Spectroscopic studies on the High-T-c superconducting La2CuO4+delta prepared by electrochemical oxidation

A superconducting phase La2CuO4+delta (T-c = 44 K) has been prepared by electrochemical oxidation which allows the oxygen to intercalate into the La2O2 layers. According to the Cu K-edge X-ray absorption near edge structure spectroscopic analysis, the oxidized phase shows an overall spectra shift of about 0.5 eV to a higher energy region compared to the as sintered one with the occurrence of an additional peak corresponding to the transition to the \1s(1)3d(n+1)L(-1)4p sigma(1)> final state, indicating the oxidation of CuO2 layer. From the X-ray photoelectron spectroscopic studies, it is found that the binding energy of La 3d(5/2) is significantly shifted from 834. 3 eV (as sintered La2CuO4) to 833.6 eV (as electrochemically oxidized La2CuO4+delta) , implying that the covalency of the (La-O) bond is decreased due to the oxygen intercalation. The O 1s spectra do not provide an evidence of the superoxide or peroxide, but the oxide (O2-) with the contaminated carbonate (CO32-) based on the peaks at 529 eV and 532 eV, respectively, which is clearly confirmed by the Auger spectroscopic analysis. Oxygen contents determined by iodometric titration (delta = 0.07) and thermogravimetry (delta = 0.09) show good coincidence each other, also giving an evidence for the ''O2-'' nature of excess oxygen. From the above results, it is concluded that ''O2-'' appeared as O 1s peak at 528.6 eV is responsible for superconductivity of La2CuO4+delta.