High entropy oxides (HEOs) are single-phase solid solutions consisting of 5 or more cations in approximately equiatomic proportions. In this study, we show the reversible control of optical properties in a rare-earth (RE) based HEO-(Ce0.2La0.2Pr0.2Sm0.2Y0.2)O2−δ and subsequently utilize a combination of spectroscopic techniques to derive the features of the electronic band structure underpinning the observed optical phenomena. Heat treatment of the HEO under a vacuum atmosphere followed by reheat treatment in air results in a reversible change in the bandgap energy, from 1.9 eV to 2.5 eV. The finding is consistent with the reversible changes in the oxidation state and related f-orbital occupancy of Pr. However, no pertinent changes in the phase composition or crystal structure are observed upon the vacuum heat treatment. Furthermore, annealing of this HEO under a H2 atmosphere, followed by reheat treatment in air, results in even larger but still a reversible change in the bandgap energy from 1.9 eV to 3.2 eV. This is accompanied by a disorder–order type crystal structure transition and changes in the O 2p–RE 5d hybridization evidenced from x-ray absorption near-edge spectra (XANES). The O K and RE M4,5/L3 XANES indicate that the presence of Ce and Pr (in 3+/4+ states) leads to the formation of intermediate 4f energy levels between the O 2p and the RE 5d gap in HEO. It is concluded that heat treatment under reducing/oxidizing atmospheres affects these intermediate levels, thus offering the possibility to tune the bandgap energy in HEOs.

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Changes of the magnetic and crystal structure on the microscopic scale in 40 nm FeRh thin films have been applied to investigate the phenomena of a disorder induced ferromagnetism at room temperature initiated through light ion-irradiation with fluences 1 arXiv:1911.11256v1 [cond-mat.mtrl-sci] 25 Nov 2019 up to 0.125 Ne + /nm −2 . Magnetometry shows an increase of magnetic ordering at low temperatures and a decrease of the transition temperature combined with a broadening of the hysteresis with rising ion fluence. 57 Fe Mössbauer spectroscopy reveals the occurrence of an additional magnetic contributions with an hyperfine splitting of 27.2 T -identical to that of ferromagnetic B2-FeRh. The appearance of an anti-site Fe-contribution can be assumed to be lower than 0.6 Fe-at%, indicating that no change of the chemical composition is evident. The investigation of the local structure shows an increase of the static mean square relative displacement determined by X-ray absorption fine structure spectroscopy, while an increase of the defect-concentration has been determined by positron annihilation spectroscopy. From the changes of the microscopic magnetic structure a similarity between the temperature induced and the structural disorder induced ferromagnetic phase can be observed. These findings emphasize the relationship between magnetic ordering and the microscopic defect structure in FeRh.

Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm2Co17 and SmCo5 phases is stabilized. While typically in the prototype Sm­(Co, Fe, Cu, Zr)7.5–8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm–Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm2Co17 and SmCo5 phases with 65% Sm2Co17 and 35% SmCo5. The SmCo5 phase is stabilized directly on an Al2O3 substrate up to a thickness of 4 nm followed by a matrix of Sm2Co17 intermixed with SmCo5. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm2Co17 and SmCo5 phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm2Co17 and SmCo5 phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets.

The adsorption of carbon dioxide (CO 2 ) in porous materials is of great importance to address current environmental issues. We propose an approach where high-resolution powder X-ray diffraction and isotherm modeling are combined to unravel the adsorption process of CO 2 in the zeolite silicalite. Four main positions where the CO 2 molecules locate in silicalite are identified, two in the straight channels and two in the sinusoidal channels, which imposes a maximum adsorption capacity of 16 molecules per unit cell (2.77 mmol•g −1 ) at 21 bar. The resulting global isotherm is successfully fitted with a Toth model, in accordance with heterogeneous adsorption and the presence of intermolecular interactions. Then, to characterize the adsorption process at the sorption site level, a parametric Rietveld refinement is implemented, where the occupancy of each site is calculated from a site-specific isotherm model. The most favored site, in the straight channels, follows a simple Langmuir model, with the homogeneous adsorption of isolated molecules. The three other sites, less favored, display a Toth adsorption behavior, in accordance with the presence of molecule−molecule interactions. This combined approach, where Rietveld refinement and adsorption modeling are intrinsically linked, is a way to gain deeper knowledge of adsorption processes in nanoporous materials.