Reversibly tuning the band structure of high entropy oxides

High entropy oxides (HEO) are a recently discovered class of oxides comprised of five or more elements in near-equiatomic proportion. Despite their high chemical complexity, they usually crystallize in single-phase solid solutions with homogenous cationic distribution.

Sarkar et al. are the first to use X-ray absorption spectroscopy to study the electronic band structure of a rare earth element-based HEO, (Ce_0.2La_0.2Pr_0.2Sm_0.2Y_0.2)O_2-δ. They demonstrated reversible tuning of this HEO system’s band gap energy between 2 and 3.2 eV through heat treatment under different atmospheres.

The authors reported that with heat treatment plus an oxidizing atmosphere, Pr was primarily present in the 4+ state, resulting in an unoccupied Pr 4f band and a band gap energy of about 2 eV. However, with heat treatment in a reducing atmosphere, Pr is reduced to the 3+ state, so the Pr 4f band becomes occupied, resulting in a band gap energy of 2.5 eV under vacuum and 3.2 eV under hydrogen. Following a reducing atmosphere with oxidizing heat treatment can revert the band gap energy to 2 eV because Pr³⁺ is oxidized to Pr⁴⁺ and Pr 4f states are again unoccupied.

The other cations, La, Sm and Y, do not affect the band gap energy because their unoccupied states are too high in energy, but the authors suggest the roles of these cations is to provide structural stability.

“This work could benefit future studies on HEOs,” said author Abhishek Sarkar. “The reversible tuning phenomenon could also be useful for sensor applications.”

One drawback is the high temperature required for the redox reactions. Next, the authors will seek room-temperature reversible tuning methods, such as an electrochemical approach.

Source: “Role of intermediate 4f states in tuning the band structure of high entropy oxides,” by Abhishek Sarkar, Benedikt Eggert, Leonardo Velasco, Xiaoke Mu, Johanna Lill, Katharina Ollefs, Subramshu S. Bhattacharya, Heiko Wende, Robert Kruk, Richard A. Brand, and Horst Hahn, APL Materials (2020). The article can be accessed at https://doi.org/10.1063/5.0007944.