N. X. Shen

Nitrogen location in R2Fe17 compounds: an NMR study

Abstract 14 N, 89 Y and 1 H spin-echo NMR experiments have been carried out in order to study the nitrogen location in the R 2 Fe 17 lattice with R = Gd, Lu and Y. The observed 14 N spectrum for the Gd-compound indicates that essentially all of the N atoms occupy the octahedral interstitial sites. However, the ‘two-peak’ 14 N spectra for the Y-compounds and, to a lesser degree the Lu-compound, indicate that some of the N atoms enter the tetrahedral interstitial sites in addition to the octahedral sites. A consideration of both 14 N and 89 Y spectra indicates that the probability for a N atom to enter a tetrahedral site in the Y 2 Fe 17 lattice is much smaller than that for octahedral site occupation. This study reveals that the local structure of Y 2 Fe 17 N x may change with the nitrogenation conditions.

Nuclear magnetic resonance study of R2Fe17 (R=Y, Sm, and Gd) hydrides

In order to study the location of hydrogen atoms and the effects of their insertion into R2Fe17, spin–echo NMR experiments have been carried out on the hexagonal Y2Fe17H x (x=0, 3.0, 4.7), rhombohedral Sm2Fe17H x (x=0, 1.7, 5.7), and mixed‐phase Gd2Fe17H x (x=0, 2.3, 5.8) compounds. 1H and 89Y spectra obtained from Y2Fe17H x clearly demonstrate that the hydrogenation process is reversible upon vacuum annealing. For both Y2Fe17H x and Sm2Fe17H x , the 1H spectra show two broad peaks; the peaks are tentatively assigned to H atoms in the tetrahedral and octahedral interstitial sites, and a hydrogen filling scheme is proposed. For Gd2Fe17H x , a single broad 1H peak near 70 MHz is observed; application of an external magnetic field indicates that the hyperfine field has the same direction as the net magnetization.

NMR study of Y2Fe17 nitrides

14 N, 57 Fe and 89 Y spin-echo NMR experiments have been carried out in Y 2 Fe 17 N x , 0 < x < 2.8. It is found that the N distribution in the octahedral sites is not random, and a preferred nitrided (with Y-2N configuration)/unnitrided configuration is present. The change in the Fe hyperfine field distribution with N content provides evidence that lattice expansion plays a major role in the Fe moment enhancement upon nitrogenation

X-ray diffraction, magnetization and nuclear magnetic resonance study of Y2Fe17-xGax

A combined x-ray diffraction, magnetization and nuclear magnetic resonance ( and ) study of the system is presented. It is found that the substitution of larger Ga atoms for Fe causes a transformation from the hexagonal to rhombohedral structure and an overall average lattice expansion of per formula unit per Ga atom. Measurements on magnetically aligned powders yielded values for the anisotropy field and the average moment per Fe atom. Consistent with a change from planar to uniaxial anisotropy, the anisotropy field decreases with increasing Ga content and extrapolates to zero for . The average moment per Fe atom as well as the hyperfine field also decrease with Ga content. Using a calculated value of for the Y moment, values for the hyperfine coupling constants of and were obtained for the on-site and transferred contributions, respectively. The experimental results are discussed in terms of a model for the substitution of Ga for Fe in this system, and its relationship to the enhanced Curie temperature.

Nitrogen diffusion in lattice: a trapping diffusion process

X-ray diffraction, thermoconductivity and nuclear magnetic resonance experiments have been carried out in order to study the diffusion mechanism of N atoms in the lattice. These experiments reveal that a small number of N atoms which enter the lattice, but do not occupy the octahedral interstitial sites, are mobile at the nitrogenation temperature, while most of the N atoms, which enter the octahedral sites, are immobilized. Based on the characteristics of nitrogenation observed in (were R is the rare-earth element) systems, a tripping diffusion model has been proposed and discussed in detail. The previously proposed free diffusion model is the high-temperature approximation of the trapping diffusion model. The experiments and theoretical analysis show that it is the N - lattice interaction, instead of N - N interaction, that leads to the formation of the observed nitrided - unnitrided configuration, causes a tremendous decrease of the apparent diffusion frequency factor and is responsible for the N uptake stability and irreversibility. Also, this work shows how the N - lattice interaction affects the N uptake in various nitrogenation conditions.

