François Fauth

Interplay of structural, magnetic and transport properties in thelayered Co-based perovskite LnBaCo 2 O 5 (Ln = Tb, Dy, Ho)

Abstract:The oxygen deficient cobaltites LnBaCo2O5 (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at TN∼340 K and at TCO∼210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co2+ and Co3+ species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ∼3.7 and ∼2.7 , the electronic configuration of the Co ions in LnBaCo2O5 remains conjectural. Two pictures, with Co3+ ions either in intermediate spin state ( t52ge1g) or in high spin state ( t42ge2g), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al. , Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo2O5 (Ln = Tb, Dy, Ho).

Cooling by adiabatic pressure application in Pr1−xLaxNiO3

A novel principle for cooling by adiabatic pressure application in the mixed crystalline compound Pr1−xLaxNiO3 is described and experimentally verified. Cooling occurs in the vicinity of the structural phase transition where the electronic ground state of the Pr3+ ions changes from a singlet to a doublet state. By properly choosing the La concentration x, the cooling effect can be achieved down to some 100 mK. Furthermore, Pr1−xLaxNiO3 can be used for second and third stage cooling down to the μK region by classical paramagnetic and nuclear demagnetization techniques, respectively.

Nonlinear effects and Joule heating in I-V curves in manganites

We study the influence of the Joule effect on the nonlinear behavior of the transport I-V curves in polycrystalline samples of the manganite Pr0.8Ca0.2MnO3 by using the crystalline unit-cell parameters as an internal thermometer in x-ray and neutron diffractions. We develop a simple analytical model to estimate the temperature profile in the samples. Under the actual experimental conditions we show that the internal temperature gradient or the difference between the temperature of the sample and that of the thermal bath is at the origin of the nonlinearity observed in the I-V curves. Consequences on other compounds with colossal magnetoresistance are also discussed.

Spinel materials for Li-ion batteries: new insights obtained by operando neutron and synchrotron X-ray diffraction

In the last few decades Li-ion batteries changed the way we store energy, becoming a key element of our everyday life. Their continuous improvement is tightly bound to the understanding of lithium (de)intercalation phenomena in electrode materials. Here we address the use of operando diffraction techniques to understand these mechanisms. We focus on powerful probes such as neutrons and synchrotron X-ray radiation, which have become increasingly familiar to the electrochemical community. After discussing the general benefits (and drawbacks) of these characterization techniques and the work of customization required to adapt standard electrochemical cells to an operando diffraction experiment, we highlight several very recent results. We concentrate on important electrode materials such as the spinels Li1 + xMn2 - xO4 (0 ≤ x ≤ 0.10) and LiNi0.4Mn1.6O4. Thorough investigations led by operando neutron powder diffraction demonstrated that neutrons are highly sensitive to structural parameters that cannot be captured by other means (for example, atomic Debye-Waller factors and lithium site occupancy). Synchrotron radiation X-ray powder diffraction reveals how LiMn2O4 is subject to irreversibility upon the first electrochemical cycle, resulting in severe Bragg peak broadening. Even more interestingly, we show for the first time an ordering scheme of the elusive composition Li0.5Mn2O4, through the coexistence of Mn(3+):Mn(4+) 1:3 cation ordering and lithium/vacancy ordering. More accurately written as Li0.5Mn(3+)0.5Mn(4+)1.5O4, this intermediate phase loses the Fd\overline 3m symmetry, to be correctly described in the P213 space group.

Structural and electrochemical studies of a new Tavorite composition: LiVPO4OH

Polyanionic materials attract strong interest in the field of Li-ion battery research thanks to the wide range of compositions, structures and electrochemical properties they offer. Tavorite-type compositions offer a very rich crystal chemistry, among which LiVPO4F has the highest theoretical energy density (i.e. 655 W h kg−1). A new Tavorite-type LiVPO4OH composition was synthesized by a hydrothermal route from three different vanadium-containing precursors. The crystal structure of this new phase was fully determined thanks to synchrotron X-ray and neutron diffraction. 1H, 7Li, and 31P magic angle spinning nuclear magnetic resonance spectroscopy as well as diffuse reflectance infra-red spectroscopy were performed in order to support further the nature of the phases formed. Galvanostatic intermittent titration technique experiments in lithium batteries and ex situ X-ray diffraction analyses revealed that during oxidation the concomitant extraction of Li+ and H+ occurs at the same equilibrium potential (3.95 V vs. Li+/Li) and leads to the formation of the Tavorite phase VPO4O at the end of the charge. LiVPO4OH is also electrochemically active in the low voltage region, upon Li+ insertion. The reversible insertion/extraction of lithium at 1.35 V vs. Li+/Li leads to the formation of Li2VPO4OH at the end of discharge.

