Features of both structural transitions and related fluctuations in La1.875-xSmxSr0.125CuO4 have been examined by means of electron diffraction in a temperature range between about 600 K and about 20 K. As the temperature is lowered, a structural phase transition from a high-temperature tetragonal (HTT) structure to a low-temperature orthorhombic (LTO) one takes place in 0≤x≤0.2 and HTT→LTO→Pccn→LTT successive transitions in 0.3≤x≤0.4 where the Pccn and low-temperature tetragonal (LTT) structures are characterized as a rotation of an oxygen octahedron about two 110 directions. An enhancement of a structural fluctuation in the LTO phase, which indicates a soft-phonon mode in the LTO-to-Pccn transition, occurs on cooling even in 0≤x≤0.2 as well as in 0.3≤x≤0.4. Because an increase in the Sm content results in two effects, a lowering of Tc and an enhancement of the structural fluctuation, the fluctuation is understood to suppress the superconductivity. Eventually, a dip in a composition dependence of Tc in La-Sr-Cu-O, which was experimentally found by Oda et al. [J. Phys. Soc. Jpn. 58, 1137 (1989)], is concluded to be due to the structural fluctuation related to the LTO-to-Pccn transition.
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