Abstract
A new n = 2-member of the Ruddlesden−Popper (RP) cobaltite series, Sr3-δCo1.9Nb0.1O6.65-δ, has been stabilized by niobium doping. This oxide reacts topotactically with atmospheric water through hydrolysis/hydration and reduction of cobalt, leading to an oxyhydroxide hydrate derivative, Sr3-δCo1.9Nb0.1O4.86-δ(OH)3.04·0.4H2O. The reaction with water at room temperature induces a magnetic transformation from a spin glass behavior with a freezing temperature Tg ≈ 50 K for the pristine RP oxide to weak ferromagnetism with an apparent Curie temperature TC ≈ 200 K for the oxyhydroxide hydrate derivative. The spectacular effect of water upon the magnetic properties of this layered oxide is explained by the partial reduction of Co4+ into Co3+ species, concomitantly inducing a coordination change for cobalt cations that form antiferromagnetic coupling between high-spin Co3+. A generalization of this phenomenon to other layered cobaltites and even iron oxides is considered.
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