Janus-faced influence of oxygen vacancy in high entropy oxide films with Mott electrons

Contrary to traditional approaches, high entropy oxides (HEOs) strategically employ cationic disorder to engineer tunable functionalities. This disorder, stemming from multiple elements at the same crystallographic site, disrupts local symmetry and induces local distortions. By examining a series of single-crystalline [La$_{0.2}$Pr$_{0.2}$Nd$_{0.2}$Sm$_{0.2}$Eu$_{0.2}$]NiO$_{3-δ}$ thin films, we demonstrate herein that the creation of oxygen vacancies (OVs) further offers a powerful means of tailoring electronic behavior of HEOs by concurrently introducing disorder in the oxygen sublattice and doping electrons into the system. Increasing OV concentration leads to a monotonic increase in room-temperature sheet resistance. A striking feature is the Janus-faced response of the metal-insulator transition (MIT) to OVs due to the interplay among correlation energy scales, electron doping, and disorder. Unlike the monotonous influence of OV observed for the MIT in VO$_2$ and V$_2$O$_3$, initial OV doping lowers the MIT temperature here, whereas higher OV levels completely suppress the metallic phase. Magnetotransport measurements further reveal weak localization, strong localization as a function of $δ$. Moreover, the disorder on both $RE$ and oxygen sublattices is responsible for the Mott-Anderson insulator state. These findings surpass the scope of the recently featured `electron antidoping' effect and demonstrate the promising opportunity to utilize OV engineering of HEOs for Mottronics and optoelectronics applications.