TY - JOUR

T1 - Fe-N System at High Pressures and Its Relevance to the Earth's Core Composition

AU - Sagatov, Nursultan E.

AU - Sagatova, Dinara N.

AU - Gavryushkin, Pavel N.

AU - Litasov, Konstantin D.

N1 - Funding Information: We thank the Information and Computing Center of Novosibirsk State University for providing access to the cluster computational resources. This study was financially supported by RFBR, project number 19-35-90043. P.N.G. and D.N.S. were supported by the state assignment of IGM SB RAS and K.D.L. was supported by the state assignment of IHPP RAS. Publisher Copyright: © 2021 American Chemical Society.

PY - 2021/11/3

Y1 - 2021/11/3

N2 - Based on ab initio calculations within the density functional theory and crystal structure prediction algorithms, the structure and stability of iron-nitrogen compounds in the pressure range of 100-400 GPa and temperatures up to 4000 K were determined. Three new iron nitrides Fe4N3-Imm2, Fe2N-Pnma, and Fe3N-C2/m were predicted. Fe4N3 was shown to be stable at pressures up to 266 GPa and then decompose into Fe2N + 2FeN. Predicted Fe2N-Pnma becomes stable with respect to the decomposition reaction 9Fe2N = Fe4N3 + 2Fe7N3 at pressures above 221 GPa. Fe3N-C2/m stabilizes with respect to decomposition into 2Fe + Fe7N3 at pressures above 265 GPa. Also, it was shown that β-Fe7N3 synthesized in diamond anvil cell experiments has an orthorhombic Pbca structure, and at pressures above ∼320 GPa decomposes into 2Fe2N + Fe3N. All predicted Fe-rich iron nitrides, except Fe4N3-Imm2, have structural analogs among iron carbides. Considering the temperature effect, we observed that FeN-P213, Fe2N-Pnma, and Fe3N-C2/m can be stable at the Earth's inner core pressures and temperatures up to 4000 K, whereas Fe4N3-Imm2 and β-Fe7N3 are thermodynamically unstable in the entire studied temperature range. Although Fe7N3-Pbca is thermodynamically unstable at inner core pressures, it shows the closest coincidence of the S- and P-wave velocities with seismic observations among the studied Fe-nitrides. Overall, Fe-nitrides cannot be the major compounds in the inner core of the Earth and can only substitute other elements such as carbon in Fe-carbides in minor amounts.

AB - Based on ab initio calculations within the density functional theory and crystal structure prediction algorithms, the structure and stability of iron-nitrogen compounds in the pressure range of 100-400 GPa and temperatures up to 4000 K were determined. Three new iron nitrides Fe4N3-Imm2, Fe2N-Pnma, and Fe3N-C2/m were predicted. Fe4N3 was shown to be stable at pressures up to 266 GPa and then decompose into Fe2N + 2FeN. Predicted Fe2N-Pnma becomes stable with respect to the decomposition reaction 9Fe2N = Fe4N3 + 2Fe7N3 at pressures above 221 GPa. Fe3N-C2/m stabilizes with respect to decomposition into 2Fe + Fe7N3 at pressures above 265 GPa. Also, it was shown that β-Fe7N3 synthesized in diamond anvil cell experiments has an orthorhombic Pbca structure, and at pressures above ∼320 GPa decomposes into 2Fe2N + Fe3N. All predicted Fe-rich iron nitrides, except Fe4N3-Imm2, have structural analogs among iron carbides. Considering the temperature effect, we observed that FeN-P213, Fe2N-Pnma, and Fe3N-C2/m can be stable at the Earth's inner core pressures and temperatures up to 4000 K, whereas Fe4N3-Imm2 and β-Fe7N3 are thermodynamically unstable in the entire studied temperature range. Although Fe7N3-Pbca is thermodynamically unstable at inner core pressures, it shows the closest coincidence of the S- and P-wave velocities with seismic observations among the studied Fe-nitrides. Overall, Fe-nitrides cannot be the major compounds in the inner core of the Earth and can only substitute other elements such as carbon in Fe-carbides in minor amounts.

UR - http://www.scopus.com/inward/record.url?scp=85117460679&partnerID=8YFLogxK

U2 - 10.1021/acs.cgd.1c00432

DO - 10.1021/acs.cgd.1c00432

M3 - Article

AN - SCOPUS:85117460679

VL - 21

SP - 6101

EP - 6109

JO - Crystal Growth and Design

JF - Crystal Growth and Design

SN - 1528-7483

IS - 11

ER -