Veerle Keppens

Localized vibrational modes in metallic solids

Filled skutterudite antimonides, are cubic compounds with the formula RM4Sb12, where R is a rare-earth element (such as La or Ce), and M is a transition metal (for example, Fe or Co). The rare-earth ion is weakly bound in an oversized atomic cage formed by the other atoms. Its presence has been shown to cause a dramatic reduction in the lattice component of the thermal conductivity, while having little effect on the electronic properties of the compound. This combination of properties makes filled skutterudites of interest as thermoelectric materials. It has been suggested that localized, incoherent vibrations of the rare-earth ion are responsible for the reduction in thermal conductivity, but no direct evidence for these local vibrational modes exists. Here we report the observation of local modes in La-filled skutterudites, using heat capacity, elastic constant and inelastic neutron scattering measurements. The La atoms show unusual thermodynamic behaviour, characterized by the presence of two low-energy localized modes. Our results suggest that consideration of local modes will play an important role in the design of the next generation of thermoelectric materials.

Extraordinary magnetoelasticity and lattice softening in bcc Fe-Ga alloys

Extraordinary magnetostrictive behavior has been observed in Fe-Ga alloys with concentrations of Ga between 4% and 27%. λ100 exhibits two peaks as a function of Ga content. At room temperature, λ100 reaches a maximum of 265 ppm near 19% Ga and 235 ppm near 27% Ga. For compositions between 19% and 27%, λ100 drops sharply to a minimum near 24% Ga and exhibits an anomalous temperature dependence, decreasing by as much as a factor of 2 at low temperatures. This unusual magnetostrictive behavior is interpreted on the basis of a single maximum in the magnetoelastic coupling |b1| of Fe with increasing amounts of nonmagnetic Ga, combined with a strongly temperature dependent elastic shear modulus (c11−c12) which approaches zero near 27% Ga. λ111 is significantly smaller in magnitude than λ100 over this composition range, and has an abrupt change in sign from negative for low Ga concentrations to positive for a concentration of Ga near 21%.

Synthesis of iron-based bulk metallic glasses as nonferromagnetic amorphous steel alloys

Iron-based amorphous metals are investigated as nonferromagnetic amorphous steel alloys with magnetic transition temperatures well below ambient temperatures. Rod-shaped amorphous samples with diameters reaching 4 mm are obtained using injection casting. Amorphous steel alloys are designed by considering atomistic factors that enhance the stability of the amorphous phase, coupled with the realization of low-lying liquidus temperatures. The present alloys are found to exhibit superior mechanical strengths. In particular, the elastic moduli are comparable to those reported for super austenitic steels.

Effects of nematic fluctuations on the elastic properties of iron arsenide superconductors

We demonstrate that the changes in the elastic properties of the FeAs systems, as seen in our resonant ultrasound spectroscopy data, can be naturally understood in terms of fluctuations of emerging nematic degrees of freedom. Both the softening of the lattice in the normal, tetragonal phase as well as its hardening in the superconducting phase are consistently described by our model. Our results confirm the view that structural order is induced by magnetic fluctuations.

Glass-like phonon scattering from a spontaneous nanostructure in AgSbTe2.

Materials with very low thermal conductivity are of great interest for both thermoelectric and optical phase-change applications. Synthetic nanostructuring is most promising for suppressing thermal conductivity through phonon scattering, but challenges remain in producing bulk samples. In crystalline AgSbTe2 we show that a spontaneously forming nanostructure leads to a suppression of thermal conductivity to a glass-like level. Our mapping of the phonon mean free paths provides a novel bottom-up microscopic account of thermal conductivity and also reveals intrinsic anisotropies associated with the nanostructure. Ground-state degeneracy in AgSbTe2 leads to the natural formation of nanoscale domains with different orderings on the cation sublattice, and correlated atomic displacements, which efficiently scatter phonons. This mechanism is general and suggests a new avenue for the nanoscale engineering of materials to achieve low thermal conductivities for efficient thermoelectric converters and phase-change memory devices.

Disparate atomic displacements in skutterudite-type LaFe3CoSb12, a model for thermoelectric behavior

Mean-square atomic displacements in lanthanum triiron cobalt dodecaantimonide, determined as a function of temperature using single-crystal neutron diffraction, show that the La atom exhibits an anomalously large displacement at room temperature, U(eq) = 0.0196 (9) Å(2), because it is too small to fill the atomic cage formed by the corner-linked octahedral framework of M(4)Sb(12), M = Fe, Co. Site-occupancy refinements show 25% vacancies on the La site and an actual Fe:Co ratio of 2.17:1. Analysis of the temperature dependence of the atomic displacements identifies a significant temperature-independent component for the La atom ascribed to static disorder, which amounts to 19% of the room-temperature value. The large-amplitude rattling of the La atom can be effectively linked to the dramatic decrease of the lattice contribution to the thermal conductivity, which is a key factor for improving the thermoelectric behavior of these materials. This structure-property relationship offers a new paradigm for the exploration of thermoelectric materials.

Absence of superconductivity in hole-doped BaFe2 xCrxAs2 single crystals

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Elastic properties of Zr-based bulk metallic glasses studied by resonant ultrasound spectroscopy

crystallizes in the pyrochlore structure and undergoes a metal-insulator transition (MIT) near 226 K. We have characterized the MIT in

Physical factors controlling the ductility of bulk metallic glasses

{\mathrm{Cd}}_{2}{\mathrm{Os}}_{2}{\mathrm{O}}_{7}