Role of intermediate 4f states in tuning the band structure of high entropy oxides

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.

High-entropy ceramics: Present status, challenges, and a look forward

“…They speculated that multivalent element Pr was responsible to the band gap narrowing, and thus, enabled HE-REOs to absorb light over the entire visible spectral range. To verify the hypothesis, they further obtained element resolved charge state and electronic structure of (CeLaPrSmY)O 2-δ by X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) [61], and the ability of Pr in tuning the bandgap energy of HE-REO by redox reaction was confirmed, as demonstrated in Fig. 2.3.…”

Magnetic properties of high entropy oxides

“…Currently, there are eight major classes of HEOs that can be categorized based on the crystallographic structures: rocksalt, 1,3,8,9 fluorite, [10][11][12][13][14][15] bixbyite, 10,12,16 perovskite (cubic, orthorhombic, rhombohedral), 5,[17][18][19] spinel, 20,21 pyrochlore, 7,[22][23][24] layered (Ruddlesden-Popper and delafossite) [25][26][27] and magnetoplumbite. 28,29 Likewise, a broad range of properties, such as electrochemical, 2,[30][31][32] optical, 11,[33][34][35] magnetic, 21,29,34,[36][37][38][39][40][41][42]43 electronic and ionic transport, [44][45]…”

High-entropy materials for energy-related applications

“…HRTEM and high-resolution STEM have been used to clarify the atomic configuration uniformity (Gao et al, 2020). Hard X-ray can detect the electron structure and bond length of HEM (Sarkar et al, 2020b). The combination of the aforementioned methods may have advantages to reveal the local structure: for example, Ding et al proposed that elements adopted greater aggregation with a wavelength of incipient concentration waves in CrFeCoNiPd bulk alloy, with the assistant of aberration-corrected TEM and EDS mapping (Ding et al, 2019).…”