X-ray diffraction and magnetization studies on Sm2Fe17 and its nitrides

Abstract Structural and magnetic characterizations of the Sm 2 Fe 17 N x system with x = 0, 0.3, 0.7, 1.3, 1.8, 2.3, and 2.6 are presented. The pure Sm 2 Fe 17 parent phase crystallizes in the Th 2 Zn 17 -like rhombohedral structure, as does ‘completely nitrided’ Sm 2 Fe 17 N 2.6 . Samples with N content 1.3 ≤ x ≤ 2.3 clearly show a two-phase structure, having both an unnitrided phase and a nitrided phase, with the nitrided phase possessing larger lattice parameters. The x = 0.3 sample does not show any nitrided phase, and has magnetic behavior unlike the samples with higher N content. As discussed, it appears that the N atoms which are initially introduced might occupy vacancies, grain boundaries, or sites other than the octahedral interstitial sites as originally proposed.

X‐ray diffraction and nuclear magnetic resonance studies of Y2Fe17

X‐ray structure analysis and 89Y nuclear magnetic resonance (NMR) measurements are presented for the hexagonal and rhombohedral phases of Y2Fe17. The rhombohedral structure has one Y site (designated 6c) and the hexagonal structure has two Y sites (designated 2b and 2d). By combining the x‐ray and NMR results, the hyperfine fields corresponding to the three Y sites in these two phases are obtained. Furthermore, a deconvolution of the NMR spectrum for 89Y in the hexagonal Y2Fe17 phase shows that a disordering exists, resulting in an occupancy for the Y 2b sites of approximately 71%.

Specific Fe site moment enhancement in Y2Fe17 upon N insertion

In order to gain information concerning the separate effects of lattice expansion and Fe–N chemical bonding on the Fe moment for each individual Fe site in R2Fe17Nx systems (R=rare earth), x-ray diffraction, magnetization, and 57Fe nuclear magnetic resonance experiments were carried out on rhombohedral Y2Fe17Nx with N content 0⩽x⩽2.8. The increase of the average 57Fe hyperfine field (HF) from the unnitrided to the nitrided phase follows the increase in the saturation magnetization, providing evidence that the 57Fe HF does scale with the Fe moment in this system. The changes of the 57Fe HF are not the same for the various Fe sites; e.g., the HF at the 18f site, which has one N atom as a nearest neighbor, has much less enhancement than that at the 9d site, which has no N atoms as a nearest neighbor. More significantly, for the Fe 6c site, the HF does not change at all with nitrogen content. These results are analyzed in conjunction with existing neutron diffraction data regarding the Fe–Fe distance variatio...

Structural and magnetic properties of ammonia-nitrided

In order to make a comparison with N2-gas nitrogenation, the structural and magnetic properties of ammonia-nitrided Y2Fe17Nx were investigated by a combination of x-ray diffraction, magnetization and nuclear magnetic resonance. It was found that by using NH3, the nitrogenation temperature and/or time are greatly reduced, and nominal N concentrations 0 6 x 6 6:3 are obtainable. For lower N content,x6 3:3, a two-phase (nitrided + unnitrided) configuration exists in the sample particles. The unnitrided phase is completely suppressed for x> 3:8; however, an amorphous-like phase appears for x> 5:6. The average value for the magnetic moment per Fe atom initially increases with the N content, reaching a maximum forx 4 before decreasing. The magnetic anisotropy, which arises solely from the Fe sublattice, is reduced for higher N content, but is still basal planar. These results indicate that the microscopic structure of the nitrided phase in ammonia-nitrided Y2Fe17Nx is different from that in the same material prepared by N2-gas nitrogenation. Evidence is provided for the existence of more than three, most likely four, nitrogen atoms per formula unit in the nitrided phase when x> 3.