Peter C. Ezekwenna

Magnetic and structural properties of Nd2Fe17−xMnx solid solutions

A series of Nd2Fe17−xMnx solid solutions with x values between 0 and 6 were prepared and analyzed using magnetic measurements, neutron diffraction, and Mossbauer spectroscopy. All of the Nd2Fe17−xMnx samples crystallized in the Th2Zn17−x-type rhombohedral structure. The lattice parameters and unit cell volumes decrease with increasing manganese content up to ∼x equal to 2, and then increase for higher manganese content. The magnetizations of Nd2Fe17−xMnx decrease with increasing manganese content and Nd2Fe17−xMnx is paramagnetic at room temperature for x greater than 3. The Curie temperature in Nd2Fe17−xMnx solid solutions is maximum for x equal to 0.5 and decreases at a rate of ∼10° per substituted manganese up to x equal to 3, after which it drops sharply. These results are discussed in terms of the manganese site occupancies in Nd2Fe17−xMnx.

Neutron diffraction and magnetic studies of Nd/sub 2/Fe/sub 17-x/T/sub x/ (T=Si, Mn) alloys

We have carried out neutron and magnetic studies on Nd/sub 2/Fe/sub 17-x/T/sub x/ (T=Si, Mn) alloys. The unit cell of the compounds contracts with both Si and Mn substitution. Si atoms avoid the 6c site, prefer the 18h site strongly, fill the 9d site steadily and fill the 18f site only at relatively high Si concentration. Mn atoms avoid the 9d sites but prefer the 6c site strongly, while the 18f site and 18h site fill slowly. The Si site occupancies appear to be dominated by the crystal environment while the Mn site occupancies are dominated by steric considerations. The Curie temperature of the compound Nd/sub 2/Fe/sub 17/ increases significantly with Si substitution but is only affected slightly by the Mn substitution. Although the unit cell contracts with increasing Si concentration, the average length of bonds to the Fe(6c) and the Fe(18f) sites increase slightly at x<4.

A magnetic and crystallographic study of (Sm/Gd)2(Fe/Si)17Cz solid solutions

The crystallographic and magnetic properties of SmyGd2−yFe17−xSix (0⩽x⩽3 and y=1 and 1.5) solid solutions and their interstitial carbides have been investigated using x-ray diffraction and magnetic measurements. The SmyGd2−yFe17−xSix samples crystallized in the rhombohedral Th2Zn17 structure with less than 5 mol % of impurities. The unit cells of the mixed rare-earth (R) samples are smaller than those of Sm2Fe17 and Gd2Fe17. The carbided samples contain up to a total of 15 mol % of free iron, an iron silicide, and/or cubic Si5C3. The unit cells of the carbided samples are 1%–4% larger than those of the parent samples. For a given silicon concentration, the Curie temperatures (Tc) of SmyGd2−yFe17−xSix intermetallics are higher than those of the two end members. For example, the Tc of SmGdFe17 (280 °C) is approximately 160° and 80° higher than that of Sm2Fe17 and Gd2Fe17, respectively. The Tc measured for the SmyGd2−yFe17−xSix samples, 280–290 °C, are among the highest values observed for a R2Fe17−xMx inter...

A Mössbauer spectral study of Tb2Fe17 and the Tb2Fe17−xSix solid solutions

The Mossbauer spectra of a series of rhombohedral Tb2Fe17−xSix solid solutions, with x equal to 0, 1, 2, and 3, have been measured as a function of temperature. Although the spectra of Tb2Fe17 change substantially upon cooling from 295 to 85 K, it has been possible to fit them with a consistent seven sextet model corresponding to a basal magnetization. The spectral analysis yields reasonable hyperfine parameters and the expected changes with temperature. The resulting weighted average effective iron recoil mass of 66 g/mol and the Mossbauer temperature of 395 K are typical of this type of intermetallic compound. In addition, the isomer shifts and the hyperfine fields observed for the crystallographically distinct iron sites in Tb2Fe17 agree well with those expected from the differences in the Wigner–Seitz cell volumes and the near-neighbor environments of the four sites. The spectra of the silicon substituted solid solutions have been fit with the same model and similar hyperfine parameters, but with a bi...

Neutron diffraction structural studies of Nd/sub 2/Fe/sub 17-x/M/sub x/C/sub y/

Four samples of Nd/sub 2/Fe/sub 17-x/M/sub x/C/sub y/ (where M=Al,Ga,Ti,V) were synthesized using rf induction melting and C was introduced from the melt. Structural studies were the carried out using neutron diffraction. All samples were confirmed to have the Th/sub 2/Zn/sub 17/-type rhombohedral structure with expanded unit cell volumes. However, the volume effect, as well as the site preference of the substitutional atoms (M) is different among these samples and is different compared to their uncarbided counterparts. The C atoms were found to reside in the 9e site for all samples. The observed Curie temperature is higher for samples with higher C concentrations.

