Cation and anion topotactic transformations in cobaltite thin films leading to Ruddlesden-Popper phases

Author(s): Chiu, IT; Lee, MH; Cheng, S; Zhang, S; Heki, L; Zhang, Z; Mohtashami, Y; Lapa, PN; Feng, M; Shafer, P; N'Diaye, AT; Mehta, A; Schuller, JA; Galli, G; Ramanathan, S; Zhu, Y; Schuller, IK; Takamura, Y | Abstract: Topotactic transformations involve structural changes between related crystal structures due to a loss or gain of material while retaining a crystallographic relationship. The perovskite oxide La0.7Sr0.3CoO3 (LSCO) is an ideal system for investigating phase transformations due to its high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and strong coupling of the magnetic and electronic properties to the oxygen stoichiometry. While the transition between cobaltite perovskite and brownmillerite (BM) phases has been widely reported, further reduction beyond the BM phase lacks systematic studies. In this paper, we study the evolution of the physical properties of LSCO thin films upon exposure to highly reducing environments. We observe the rarely reported crystalline Ruddlesden-Popper phase, which involves the loss of both oxygen anions and cobalt cations upon annealing where the cobalt is found as isolated Co ions or Co nanoparticles. First-principles calculations confirm that the concurrent loss of oxygen and cobalt ions is thermodynamically possible through an intermediary BM phase. The strong correlation of the magnetic and electronic properties to the crystal structure highlights the potential of utilizing ion migration as a basis for emerging applications such as neuromorphic computing.