First-principles simulations of liquid iron-heavy element alloys at high pressure

Abstract

Iron-rich metallic liquid in the Earth's outer core is thought to contain many elements. While the incorporation of light elements in liquid iron has been widely studied, liquid iron-heavy element mixtures, particularly considering Co, Mo, and W which are relevant in constraining core-forming conditions need more studies. Here we investigate the thermodynamic and structural behavior of these siderophile elements and Ni dissolved in liquid iron in the amount of 2.67 atom% using first-principles molecular dynamics over the pressure range of the entire Earth's interior at temperatures 4000 to 7000 K. The calculated pressure-volume-temperature results of these iron-rich alloys are accurately described by adding the appropriative terms to the equation of state of pure iron liquid to account for the effects of temperature and impurity. 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 substitutional incorporation mechanism. The lack of clustering activity among the impurity atoms means that each of these impurity elements is soluble in liquid iron. Our analysis also shows that Mo and W increase the liquid density much more than Ni and Co mainly for mass reason while the bulk modulus remains essentially unaffected in all cases. The presence of heavy elements widens the apparent density gap between iron-rich liquid and the outer core so additional amounts of lighter impurity elements should be considered when constraining the core composition.