Ternary Metastable Nitrides
ε‑Fe<sub>2</sub><i>TM</i>N (<i>TM</i> = Co, Ni): High-Pressure,
High-Temperature Synthesis, Crystal Structure, Thermal Stability,
and Magnetic Properties
- Publication date
- Publisher
Abstract
High-pressure, high-temperature synthesis gives access
to ternary
metastable nitrides ε-Fe<sub>2</sub><i>TM</i>N (<i>TM</i> = Co, Ni) as bulk materials for the first time. Both
ε-Fe<sub>2</sub>CoN and ε-Fe<sub>2</sub>NiN crystallize
isostructural to ε-Fe<sub>3</sub>N as evidenced by X-ray powder
diffraction data. The lattice parameters of the new compounds are
slightly smaller than those of ε-Fe<sub>3</sub>N 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 Fe<sub>1.99(6)</sub>Co<sub>1.01(6)</sub>N and Fe<sub>1.97(2)</sub>Ni<sub>1.03(2)</sub>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-Fe<sub>2</sub>CoN disintegrates into N<sub>2</sub> and iron–cobalt alloy, while ε-Fe<sub>2</sub>NiN decays
into N<sub>2</sub>, 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 μ<sub>B</sub>/f.u.
for ε-Fe<sub>3</sub>N to nearly 4.3 μ<sub>B</sub>/f.u.
for ε-Fe<sub>2</sub>CoN and 3.1 μ<sub>B</sub>/f.u. for
ε-Fe<sub>2</sub>NiN. In parallel, the Curie temperature decreases
from 575(3) K for ε-Fe<sub>3</sub>N to 488(5) K for ε-Fe<sub>2</sub>CoN and 234(3) K for ε-Fe<sub>2</sub>NiN. Calculations
of the formation enthalpies illustrate that the phases ε-Fe<sub>2</sub><i>TM</i>N (<i>TM</i> = 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 <i>TM</i> atoms
