Synthesis and structural characterization of gadolinium barium cobalt oxide Gd₈Ba₅Co₄O₂₁ with CoO₅ square pyramids

抄録

We have determined the crystal structure of Gd₈Ba₅Co₄O₂₁ by single-crystal X-ray diffraction and transmission electron microscopy (TEM). Gd₈Ba₅Co₄O₂₁ shows a tetragonal I4/m structure with a = 13.8982(3) Å and c = 5.76510(10) Å. This compound is isostructural with R₈Ba₅Ni₄O₂₁ (R = rare-earth ions) with two types of BaO₁₀, GdO₇ and CoO₅ polyhedra. Using scanning TEM, we directly observed the atomic arrangements of Gd₈Ba₅Co₄O₂₁, which was consistent with crystal structure analysis. Isolated CoO₅ square pyramids are placed with the relationship to 4-fold symmetry about the c-axis. Gd₈Ba₅Co₄O₂₁ has a similar local structure to R₂BaCoO₅ with a Y₂BaNiO₅-type structure. The first neighboring Co–Co atomic distances are shorter than those of R₂BaCoO₅ because of changes in the framework structure composed of RO₇ and CoO₅ polyhedra.