Effects of Ni Substitution on the Superconducting and Raman Spectra in YBa₂Cu₄O₈

Abstract

The effects of Ni-substitution on YBa₂Cu₄O₈ have been investigated by Raman scattering, X-ray diffraction, resistivity and susceptibility measurements. Samples of YBa₂(Cu _1−xNi _x )₄O₈ with x = 0 to 0.1 were prepared using a highoxygen-pressure technique. As the Ni content is increased, the lattice parameter of the a axis increases, but that of the c axis decreases. The structure of the YBa₂(Cu _1−xNi _x )₄O₈ samples remain orthorhombic up to the highest doping concentration. The superconducting transition temperature T _c decreases monotonically with increasing Ni concentration for x<0.03, with a coefficient of -32 K/at.%. It is found that the linewidth of Cu(2) A _g mode at 150 cm⁻¹ increases, while the frequency of the axial stretching vibration of 0(4) at 500 cm⁻¹ remains constant for increasing Ni concentration. These facts suggest that Ni atoms will preferentially occupy the Cu(2) sites. The depression of T _c is also discussed based on this substitution model.