First-principles Study On The Structures Of Ice And Hydrous Minerals Under High Pressure

Water(ice)is widely present on the earth's surface and interior,influencing geophysical properties importantly.The distribution of water on the earth continues to alter over time,constantly circulating between the earth's surface and interior.The water inside the earth could lower the melting temperature of silicate to produce magma,which enhances the mantle convection;and could also cause earthquakes.It is an important scientific significance to study water cycle for understanding the geophysical properties of earth.In 2020,the Earth in Time reported that studying“water cycle changing”is one of scientific topics for the next ten years.It is generally believed that water is in the form of ice or hydrous minerals in the earth's interior.Therefore,the structures of ice and hydrous minerals stable under pressure have always been important topics.However,traditional experimental methods such as X-ray diffraction face huge challenges in determining structures of substances that contain hydrogen and oxygen elements.Thus,we used the structure prediction method and its same name code CALYPSO,combined with first-principles calculation methods,to perform a series of studies on structures and properties of ice and hydrous minerals under pressure.The obtained innovative results are as follows:1.The existence of strong and weak hydrogen bonds is the reason for the formation of a large number of ice structures.At least 21 stable or metastable ice structures have been found in experiments.However,there are still unknown ice structures having not been found,and the ice phase diagram is still being updated.Here,a large-scale exploration on the potential energy surface of ice under low pressure is performed by the CALYPSO structure prediction method accelerated by machine learning potential.25 new ice structures are discovered.Among them,the P2₁2₁2₁ phase with 60 atoms per unit cell and the ice XV phase are energy degenerate in the pressure range of 0.5?2.0GPa.Therefore,we believed that the P2₁2₁2₁ phase might be a potential ground-state structure.Meanwhile,two independent experimental teams successfully synthesized a new ice phase,named XIX.The configuration of the XIX phase is similar to P2₁2₁2₁phase.However,the position of the hydrogen atoms for XIX phase cannot be determined experimentally.The multiple ice structures with same configuration as XIX phase proposed in this work could provide candidate structures to help in determining the structure of the XIX phase.Our work also provides a demonstrative study for large-scale exploration of ice potential energy surface using structure prediction method combined with machine learning potential.2.A large amount of water(?10¹¹ kg/year)could enter the earth's interior through slab subduction every year and return to the surface through geological activities such as volcanic eruptions,which forms a water cycle between the earth's interior and surface.It is a concerned question that how deep the water could be transported into the earth's interior.The key to solving this question is to study the hydrous minerals which are stable at the conditions of subducting plates.Iron silicate is one of the main components of subducting slab.Whether it could react with ice to form hydrated iron silicate is still unknown.Therefore,we studied a series of reactions for iron silicate and ice,and designed a new hydrated iron silicate Fe Si O₄H₂,proposing two high-pressure phases P2₁/c-l and P2₁/c-h.We found that it could be stable at high pressure and temperature(<45 GPa and 1450 K),indicates that Fe Si O₄H₂ might also be one of the important carriers for water transport into earth's interior.Moreover,through element substitution,we found that the energy of the P2₁/c-l and P2₁/c-h phases is lower than that of the Phase H(P2/m)phase for Mg Si O₄H₂ proposed in the previous study,indicating that the P2₁/c-l and P2₁/c-h are also the ground-state structures of Mg Si O₄H₂.The Fe Si O₄H₂ proposed in this work provides a new way for water transported into the earth's interior.3.The earth is the only planet with an estimated water content.If all ocean on the surface is recorded as one earth ocean,then the accumulated water content is within ten earth oceans during the earth's formation.Thus,a large amount of water might be stored in the earth's interior.Although many hydrous minerals have been found to be stable at high temperature and pressure conditions,most of the them(Mg Si O₄H₂ etc)will decomposed at the conditions of the lower mantle.Exploring the hydrous minerals that are stable at the conditions of the lower mantle is of important scientific significance for understanding water storage of the earth's mantle.Therefore,this work conducted a systematic study on the structures of Ca(OH)₂ at high pressure and predicted two new high-pressure phases P2₁/c and Pnma.They are stable at high pressure and temperature relative to Ca O and H₂O.The sound velocity of Ca(OH)₂ is lower than that of the one-dimensional seismic wave model.At the core-mantle boundary pressure(134 GPa),the sound velocity of Ca(OH)₂ is within the range of sound velocity of the ultra-low sound velocity zone.Therefore,if Ca(OH)₂ does exist in the earth's interior,more water resources might be stored in the lower mantle and contribute to the formation of the ultra-low sound velocity zone.