Year-end Sale % OFF 
Abstract
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.
Highlights
High entropy oxides (HEOs) were first reported in 2015,1 and since the topic has gained significant interest, which is evident from numerous published reports focusing on different aspects of HEOs.2–10 HEOs can be broadly defined as single-phase solid solution oxides containing 5 or more cations in near-equiatomic compositions
The results obtained for heat treatment of (Ce0.2La0.2Pr0.2Sm0.2 Y0.2)O2−δ-fluorite type HEOs (F-HEOs) in vacuum differ significantly to those obtained when heat-treated in a hydrogen atmosphere
In the scitation.org/journal/apm case of the F-HEO, the analysis of the O K x-ray absorption near-edge spectra (XANES) edge provides a comprehensive understanding of the electronic band structure and valence states
Summary
High entropy oxides (HEOs) were first reported in 2015,1 and since the topic has gained significant interest, which is evident from numerous published reports focusing on different aspects of HEOs. HEOs can be broadly defined as single-phase solid solution oxides containing 5 or more cations in near-equiatomic compositions. High entropy oxides (HEOs) were first reported in 2015,1 and since the topic has gained significant interest, which is evident from numerous published reports focusing on different aspects of HEOs.. HEOs can be broadly defined as single-phase solid solution oxides containing 5 or more cations in near-equiatomic compositions. The presence of multiple cations in comparable amounts leads to an enhanced configurational entropy of mixing (Sconfig), which is calculated using the Boltzmann statistical entropy equation.. One of the most intriguing characteristics of HEOs is the phase purity despite their compositional complexity. The underlying principle for phase purity is distinct in different HEOs. For instance, in some systems a dominant role of entropy has been evidenced, which scitation.org/journal/apm effectively overcomes the related enthalpic penalties and stabilizes a single-phase solid solution at high temperatures.
Sign-in/Register to view highlights & summary 
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
