Structure, superconductivity and ¹⁵¹Eu Mössbauer studies on Eu(Ba_2-xEu_x)Cu₃O_7+z oxide system

The compounds of the type Eu(Ba_2-xEu_x)Cu₃O_7+z with x ranging from 0.0 to 0.5 have been prepared and investigated from structure and superconductivity view point. The compound with x = 0 is the well known 1:2:3 orthorhombic superconducting compound EuBa₂Cu₃O_7-y with mid-point T_c of 87K. It is observed that in the Eu excess system, the orthorhombicity decreases with increasing x, and the compounds with x≥0.2 appear tetragonal. The superconducting transition temperature decreases with increasing x: the sample with x = 0.4 is non-superconducting down to 4.2K. ¹⁵¹Eu Mossbauer studies reveal that all the europium ions are in a trivalent non-magnetic state. The Eu at the regular site and also the one substituting at the divalent Ba site show nearly the same isomer shift. In this respect the two sites are very similar. The decrease in T_c with increasing x is not due to the magnetism of the rare earth ion but, has to be attributed to the structure and electronic effects.