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Various high and medium entropy oxides with rock salt structures were prepared and studied as anodes for lithium batteries. All the systems had complex reaction mechanisms involving conversion reactions. Their capacity and reduction potential depend on the number of components and microstructure of the initial materials. However, the dependence is difficult to rationalize based on simple stability logic. This paper discusses the implication of our findings in the wider context of the science of high entropy materials.

In the orbitally ordered, quasi-one dimensional Heisenberg antiferromagnet KCuF3 the low-energy Eg and B1g phonon modes show an anomalous softening (25% and 13%) between room temperature and the characteristic temperature T_S = 50 K. In this temperature range a freezing-in of F ion dynamic displacements is proposed to occur. In addition, the Eg mode at about 260 cm-1 clearly splits below T_S. The width of the phonon lines above T_S follows an activated behavior with an activation energy of about 50 K. Our observations clearly evidence a reduction of the structural symmetry below T_S and indicate a strong coupling of lattice and spin fluctuations for T>T_S., 7 pages, 9 figures

The spin dynamics of the prototypical quasi-one-dimensional antiferromagnetic Heisenberg spin S=1/2 chain KCuF3 is investigated by electron spin resonance spectroscopy. Our analysis shows that the peculiarities of the spin dynamics require a new dynamical form of the antisymmetric anisotropic spin-spin interaction. This dynamical Dzyaloshinsky-Moriya interaction is related to strong oscillations of the bridging fluorine ions perpendicular to the crystallographic c axis. This new mechanism allows us to resolve consistently the controversies in observation of the magnetic and structural properties of this orbitally ordered perovskite compound. © 2008 The American Physical Society.

The phonon dispersions of $\mathrm{Nd_{1.86}Ce_{0.14}CuO_{4+��}}$ along the $[��,0,0]$ direction have been determined by inelastic x-ray scattering. Compared to the undoped parent compound, the two highest longitudinal phonon branches, associated with the Cu-O bond-stretching and out-of-plane oxygen vibration, are shifted to lower energies. Moreover, an anomalous softening of the bond-stretching band is observed around $\mathbf{q}=(0.2,0,0)$. These signatures provide evidence for strong electron-phonon coupling in this electron-doped high-temperature superconductor., 4 pages, 4 figures

We present the results of resonant x-ray scattering experiments on KCuF3. Structurally forbidden reflections, corresponding to magnetic and 3d-orbital long-range order, have been observed. Integrated intensities have been measured as a function of incident energy, polarization, azimuthal angle, and temperature. The results give evidence for a strong coupling between orbital and spin degrees of freedom. The interplay between magnetic and orbital order parameters is revealed by the temperature dependence of the intensity of orbital Bragg peaks.

In this paper we introduce the use of molecular metal clusters as precursors for the in situ formation of nanoparticles with a high activity and surprising stability as electrocatalysts in energy conversion. The work is relevant in the present quest for remediation of greenhouse gas production and global warming, where fuel cells and metal and lithium air batteries are intended to play a crucial role. The research for an increased efficiency of these devices needs to solve the problems linked to the Oxygen Reduction Reaction (ORR), which plays a key role in these processes, due to its sluggish electrochemical kinetics that must be increased by a suitable electrocatalyst. Here we describe the facile synthesis of a new cathode material for fuel cells based on CO–Pt molecular clusters that are strongly bound to Vulcan XC72R®. The Pt–CO bond can be removed just after the synthesis, by thermal treatment at 450 K, or once in solution, leaving low-coordinated, highly reactive surface Pt atoms. The materials are investigated thoroughly using identical-location transmission electron microscopy, X-ray diffraction (with pair distribution function) and X-ray absorption measurements, revealing an extremely small, yet controlled, particle size (1–2 nm), with high structural and morphological stability, and lack of any aggregation process after thermal treatment. The surprising stability is related to the quite large number of Pt–C bonds that the nanoparticles form with the support, allowing them to attach stably onto the carbonaceous moiety. At the same time, the nanoparticles are particularly strained, and enclose a high defect density, thus enhancing their catalytic properties. The best performing composite presents a mass activity of 225 mA mgPt−1 @ 0.9 V vs. RHE for the ORR, and a less cathodic onset potential compared with a conventional Pt nanoparticle cathode, used as a benchmark under the same experimental conditions, while in turn having a lower noble metal content.

High-entropy oxides based on transition metals, such as Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O (TM-HEO), have recently drawn special attention as potential anodes in lithium-ion batteries due to high specific capacity and cycling reversibility. However, the lithiation/delithiation mechanism of such systems is still controversial and not clearly addressed. Here, we report on an operando XAS investigation into TM-HEO-based anodes for lithium-ion cells during the first lithiation/delithiation cycle. This material showed a high specific capacity exceeding 600 mAh g-1 at 0.1 C and Coulombic efficiency very close to unity. The combination of functional and advanced spectroscopic studies revealed complex charging mechanisms, developing through the reduction of transition-metal (TM) cations, which triggers the conversion reaction below 1.0 V. The conversion is irreversible and incomplete, leading to the final collapse of the HEO rock-salt structure. Other redox processes are therefore discussed and called to account for the observed cycling behavior of the TM-HEO-based anode. Despite the irreversible phenomena, the HEO cubic structure remains intact for ∼60% of lithiation capacity, so proving the beneficial role of the configuration entropy in enhancing the stability of the HEO rock-salt structure during the redox phenomena.

The most common MXene composition Ti3C2Tx(T = F, O) shows outstanding stability as anode for sodium ion batteries (100% of capacity retention after 530 cycles with charge efficiency >99.7%). However, the reversibility of the intercalation/deintercalation process is strongly affected by the synthesis parameters determining, in turn, significant differences in the material structure. This study proposes a new approach to identify the crystal features influencing the performances, using a structural model built with a multitechnique approach that allows exploring the short-range order of the lamella. The model is then used to determine the long-range order by inserting defective elements into the structure. With this strategy it is possible to fit the MXene diffraction patterns, obtain the structural parameters including the stoichiometric composition of the terminations (neutron data), and quantify the structural disorder which can be used to discriminate the phases with the best electrochemical properties.

Transition metal-based high entropy oxides (TM-HEO) have recently been gaining interest for their potential use as anodes in next-generation Li-ion batteries. In such systems, however, the lithiation/delithiation mechanisms remain a matter of ongoing investigation. In this work we demonstrate the suitability of Multivariate Curve Resolution (MCR) of operando X-ray absorption spectroscopy (XAS) data to study the lithiation mechanisms in the TM-HEO (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)O. Through the MCR analysis, the number, nature and concentration evolution as a function of stored charge of all Cu-, Ni- and Co-related active species present during the first lithiation cycle is obtained.