Crystal Growth, Transport, and the Structural and Magnetic Properties of Ln4FeGa12 with Ln = Y, Tb, Dy, Ho, and Er

in Inorganic Chemistry (2010), 49

Ln4FeGa12, where Ln is Y, Tb, Dy, Ho, and Er, prepared by flux growth, crystallize with the cubic Y4PdGa12 structure with the space group and with a = 8.5650(4), 8.5610(4), 8.5350(3), 8.5080(3), and 8 ... [more ▼]

Ln4FeGa12, where Ln is Y, Tb, Dy, Ho, and Er, prepared by flux growth, crystallize with the cubic Y4PdGa12 structure with the space group and with a = 8.5650(4), 8.5610(4), 8.5350(3), 8.5080(3), and 8.4720(4) Å, respectively. The crystal structure consists of an iron-gallium octahedra and face sharing rare-earth cuboctahedra of the Au3Cu type. Y4FeGa12 exhibits weak itinerant ferromagnetism below 36 K. In contrast, Tb4FeGa12, Dy4FeGa12, Ho4FeGa12, and Er4FeGa12 order antiferromagnetically with maxima in the molar magnetic susceptibilities at 26, 18.5, 9, and 6 K. All the compounds exhibit metallic electric resistivity and their iron-57 Mössbauer spectra, obtained between 4.2 and 295 K, exhibit a single-line absorption with a 4.2 K isomer shift of ca. 0.50 mm/s, a shift that is characteristic of iron in an iron-gallium intermetallic compound. A small but significant broadening in the spectral absorption line width is observed for Y4FeGa12 below 40 K and results from the small hyperfine field arising from its spin-polarized itinerant electrons [less ▲]

Fading of Modern Prussian Blue Pigments : Preliminary Study

Poster (2009, June)

A study of the high temperature spin reorientation in YCoFe/sub 3/B

in Journal of Physics : Condensed Matter (2009), 21(18), 1860017-1860017

The iron-57 Mossbauer spectra of YCoFe /sub 3/ B have been measured between 4.2 and 480 K and reveal that YCoFe /sub 3/ B exhibits an axial orientation of the iron magnetic moments below 450 K and a basal ... [more ▼]

The iron-57 Mossbauer spectra of YCoFe /sub 3/ B have been measured between 4.2 and 480 K and reveal that YCoFe /sub 3/ B exhibits an axial orientation of the iron magnetic moments below 450 K and a basal orientation above 450 K. This spin reorientation, also observed in the thermomagnetic curves, results from the different signs of the contributions to the magnetic anisotropy of the 2c and 6i sites that are occupied by iron. The neutron diffraction patterns of YCoFe /sub 3/ B have been measured at 2 K and between 290 and 770 K and have been successfully analyzed with a model compatible with the magnetic orientation obtained from the Mossbauer spectra. The hybridization between the cobalt or iron 3d orbitals and the boron 2p orbitals leads to a larger magnetic moment and hyperfine field on the 2c site as compared to the 6i site. [less ▲]

Structural and magnetic properties of Pr18Li8Fe5-xMxO39 (M = Ru, Mn, Co)

in Journal of Solid State Chemistry (2009), 182(7), 1638-1648

A polycrystalline sample of Pr18Li8Fe4RuO39 has been synthesized by a solid state method and characterized by neutron powder diffraction, magnetometry and Mossbauer spectroscopy; samples of Pr18Li8Fe5 ... [more ▼]

