Static compression of Fe4N to 77 GPa and its implications for nitrogen storage in the deep Earth

Compression and decompression experiments on face-centered cubic (fcc) gamma'-Fe4N 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, gamma'-Fe4N 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-0 = 169 (6) GPa, K' = 4.1 (4), with a fixed V-0 = 54.95 angstrom at 1 bar. A quantitative Schreinemakers web was obtained at 15-60 GPa and 300-1600 K by combining the EoS for gamma'-Fe4N with reported phase stability data at low pressures. The web indicates the existence of an invariant point at 41 GPa and 1000 K where gamma'-Fe4N, hexagonal closed-packed (hcp) epsilon-Fe7N3, double hexagonal closed-packed beta-Fe7N3, and hcp Fe phases are stable. From the invariant point, a reaction gamma'-Fe4N = beta-Fe7N3 + hcp Fe originates toward the high-pressure side, which determines the high-pressure stability of gamma'-Fe4N at 56 GPa and 300 K. Therefore, the gamma'-Fe4N phase observed in the experiments beyond this pressure must be metastable. The obtained results support the existing idea that beta-Fe7N3 would be the most nitrogen-rich iron compound under core conditions. An iron carbonitride Fe-7(C,N)(3) found as a mantle-derived diamond inclusion implies that beta-Fe7N3 and Fe7C3 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-7(C,N)(3) in diamond can be an indicator of water transportation to the deep mantle.