First-Principles Molecular Dynamics Study of Liquid Iron-Rich Alloys Under Conditions of the Earth’s Core

In this dissertation, we report the results of the first-principles molecular dynamics (FPMD) simulations and data analysis on the thermodynamic, structural, and transport properties of iron-rich metallic liquids considering several light and heavy elements under wide ranges of pressure and temperature that are relevant to the earth’s core. Our simulations of pure liquid iron cover the pressure range from 0 GPa at 2000 K to 380 GPa at 7000 K perhaps representing the most extensive computational study to date. We studied four molten iron-rich alloys corresponding to 2.67 atom% of Ni, Co, Mo, and W at pressures up to 380 GPa and at temperatures 4000 to 7000 K. We also simulated H- and C-bearing metallic systems at similar pressure-temperature conditions. The calculated pressure-temperature-volume (P-V-T) results of all liquid alloys can be accurately described with a three-term form of the equation of state consisting of the reference pressure-volume isotherm, the thermal pressure, and the impurity pressure. Moreover, pressure is corrected for the difference in calculation and recent experimental results by adding the fourth term to the equation of state. The resulting density-pressure profile of pure iron liquid along a geotherm shows that the outer core suffers from a density deficient of ~ 6.8%. Our results show that the addition of Mo and W in liquid iron in any amount widens the density gap whereas the addition of H and C reduces the gap. Both Ni and Co do not affect the liquid density significantly and they behave as host iron atoms showing similar bond distances and local coordination. The calculated mean iron coordination number of Mo and W is somewhat larger than that of Ni and Co and host iron atoms thus implying a substitutional incorporation mechanism, whereas both H and C are undercoordinated consistent with their interstitial incorporation. Our results from extended FPMD simulations for pure iron and FeW liquids show that the diffusion coefficient of Fe varies modestly over the outer core regime taking values of ~3 to 5 x 10-9 m2s-1. The heavy impurity (W) atoms tend to diffuse slower than host iron atoms by almost a factor of two, unlike highly mobile H atoms. The calculated viscosity of pure and alloyed iron liquids remains almost unchanged taking a low value of 12±2 mPa.s at the outer core conditions. It implies that outer core convection may involve small-scale turbulent circulation and barodiffusion possibly causing heavy elements to accumulate near the inner core boundary

First-principles studies of electronic, transport and bulk properties of pyrite FeS2

We present results from first principle, local density approximation (LDA) calculations of electronic, transport, and bulk properties of iron pyrite (FeS2). Our non-relativistic computations employed the Ceperley and Alder LDA potential and the linear combination of atomic orbitals (LCAO) formalism. The implementation of the LCAO formalism followed the Bagayoko, Zhao, and Williams (BZW) method, as enhanced by Ekuma and Franklin (BZW-EF). We discuss the electronic energy bands, total and partial densities of states, electron effective masses, and the bulk modulus. Our calculated indirect band gap of 0.959 eV (0.96), using an experimental lattice constant of 5.4166 Å, at room temperature, is in agreement with the measured indirect values, for bulk samples, ranging from 0.84 eV to 1.03 ± 0.05 eV. Our calculated bulk modulus of 147 GPa is practically in agreement with the experimental value of 145 GPa. The calculated, partial densities of states reproduced the splitting of the Fe d bands to constitute the dominant upper most valence and lower most conduction bands, separated by the generally accepted, indirect, experimental band gap of 0.95 eV

Mixed incorporation of carbon and hydrogen in silicate melts under varying pressure and redox conditions

Volatiles including carbon and hydrogen are generally considered to be more soluble in silicate melts than in mantle rocks. How these melts contribute to the storage and distribution of key volatiles in Earth\u27s interior today and during its early evolution, however, remains largely unknown. It is essential to improve our knowledge about volatiles-bearing silicate magmas over the entire mantle pressure regime. Here we investigate molten Mg Fe SiO (x=0, 0.25) containing both carbon and hydrogen using first-principles molecular dynamics simulations. Our results show that the dissolution mechanism of the binary volatiles in melts varies considerably under different conditions of pressure and redox. When incorporated as CO and H O components (corresponding to oxidizing conditions) almost all carbon and hydrogen form bonds with oxygen. Their speciation at low pressure consists of predominantly isolated molecular CO , carbonates, and hydroxyls. More oxygenated species, including tetrahedrally coordinated carbons, hydrogen (O-H-O) bridges, various oxygen-joined complexes appear as melt is further compressed. When two volatiles are incorporated as hydrocarbons CH and C H (corresponding to reducing conditions), hydroxyls are prevalent with notable presence of molecular hydrogen. Carbon-oxygen bonding is almost completely suppressed. Instead carbon is directly correlated with itself, hydrogen, and silicon. Both volatiles also show strong affinity to iron. Reduced volatile speciation thus involves polymerized complexes comprising of carbon, hydrogen, silicon, and iron, which can be mostly represented by two forms: C H Si O (iron-free) and C H Si Fe O . The calculated partial molar volumes of binary volatiles in their oxidized and reduced incorporation decrease rapidly initially with pressure and then gradually at higher pressures, thereby systematically lowering silicate melt density. Our assessment of the calculated opposite effects of the volatile components and iron on melt density indicates that melt-crystal density crossovers are possible in the present-day mantle and also could have occurred in early magma ocean environments. Melts at upper mantle and transition zone conditions likely dissolve carbon and hydrogen in a wide variety of oxidized and non-oxygenated forms. Deep-seated partial melts and magma ocean remnants at lower mantle conditions may exsolve carbon as complex reduced species possibly to the core during core-mantle differentiation while retaining a majority of hydrogen as hydroxyls-associated species. 1−x x 3 2 2 2 4 2 6 1−4 1−5 0−5 0−2 5−8 1−8 0−6 5−8 0−