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

Abstract

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 YSr 2 Cu 2 FeO 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 YSr 2 Cu 2 FeO 8 ) and insulating (idealized YSr 2 Cu 2 FeO 7 ) phases of the system. The analysis of the DFT results is supported by DC resistivity measurements that denote the metal character of the synthesized YSr 2 Cu 2 FeO 7.86 and the semiconducting character of YSr 2 Cu 2 FeO 7.08 prepared under reducing conditions. In addition, the band gap of YSr 2 Cu 2 FeO 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 YSr 2 Cu 2 FeO 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 YSr 2 Cu 2 FeO 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 x 2 −y 2 states of the conduction band. These results validate the SCAN functional for future DFT investigations of complex functional oxides that combine several transition metals.

The SCAN+U method in the investigation of complex transition metal oxides: a case study on YSr2Cu2FeO7+δ (δ = 0, 1)

“…17 Noteworthily, the inter-plane magnetic interactions are weak throughout the YSr 2 Cu 2 FeO 7+d system. 8,17,32,34 This is consistent with the small energy differences found between the inter-plane ferromagnetic and antiferromagnetic couplings in YSr 2 Cu 2 FeO 7 within the PBE, PBE+U and SCAN approximations. 8 For this oxide, the SCAN+U (U Fe = U Cu = 4 eV) yields a negligible energy (E TotalFM -E TotalA-AFM ) difference of 0.004 eV per formula unit.…”

“…8,17,32,34 This is consistent with the small energy differences found between the inter-plane ferromagnetic and antiferromagnetic couplings in YSr 2 Cu 2 FeO 7 within the PBE, PBE+U and SCAN approximations. 8 For this oxide, the SCAN+U (U Fe = U Cu = 4 eV) yields a negligible energy (E TotalFM -E TotalA-AFM ) difference of 0.004 eV per formula unit. Similarly, for the metallic YSr 2 Cu 2 FeO 8 , the A-type AFM configuration is only 0.018 eV per formula unit more stable than the FM configuration within the SCAN +U (U Fe = U Cu = 4 eV).…”

“…ordering within the Cu/Fe plane layer arising from superexchange interactions via oxygen bridges. 8,34 In contrast, in-plane AFM ordering is not observed in the metallic YSr 2 Cu 2 FeO 8 , for which an A-type AFM magnetic structure is reported. 17 Noteworthily, the inter-plane magnetic interactions are weak throughout the YSr 2 Cu 2 FeO 7+d system.…”

“…The recently developed strongly constrained and appropriately normed (SCAN) meta-GGA functional 1 has demonstrated a superior performance in reproducing the fundamental physical properties and phenomena associated with correlated oxides. [2][3][4][5][6][7][8] Furthermore, studies have shown that incorporating the SCAN functional into the DFT+U framework (SCAN+U) can enhance the accuracy in reproducing certain physical properties of transition metal oxides. [9][10][11][12][13] For example, Carter and coworkers demonstrated that the introduction of the Hubbard term improves the reproduction of the ground-state properties of binary 3d TM oxides, being critical for the prediction of the most stable polymorphs of Mn 2 O 3 and Fe 3 O 4 .…”

“…The synthetized compounds with d B 0 are insulators with mainly Fe 3+ and Cu 2+ ions. 8,16,18,19 The combination of Cu and Fe ions in different oxidations states, and the evolution of the electronic properties of the YSr 2 Cu 2 FeO 7+d series, provides an excellent framework to examine the applicability of the SCAN+U approach in the DFT investigation of complex oxides. This study delves into the effects of U Cu and U Fe correction terms on the crystallographic, magnetic and electronic structures of the idealized endmembers of YSr 2 Cu 2 FeO 7+d (d = 0 and 1).…”