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

Abstract

The complex electromagnetic phase diagram of iron-based perovskites, e.g., Re 0.33 Sr 0.67 FeO 3 (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 Re 0.33 Sr 0.67 FeO 3 with light or middle rare-earth compositions, Re 0.33 Sr 0.67 FeO 3 containing heavy rare-earth elements beyond Gd have not yet been synthesized owing to their larger intrinsic metastability. Herein, we effectively synthesize Re 0.33 Sr 0.67 FeO 3 covering a large variety of rare-earth elements (e.g., Re = La−Dy) by first forming an oxygen-deficient Re 0.33 Sr 0.67 FeO 3-δ framework via conventional solid-state reactions in air and afterward high-oxygenpressure 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 Re 0.33 Sr 0.67 FeO 3 containing heavy rare-earth compositions (e.g., Dy) change from the space group Imma to R3̅ c, while a larger amount of oxygen vacancy is also expected. Consequently, the potentially more distorted FeO 6 octahedron is expected to be balanced by the tendency of generating the oxygen vacancy within Re 0.33 Sr 0.67 FeO 3 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 finestructure 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 Re 0.33 Sr 0.67 FeO 3 when varying the rare-earth compositions. Therefore, pronounced MIT performance is achieved in Re 0.33 Sr 0.67 FeO 3 with light rare-earth compositions and a low oxygen vacancy.