Raman and Mössbauer study of the pseudo-orthorhombic-to-tetragonal phase transition in YBa2(Cu1−xFex)3O7−δ (0.02⩽x⩽0.15)

Abstract

The polarized Raman spectra from microcrystals of YBa2(Cu1-xFex)3O7-δ (0.02≤x≤0.15) were studied in various scattering configurations allowing one to follow the variations with x of both diagonal (Ag) and non-diagonal (B2g and B3g) Raman modes. It was found that the splitting of the strongest in intensity B2g, B3g Raman pair at 210 and 300 cm-1 associated with O(4) vibrations along a and b, respectively, decreases slightly with x, thus indicating that in a microscopic scale the structure remains orthorhombic over the whole substitutional range. The Mössbauer spectra for x=0.05, 0.10, and 0.15 showed a superlinear increase of the number of five-fold oxygen-coordinated Fe-atoms at the Cu(1)-sites. This is consistent with the assumption that Fe-clusters are formed along the <110> microtwin boundaries at higher x. In this sense YBa2(Cu1-xFex)3O7-δ could be considered as a two-phase system. The observed splitting of the Ag Raman mode of Ba at x≥0.07 supports such an assumption. The Fe substitution increases the local disorder thus including additional Raman scattering of one-phonon density-of-states origin with a maximum at 580 cm-1. © 1992.