ANOMALOUS X DEPENDENCE OF TC AND POSSIBILITY OF LOW-TEMPERATURE STRUCTURAL PHASE-TRANSITION IN LA2-XSRXCU0.99M0.01O4(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 La2-xSrxCuO4. The T(C) decreases by the Cu-site substitution in all composition of x. The local minimum of T(C) as a function of x, appearing slightly near x = 0.115 for La2-xSrxCuO4, 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 La2-xBaxCuO4.