Effect of Cation Substitution on the Lattice Vibration and Crystal Structure of Magnetic RuSr_1.9A_0.1GdCu₂O₈(A = Ca, Sr, and Ba) Superconductors

Abstract

The lattice vibration and crystal structure of alkaline earth metal-substituted RuSr1.9A0.1GdCu2O8 (A = Ca, Sr, and Ba) have been investigated with micro-Raman spectroscopy. The present RuSr1.9A0.1GdCu2O8 materials show not only several weak Raman peaks corresponding to the vibrations of OCu and ORu but also strong characteristic phonon lines related to OSr vibration mode. A comparison between the frequency of OSr vibration and the bond distances of (Ru-OSr) and (Cu-OSr) in the present ruthenocuprates reveals that the vibration energy of OSr is mainly dependent on the bond distance of (Ru-OSr). The peak splitting of the OSr phonon lines was observed for the unsubstituted RuSr2GdCu2O8, suggesting the existence of two different (Ru-OSr) 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 OSr 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 RuSr1.9A0.1GdCu2O8 (A = Ca, Ba) would be responsible for the depression of Tc upon the Ca/Ba substitution.