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 Å. However, the distances between other near neighbor iron pairs separated by less than 2.45 Å increases by about 0.04 Å. The nitrogenation of Nd2Fe17 effectively reduces the number of near neighbor iron pain separated by less than 2.45 Å by 92%. However, near neighbor interatomic distances involving the 18h sites are the most affected by nitrogenation. Consequently, the 18h site may play a major role in the enhancement of the Curie temperature due to nitrogenation

A Magnetic and Crystallographic Study of (Sm/Gd)₂(Fe/Si)₁₇C\u3csub\u3ez\u3c/sub\u3e 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 suicide, 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-xSx samples, 280-290°C, are among the highest values observed for a R2Fe17-xMx intermetallic where M is a substituent other than cobalt. Except in the case of SmGdFe16SiCz (z unknown), the Tc of the carbided samples are 20%-25% higher than those of the parent samples. A Tc of 426°C and a magnetization of 120.6 emu/g observed for SmGdFe16SiCz are the highest values measured for the intermetallics investigated herein. As determined by x-ray diffraction studies of magnetically aligned samples, the easy axis of magnetization is parallel to the c axis

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

Magnetic and Structural Properties of Nd₂Fe₁₇₋ₓMnₓ Solid Solutions

A series of Nd2Fe17-xMnx solid solutions with x values between 0 and and 6 were prepared and analyzed using magnetic measurements, neutron diffraction, and Mössbauer 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 she occupancies in Nd2Fe17-xMnx

Neutron Diffraction and Magnetic Studies of Nd₂Fe₁₇₋ₓTₓ (T=Si, Mn) Alloys

We have carried out neutron and magnetic studies on Nd2Fe17-xTx (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 Nd2Fe17-xTx 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\u3c4

Thermal Expansion Anomalies in R₂Fe₁₇ Compounds Before and after Nitrogenation (R: Y and Sm)

The lattice expansion of R2Fe17 and R2Fe17N3 compounds (R: Y and Sm) have been measured by X-ray diffractometry from 12 K up to 873 K. Upon nitrogenation, the low temperature anomalous expansion along the c-axis disappears; nevertheless, the spontaneous volume magnetostriction is larger

A Mössbauer Spectral Study of the Sm₂Fe₁₇₋ₓAlₓ Solid Solutions

The Mössbauer spectra of the Sm2Fe17-xAlx solid solutions, where x is 0, 0.5, 0.75, 1, 3, and 4, which crystallize in the rhombohedral Th2Zn17 structure, have been measured between 85 and 295 K. The spectra have been successfully analyzed with a model based on the binomial distribution of aluminum near neighbors calculated from the aluminum site occupancies derived from neutron diffraction measurements on Nd2Fe17-xAlx solid solutions. This analysis indicates that the Mössbauer spectra are sensitive to the spin reorientation occurring between x values of one and three, confirms that the aluminum site occupancies are independent of the rare-earth element in the R2Fe17-xAlx solid solutions, gives values of the iron site magnetic moments, and reveals an important covalent bonding between iron and its aluminum near neighbors

Neutron Diffraction Structural Studies of Nd₂Fe₁₇₋ₓMₓCy

Four samples of Nd2Fe17MxCy (where M = Al, Ga, Ti, V) were synthesized using rf induction melting and C was introduced from the melt. Structural studies were then carried out using neutron diffraction. All samples were confirmed to have the Th2Zn17-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

A Mössbauer Effect Study of the Magnetic Properties of a Series of Tb₂Fe₁₇₋ₓSiₓ Solid Solutions

The Mössbauer 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 basal magnetization. The spectral analysis yields reasonable hyperfine parameters and the expected changes with temperature. The resulting weighted average effective recoil iron mass of 67 g/mol and the Mössbauer temperature of 396 K are typical of this type of intermetallic compound. In addition, the isomer shifts and hyperfine fields observed for the four 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 binomial distribution of near-neighbor environments. At 295 K the weighted average hyperfine field remains virtually constant for x=0, 1, and 2, and decreased slightly for x= 3. In contrast, the weighted average isomer shift increases with increasing x