Neutron diffraction and Mössbauer spectral study of Nd2Fe16Ti and its nitride

The 295 K powder neutron diffraction patterns and the temperature dependence of the Mossbauer spectra of Nd2Fe17−xTix and Nd2Fe17−xTixNy have been measured. A Rietveld refinement of the neutron diffraction patterns yields the Nd2Fe16.32Ti0.68 and Nd2Fe16.32Ti0.68N2.7 stoichiometries for the two compounds and indicates that titanium occupies only the 6c crystallographic site and nitrogen only the 9e site in the rhombohedral Th2Zn17 structure. The insertion of interstitial nitrogen into Nd2Fe16.32Ti0.68 produces a 6.8% increase in the unit cell volume. The relative areas of the Mossbauer spectral components indicate that iron–iron, iron–titanium, and titanium–titanium 6c–6c dumbbell pairs exist in both of these materials. The magnetic hyperfine fields observed for the iron on the 6c site in the iron–titanium dumbbell pair in both compounds is substantially reduced from that found in Nd2Fe17 and Nd2Fe17N2.6 because of the titanium dumbbell near neighbor. For all the remaining hyperfine parameters there is a ...

Neutron diffraction and Mössbauer spectral study of the Nd2Fe16TiCx solid solutions

Three samples of Nd2Fe16TiCx, with x equal to 0.0, 0.3, and 2.8, with the Th2Zn17‐type rhombohedral structure, have been studied by powder x‐ray and neutron diffraction, magnetic measurements, and Mossbauer spectroscopy. Nd2Fe16Ti and Nd2Fe16TiC0.3 were synthesized by induction melting stoichiometric amounts of the constituent elements, whereas Nd2Fe16TiC2.8 was synthesized by methane‐derived gas phase insertion of carbon into finely ground Nd2Fe16Ti at 600 K. The neutron diffraction determined titanium site occupancies are similar in both Nd2Fe16Ti and Nd2Fe16TiC2.8 in which titanium preferentially occupies the 6c transition metal site. In contrast, the titanium occupancies in Nd2Fe16TiC0.3 are markedly different in that titanium avoids the 6c transition metal site and randomly occupies the other three transition metal sites. This difference in occupancies most likely occurs because the titanium diffusion rate during the quenching of Nd2Fe16TiC0.3 is affected by the presence of carbon in the melt. Even t...

Studies of V, Nb, Cr, and Zr substituted 2:17 compounds and their carbides using neutron diffraction

Samples of Nd2Fe17−xTxCy with T=V, Nb, Cr, and Zr were prepared by melting of the constituent elements including C and studied by neutron diffraction. Comparing with their uncarbided counterparts, we found that the substituents transfer, more or less, from the 6c site to the 18f and 18h sites with the introduction of C atoms. This behavior appears to relate to the electronegativities between the C atoms and the early transition series elements because the 18f and 18h sites are near neighbors of the interstitial C site. However, the C effects in the V, Nb, Cr, and Zr samples are not as strong as those in Ti samples. SQUID measurements show that the Curie temperatures of these samples depend on both the interstitial C atoms and the substituents.

Neutron Diffraction and Magnetic Studies of Nd 2 Fe 17-x T x (T=Si, Mn) Alloys

We have carried out neutron and magnetic studies on Nd/sub 2/Fe/sub 17-x/T/sub x/ (T=Si, Mn) alloys. The unit cell of the compounds contracts with both Si and Mn substitution. Si atoms avoid the 6c site, prefer the 18h site strongly, fill the 9d site steadily and fill the 18f site only at relatively high Si concentration. Mn atoms avoid the 9d sites but prefer the 6c site strongly, while the 18f site and 18h site fill slowly. The Si site occupancies appear to be dominated by the crystal environment while the Mn site occupancies are dominated by steric considerations. The Curie temperature of the compound Nd/sub 2/Fe/sub 17/ increases significantly with Si substitution but is only affected slightly by the Mn substitution. Although the unit cell contracts with increasing Si concentration, the average length of bonds to the Fe(6c) and the Fe(18f) sites increase slightly at x<4.

An analysis of iron-iron interatomic distances in several rare earth transition metal intermetallics

As a part of a systematic study of the dependence of iron-iron exchange interactions on near neighbor iron-iron distances in rare earth transition metal magnetic materials, we have conducted a statistical analysis of interatomic distances in Nd2Fe17, Nd2Fe17N3, and Nd2Fe14B. Results, in general, support the notion that larger near neighbor iron-iron distances promote higher Curie temperatures. In this work, special attention has been paid to the expansion of the Nd2Fe17 lattice due to interstitial nitrogenation and the accompanying increase in the Curie temperature. Within the unit cell, the expansion of the Nd2Fe17 lattice due to nitrogenation is highly nonuniform. When nitrided, the distance between near neighbor 6c iron sites in Nd2Fe17 increases only slightly, by 0.021 A. However, the distances between other near neighbor iron pairs separated by less than 2.45 A increases by about 0.04 A. The nitrogenation of Nd2Fe17 effectively reduces the number of near neighbor iron pairs separated by less than 2.4...