A Second-Generation Janus Scorpionate Ligand: Controlling Coordination Modes in Iron(II) Complexes by Steric Modulation

The second-generation Janus scorpionate ligand [HB(mtdaMe)3−] containing methyl-mercaptothiadiazolyl (mtdaMe) heterocyclic rings and (N,N,N-) and (S,S,S-) binding pockets has been prepared. The effect of methyl substitution versus the unsubstituted first-generation Janus scorpionate [HB(mtda)3]− on the coordination chemistry with alkali metals and on the binding preferences and on the ground spin state of iron(II) complexes has been studied structurally and by 57Fe Mossbauer Spectroscopy

Pyrazolyl Methyls Prescribe the Electronic Properties of Iron(II) Tetra(pyrazolyl)lutidine Chloride Complexes

A series of iron(II) chloride complexes of pentadentate ligands related to α,α,α′,α′-tetra(pyrazolyl)-2,6-lutidine, pz4lut, has been prepared to evaluate whether pyrazolyl substitution has any systematic impact on the electronic properties of the complexes. For this purpose, the new tetrakis(3,4,5-trimethylpyrazolyl)lutidine ligand, pz**4lut, was prepared via a CoCl2-catalyzed rearrangement reaction. The equimolar combination of ligand and FeCl2 in methanol gives the appropriate 1:1 complexes [FeCl(pzR4lut)]Cl that are each isolated in the solid state as a hygroscopic solvate. In solution, the iron(II) complexes have been fully characterized by several spectroscopic methods and cyclic voltammetry. In the solid state, the complexes have been characterized by X-ray diffraction, and, in some cases, by Mössbauer spectroscopy. The Mössbauer studies show that the complexes remain high spin to 4 K and exclude spin-state changes as the cause of the surprising solid-state thermochromic properties of the complexes. Non-intuitive results of spectroscopic and structural studies showed that methyl substitution at the 3- and 5- positions of the pyrazolyl rings reduces the ligand field strength through steric effects whereas methyl substitution at the 4-position of the pyrazolyl rings increases the ligand field strength through inductive effects

A Mossbauer spectral study of some iron nitride-based nanocomposites prepared by ball milling

Powder mixtures of (FeyN)(x) and (Al2O3)(1-x) or (SiO2)(1-x) with x = 0.2 and 0.6 and y = 3.8 have been ball milled for 4, 8, 16, 32, 64, and 128 h and their magnetic properties and Mossbauer spectra have been measured. The 5 and 295 K saturation magnetizations decrease with increasing milling time as a result of decreasing particle sizes. Fits with the Langevin function of the field dependence of the magnetization yield particle sizes that are slightly smaller than those determined from a Scherrer analysis of the broadening of the X-ray diffraction peaks. The Mossbauer spectra have been fit with a distribution of hyperfine fields between 0 and 40 T and the peaks in the distribution assigned to the different iron nitride phases present in the nanocomposites. This analysis of the Mossbauer spectra indicates that the gamma'-Fe4N phase present in FeyN is transformed into epsilon-Fe3+xN and alpha-iron by the milling process. The weighted average field decreases and the weighted average isomer shift increases with milling time, as expected for the simultaneous effects of size reduction, the above phase transformation, and mixing with the oxides. A peak in the hyperfine field distribution at 4 T indicates the presence of small superparamagnetic particles, a presence which is also confirmed by the temperature dependence of the Mossbauer spectra and the failure of the magnetization to saturate even at applied fields of 5.5T. (c) 2004 Elsevier B.V. All rights reserved

An Electrical Resistivity Study of Ce₂Fe₁₆.₈ and the Ce₂Fe₁₇₋ₓAlₓ and Ce₂Fe₁₇₋ₓSiₓ Solid Solutions

Electrical resistivity measurements have been carried out between 20 and 300 K on Ce2Fe16.8, on the Ce2Fe17-xAlx. solid solutions, with x = 0.4, 1, 2, 5, 6, 8, and 9, and on the Ce2Fe17-xSix solid solutions, with x = 0.2, 0.4, 1, and 2, with the four probe method. The temperature dependence of the resistivity of Ce2Fe16.8 shows an inflection at 110 K, a feature which is also observed in the temperature dependence of the magnetization and is related to a magnetic phase transition from a helical structure, above 110 K, to a fan structure, below 110 K. The temperature dependence of the resistivity of Ce2Fe16.8 is characteristic over the investigated temperature range of weak antiferromagnetic behavior with incommensurate periodicity as has been observed in the earlier neutron diffraction study. The resistivity of Ce2Fe17-x-Alx. increases with temperature in agreement with a density of states at the Fermi level dominated by the d band. For a given temperature, the resistivity increases substantially and regularly with x for both series of solid solutions because of an increase in the number of conduction electron scattering potentials. This increase in resistivity also corresponds to a decrease in the number of conduction electrons due to their increasing transfer into the localized cerium 4f orbitals as the cerium valence state changes from a mixture of trivalent and tetravalent in Ce2Fe17 to predominantly trivalent in Ce2Fe17-xAlx. and Ce2Fe17-xSix with increasing x

