Investigating Complex Oxides of Geophysical Importance with First-Principles Calculations(1/3)

We propose a three-year project to investigate the structural, electronic, magnetic, elastic, andthermodynamic properties of complex oxides of importance in geophysics, using state of the art firstprinciplescalculation techniques. Our research will be focused on two categories of materials, as describedbelow.One category of materials in this project includes iron-bearing magnesium carbonates [(Mg,Fe)CO3]under the Earth’s lower-mantle pressure (23-135 GPa). Given the crucial role played by carbon in theecosystem on our planet, the global carbon cycle is a topic of highly interdisciplinary interest. Whiletremendous progress has been made for our understanding of the carbon cycle on the Earth, knowledge ofcarbon cycle in the Earth’s deep interior is still very limited, and yet, the Earth’s deep interior may containmore than 90% of the Earth’s carbon. Some experiments have suggested that (Mg,Fe)CO3 could be the majorcarbon reservoir in the Earth’s lower mantle; it has also been shown that (Mg,Fe)CO3 can go throughcomplicated transitions in both atomic and electronic structure under high pressure. These transitions,however, are still poorly understood so far. Rigorous theoretical works on this class of materials are thushighly desirable. We believe our work can shed light on the physical properties of these complicatedcompeting polymorphs and help developing a more accurate picture of the Earth’s deep carbon cycle.The other category of materials in this project includes orthosilicates, in particular, (Co,Fe)2SiO4systems under high pressure and high temperature. The pressure-induced structural transition of iron-bearingMg2SiO4 from olivine (α) to modified spinel (β) to spinel (γ) structure causes the seismic wave velocitydiscontinuity in the Earth’s upper mantle. Interestingly, Fe2SiO4 and Ni2SiO4 transform directly from α- toγ-phase without going through β-phase. In contrast, Co2SiO4 goes through the β-phase, but with atemperature-dependent stability region. The reason is still unclear. We plan to study the thermodynamic andelastic properties of the high-pressure phase of (Co,Fe)2SiO4 systems. We believe this project can provide adifferent prospective to understand the formation of various transition-metal bearing orthosilicates inplanetary interiors.