Sm2Fe17Nx: Site and valence of the interstitial nitrogen (abstract)
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abstract
The discovery that nitriding Sm2Fe17 greatly enhances its Curie temperatures and the magnetic anisotropy, making it potentially attractive for permanent magnet applications, has intensified research efforts to understand the physical and chemical properties of this material. In this work we have combined x-ray diffraction, x-ray absorption near edge structure (XANES), and extended x-ray absorption fine structure (EXAFS) to determine the position and valence of the nitrogen atoms in the expanded Sm2Fe17 lattice following nitriding. The XANES from the Sm L3 edge and Fe K edge of Sm2Fe17 before and after nitriding reveals that the valences of both Sm and Fe are unchanged. This implies that the nitrogen is incorporated covalently into this material, i.e., there is no net charge transfer between the N atoms and the Sm or Fe. From the observed changes in the Sm EXAFS, we determine the changes in the local Sm environment produced by nitriding. It is determined that in Sm2Fe17Nx there are 2.8±1.0 nitrogen atoms at a distance of 0.254 nm from the Sm site. X-ray diffraction patterns of the Sm2Fe17 before and after nitriding show the original Th2Zn17 lattice expands such that c and a lattice parameters change from 1.2435 and 0.8548 nm to 1.2639 and 0.8731 nm, respectively. The above valence, coordination number, Sm-N distance, and lattice parameters are consistent with N atoms occupying the 9(e) interstitial sites of the expanded lattice.
