Hidden metal-insulator transition in manganites synthesized via a controllable oxidation

Oxygen usually plays crucial roles in tuning the phase structures and functionalities of complex oxides such as high temperature superconductivity, colossal magnetoresistance, catalysis, etc. Effective and considerable control of the oxygen content in those functional oxides could be highly desired. Here, using perovskite manganite (La _0.5 Sr _0.5 )MnO ₃ as a paradigm, we develop a new pathway to synthesize the epitaxial thin films assisted by an in-situ chemical process, where the oxygen content can be precisely controlled by varying oxidative activity tuned by the atmospheric temperature (T _atm ) during the growth. A hidden metal-insulator transition (MIT) emerges due to the phase competition, which is never shown in the phase diagram of this classic manganite. The oxygen-mediated interaction between Mn ions together with the change of carrier density might be responsible for this emerging phase, which is compatible with the results of first-principle calculations. This work demonstrates that, apart from traditional cation doping, a precise modulation of anion (O ²⁻ , S ²⁻ , etc.) may provide a new strategy to control phase structures and functionalities of epitaxial compound thin films.