The Role of Oxygen Stoichiometry on Phase Stability, Structure, and Magnetic Properties of Sr2CoIrO6−δ

The phase stability, crystal structure, and magnetic properties of perovskite-like nonstoichiometric Sr2CoIrO6−δ were studied. Oxygen deficiency can be well controlled and reversibly varied up to δ = 0.33. A single phase exists at least for partial oxygen pressures between 10−5 and 1 bar at 1273 K, followed by phase decomposition at higher temperature with the elimination of metallic Ir and the formation of a new phase with approximately Sr3CoIrO6 composition crystallizing in K4CdCl6 structure type. The structural features of Sr2CoIrO6−δ are dependent on both temperature and oxygen content and were determined by synchrotron and neutron powder diffraction. Both the increasing amount of oxygen vacancies at constant temperature and increasing temperature at constant oxygen content result in the same higher crystal symmetry of Sr2CoIrO6−δ: (1) The oxygen-stoichiometric phase Sr2CoIrO6.00 is monoclinic (I2/m or P21/n) at room temperature but cubic (Fm-3m) for Sr2CoIrO5.67. (2) A sequence of phase transitions I 2 / m + P 2 1 / n ↔ 220 K I 2 / m ↔ 620 K I 4 / m ↔ 700 K F m - 3 m was observed for Sr2CoIrO6.00 in air. All Sr2CoIrO6−δ compositions show weak ferromagnetism at low temperature with a canted but predominantly antiferromagnetic ground state. The magnetic ordering temperature decreases monotonously with increasing oxygen deficiency, while pronounced extrema are observed for the paramagnetic moment and the Curie−Weiss temperature at an oxygen deficiency δ ≈ 0.10, which corresponds to the P21/n ↔ I2/m phase transformation.