Determination of valence of Cu in superconductingLa2−x(Sr,Ba)xCuO4

The measurement of Cu K-edge x-ray absorption near-edge structure of monovalent, divalent, and trivalent copper oxides has been used to identify the presence of different valence of copper in the La/sub 2-//sub x/(Sr,Ba)/sub x/CuO/sub 4/ (x = 0.0--0.3) system. The results indicate the coexistence of Cu II and Cu III states in these compounds. Theoretical calculations based on a cluster of CuO/sub 4/O/sub 2/La/sub 8/LaSr predict the presence of nearly degenerate electronic ground states representing nominally Cu II and Cu III configurations.