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 ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$${\mathrm{CuO}}_4$ (x=0.0–0.3) system. The results indicate the coexistence of Cu ii and Cu iii states in these compounds. Theoretical calculations based on a cluster of ${\mathrm{CuO}}_4$${\mathrm{O}}_2$${\mathrm{La}}_8$LaSr predict the presence of nearly degenerate electronic ground states representing nominally Cu ii and Cu iii configurations.

• Received 9 March 1987

DOI:https://doi.org/10.1103/PhysRevB.35.7199