Raphaël P. Hermann

Shape Induced Symmetry in Self-Assembled Mesocrystals of Iron Oxide Nanocubes

Grazing incidence small-angle scattering and electron microscopy have been used to show for the first time that nonspherical nanoparticles can assemble into highly ordered body-centered tetragonal mesocrystals. Energy models accounting for the directionality and magnitude of the van der Waals and dipolar interactions as a function of the degree of truncation of the nanocubes illustrated the importance of the directional dipolar forces for the formation of the initial nanocube clusters and the dominance of the van der Waals multibody interactions in the dense packed arrays.

Charge order in LuFe2O4: antiferroelectric ground state and coupling to magnetism

X-ray scattering by multiferroic LuFe2O4 is reported. Below 320 K, superstructure reflections indicate an incommensurate charge order with propagation close to (1/3 1/3 3/2). The corresponding charge configuration, also found by electronic structure calculations as most stable, contains polar Fe/O double layers with antiferroelectric stacking. Diffuse scattering at 360 K, with (1/3 1/3 0) propagation, indicates ferroelectric short-range correlations between neighboring double layers. The temperature dependence of the incommensuration indicates that charge order and magnetism are coupled.

Influence of the rare-earth element on the effects of the structural and magnetic phase transitions in CeFeAsO, PrFeAsO and NdFeAsO

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 Mossbauer 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 with 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 the 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 Mossbauer 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.

Physico-chemical and NMR relaxometric characterization of gadolinium hydroxide and dysprosium oxide nanoparticles

Gadolinium hydroxide and dysprosium oxide nanoparticles, which constitute a new interesting class of magnetic nanoparticles, are characterized by different methods, using x-ray diffraction, magnetometry and NMR relaxometry at multiple fields. The rod-like particles are first shown to have a simple paramagnetic behavior, like the bulk compound, without any influence of the nanometric size of the particles. Because of their paramagnetic moment, these particles considerably shorten water relaxation times, especially the transverse relaxation time at high fields. The relaxation induced by gadolinium hydroxide particles is due to a proton exchange between the particle surface and bulk water, while the transverse relaxation caused by dysprosium oxide particles is governed by the diffusion of water protons around the magnetized particles. 1/T(2) increases linearly with the magnetic field for gadolinium hydroxide particles while a quadratic increase is observed for dysprosium oxide nanoparticles. The relaxation results are compared with those from previous studies and interpreted using different theories for the relaxation induced by magnetic particles.

Einstein oscillators that impede thermal transport

The Einstein model of a solid usually lacks a clear illustration in introductory solid-state physics courses because most solids are much better described by the Debye model. Filled antimony skutterudites, materials that have recently attracted much attention because of their potential for thermoelectric applications, provide a canonical illustration of the Einstein model. The filling atoms are loosely bound in the atomic cage formed by their neighbors, and hence their description as independent harmonic oscillators is adequate. Simple models for the heat capacity and thermal conductivity of a solid are introduced, with emphasis on the density of vibrational states. These models are used in conjunction with experimental results obtained from heat capacity and inelastic neutron scattering measurements to demonstrate the applicability of the concept of the Einstein oscillator to the filling guests in antimony skutterudites. The importance of these Einstein oscillators for impeding thermal transport is discussed and some simple problems involving the heat capacity, thermal conductivity, and inelastic neutron scattering are proposed.

Structural diversity in iron oxide nanoparticle assemblies as directed by particle morphology and orientation

The mesostructure of ordered arrays of anisotropic nanoparticles is controlled by a combination of packing constraints and interparticle interactions, two factors that are strongly dependent on the particle morphology. We have investigated how the degree of truncation of iron oxide nanocubes controls the mesostructure and particle orientation in drop cast mesocrystal arrays. The combination of grazing incidence small-angle X-ray scattering and scanning electron microscopy shows that mesocrystals of highly truncated cubic nanoparticles assemble in an fcc-type mesostructure, similar to arrays formed by iron oxide nanospheres, but with a significantly reduced packing density and displaying two different growth orientations. Strong satellite reflections in the GISAXS pattern indicate a commensurate mesoscopic superstructure that is related to stacking faults in mesocrystals of the anisotropic nanocubes. Our results show how subtle variation in shape anisotropy can induce oriented arrangements of nanoparticles of different structures and also create mesoscopic superstructures of larger periodicity.

Milli-electronvolt monochromatization of hard X-rays with a sapphire backscattering monochromator.

Monochromatization of hard X-rays in the 20–40 keV energy range to ∼1 meV bandwidth using a sapphire backscattering monochromator is demonstrated.

Numerical simulation of a quantum particle in a box

It is shown how one can get numerical prediction of quantum mechanical particle behaviour without using the Schrodinger equation. The main steps of this development are the non-differentiability hypothesis, the equations of motion entailed by this hypothesis, and the numerical formulation of a simple one-dimensional problem: the particle in a box.

An X-ray Rietveld, infrared, and Mössbauer spectral study of the NaMn(Fe1−xInx)2(PO4)3 alluaudite-type solid solution

Abstract Several compounds of the NaMn(Fe1-xInx)2(PO4)3 solid solution were synthesized by solid state reaction in air; pure alluaudite-like compounds were obtained for x = 0.00 to 1.00. X-ray Rietveld refinements indicate the presence of Na+ at the A1 and A2’ sites, Mn2+ at the M1 site, and Fe2+, Fe3+, and In3+ at the M2 site. The presence of small amounts of In3+ at the M1 site, and Mn2+ at the M2 site, indicates a partially disordered distribution between these cations. A good correlation was also established between the M1-M2 bond distance and the β angle of the alluaudite-like compounds. The disordered distribution of Fe2+, Fe3+, and In3+ at the M2 site is confirmed by the broadness of the infrared absorption bands. The Mossbauer spectra, measured between 90 and 295 K, were analyzed in terms of a model that takes into account the next-nearest neighbor interactions around the M2 crystallographic site. In all cases these spectra reveal the unexpected presence of small amounts of Fe2+ at the M2 site, an amount that decreases as the In3+ content increases. The Fe2+ and Fe3+ isomer shifts are typical of the alluaudite structure and vary with temperature, as expected from a second-order Doppler shift. The derived iron vibrating masses and Mossbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The Fe2+ and Fe3+ quadrupole splittings are also typical of the alluaudite structure and the temperature dependence of the Fe2+ quadrupole splitting was fit with the model of Ingalls (1964), which yielded a ground state orbital splitting of ca. 380 to 570 cm-1 for the Fe2+ sites.

Electronegative guests in CoSb3

Introducing guests into a host framework to form a so called inclusion compound can be used to design materials with new and fascinating functionalities. The vast majority of inclusion compounds have electropositive guests with neutral or negatively charged frameworks. Here, we show a series of electronegative guest filled skutterudites with inverse polarity. The strong covalent guest–host interactions observed for the electronegative group VIA guests, i.e., S and Se, feature a unique localized “cluster vibration” which significantly influences the lattice dynamics, together with the point-defect scattering caused by element substitutions, resulting in very low lattice thermal conductivity values. The findings of electronegative guests provide a new perspective for guest-filling in skutterudites, and the covalent filler/lattice interactions lead to an unusual lattice dynamics phenomenon which can be used for designing high-efficiency thermoelectric materials and novel functional inclusion compounds with open structures.