A Comparison of the Mössbauer Effect Spectra of R₂Fe₁₄B and R₂Fe₁₄C

The Mössbauer spectra of R2Fe14B, where R is La, Tb, Dy, Ho and Lu, have been measured at 295 K and the Mössbauer spectra of R2Fe14C, where R is Nd, Ho and Gd, have been measured at various temperatures between 85 and 295 K. All of the resulting spectra have been successfully analyzed with the same model used earlier to fit the spectra of various related compounds. The results indicate that the weighted average hyperfine field increases uniformly from La to Gd and decreases uniformly from Gd to Lu in R2Fe14B, as was found earlier for R2Fe14C. The weighted average isomer shift decreases by 0.04 mm/s in going from La2Fe14B to Lu2Fe14B, because of a decrease in the unit cell volume and an increase of the number of electrons in the unit cell, a decrease which corresponds to a 13.5% increase in the electron density. The use of the same model for both the Nd, Gd, Tb, Dy, Ho and Lu borides and carbides permits a detailed comparison of the Mössbauer hyperfine parameters in the two series of compounds. There is a linear correlation of the hyperfine fields at each of the sites in these two series of compounds, indicating that the replacement of boron by carbon has a long-range effect on the magnetic exchange within these materials. There is a correlation of the isomer shifts of the 8j1, 8j2 and 16k2 sites in the two series of compounds. In contrast the isomer shift of the 16k1 site show the chemical influence of replacing boron by carbon. With the expected exception of the 4c site there is a correlation between the quadrupole interactions in the two series of compounds. The Nd2Fe14B and Nd2Fe14C compounds have relatively low effective Mössbauer temperatures as compared to the borides and carbides with the heavier rare-earth elements

A structural, infrared, and Mossbauer spectral study of rosemaryite, NaMnFe3+Al(PO4)(3)

Rosemaryite, ideally NaMnFe(3+)AI(PO4)(3), has been collected in the Buranga pegmatite, Rwanda. A single-crystal structure refinement was performed to R-1 = 4.01 %, in the P2(1)/n space group, with a = 12.001(2), b = 12.396(1), c = 6.329(1) angstrom, beta 114.48(1)degrees, Vol. = 856.9(2) angstrom(3), Z = 4. The crystal structure and cation distributions are similar to those of ferrorosemaryite, NaFe2+Fe3+Al(PO4)(3), and qingheiite, Na2MnMgAl(PO4)(3), but aluminium predominantly occurs in the M(2a) site, not in the M(2b) site as observed in ferrowyllieite, Na2Fe22+Al(PO4)(3). The topologies of the X(1a) and X(1b) crystallographic sites are identical to those found in ferrorosemaryite, and correspond to a distorted octahedron and to a distorted cube, respectively. The [7+1]-coordinated X(2) site is a very distorted gable disphenoid, similar to the A(2)' site of the alluaudite structure. Mossbauer spectra have been obtained from 4.2 to 295 K, and fitted with a model including two Fe3+ and two Fe2+ doublets. The Fe2+ component corresponding to 2/3 of the Fe2+ spectral area and having a smaller quadrupole splitting of 2.63 mm/s at 15 K, is assigned to the Fe2+ on the M(2a) site, and the Fe2+ component with the larger quadrupole splitting of 3.17 mm/s at 15 K, is assigned to the Fe2+ on the M(1) site. Fe3+ is located only at the M(2a) and M(2b) sites, and the Fe3+ component corresponding to 3/4 of the Fe3+ and exhibiting the larger quadrupole spitting of 0.77 mm/s at 15 K, is most likely associated with Fe3+ on the M(2b) site. The infrared spectrum of rosemaryite shows absorption bands at 3450 and 1624 cm(-1), bands that arise from the vibrational modes of H2O and confirm the presence of water in the channels of the wyllieite structure. A comparison of both the Mossbauer spectra and structural data of rosemaryite with those of other phosphates of the alluaudite and wyllieite groups, is also presented

Comparative Mössbauer Effect Study of Several R₂Fe₁₇ and R₂Fe₁₇Nₓ Compounds

The Mössbauer spectra of Sm2Fe17 and Ho 2Fe17 and their nitrides have been measured between 295 and 85 K and analyzed with a model which is consistent with our earlier work on R2Fe17 and R2Fe17Nx compounds, where R is Pr, Nd, and Th. This model is completely consistent throughout these rare-earth compounds and is in agreement with the crystallographic changes occurring upon nitrogenation and with the prediction of band structure calculations. The dramatic increase in Curie temperature in the nitrides results from the expansion of the crystallographic lattice, an expansion which is mainly centered on the 9d and 18h iron sites as is indicated by the increase of their Wigner-Seitz cell volumes upon nitrogenation. The 9d and 18h sites show a larger enhancement of their hyperfine fields as compared to the 6c and 18f sites as a result of improved ferromagnetic exchange between these sites and their near neighbors because of the lattice expansion and the consequent reduced iron 3d-iron 3d overlap