An x-ray scattering study of the structural phase transition in La _2-xSr_xCu_0.99Fe_0.01O₄ (x = 0.20)

We report a comprehensive x-ray scattering study of the low-temperature orthorhombic (LTO)high-temperature tetragonal (HTT) structural phase transition in 1% iron-doped La_2-xSr_xCuO₄ (x = 0.2). The superlattice (032) peak intensity and the width are investigated in detail for a wide temperature range. We found that the structural phase transition is not sharp and the tilt ordering of the CuO₆ octahedra persists above the transition temperature T_S (≈77K). Even at room temperature, the superlattice peak is still observable. The structural phase transition is identified as an orderdisorder type phase transition. We found that the tilt ordering in our iron-doped material is always short-ranged, and in the HTT phase the correlation between the tilts along the b axis is better preserved than that along the a axis. Moreover, we identify the role of the Fe as the nucleation centers of the LTO domains in the structural phase transition.