Studies of the Cu-O bond in (Pb, Cd)-1212 and (Pb, Cd)-1201 cuprates by X-ray photoelectron spectroscopy

It is shown that in (Pb_0.5Cd_0.5)Sr₂(Y_0.5Ca _0.5)Cu₂O₇ and (Pb_0.5Cd_0.5)SrLaCuO₅ cuprates, where Cu²⁺ ions are in pyramidal and octahedral geometry with five and six neighboring oxygens, respectively, the Cu2p_3/2(2p⁵3d¹⁰L) XPS core line is broader than that in Nd₂CuO₄ where Cu²⁺ ions are in a square planar geometry with four coordination oxygen ions, and that for (Pb_0.5Cd_0.5)SrLaCuO₅ is the broadest. To explain these changes in the XPS Cu2p_3/2 main line, two different charge transfer mechanisms from the O2p to the Cu3d orbitals are considered. For the square planar geometry (CuO₄) only the O2p_{x, y} → Cu3d_x2-y2 charge transfer is allowed, whereas for pyramidal and octahedral geometries an additional apical O2p_z → Cu3d_3z2-r2 transition could take place, and in octahedral geometry the latter transition is more significant. The valence band spectra of the cuprates also confirm this proposition.