Structural phase transition and structural properties in oxygenated La₂CuO_4+y; y ∼ 0.02

The crystal structure of the oxygenated La₂CuO_4+y; y ∼ 0.02 has been studied by X-ray diffraction. It is found that the structural phase transition from tetragonal to orthorhombic symmetry occurring at about 450 K (Tc) is of second order and shows diffuse scattering above Tc. The diffuse scattering is related to static and/or dynamic short-range order of cooperative tilts of rigid-like elongated CuO₆ octahedra, which make two-dimensional networks stacked along the [001] axis. The structures at room temperature and 210 K are determined using a 4-circle X-ray diffractometer and have the orthorhombic space group ¹⁰D_2h-Pccn. The structure is mainly characterized by anisotropic cooperative tilts of the CuO₆ octahedra around the [100]_p and [010]_p primitive axes in two-dimensional octahedron-networks, where suffix "p" means pseudo K₂NiF₄-type structure. The anisotropy of the tilting-angles results from pushing by excess oxygen atoms to apical oxygen atoms of the CuO₆ octahedra. It is also found by refinement of the structure that the excess oxygen atoms are randomly sited at the atomic coordinates (1/4, 1/4, 0.244) and/or (1/4, 1/4, 0.279).