Raman Scattering from Triple Perovskites with the Ba2YCu3Ox Structure: Normal Mode Assignments from Substitutions On the Ba Site

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Raman spectra are reported for Ba1.5La1.5Cu3O7+ ceramics and Sr2YCu3-xAlxO7- ceramics and single crystals. The shifts in the frequencies of the Raman features for these triple perovskites, as compared to Ba2YCu3Ox, provide additional evidence for the assignments of the normal modes. There are large changes in the frequency of the prominent 500-cm-1 feature that are well correlated to changes in the lengths of the axial Cu-O bonds. This correlation supports the assignment of the 500-cm-1 feature to the axial stretching of the oxygen atoms that bridge the Cu sites. The substitution of Sr for Ba does not change the frequency of the 140-cm-1 mode of Ba2YCu3Ox, thereby supporting its assignment to axial stretching of the planar Cu atoms rather than to a Ba mode. With these new data and the previous, definitive assignment of the feature at 340 cm-1, the assignments of the five prominent Raman modes are well supported by experiment and agree with lattice-dynamics calculations.



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