Syntheses and structures of compounds with YBaCo 4O 7-type structure

Abstract

More than 20 different isostructural (YBaCo 4O 7-type, P6 3 mc, a=6.3–6.4, c=10.1–10.3 Å) compounds in the system RBaX 4O 7 (R=Ca, Y, In, Dy–Lu, X=Co, Al, Zn, Fe) have been synthesised through solid-state reaction and characterised using either X-ray single crystal or powder diffraction data. The R atom occupies an octahedral space in the close-packed oxygen lattice, the X atoms form together with oxygen a defect wurtzite like net of tetrahedra, and the Ba replaces one of the oxygen in the close packing ending up in an anticuboctahedral 12-coordination. Unit cell expansions, especially in the ab-plane, are seen with the presence of Zn 2+ and larger rare-earth ( RE) ions at the same time as Al 3+ causes the c-axis to shrink. The distribution of Al, Co, Fe and Zn at the two crystallographic X positions is discussed. It was possible to conclude, from displacement parameters, that one certain oxygen position has significantly lower freedom to move than the others, which might affect the materials properties since the very same oxygen is also part in the metal–oxygen–metal bridge with the angle best suited for magnetic super-exchange coupling.