in the core of Neptune and other solar system giant planets than the known Cotunnite type silica,and it may exist in the core or mantle of super-Earths.In addition,we also found that silicon and oxygen can form some superoxides in an oxygen-rich environment,such as Cmcm Si O₃and Ccce Si O₆.The above studies on helium water and helium ammonia compounds predict that helium in the atmosphere of Uranus and Neptune may penetrate into the inner regions of the hot ice layer and form new compounds with water and ammonia in the hot ice layer.It is also revealed that these compounds may present special state between solid and liquid under high temperature and high pressure inside the planets,namely supe-rionic state and plastic state.These results reveal that helium,which is very inert at atmospheric pressure,can react with other substances at high pressure,and are of great significance in understanding the magnetic field of Uranus and Neptune and in con-structing more reliable planetary models.Moreover,in our study of silica,we found a mixed coordination stable phase for the first time,which greatly enriched the high pressure phase diagram of silicon oxide.These innovative achievements have greatly enriched our understanding of planetary matter,and are of great scientific significance for us to deeply understand the internal structure of the solar system and extrasolar giant planets,and to explore the unknown world.