Fe–N System at High Pressures and Its Relevance to the Earth’s Core Composition

Abstract

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 Fe 4 N 3 -Imm2, Fe 2 N-Pnma, and Fe 3 N-C2/m were predicted. Fe 4 N 3 was shown to be stable at pressures up to 266 GPa and then decompose into Fe 2 N + 2FeN. Predicted Fe 2 N-Pnma becomes stable with respect to the decomposition reaction 9Fe 2 N = Fe 4 N 3 + 2Fe 7 N 3 at pressures above 221 GPa. Fe 3 N-C2/m stabilizes with respect to decomposition into 2Fe + Fe 7 N 3 at pressures above 265 GPa. Also, it was shown that β-Fe 7 N 3 synthesized in diamond anvil cell experiments has an orthorhombic Pbca structure, and at pressures above ∼320 GPa decomposes into 2Fe 2 N + Fe 3 N. All predicted Fe-rich iron nitrides, except Fe 4 N 3 -Imm2, have structural analogs among iron carbides. Considering the temperature effect, we observed that FeN-P2 1 3, Fe 2 N-Pnma, and Fe 3 N-C2/m can be stable at the Earth's inner core pressures and temperatures up to 4000 K, whereas Fe 4 N 3 -Imm2 and β-Fe 7 N 3 are thermodynamically unstable in the entire studied temperature range. Although Fe 7 N 3 -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.

First Finding of High-Pressure Modifications of Na2CO3 and K2CO3 with sp3-Hybridized Carbon Atoms

“…Speaking about the sp 3 structures of carbonates, it should be also mentioned the recently found intermediate compounds in the MO−CO 2 (M = Mg, Ca, Sr, Ba) and M′ 2 O−CO 2 (M′ = Li, Na, K) systems, including Mg 2 CO 4 , 14,15 Ca 2 CO 4 , 16,17 Sr 2 CO 4 , 6,18 6,20 Sr 3 CO 5 , 6,21 Ba 3 CO 5 6 and CaC 2 O 5 . 20,22,23 The sp 3 -hybridized structures of these compounds became thermodynamically stable at much lower pressures than sp 3 structures of classical carbonates. The pressures of stabilization vary in the range of 5−25 GPa.…”

Synthesis and Structure of Two New Cadmium Carbonates at Extreme Conditions

“…In CaC 2 O 5 (I4̅ 2d), the [CO 4 ] 4− tetrahedra polymerize to form pyramidal [C 4 O 10 ] 4− anionic groups. 16,17 In order to better understand the systematics of phase relations in carbonates, it is therefore of interest to explore correlations between the constituents and phases obtained by reactions in closely related systems. This will allow us to distinguish between general trends common to several carbonates in contrast to features specific to the CaCO 3 system.…”