Fig. 3.

In-plane Cu–O and apical Cu–O bond lengths. (Upper Left) In-plane Cu–O bond length for various single-layer cuprates: LSCO (21), Bi₂Sr₂CuO_6+δ, Tl₂Ba₂CuO_6+δ (22), HgBa₂CuO_4+δ (23), and Ba214. (Lower Left) The same set of the data for Cu apical O bond length (apical O distance). In Ba214, the bond-length ratio is smaller than one, in which case the 3dz² orbital level is expected to be located above the 3dx²-y² orbital level in contrast to the case where the ratio is significantly larger than one as in the case of conventional high-T_c cuprates sketched in Right. A schematic crystal structure with a compressed “oxygen octahedron” is also shown (exact positions of oxygen vacancies in the plane are not known at present).