Papers by Fernande Grandjean
Inorganic Chemistry, 2012
A new compound, Na(1.515)EuGeS(4), has been synthesized at 750 °C from a reaction of elemental eu... more A new compound, Na(1.515)EuGeS(4), has been synthesized at 750 °C from a reaction of elemental europium, germanium, and sulfur and Na(2)S. The compound crystallizes in the trigonal system with Z = 18 and the R3c space group with a = 23.322(3) Å, c = 6.838(1) Å, and V = 3221.2(9) Å(3). Na(1.515)EuGeS(4), which is isostructural with Na(2)EuSiSe(4), contains quasi-infinite nanoscale (∞)[EuGeS(4)](2-) tubules that are held together by sodium cations through electrostatic interactions. The tubules consist of a complex network of monoface-capped EuS(7) trigonal prisms and GeS(4) tetrahedra. The most striking structural feature of Na(1.515)EuGeS(4) is the absence of sodium cations inside the tubules, an absence that is balanced by the presence of mixed valence europium(II/III) ions. This mixed valence is confirmed by europium-151 Mössbauer spectroscopy, which indicates discrete mixed-valence europium ions at least up to 295 K. The stoichiometry has been determined by a fit of χ(M)T measured between 20 and 300 K with a combination of europium(II) ions, with a Curie constant of 7.877 emu K/mol, and europium(III) ions whose contribution to χ(M)T has been fit by using the Van Vleck expression for its molar susceptibility. The best fit corresponds to 51.5% of europium(II), 48.5% of europium(III), a stoichiometry of Na(1.515(5))EuGeS(4), and a splitting, E, between the J = 0 and the first excited J = 1 state of europium(III) of 360(6) cm(-1). The field dependence of the 1.8 K magnetization is in perfect agreement with a S = 7/2 Brillouin function with g = 2.00 and yields a saturation magnetization of 7 Nβ at 5 T.
Physical Review B, 2007
Weak ferromagnetism in Fe1−xCoxSb2 is studied by magnetization and Mössbauer measurements. A smal... more Weak ferromagnetism in Fe1−xCoxSb2 is studied by magnetization and Mössbauer measurements. A small spontaneous magnetic moment of the order of ∼ 10 −3 µB appears along the b-axis for 0.2 ≤ x ≤ 0.4. Based on the structural analysis, we argue against extrinsic sources of weak ferromagnetism. We discuss our results in the framework of the nearly magnetic electronic structure of the parent compound FeSb2.
MRS Proceedings, 2005
ABSTRACT In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the gu... more ABSTRACT In the filled gallium-germanium clathrates, R8Ga16Ge30, where R is Ba, Sr, or Eu, the guests are located in two large cages and are weakly bound to the crystalline clathrate framework. The caged guests exhibit a localized “rattling” vibrational mode that provides an efficient mechanism for reducing the thermal conductivity. Inelastic neutron scattering and nuclear inelastic scattering measurements have yielded the phonon density of states in R8Ga16Ge30; the line width of the localized vibrational modes is found to be an important parameter in determining the lattice thermal conductivity. Neutron diffraction studies on R8Ga16Ge30 have shown that the guests in the larger cage are located off-center, and it was proposed that their jumping about the four off-center locations is responsible for the observed glass-like thermal conductivity at temperatures below 10 K. The detection of such slow guest motion is challenging because the typical time and energy scales involved are ca. 4 ns and 1 µeV, respectively. We have studied the slow europium tunneling dynamics in Eu4Sr4Ga16Ge30 by both Mössbauer and microwave absorption spectroscopy.
Journal of Applied Physics, 2007
Solid State Sciences, 2002
The Mössbauer spectra of the Na 1−x Li x MnFe 2 (PO 4 ) 3 compounds, with x = 0.00, 0.25, 0.50, a... more The Mössbauer spectra of the Na 1−x Li x MnFe 2 (PO 4 ) 3 compounds, with x = 0.00, 0.25, 0.50, and 0.75, have been measured between 90 and 295 K and analyzed in terms of a model which takes into account the next-nearest neighbor interactions within the alluaudite structure. Surprisingly, the spectra reveal an unexpected presence of iron(II) in these compounds; the amount of iron(II) is observed to decrease from 19 to 15 atomic percent of the total iron content with increasing x. The temperature dependence of the Fe 2+ and Fe 3+ isomer shifts agrees with that expected from a second-order Doppler shift and the resulting iron vibrating masses and Mössbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The temperature dependence of the Fe 2+ quadrupole splitting has been fit with the Ingalls' model and the results yield a ground state orbital splitting of ca. 500 cm −1 for the iron(II) sites. The compositional dependence of the isomer shifts and Fe 2+ content can be understood in terms of a decrease in the unit-cell volume with increasing substitution of sodium by lithium, a substitution which does not influence the observed quadrupole splittings.
