Effect of Silicon, Carbon, and Sulfur on Structure of Liquid Iron and Implications for Structure-Property Relations in Liquid Iron-Light Element Alloys

Abstract

Planetary cores are considered to be composed of iron (Fe) and light elements. Extensive studies on structure and properties of crystalline Fe-light element alloys have been carried out to discuss composition and structure of the solid inner core. In contrast, structure and properties of liquid Fe-light element alloys, which are important to discuss nature and dynamics of the liquid outer core, remain poorly understood. Here we conducted series of liquid structure measurements to systematically understand effect of silicon (Si), carbon (C), and sulfur (S) on structure of liquid Fe. We found that incorporation of Si in liquid Fe shortens the nearest (r1) and second (r2) neighbor distances, while incorporation of C and small amount of S expands these distances. The different structural behavior is interpreted in view of different light-element incorporation mechanisms: substitutional incorporation of Si and interstitial incorporation of C and S. All light elements lower density of liquid Fe regardless of shortening or expansion of the r1 and r2 distances, while P-wave velocities of liquid Fe-light element alloys show linear increase with shortening of the r1 and r2. The different light-element incorporation behavior in the structure of liquid Fe and the resultant variation in the properties of liquid Fe-light element alloys should be important in understanding behavior of liquid Fe-light element alloys in the planetary cores.