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Effect of Cation Substitution on the Lattice Vibration and Crystal Structure of Magnetic RuSr1.9A0.1GdCu2O8 (A = Ca, Sr, and Ba) Superconductors

The lattice vibration and crystal structure of alkaline earth metal-substituted RuSr 1.9 A 0.1 GdCu 2 O 8 (A= Ca, Sr, and Ba) have been investigated with micro-Raman spectroscopy. The present RuSr 1.9 A 0.1 GdCu 2 O 8 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 RuSr 2 GdCu 2 O 8 , 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 RuO 2 plane to the superconductive CuO 2 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 RuO 2 and CuO 2 planes. The weakened role of RuO 2 layer as charge reservoir in the RuSr 1.9 Ao 0.1 GdCu 2 O 8 (A = Ca, Ba) would be responsible for the depression of T c upon the Ca/Ba substitution.