Physical Review Letters, 2003
all show a sharp peak in the vibrational density of states at 57 2 K, which is absent in the unfi... more all show a sharp peak in the vibrational density of states at 57 2 K, which is absent in the unfilled skutterudite CoSb 3 . Heat capacity measurements on Tl 0:8 Co 4 Sb 11 Sn as compared to CoSb 3 are consistent with the presence of a localized vibrational mode associated with the ''rattling'' thallium atoms in this filled skutterudite compound. Both results are well described by a localized Einstein mode model with an Einstein temperature E of 53 1 K. These data provide perhaps the clearest example of local mode behavior in a concentrated metallic system.
Physical Review Letters, 2006
Mössbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the pr... more Mössbauer-effect and microwave absorption experimental evidence unambiguously demonstrates the presence of slow, 450 MHz, tunneling of magnetic europium between four equivalent sites in Eu 8 Ga 16 Ge 30 , a stoichiometric clathrate. Remarkably, six of the eight europium atoms, or 11% of the constituents in this solid, tunnel between these four sites separated by 0.55 Å . The off centering of the atoms or ions in crystalline clathrates appears to be a promising route for producing Rabi oscillators in solid-state materials.
Physical Review B, 2008
We present results from a detailed experimental investigation of LaFeAsO, the parent material in ... more We present results from a detailed experimental investigation of LaFeAsO, the parent material in the series of "FeAs" based oxypnictide superconductors. Upon cooling this material undergoes a tetragonal-orthorhombic crystallographic phase transition at ∼160 K followed closely by an antiferromagnetic ordering near 145 K. Analysis of these phase transitions using temperature dependent powder X-ray and neutron diffraction measurements is presented. A magnetic moment of ∼0.35 µ B per iron is derived from Mössbauer spectra in the low temperature phase. Evidence of the structural transition is observed at temperatures well above the structural transition (up to near 200 K) in the diffraction data as well as the polycrystalline elastic moduli probed by resonant ultrasound spectroscopy measurements. The effects of the two phase transitions on the transport properties (resistivity, thermal conductivity, Seebeck coefficient, Hall coefficient), heat capacity, and magnetization of LaFeAsO are also reported, including a dramatic increase in the magnitude of the Hall coefficient below 160 K. The results suggest that the structural distortion leads to a localization of carriers on Fe, producing small local magnetic moments which subsequently order antiferromagnetically upon further cooling. Evidence of strong electron-phonon interactions in the high-temperature tetragonal phase is also observed. PACS numbers: 61.05.cp 61.05.fm 61.50.Ks 63.20.kd 65.40.Ba 72.15.Eb 72.15.Jf 74.10.+v 75.25.+z 76.80.+y
Physical Review B, 2007
The Mössbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11... more The Mössbauer spectra of monodispersed iron oxide nanoparticles with diameters of 4, 7, 9, and 11nm have been measured between 4.2 and 315K 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
Physical Review B, 2005
The europium partial vibrational density of states ͑DOS͒ in EuFe 4 Sb 12 and the iron partial vib... more The europium partial vibrational density of states ͑DOS͒ in EuFe 4 Sb 12 and the iron partial vibrational DOS in both EuFe 4 Sb 12 and Ce 57 Fe 4 Sb 12 have been obtained by nuclear inelastic scattering. The results reveal the strong independence of the iron and rare-earth vibrational modes. The cage filling europium only participates significantly to the low-energy local vibrational modes. The force constants have been obtained from the measured probability of nuclear absorption. The energies of the peaks in the europium and iron DOS are in excellent agreement with the calculated DOS in LaFe 4 Sb 12 . The results indicate that nuclear inelastic scattering is the technique of choice for the study of the localized vibrational modes in thermoelectric "phonon glass" materials.