A polycrystalline sample of Pr18Li8Fe4RuO39 has been synthesized by a solid state method and characterized by neutron powder diffraction, magnetometry and Mossbauer spectroscopy; samples of Pr18Li8Fe5-xMxO39 and Pr18Li8Fe5-xCoxO39 (x = 1, 2) have been studied by magnetometry. All these Compounds adopt a cubic structure (space group Pm (3) over barn, a(o)similar to 11.97 angstrom) based on intersecting < 111 > chains made up of alternating octahedral and trigonal-prismatic coordination sites. These chains occupy channels within a Pr-O framework. The trigonal-prismatic site in Pr18Li8Fe4RuO39 is occupied by Li+ and high-spin Fe3+. The remaining transition-metal cations occupy the two crystallographically-distinct octahedral sites in a disordered manner. All five compositions adopt a spin-glass-like state at 7 K (Pr18Li8Fe4RuO39) or below. (C) 2009 Elsevier Inc. All rights reserved. [less ▲]

Structural, Magnetic and Mössbauer Spectral Study of the Electronic Spin-state Transition in {Fe[HC(3-Mepz)2(5-Mepz)]2}(BF4)2

in Inorganic Chemistry (2009), 48

The complex {Fe[HC(3-Mepz)2(5-Mepz)]2}(BF4)2 (pz = pyrazolyl ring) has been prepared by the reaction of HC(3-Mepz)2(5-Mepz) with Fe(BF4)2·6H2O. The solid state structures obtained at 294 and 150 K show a ... [more ▼]

The complex {Fe[HC(3-Mepz)2(5-Mepz)]2}(BF4)2 (pz = pyrazolyl ring) has been prepared by the reaction of HC(3-Mepz)2(5-Mepz) with Fe(BF4)2·6H2O. The solid state structures obtained at 294 and 150 K show a distorted iron(II) octahedral N6 coordination environment with the largest deviations arising from the restrictions imposed by the chelate rings. At 294 K the complex is predominately high-spin with Fe–N bond distances averaging 2.14 Å, distances that are somewhat shorter than expected for a purely high-spin iron(II) complex because of the presence of an admixture of ca. 70 (I get 80 from both mag and X-ray, 3/15, where 3 is the subtraction of 2.14 and 2.17 and 15 1.99 and 2.14 and from Figure 3b) percent high-spin and 30 (20) percent low-spin iron(II). At 294 K the twisting of the pyrazolyl rings from the ideal C3v symmetry averages only 2.2o, a much smaller twist than has been observed previously in similar complexes. At 150 K the Fe–N bond distances average 1.99 Å, indicative of an almost fully low-spin iron(II) complex; the twist angle is only 1.3o, as expected for a complex with these Fe–N bond distances. The magnetic properties show that the complex undergoes a gradual change from low-spin iron(II) below 85 K to high-spin iron(II) at 400 K. The 4.2 to 60 K Mössbauer spectra correspond to a fully low-spin iron(II) complex but, upon further warming, the iron(II) begins to undergo spin-state relaxation on the Mössbauer time scale such that, at 155 and 315 K, the complex is 7.5 and 65 percent high-spin in the absence of any adjustment for the differing low-spin and high-spin recoil-free fractions. I would replace the previous sentence with the red. I see no reason to give the % from the Mössbauer in the abstract as it is likely a bit low as discussed in detail – neither the mag data nor X-ray data have the recoil issue. The last sentence in the abstract is the key information. OK The 4.2 to 60 K Mössbauer spectra correspond to a fully low-spin-iron(II) complex but, upon further warming above 85 K the iron(II) begins to undergo spin-state relaxation between the low- and high-spin forms on the Mössbauer time scale. At 155 and 315 K the complex exhibits spin-state relaxation rates of 0.36 and 7.38 MHz, respectively, and an Arrhenius plot of the logarithm of the relaxation rate yields an activation energy of 670 ± 40 cm–1 for the spin-state relaxation. [less ▲]

A structural, magnetic and Mössbauer spectral study of the magnetocaloric Mn1.1Fe0.9P1-xGex compounds

in Journal of Physics : Condensed Matter (2008), 20

The structural, magnetic and Mössbauer spectral properties of the magnetocaloric Mn1.1Fe0.9P1−xGex compounds, with 0.19 < x < 0.26, have been measured between 4.2 and 295 K. The 295 K unit-cell volume ... [more ▼]

