Metal-to-Insulating Transition in the Perovskite System YSr₂Cu₂FeO_8-δ (0 < δ < 1) Modeled by DFT Methods

Progress in the design of functional perovskite oxides relies on advances in density functional theory (DFT) methods to efficiently and effectively model complex systems composed of several transition-metal ions. This work reports the application of DFT methods to investigate the electronic structure of the YSr₂Cu₂FeO_8-δ (0 < δ < 1) family in which the insulating, metal, or superconducting behaviors and even anion conductivity can be tuned by modifying the oxygen content. In particular, we assess the performance of the generalized gradient approximation (GGA), its Hubbard-U correction (GGA + U), and the strongly constrained and appropriately normed (SCAN) to model the metallic (idealized YSr₂Cu₂FeO₈) and insulating (idealized YSr₂Cu₂FeO₇) phases of the system. The analysis of the DFT results is supported by DC resistivity measurements that denote the metal character of the synthesized YSr₂Cu₂FeO_7.86 and the semiconducting character of YSr₂Cu₂FeO_7.08 prepared under reducing conditions. In addition, the band gap of YSr₂Cu₂FeO_7.08, in the range of 0.73-1.2 eV, has been extracted from diffuse reflectance spectroscopy (DRS). While the three methodologies (GGA, GGA + U, SCAN) permit the reproduction of the crystal structures of the synthetized oxides (determined here in the case of YSr₂Cu₂FeO_7.08 by neutron powder diffraction (NPD)), the SCAN emerges as the only one capable to predict the basic electronic and magnetic properties across the YSr₂Cu₂FeO_8-δ (0 < δ < 1) series. The picture that emerges for the metal (δ = 0) to insulating (δ = 1) transition is the one in which oxygen vacancies contribute electrons to the filling of the Cu/Fe-3d states of the conduction band. These results validate the SCAN functional for future DFT investigations of complex functional oxides that combine several transition metals.