Oxygen-nonstoichiometry-driven phase transition in Sr₁₋ₓNdₓCoO₃₋δ (x = 0.1, 0.2, 0.3) perovskites

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We report a systematic study of the interplay between oxygen nonstoichiometry, crystal structure, and magnetic/electrotransport properties in Sr₁₋ₓNdₓCoO₃₋δ (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 (2aₚ × 2aₚ × 4aₚ) to an oxygen-deficient orthorhombic Pmmm (aₚ × aₚ × 2aₚ) 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 Co³⁺/Co⁴⁺ 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.