An investigation of the polytypical structure of Sr_0.2Ba_0.8CoO_3–δ by neutron powder diffraction

Abstract

The preparation and characterization of two polymorphs of the title composition are described. One hexagonal perovskite, labeled as “H”, was synthesized by thermal treatment of reactive citrate precursor at 900 °C in high oxygen pressure (20 MPa) followed by slow cooling (10 °C/min) to room temperature. This 1D-structure displays a P6₃/mmc hexagonal space group with a thermal expansion of 13 × 10^–6 K^–1 between 100 and 900 °C. On the other hand, for the first time in such a Ba-rich composition, a highly oxygen-defective perovskite oxide cubic polymorph, labeled as “C”, was obtained from the citrate precursor heated in air and quenched in liquid N₂ from 1000 °C and completely characterized. Neutron powder diffraction evidence that the obtained “C” phase posses a cubic Pm-3m space group and antiferromagnetic order at room temperature. The material can be formulated as Sr_0.2Ba_0.8CoO_2.27(1) where the high number of oxygen vacancies are random distributed with large thermal factors of 3.6 Ų suggesting a considerable mobility.

The evolution of the crystal structure of the 3C phase has been explored in situ by XRD thermo-diffractometry in complement with thermal analysis and thermal expansion. At approx.500 °C this phase irreversibly transforms to a hexagonal “H” phase and at 925 °C a new cubic perovskite “C” is identified, which is transformed again, upon cooling, into the “H” phase at 840 °C.