Raman scattering from triple perovskites with the ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$ structure: Normal mode assignments from substitutions on the Ba site

Raman spectra are reported for ${\mathrm{Ba}}_{1.5}{\mathrm{La}}_{1.5}{\mathrm{Cu}}_3{\mathrm{O}}_{7+\delta }$ ceramics and ${\mathrm{Sr}}_2\mathrm{Y}{\mathrm{Cu}}_{3-x}{\mathrm{Al}}_x{\mathrm{O}}_{7-\delta }$ ceramics and single crystals. The shifts in the frequencies of the Raman features for these triple perovskites, as compared to ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$, provide additional evidence for the assignments of the normal modes. There are large changes in the frequency of the prominent 500-${\mathrm{cm}}^{-1\mathrm{}}$ feature that are well correlated to changes in the lengths of the axial Cu-O bonds. This correlation supports the assignment of the 500-${\mathrm{cm}}^{-1\mathrm{}}$ 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-${\mathrm{cm}}^{-1\mathrm{}}$ mode of ${\mathrm{Ba}}_2\mathrm{Y}{\mathrm{Cu}}_3{\mathrm{O}}_x$, 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 ${\mathrm{cm}}^{-1\mathrm{}}$, the assignments of the five prominent Raman modes are well supported by experiment and agree with lattice-dynamics calculations.