The structural, magnetic and Mössbauer spectral properties of the magnetocaloric Mn1.1Fe0.9P1−xGex compounds, with 0.19 < x < 0.26, have been measured between 4.2 and 295 K. The 295 K unit-cell volume increases from x = 0.19 to 0.22 and is substantially smaller in the ferromagnetic Mn1.1Fe0.9P0.74Ge0.26. The temperature dependence of the magnetization reveals a ferromagnetic to paramagnetic transition with a Curie temperature between approximately 250 and 330 K and hysteresis width of 10 to 4 K, for 0.19 < x < 0.25. The composition Mn1.1Fe0.9P0.78Ge0.22 shows the largest isothermal entropy change of approximately 10 J/(kgKT) at 290 K. The M¨ossbauer spectra have been analysed with a binomial distribution of hyperfine fields correlated with a change in isomer shift and quadrupole shift, a distribution that results from the distribution of phosphorus and germanium among the near neighbours of the iron. The coexistence of paramagnetic and magnetically ordered phases in ranges of temperature of up to 50 K around the Curie temperature is observed in the Mössbauer spectra and is associated with the first-order character of the ferromagnetic to paramagnetic transition. The temperature dependence of the weighted average hyperfine field is well fitted within the magnetostrictive model of Bean and Rodbell. Good fits of the Mössbauer spectra could only be achieved by introducing a difference between the isomer shifts in the paramagnetic and ferromagnetic phases, a difference that is related to the magnetostriction and electronic structure change. [less ▲]

Iron(III) species dispersed in porous silica through sol-gel chemistry

in Journal of Non-Crystalline Solids (2008), 354(2-9), 665-672

Fe/SiO2 catalysts have been prepared by two different sol-gel methods, cogelation and dissolution. The cogelation and dissolution preparative methods lead to xerogels with fundamentally different pore ... [more ▼]

Fe/SiO2 catalysts have been prepared by two different sol-gel methods, cogelation and dissolution. The cogelation and dissolution preparative methods lead to xerogels with fundamentally different pore width distributions. The nature of the iron species obtained has been examined in detail by UN-visible and Mossbauer spectroscopy, and magnetic, transmission electron microscopy, and X-ray diffraction studies. There is no evidence for the presence of any ordered iron(III) oxides in the samples but all three contain two types of iron species, specifically paramagnetic high-spin iron(III) ions isolated in silica and iron(III) containing nanoparticles with a broad width distribution centered on 1.5 nm, nanoparticles that contain antiferromagnetically coupled clusters of a few ligated bridged iron(III) ions. (C) 2007 Elsevier B.V. All rights reserved. [less ▲]

Antimony vibrations in skutterudites probed by Sb-121 nuclear inelastic scattering

in Physical Review. B, Condensed Matter and Materials Physics (2007), 76(14),

The specific lattice dynamic properties of antimony in the unfilled CoSb3 and filled EuFe4Sb12 skutterudites have been determined by nuclear inelastic scattering at the Sb-121 nuclear resonance energy of ... [more ▼]

The specific lattice dynamic properties of antimony in the unfilled CoSb3 and filled EuFe4Sb12 skutterudites have been determined by nuclear inelastic scattering at the Sb-121 nuclear resonance energy of 37.1298(2) keV with a 4.5 meV high-resolution backscattering sapphire monochromator. The Sb partial vibrational density of states (DOS) shows a maximum centered at 17 and 16 meV in CoSb3 and EuFe4Sb12, respectively. The difference between the Sb DOSs of CoSb3 and EuFe4Sb12 reveals that upon filling there is a transfer of 10% of the vibrational states toward lower energy. Further, a likely indication of the coupling between the guest and the host lattice in rattler systems is observed, a coupling that is required to reduce the lattice thermal conductivity. [less ▲]

