High-Temperature Superconductors and Related Compounds Studied by Mössbauer Spectroscopy

Abstract

⁵⁷Fe, ¹¹⁹Sn, and ¹⁵¹Eu Mössbauer spectroscopy was used to study the chemical structure, phonon mode changes, and low-temperature phase transformation around the T_c as well as suppression of superconductivity in high-temperature superconductors and related compounds. Anomalous temperature-dependent changes in the total ⁵⁷Fe spectral area fraction and in the Mössbauer line shift were simultaneously found around the T_c in a EuBa₂(Cu_0.99⁵⁷Fe_0.01)₃O_7-d superconductor. These anomalous changes were attributed to phonon softening and low-temperature phase transformation occurring around the superconducting transition. Significant differences were observed between the ⁵⁷Fe Mössbauer spectra of superconducting EuBa₂(Cu_0.99⁵⁷Fe_0.01)₃O_7-d and the isostructural non-superconducting PrBa₂(Cu_0.99⁵⁷Fe_0.01)₃O_7-d. The differences were interpreted in connection with the suppression of superconductivity (by hole filling or hybridization of Pr, Cu and O states) in the Pr-containing compound. The unusually high isomer shift value observed in the Pr-containing material can give evidence for the charge transfer mechanism between the Cu(1) chains and the Cu(2) planes and for its role in the suppression of superconductivity.