Magnetic dioxygen molecules can be used as building blocks of model systems to investigate spin-polarization that arises from unpaired p-electrons, the scientific potential of which is evidenced by phenomena such as spin-polarized transport in graphene. In solid elemental oxygen and all of the known ionic salts comprised of magnetic dioxygen anions and alkali metal cations, the dominant magnetic interactions are antiferromagnetic. We have induced novel ferromagnetic interactions by introducing oxygen deficiency in rubidium superoxide (RbO 2 ). The anion vacancies in the resulting phase with composition RbO 1.72 provide greater structural flexibility compared to RbO 2 and facilitate a Jahn-Teller-driven order-disorder transition involving the anion orientations at ∼230 K, below which their axes become confined to a plane. This reorganization gives rise to short-range ferromagnetic ordering below ∼50 K. A ferromagnetic cluster-glass state then forms below ∼20 K, embedded in an antiferromagnetic matrix that orders at ∼5 K. We attribute this inhomogeneous magnetic order to either subtly different anion geometries within different structural nanodomains or to the presence of clusters in which double exchange takes place between the anions, which are mixed-valence in nature. We thus demonstrate that nonstoichiometry can be employed as a new route to induce ferromagnetism in alkali metal oxides.
We have synthesized members of an isostructural solid solution series K x Ba 1−x O 2−δ (x < 0.41, δ < 0.11) containing mixed-valent dioxygen anions. Synthesis in liquid ammonia solution allows a continuous range of compounds to be prepared. X-ray and neutron diffraction show that K x Ba 1−x O 2−δ adopts the tetragonal rocksalt-derived structure of the end members KO 2 and BaO 2 , without any structural phase transition down to 5 K, the lowest temperature studied here. We identify four oxygen−oxygen stretching modes above 750 cm −1 in the measured Raman spectra, unlike the spectra of KO 2 and BaO 2 which both contain just a single mode. We use density functional theory calculations to show that the stretching modes in K x Ba 1−x O 2−δ arise from in-phase and anti-phase coupling of the stretching of nearest-neighbor oxygen dimers when the valence state of the dimers lies between −1 and −2 because of mixed cation coordination. This coupling is a direct signature of a novel type of anionogenic mixed valency.
The solid solution series KxBa1‐xO2‐δ (x < 0.41) is synthesized from stoichiometric amounts of Ba(NO3)2 and KO2 in liquid NH3 (‐40 to ‐60 °C, 30 min).
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