Structural phase transition and structural properties in oxygenated La2CuO4+y; y ∼ 0.02

The crystal structure of the oxygenated La2CuO4+y; ysimilar to0.02 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 D-10(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).