Effects of the Cu-site substitution on the anomalous x dependence of T_c in La_2-xBa_xCuO₄

Effects of the partial substitution of Ni, Zn and Ga for Cu on the anomalous x dependence of T_c and the resistance anomaly around 60 K have been investigated in La_2-xBa_xCuO₄. T_c decreases by the substitution in comparison with non-subsituted samples in almost all compositions of x except compositions near x = 0.125. For 1% Ga-substituted samples for Cu, the local minimum of T_c as a function of x, appearing near x = 0.125 for non-substituted samples, clearly shifts to x ≈ 0.135. This suggests that the low-temperature structural phase transition around x = 0.125 in La_2-xBa_xCuO₄ is primarily due to the optimum hole concentration of 0.125 per Cu atom rather than the atomic ordering of La and Ba. The resistance anomaly near 60 K does not change clearly by the substitution.