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

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• 作者：Kai Guo ; Dieter Rau ; Lorenzo Toffoletti ; Carola M眉ller ; Ulrich Burkhardt ; Walter Schnelle ; Rainer Niewa ; Ulrich Schwarz
• 刊名：Chemistry of Materials
• 出版年：2012
• 出版时间：December 11, 2012
• 年：2012
• 卷：24
• 期：23
• 页码：4600-4606
• 全文大小：450K
• 年卷期：v.24,no.23(December 11, 2012)
• ISSN：1520-5002

文摘

High-pressure, high-temperature synthesis gives access to ternary metastable nitrides 蔚-Fe₂TMN (TM = Co, Ni) as bulk materials for the first time. Both 蔚-Fe₂CoN and 蔚-Fe₂NiN crystallize isostructural to 蔚-Fe₃N as evidenced by X-ray powder diffraction data. The lattice parameters of the new compounds are slightly smaller than those of 蔚-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 Fe_1.99(6)Co_1.01(6)N and Fe_1.97(2)Ni_1.03(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. 蔚-Fe₂CoN disintegrates into N₂ and iron鈥揷obalt alloy, while 蔚-Fe₂NiN decays into N₂, iron鈥搉ickel alloy as well as 伪-Fe. The replacement of iron by cobalt or nickel essentially lowers the saturation magnetization from roughly 6.0 渭_B/f.u. for 蔚-Fe₃N to nearly 4.3 渭_B/f.u. for 蔚-Fe₂CoN and 3.1 渭_B/f.u. for 蔚-Fe₂NiN. In parallel, the Curie temperature decreases from 575(3) K for 蔚-Fe₃N to 488(5) K for 蔚-Fe₂CoN and 234(3) K for 蔚-Fe₂NiN. Calculations of the formation enthalpies illustrate that the phases 蔚-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.

Keywords:

nitride materials; transition-metal substitution; high-pressure synthesis; magnetization; first-principles calculations