LOW-TEMPERATURE STRUCTURAL PHASE-TRANSITION AND ELECTRONIC ANOMALIES IN LA1.775R0.10SR0.125CUO4 (R=ND,SM,GD,TB)

Sound velocity and powder x-ray diffraction in La1.9-xR0.1SrxCuO4 (R=Nd,Sm,Gd,Tb) reveal that the transition temperature to the tetragonal low-temperature (TLT) phase increases with decreasing the ionic radius of R up to 150 K (R=Tb around x approximately 0.125). Prominent change of the resistivity rho and the thermoelectric power S is observed in the narrow region of x around 0.125 and at almost the same temperature near 80 K in the Tb-, Gd-, and Sm-substituted samples whose crystal structure is the TLT phase. In the Nd-substituted sample, less prominent but significant change of S is also observed below approximately 50 K where the crystal structure is the orthorhombic midtemperature phase. These results suggest that the change of the transport properties is responsible for the change of the electronic state itself rather than for the structural phase transition and this modification of the electronic state is more pronounced in the TLT phase.