Extra-low thermal conductivity in unfilled CoSb3-δ skutterudite synthesized under high-pressure conditions

Thermoelectric CoSb3-δ skutterudite was synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. Structural analysis carried out from synchrotron x-ray powder diffraction data reveals a significant Sb deficiency in this material. The introduction of point defects in the form of Sb vacancies distributed at random in the structure leads to an impressive reduction (>50%) of the total thermal conductivity, κ, which is one of the main ingredients of good thermoelectric materials. This suggests phonon scattering effects originated in the Sb defects, which drives to a better improvement in κ than that achieved by the conventional strategy of filling the cages of the skutterudite structure with rare earths or other heavy cations. In parallel, changes in the electronic band structure caused by point variation of the stoichiometry produce an undesired increment in the electrical resistivity. Nevertheless, the low thermal conductivity combined with a high...

On the dynamics of transition metal migration and its impact on the performance of layered oxides for sodium-ion batteries: NaFeO2 as a case study

Transition metal (TM) layered oxides constitute one of the most promising families of compounds for the cathode of Na-ion batteries. However, their structural stability at the charged state is a critical performance limiting factor, which is believed to be closely related to irreversible TM migration into the Na layers. Nevertheless, experimental evidence of this TM migration and its influence on the electrochemical performance is still scarce, while the understanding of such a phenomenon constitutes a key step for developing better performing TM layered oxides. Here NaFeO2 has been studied as a model system, since it is expected to produce one of the most pronounced TM migrations and provide possibly one of the highest theoretical energy densities of TM layered oxides. By combining the Potential Intermittent Titration Technique (PITT), Electrochemical Impedance Spectroscopy (EIS) and operando X-ray diffraction, it has been possible to analyze the structural evolution of NaxFeO2, track the iron migration and observe its influence on the insertion capacity and Na diffusivity.

Ag3V2(PO4)2F3, a new compound obtained by Ag+/Na+ ion exchange into the Na3V2(PO4)2F3 framework

Phosphate polyanionic compounds have been used for several technological applications and are especially widespread in battery research. Na3V2(PO4)2F3 is a material that holds great promise as a positive electrode for Na-ion batteries. We study here the Ag+/Na+ ion exchange that is possible due to the high ionic mobility of Na+ in this material. A nearly complete ion exchange was obtained and we report the crystal structure of the new orthorhombic phase Ag3V2(PO4)2F3, determined by synchrotron X-ray and neutron powder diffraction. Silver occupies the same crystallographic sites as sodium and, except for the differences caused by steric effects, Ag3V2(PO4)2F3 preserves the symmetry constraints already present in the parent compound Na3V2(PO4)2F3. We also followed the evolution of the crystal structure upon heating/cooling, to observe an order/disorder transition analogous to the one already reported for Na3V2(PO4)2F3, but occurring at a significantly higher temperature.

Low thermal conductivity in La-filled cobalt antimonide skutterudites with an inhomogeneous filling factor prepared under high-pressure conditions

La-filled skutterudites LaxCo4Sb12 (x = 0.25 and 0.5) have been synthesized and sintered in one step under high-pressure conditions at 3.5 GPa in a piston-cylinder hydrostatic press. The structural properties of the reaction products were characterized by synchrotron X-ray powder diffraction, clearly showing an uneven filling factor of the skutterudite phases, confirmed by transmission electron microscopy. The non-homogeneous distribution of La filling atoms is adequate to produce a significant decrease in lattice thermal conductivity, mainly due to strain field scattering of high-energy phonons. Furthermore, the lanthanum filler primarily acts as an Einstein-like vibrational mode having a strong impact on the phonon scattering. Extra-low thermal conductivity values of 2.39 W m−1 K−1 and 1.30 W m−1 K−1 are measured for La0.25Co4Sb12 and La0.5Co4Sb12 nominal compositions at 780 K, respectively. Besides this, lanthanum atoms have contributed to increase the charge carrier concentration in the samples. In the case of La0.25Co4Sb12, there is an enhancement of the power factor and an improvement of the thermoelectric properties.

Synthesis, Structure, and Physical Properties of the Polar Magnet DyCrWO6

It has recently been reported that the ordered aeschynite-type polar ( Pna21) magnets RFeWO6 (R = Eu, Tb, Dy, Y) exhibit type II multiferroic properties below TN ∼ 15-18 K. Herein, we report a comprehensive investigation of the isostructural oxide DyCrWO6 and compare the results with those of DyFeWO6. The cation-ordered oxide DyCrWO6 crystallizes in the same polar orthorhombic structure and undergoes antiferromagnetic ordering at TN = 25 K. Contrary to DyFeWO6, only a very weak dielectric anomaly and magnetodielectric effects are observed at the Néel temperature and, more importantly, there is no induced polarization at TN. Furthermore, analysis of the low-temperature neutron diffraction data reveals a collinear arrangement of Cr spins but a noncollinear Dy-spin configuration due to single-ion anisotropy. We suggest that the collinear arrangement of Cr spins may be responsible for the absence of electric polarization in DyCrWO6. A temperature-induced magnetization reversal and magnetocaloric effects are observed at low temperatures.