Physical Review B, 2007
The specific lattice dynamic properties of antimony in the unfilled CoSb3 and filled EuFe4Sb12 sk... 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 Sb121 nuclear resonance energy of 37.1298(2)keV with a 4.5meV high-resolution backscattering sapphire monochromator. The Sb partial vibrational density of states (DOS) shows a maximum centered at 17 and 16meV in CoSb3 and EuFe4Sb12 , respectively. The
Physical Review B, 2004
The magnetic phase transitions in the FeMnP1-xAsx compounds with x= 0.25, 0.35, 0.45, 0.50, and 0... more The magnetic phase transitions in the FeMnP1-xAsx compounds with x= 0.25, 0.35, 0.45, 0.50, and 0.55, have been studied by iron-57 Mössbauer spectroscopy. The ferromagnetic and antiferromagnetic spectra have been analyzed with a model that takes into account the random distribution of the P and As near-neighbor anions of a given iron site. This distribution is a binomial distribution of
Physical Review B, 2011
Thin films of FeSb were characterized by electronic transport, magnetometry, x-ray diffraction, F... more Thin films of FeSb were characterized by electronic transport, magnetometry, x-ray diffraction, Fe and ¹²¹Sb nuclear inelastic scattering, and Fe Moessbauer spectroscopy. Resistivity and magnetometry measurements reveal semiconducting behavior with a 16.3(4) meV band gap and an effective paramagnetic moment of 0.57(6) B, respectively. A systematic comparison of the lattice dynamics with CoSb and EuFeSb reveals that the FeSb framework
Physical Review B, 2008
We report on a positive colossal magnetoresistance (MR) induced by metallization of FeSb2, a near... more We report on a positive colossal magnetoresistance (MR) induced by metallization of FeSb2, a nearly magnetic or "Kondo" semiconductor with 3d ions. We discuss contribution of orbital MR and quantum interference to enhanced magnetic field response of electrical resistivity.
New Journal of Physics, 2009
We present results of transport and magnetic properties and heat capacity measurements on polycry... more We present results of transport and magnetic properties and heat capacity measurements on polycrystalline CeFeAsO, PrFeAsO, and NdFeAsO. These materials undergo structural phase transitions, spin density wave-like magnetic ordering of small moments on iron, and antiferromagnetic ordering of rare earth moments. The temperature dependence of the electrical resistivity, Seebeck coefficient, thermal conductivity, Hall coefficient, and magnetoresistance are reported. The magnetic behavior of the materials have been investigated using Mössbauer spectroscopy and magnetization measurements. Transport and magnetic properties are affected strongly by the structural and magnetic transitions, suggesting significant changes in the band structure and/or carrier mobilities occur, and phonon−phonon scattering is reduced upon transformation to the low temperature structure. Results are compared to recent reports for LaFeAsO, and systematic variations in properties as the identity of Ln is changed are observed and discussed. As Ln progresses across the rare-earth series from La to Nd, an increase in the hole contributions to Seebeck coefficient, and increases in magnetoresistance and the Hall coefficient are observed in the low temperature phase. Analysis of hyperfine fields at the iron nuclei determined from Mössbauer spectra indicates that the moment on Fe in the orthorhombic phase is nearly independent of the identity of Ln, in apparent contrast to reports of powder neutron diffraction refinements.
Nature Chemistry, 2013
Single-molecule magnets that contain one spin centre may represent the smallest possible unit for... more Single-molecule magnets that contain one spin centre may represent the smallest possible unit for spin-based computational devices. Such applications, however, require the realization of molecules with a substantial energy barrier for spin inversion, achieved through a large axial magnetic anisotropy. Recently, significant progress has been made in this regard by using lanthanide centres such as terbium(III) and dysprosium(III), whose anisotropy can lead to extremely high relaxation barriers. We contend that similar effects should be achievable with transition metals by maintaining a low coordination number to restrict the magnitude of the d-orbital ligand-field splitting energy (which tends to hinder the development of large anisotropies). Herein we report the first two-coordinate complex of iron(I), [Fe(C(SiMe3)3)2](-), for which alternating current magnetic susceptibility measurements reveal slow magnetic relaxation below 29 K in a zero applied direct-current field. This S = complex exhibits an effective spin-reversal barrier of Ueff = 226(4) cm(-1), the largest yet observed for a single-molecule magnet based on a transition metal, and displays magnetic blocking below 4.5 K.
Uploads
Papers by Fernande Grandjean