Oxygen-nonstoichiometry-driven phase transition in $\mathrm{Sr}_{1-x}\mathrm{Nd}_{x}\mathrm{CoO}_{3-δ}$ ($x = 0.1, 0.2, 0.3$) perovskites

We report a systematic study of the interplay between oxygen nonstoichiometry, crystal structure, and magnetic/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 $I4/mmm$ ($2a_p \times 2a_p \times 4a_p$) to an oxygen-deficient orthorhombic $Pmmm$ ($a_p \times a_p \times 2a_p$) phases with preferential oxygen-vacancy occupation. This transition is accompanied by a crossover from G-type antiferromagnetic 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 Co $3d$ - O $2p$ orbital overlap upon oxygen uptake, and a magnetically inhomogeneous ferromagnetic state associated with residual oxygen vacancies and mixed $\mathrm{Co}^{3+}/\mathrm{Co}^{4+}$ 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.