Static compression of FeN to 77 GPa and its implications for nitrogen storage in the deep Earth

Abstract

Compression and decompression experiments on face-centered cubic (fcc) γʹ -FeN to 77 GPa at room temperature were conducted in a diamond-anvil cell with in situ X ray diffraction (XRD) to examine its stability under high pressure. In the investigated pressure range, γʹ -FeN did not show any structural transitions. However, a peak broadening was observed in the XRD patterns above 60 GPa. The obtained pressure-volume data to 60 GPa were fitted to the third-order Birch-Murnaghan equation of state (EoS), which yielded the following elastic parameters: K = 169 (6) GPa, K′ = 4.1 (4), with a fixed V = 54.95 Å at 1 bar. A quantitative Schreinemakers' web was obtained at 15-60 GPa and 300-1600 K by combining the EoS for γʹ -FeN with reported phase stability data at low pressures. The web indicates the existence of an invariant point at 41 GPa and 1000 K where γʹ -FeN, hexagonal closed-packed (hcp) ϵ-FeN, double hexagonal closed-packed β-FeN, and hcp Fe phases are stable. From the invariant point, a reaction γʹ -FeN = β-FeN + hcp Fe originates toward the high-pressure side, which determines the high-pressure stability of γʹ -FeN at 56 GPa and 300 K. Therefore, the γʹ -FeN phase observed in the experiments beyond this pressure must be metastable. The obtained results support the existing idea that β-FeN would be the most nitrogen-rich iron compound under core conditions. An iron carbonitride Fe(C,N) found as a mantle-derived diamond inclusion implies that β-FeN and FeC may form a continuous solid solution in the mantle deeper than 1000 km depth. Diamond formation may be related to the presence of fluids in the mantle, and dehydration reactions of high-pressure hydrous phase D might have supplied free fluids in the mantle at depths greater than 1000 km. As such, the existence of Fe(C,N) in diamond can be an indicator of water transportation to the deep mantle.