Rare-Earth Regulation in the Crystal Structure, Electronic Structure, and Metal-To-Insulator Transitions of the High Oxygen Pressure-Annealed Re _0.33Sr_0.67FeO₃

The complex electromagnetic phase diagram of iron-based perovskites, e.g., Re0.33Sr0.67FeO3 (Re stands for rare earth), exhibits a charge/spin ordering transition that enables metal insulator transitions (MIT) beyond conventional oxide semiconductors. While the previous investigations focused on Re0.33Sr0.67FeO3 with light or middle rare-earth compositions, Re0.33Sr0.67FeO3 containing heavy rare-earth elements beyond Gd have not yet been synthesized owing to their larger intrinsic metastability. Herein, we effectively synthesize Re0.33Sr0.67FeO3 covering a large variety of rare-earth elements (e.g., Re = La-Dy) by first forming an oxygen-deficient Re0.33Sr0.67FeO3-delta framework via conventional solid-state reactions in air and afterward high-oxygen-pressure post annealing that compensates the oxygen composition. Compared to the ones with light or middle rare-earth compositions (e.g., Nd-Eu), the crystal structures of Re0.33Sr0.67FeO3 containing heavy rare-earth compositions (e.g., Dy) change from the space group Imma to R3c, while a larger amount of oxygen vacancy is also expected. Consequently, the potentially more distorted FeO6 octahedron is expected to be balanced by the tendency of generating the oxygen vacancy within Re0.33Sr0.67FeO3 containing heavy rare-earth compositions. The heavy rare-earth compositions elevate the Mott temperature T-0 and activation energy E-A in their carrier transportations and eliminate their MIT property. Further, on combining with the near-edge X-ray absorption fine-structure analysis, an abrupt variation is observed in the Fe-L edge and O-K edge across Re = Sm, and this reflects the boundary of MIT in the material family of Re0.33Sr0.67FeO3 when varying the rare-earth compositions. Therefore, pronounced MIT performance is achieved in Re0.33Sr0.67FeO3 with light rare-earth compositions and a low oxygen vacancy.