RAMAN AND MOSSBAUER STUDY OF THE PSEUDO-ORTHORHOMBIC-TO-TETRAGONAL PHASE-TRANSITION IN YBA2(CU1-XFEX)3O7-DELTA (0.02-LESS-THAN-OR-EQUAL-TO-X-LESS-THAN-OR-EQUAL-TO-0.15)

The polarized Raman spectra from microcrystals of YBa2(Cu1-xFex)3O7-delta (0.02 less-than-or-equal-to x less-than-or-equal-to 0.15) were studied in various scattering configurations allowing one to follow the variations with x of both diagonal (A(g)) 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 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 <110> microtwin boundaries at higher x. In this sense YBa2(Cu1-xFe)3O7-delta could be considered as a two-phase system. The observed splitting of the A(g) Raman mode of Ba at x greater-than-or-equal-to 0.07 supports such an assumption. The Fe substitution increases the local disorder thus inducing additional Raman scattering of one-phonon density-of-states origin with a maximum at 580 cm-1.