It has been shown that oxide ceramics containing multiple transition and/or rare-earth elements in equimolar ratios have a strong tendency to crystallize in simple single phase structures, stabilized by the high configurational entropy. In analogy to the metallic alloy systems, these oxides are denoted high entropy oxides (HEOs). The HEO concept allows to access hitherto uncharted areas in the multi-element phase diagram. Among the already realized structures there is the highly complex class of rare earth -transition element perovskites. This fascinating class of materials generated by applying the innovative concept of high entropy stabilization provides a new and manyfold research space with promise of discoveries of unprecedented properties and phenomena. The present study provides a first investigation of the magnetic properties of selected compounds of this novel class of materials. Comprehensive studies by DC and AC magnetometry are combined with element specific spectroscopy in order to understand the interplay between magnetic exchange and the high degree of chemical disorder in the systems. We observe a predominant antiferromagnetic behavior in the single phase materials, combined with a small ferromagnetic contribution. The latter can be attributed to either small ferromagnetic clusters or configurations in the antiferromagnetic matrix or a possible spin canting. In the long term perspective it is proposed to screen the properties of this family of compounds with high throughput methods, including combined experimental and theoretical approaches.

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.

Hard x-ray absorption and magnetic circular dichroism spectroscopy have been applied to study the consequential changes of the local environment around Fe atoms and their orbital polarizations in 40 nm thick Fe 60 Al 40 thin films along the order-disorder (B2 → A2) phase transition initiated by 20-keV Ne + ion irradiation with fluences of (0.75-6) ×10 14 ions cm −2. The analysis of the extended x-ray absorption fine structure spectra measured at the Fe K edge at room temperature revealed an increased number of Fe-Fe nearest neighbors from 3.47(7) to 5.0(1) and ∼1% of volume expansion through the transition. The visualization of the Fe and Al nearest-neighbor rearrangement in the first coordination shell of Fe absorbers via the transition was carried out by wavelet transformations. The obtained changes in Fe coordination are evidently reflected in the x-ray magnetic circular dichroism spectra which show an increased orbital magnetic moment of Fe atoms and a pronounced magnetic multielectronic excitations peak at ∼60 eV above the edge. The amplitudes of both peaks demonstrated similar dependencies on the irradiation fluence. The results of self-consistent density functional calculations on relaxed Fe 60 Al 40 model structures for the ordered (B2) and the disordered (A2) phases are consistent with the experimental findings and point to the formation of Fe-rich regions in the films studied.

High entropy oxides (HEOs) are a recently introduced class of oxide materials, which are characterized by a large number of elements (i.e., five or more) sharing one lattice site, which crystallize in a single phase structure. One complex example of the rather young HEO family is the rare-earth transition metal perovskite high entropy oxides. In this comprehensive study, we provide an overview of the magnetic properties of three perovskite type high entropy oxides. The compounds have a rare-earth site that is occupied by five different rare-earth elements, while the transition metal site is occupied by a single transition metal. In this way, a comparison to the parent binary oxides, namely, the orthocobaltites, -chromites, and -ferrites, is possible. X-ray absorption near edge spectroscopy, magnetometry, and Mössbauer spectroscopy are employed to characterize these complex materials. In general, we find surprising similarities to the magnetic properties of the binary oxides despite the chemical disorder on the rare-earth site. However, distinct differences and interesting magnetic properties are also observed such as noncollinearity, spin reorientation transitions, and large coercive fields of up to 2 T at ambient temperature. Both the chemical disorder on the rare-earth A-site and the nature of the transitional metal on the B-site play an important role in the physical properties of these high entropy oxides.

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