Near-room temperature ferromagnetic insulating state in highly distorted LaCoO_2.5 with CoO₅ square pyramids

Dedicated control of oxygen vacancies is an important route to functionalizing complex oxide films. It is well-known that tensile strain significantly lowers the oxygen vacancy formation energy, whereas compressive strain plays a minor role. Thus, atomic reconstruction by extracting oxygen from a compressive-strained film is challenging. Here we report an unexpected LaCoO_2.5 phase with a zigzag-like oxygen vacancy ordering through annealing a compressive-strained LaCoO₃ in vacuum. The synergetic tilt and distortion of CoO₅ square pyramids with large La and Co shifts are quantified using scanning transmission electron microscopy. The large in-plane expansion of CoO₅ square pyramids weaken the crystal field splitting and facilitated the ordered high-spin state of Co²⁺, which produces an insulating ferromagnetic state with a Curie temperature of ~284 K and a saturation magnetization of ~0.25 μ_B/Co. These results demonstrate that extracting targeted oxygen from a compressive-strained oxide provides an opportunity for creating unexpected crystal structures and novel functionalities.