Mössbauer study of the Eu substitution in (Tl,Pb)-1212 superconductors

Eu-substituted (Tl0.6Pb0.4)(Sr0.9Ba0.1)2(Ca1−xEux)Cu2Oy, for x = 0.2, 0.4, and 0.6 compounds were prepared and investigated by 151Eu Mössbauer spectroscopy, XRD, SEM, EDAX and electrical resistance measurements. The 151Eu isomer shift reflected trivalent Eu in all cases, but it decreased with increasing Eu content. The monotonous change of the principal component, Vzz of electric field gradient with Eu content can be attributed mainly to the change in the lattice parameters due to substitution of Eu for Ca. The correlation between the 151Eu isomer shift and the superconducting critical temperature supports the hole filling mechanism (and change in the formal valence state of copper) to explain the suppression of superconductivity upon Eu substitution in this system.