Raman and Mössbauer study of the pseudo-orthorhombic-to-tetragonal phase transition in YBa 2(Cu 1− xFe x) 3O 7− δ (0.02⩽ x⩽0.15)

Abstract

The polarized Raman spectra from microcrystals of YBa 2(Cu 1− x Fe x ) 3O 7− δ (0.02⩽ x⩽0.15) were studied in various scattering configurations allowing one to follow the variations with x of both diagonal (A g) and non-diagonal (B 2g and B 3g) Raman modes. It was found that the splitting of the strongest in intensity B 2g, B 3g 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 YBa 2(Cu 1− x Fe x ) 3O 7− δ could be considered as a two-phase system. The observed splitting of the A g 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.