Polymorphism of Ba1–xSrxCoO3–δ (0≤ x≤ 1) Perovskites: A Thermal and Structural Study by Neutron Diffraction

Abstract

The preparation of different hexagonal, orthorhombic and cubic polymorphs of the solid solution Ba1−xSrxCoO3−δ (0 ≤ x ≤ 1) is described. The samples have been studied by thermal analysis (TG and DTA) to identify the phase transitions; the thermal structural evolution and the structural characterization of different phases were analyzed by X-ray and neutron powder diffraction and refined by the Rietveld method. A series of hexagonal perovskites Ba1−xSrxCoO3−δ (0 ≤ x < 0.5), labelled as “H”, were synthesized by thermal treatment of reactive citrate precursors at 900 °C in high oxygen pressure followed by slow cooling to r. t. The hexagonal perovskites with 0.5 ≤ x ≤ 1 were obtained from the citrate precursors heated twice at 900 °C in air and slowly cooled in the furnace. Orthorhombic brownmillerite-like structures, labelled “O”, were obtained from precursors with composition 0.5 ≤ x ≤ 1 by quenching in liquid N2 from 900 °C. For x < 0.5, quenching from high temperatures does not stabilize the “O” phases. The crystal structure for both terms of the solid solution (x = 0 and x = 1) has been investigated by neutron powder diffraction. DTA and X-ray thermo-diffractometry show that “H” phases experience a reconstructive transition at ca. 900 °C to give cubic “C” polymorphs.