Anomalous x dependence of T_c and possibility of low-temperature structural phase transition in La_2-xSr_xCu_0.99M_0.01O₄ (M Ni, Zn, Ga)

Effects of the 1% substitution of Ni, Zn and Ga for Cu on the x dependence of T_c have been investigated in La_2-xSr_xCuO₄. The T_c increases by the Cu-site substitution in all compositions of x. The local minimum of T_c as a function of x, appearing slightly near x = 0.115 for La_2-xSr_xCuO₄, becomes conspicuous with decreasing T_c. However, structural phase transition from the orthorhombic mid-temperature phase to the tetragonal low-temperature phase has not been detected from powder X-ray diffraction measurements at low temperatures down to 12 K. The local minimum of T_c is attributable to the fluctuation of the undiscovered structural phase transition, which will occur at a low temperature below 12 K as long as samples are in the normal-conducting state. In the 1% Ga-substituted system for Cu, the local minimum clearly shifts from x = 0.115 to 0.125. This suggests that the undiscovered low-temperature structural phase transition is due to the optimum hole concentration of 0.115 per Cu atom, analogously to La_2-xBa_xCuO₄.