Topotactic phase transformations by concerted dual-ion migration of B-site cation and oxygen in multivalent cobaltite La–Sr–Co–O_x films

Manipulating topotactic phase transformations via orderly ion transfer in complex oxides ABO_x is ubiquitous in advanced applications such as ionotronics, ion-batteries and catalysts. Most of such ion-mediated transformations are accomplished by the transfer of oxygen or A-site ions. However, implementing the transformation via the transfer of B-site ions, despite the great challenge to overcome a large cohesive energy, has unique advantage since they host most functional properties of materials. Here, we present a tri-state phase transformation from perovskite (P) to brownmillerite (BM) and to single-layered perovskite (SL) structure via the concerted migration of oxygen and B-site Co-ions in La_0.7Sr_0.3CoO₃ thin films. Ac-STEM, XPS, XAS, PNR, magnetic and electric measurements demonstrated that presented B-site Co-cation transfer is along the CoO₄ tetrahedral sub-layer of the BM film, which leads to the reconfiguration of 3d-electrons and spin state in remanent Co ions and causes tremendous changes in magnetic and electric properties: from canted-antiferromagnetic insulator in BM phase to ferromagnetic insulator in SL phase. First-principles calculations revealed that the La³⁺-doping at A-site largely reduces the cohesive energy of Co-ions in CoO₄ and destabilize the CoO₄ tetrahedron of BM phase, which explains the formation of Co-ions transfer channel in the CoO₄ tetrahedral sub-layer. The present study highlights the effectiveness of regulating topotactic transformation via B-site ions transfer and provides a new pathway for manipulating the topotactic transformation with diverse functionalities.