Anomalous Thermal Expansion in La_2-xSr_xCuO₄

Lattice constants of La2-xSrxCuO4 (x = 0.05, 0.10 and 0.15) are precisely determined as a function of temperature from 13 K to room temperature by Rietveld analysis using X-ray powder diffraction data. In La1.95Sr0.05CuO4 and La1.90Sr0.10CuO4, unit cell volumes show a small change below 50 K and 70 K, respectively. Anomalous volume change around T(c) found in La1.85Sr0.15CuO4 and La1.90Sr0.10CuO4 cannot be explained only by superconductivity. The comparison between lattice constants in all samples is suggestive of the structural phase transition preceding the appearance of superconductivity.