X-ray-photoelectron-spectroscopy evidence for peroxide in 1:2:3 copper oxides containing disordered or excess oxygen

The x-ray-photoelectron-spectroscopy of several layered oxides provide no Cu(2p) evidence for Cu(III), but exhibit an extra O(1s) peak where direct contact between oxygen atoms occurs within the Cu(1) planes of the YBa₂{Cu}₃O_7-δ structure. The extra peak has a binding energy typical of that for a peroxide ion (O₂{)}²$. This observation, in conformity with earlier prediction, provides compelling evidence for holes in the O 2p bands of the high-${T}c$ copper-oxide superconductors.