Binding Strength and Oxygen Coordination of the Fe Sites in High T_c GdBa₂Cu₃O_7-δ

Abstract：

The GklBa₂(Cu_1-xFe_x)₃O_7-δ system with x=0.005-0.01 and various oxygen concentrations was investigated by ⁵⁷Fe-Mössbauer spectroscopy in the temperature range 20 K to 300 K. The spectra of the orthorhombic phase were analyzed with four quadrupole doublets. Three of them are attributed to substitutional Cu(l) sites and one (with the smallest quadrupole splitting) to the substitutional Cu(2) site. From an analysis of the temperature dependence of the relative site intensities, different local Debye temperatures for the Cu(l) sites were derived. Highly texturized absorbers were investigated to study the polarization dependence of the four quadrupole doublets. Measurements at the magic angle give evidence for a vibrational anisotropy (Goldanskii-Karyagin effect) of the Cu(l) sites. The Cu(l) site with the largest quadrupole splitting exhibits the lowest Debye temperature and the largest vibrational anisotropy.