Synthesis and crystal structure of the oxygen defect perovskites containing copper and tantalum

Perovskite-related solid solutions, Sr 2− x La x Cu 1− y Ta 1+ y O z , were synthesized by solid state reaction at 1050–1080°C in air. The crystalline phases were identified by powder X-ray diffraction. Three types of crystal structure were observed including simple cubic perovskite-type, tetragonal perovskite-type and tetragonally distorted (NH 4) 3FeF 6-type structure in which Cu 2+ and Ta 5+ were ordered alternatively. The average valency state of copper in these solid solutions was nearly equal to +2. The tetragonal phases were observed in a compositional range, z≥5.85, regardless of cation composition. The tetragonality ( c/ a) for both the perovskite-type and (NH 4) 3FeF 6-type phases increased with increasing oxygen content. Ordering between Cu 2+ and Ta 5+ was not observed in the Sr-rich composition. These facts suggest that oxygen content affects the appearance of the Jahn–Teller effect of Cu 2+ leading to the tetragonal distortion and that the cation on the A-site affects the ordering of the cation on the B-site.