We report a systematic study of the interplay between oxygen nonstoichiometry, crystal structure, and magnetic and electrotransport properties in ${\mathrm{Sr}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CoO}}_{3-\delta }$ $(x=0.1,0.2,0.3)$. High-resolution neutron powder diffraction combined with synchrotron x-ray powder diffraction reveals that increasing the oxygen content induces a structural transition from a layered $I4/mmm$ $(2a_p\times 2a_p\times 4a_p)$ phase to an oxygen-deficient orthorhombic $Pmmm$ $(a_p\times a_p\times 2a_p)$ phase with preferential oxygen-vacancy occupation. This transition is accompanied by a crossover from a G-type antiferromagnetic state with a weak ferromagnetic component to a ferromagnetic state, and a drastic decay in resistivity. The evolution of the magnetic and transport properties is discussed in terms of changes in the Co spin state, enhanced $\mathrm{Co}3d$–O $2p$ orbital overlap upon oxygen uptake, and a magnetically inhomogeneous ferromagnetic state associated with residual oxygen vacancies and mixed ${\mathrm{Co}}^{3+}$ and ${\mathrm{Co}}^{4+}$ valence. Our findings experimentally confirm that the stabilization of the layered “314” structure is driven by the presence and ordering of oxygen vacancies rather than A-site cation ordering, whereas the oxygen-deficient oxidized compounds represent an intermediate orthorhombic state preceding fully stoichiometric phases.

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• Crystal structure
• Exchange interaction
• Magnetism
• Perovskites
• Neutron diffraction