X-ray absorption studies of ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$(Ba,Sr${)}_{\mathrm{x}}$${\mathrm{CuO}}_4$ superconductors

X-ray absorption spectroscopy has been used to study superconducting compounds with the composition ${\mathrm{La}}_{2\mathrm{-}\mathrm{x}}$(Ba,Sr${)}_{\mathrm{x}}$ ${\mathrm{CuO}}_4$ for x=0 to 0.3 X-ray absorption measurements at the Cu K edge indicate that the copper valence is 2+ for all dopant concentrations, and that the mean square relative displacements for Cu-O nearest neighbors are approximately the same for all compositions, with an average characteristic temperature of 570±30 K. Measurements at the La $L_3$ edge suggest that the number of empty states near the Fermi energy with O 2p character increases with dopant concentration.