Determination of valence of Cu in superconducting ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$(Sr,Ba${)}_{\mathrm{x}}$${\mathrm{CuO}}_4$

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 i i and Cu i i i 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 i i and Cu i i i configurations.