  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, atomically reconstruction by extracting oxygen
from a compressive-strained film is challenging. Here we report an unexpected
LaCoO2.5 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
{\mu}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.
