The thermal conductivity and the sound velocity in La2CuO4 are found to be significantly decreased by Nd, Zn, and Ni doping, reflecting the lattice instability of this compound. The rapid decrease of with x in La2-xMxCuO4 (M=Sr,Ba) in a small-x region that was previously reported by us is understood to be the result of an enhancement of the lattice instability induced by M doping, just as in the Nd-, Zn-, and Ni-doped insulating systems. The y dependence of the transition temperature from the LTO1 (low-temperature orthorhombic 1) to LTT (low-temperature tetragonal) phase in La1.875-yRySr0.125CuO4 (R=Nd,Sm,Eu,Gd) indicates that the larger the difference of the ionic radii between La3+ and R3+ ions, the more easily the transition to the LTT phase is induced. We propose that the LTT region exists around the R3+ ion even in the LTO1 phase and extends with the decrease of temperature and the structural transition to the LTT phase is induced when the LTT region becomes connected in a whole area of the crystal. We also point out the importance of the local lattice distortion for the Cu-site-substitution effects on La2-xMxCuO4.
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