Effect of cation substitution on the lattice vibration and crystal structure of magnetic RuSr_1.9A_0.1GdCu₂O₈ (A = Ca, Sr, and Ba) superconductors

The lattice vibration and crystal structure of alkaline earth metal-substituted RuSr_1.9A_0.1GdCu₂O₈ (A = Ca, Sr, and Ba) have been investigated with micro-Raman spectroscopy. The present RuSr_1.9A_0.1GdCu₂O₈ materials show not only several weak Raman peaks corresponding to the vibrations of O_Cu and O_Ru but also strong characteristic phonon lines related to O_Sr vibration mode. A comparison between the frequency of O_Sr vibration and the bond distances of (Ru-O_Sr) and (Cu-O_Sr) in the present ruthenocuprates reveals that the vibration energy of O_Sr is mainly dependent on the bond distance of (Ru-O_Sr). The peak splitting of the O_Sr phonon lines was observed for the unsubstituted RuSr₂GdCu₂O₈, suggesting the existence of two different (Ru-O_Sr) bond distances. Such a peak splitting caused by the appearance of low-energy shoulder reflects the presence of internal charge transfer pathway from the RuO₂ plane to the superconductive CuO₂ one. After the substitution of Sr with Ca or Ba, the low-energy shoulder peak of O_Sr vibration becomes suppressed, underscoring the depression of internal charge transfer between the RuO₂ and CuO₂ planes. The weakened role of RuO₂ layer as charge reservoir in the RuSr_1.9A_0.1GdCu₂O₈ (A = Ca, Ba) would be responsible for the depression of T_c upon the Ca/Ba substitution.