LIQUID STRUCTURE OF IRON AND IRON-NITROGEN ALLOYS WITHIN THE CORES OF SMALL TERRESTRIAL BODIES

The atomic structure of molten iron alloys at high pressures and temperatures is important to interpreting and predicting the physical properties of planetary cores. Observable properties of planetary cores, such as density, are determined by the structure of iron mixed with other light elements. Although nitrogen and carbon are candidate light elements that could be present in planetary cores, no previous studies have investigated the physical properties of Fe-N liquids to constrain the abundance of nitrogen in cores and help distinguish nitrogen from other light elements. To obtain data about the structure of liquid Fe-N under high pressures and temperatures, metallic iron or Fe-N alloy powders with compositions ranging from 0 – 9 wt.% nitrogen were loaded into the Paris-Edinburgh Cell (PEC) at Argonne National Laboratory, the Advanced Photon Source, 16 BM-B. Our results on the liquid structure of pure Fe produce a nearest neighbor Fe-Fe spacing of 2.52-2.49 Angstroms from 0-6.5 GPa and 2100 K, ~2% lower than nearest neighbor Fe-Fe distances observed in previous studies that employed boron nitride capsules. With increasing N content in the melt up to 9 wt.%, the atomic spacing between Fe atoms increased by ~1% at 5.5 GPa, decreasing the density of the melt. This study is the first to systematically determine the liquid structure of Fe-N alloys with variable nitrogen contents relevant to planetary bodies such as the moon, Mercury, and Ganymede.