Ternary Metastable Nitrides ε-Fe2TMN (TM = Co, Ni): High-Pressure, High-Temperature Synthesis, Crystal Structure, Thermal Stability, and Magnetic Properties

Abstract

High-pressure, high-temperature synthesis gives access to ternary metastable nitrides ε-Fe2TMN (TM = Co, Ni) as bulk materials for the first time. Both ε-Fe2CoN and ε-Fe2NiN crystallize isostructural to ε-Fe3N as evidenced by X-ray powder diffraction data. The lattice parameters of the new compounds are slightly smaller than those of ε-Fe3N owing to the reduced atomic radii of the metal atoms. Energy-dispersive X-ray spectroscopy of metallographic samples show homogeneous metal ratios corresponding to compositions Fe1.99(6)Co1.01(6)N and Fe1.97(2)Ni1.03(2)N. Extended X-ray absorption fine spectra indicate that cobalt and nickel occupy iron positions. Thermal analysis measurements reveal decomposition of both ternary nitrides above 920 K. ε-Fe2CoN disintegrates into N2 and iron–cobalt alloy, while ε-Fe2NiN decays into N2, iron–nickel alloy as well as α-Fe. The replacement of iron by cobalt or nickel essentially lowers the saturation magnetization from roughly 6.0 μB/f.u. for ε-Fe3N to nearly 4.3 μB/f.u. for ε-Fe2CoN and 3.1 μB/f.u. for ε-Fe2NiN. In parallel, the Curie temperature decreases from 575(3) K for ε-Fe3N to 488(5) K for ε-Fe2CoN and 234(3) K for ε-Fe2NiN. Calculations of the formation enthalpies illustrate that the phases ε-Fe2TMN (TM = Co, Ni) are thermodynamically unfavorable at ambient conditions which is consistent with our experimental observations. The substitution of one Fe by Co (Ni) yields one (two) more electrons per formula unit which reduces the magnetic interactions. First-principles analysis indicate that the replacement has a negligible influence on the electron occupation numbers and spin moments of the N and unsubstituted Fe sites, but decreases the local magnetic moments on the substituted Fe positions because the extra electrons occupy the minority-spin channel formed by states of the TM atoms.