A Mossbauer spectral study of the GdCo4-xFexB compounds

in Journal of Applied Physics (2007), 101(2),

The iron-57 Mossbauer spectra of the GdCo4-xFexB compounds, where x is 0.10, 0.15, 0.20, 0.25, 1, 2, 2.5, and 2.6, have been measured at room temperature and reveal relatively small iron hyperfine fields ... [more ▼]

The iron-57 Mossbauer spectra of the GdCo4-xFexB compounds, where x is 0.10, 0.15, 0.20, 0.25, 1, 2, 2.5, and 2.6, have been measured at room temperature and reveal relatively small iron hyperfine fields of approximately 12-18 T, relatively large quadrupole interactions of approximately +0.9 and -1 mm/s, and three very different types of spectra for x=0.10 and 0.15, x=0.25, 1, and 2, and x=2.5 and 2.6. The differences result from both the different easy magnetization directions in these compounds and the different cobalt and/or iron occupancies of the crystallographic 2c and 6i sites. The spectra have been fitted by calculating the spectral absorption with the complete iron-57 nuclear excited state Hamiltonian for the iron 2c and 6i sites. The fits have used an asymmetry parameter eta and Euler angles theta and phi that relate the hyperfine field to the iron electric field gradient axes of each crystallographic site in an orientation that is consistent with the structural and magnetic properties of the site. The results of the fits indicate both that the full Hamiltonian approach is required for physically reasonable spectral fits and that the small observed fields result from the presence of large orbital contributions which subtract from the Fermi contact contributions to the magnetic hyperfine fields of the two sites. The iron 2c occupancy obtained from the Mossbauer spectral area has been used to model the compositional dependence of the magnetic anisotropy constant in the GdCo4-xFexB compounds. (c) 2007 American Institute of Physics. [less ▲]

Fe-57 Mossbauer spectral and muon spin relaxation study of the magnetodynamics of monodispersed gamma-Fe2O3 nanoparticles

in Physical Review b (2007), 76(17),

The Mossbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of ... [more ▼]

The Mossbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11 nm have been measured between 4.2 and 315 K and fitted within the formalism for stochastic fluctuations of the hyperfine Hamiltonian. In this model, the hyperfine field is assumed to relax between the six +/- x, +/- y, and +/- z directions in space with a distribution of relaxation rates that is temperature dependent. Muon spin relaxation measurements have been carried out on the 9 nm particles between 4.2 and 295 K. Both techniques reveal three regimes in the magnetic dynamics of these nanoparticles. In the low-temperature regime, between 4.2 and similar to 30 K, the nanoparticle magnetic moments are blocked and a spin-glass-like state is observed with nearly static hyperfine fields, as is indicated by the well resolved magnetic Mossbauer spectra and the slow exponential decay of the muon asymmetry functions. In the high-temperature regime, above similar to 125 K, the nanoparticle magnetic moments and, hence, the hyperfine fields, relax rapidly and a typical thermally activated superparamagnetic behavior is observed, as is indicated by the Mossbauer doublet line shape and the muon asymmetry functions that are unquestionably characteristic of monodispersed nanoparticles. In the intermediate regime between similar to 30 and 125 K, the Mossbauer spectra are the superposition of broad sextets and doublets and the muon asymmetry functions have been fitted with a sum of two terms, one relaxing term similar to that observed at and above 125 K and one term characteristic of static local fields. Hence, in this intermediate regime, the sample is magnetically inhomogeneous and composed of nanoparticles rapidly and slowly relaxing as a result of interparticle interactions. The magnetic anisotropy constants determined from both the Mossbauer spectral and magnetic susceptibility results decrease by a factor similar to 4 with increasing diameter from 4 to 22 nm and increase linearly with the percentage of iron(III) ions present at the surface of the nanoparticles. The interparticle interaction energy is estimated to be between 89 and 212 K from the temperature dependence of the magnetic hyperfine field measured on the 9 nm nanoparticles. [less ▲]