High-Tc superconductors studied by57Co,57Fe,119Sn, and151Eu m�ssbauer spectroscopy

Abstract

151Eu,119Sn,57Fe, and57Co Mossbauer spectroscopy was used to study YBa2Cu3O7−δ high-Tc superconductors in which sites were replaced by various Mossbauer nuclides in order to get information about site preference and structural changes. By decomposition of the Mossbauer spectra, the Y site substituted by Eu and the Cu sites occupied by Sn, Fe, and Co substituents were identified. It was found that Fe and Co prefer the Cu(1) site while Sn prefers the Cu(2) site in the orthorhombic perovskite lattice of these superconductors. The site preference is enhanced in the presence of additional substituents. A sequence of thermal heat treatments permits reversible cycling of57Co between the two copper sites in the YBa2[Cu(57Co)]3O7−δ perovskite. In all cases Eu(III) and Sn(IV) states are determined, but the valence state of Fe and Co can be both three and four. BelowT c, low-temperature phase transformation and phonon softening were shown from the anomalous temperature dependence of isomer shifts and the area fractions of151Eu and119Sn Mossbauer spectra of EuBa2(Cu1-xSnx)3O7-δ cuprates.