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 YBa 2 (Cu 1− x Fe x ) 3 O 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 Mossbauer 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 microtwin boundaries at higher x . In this sense YBa 2 (Cu 1− x Fe x ) 3 O 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 .