X-ray-absorption studies of the high-Tc superconductors La1.8Sr0.2CuO4 and La1.8Ba0.2CuO4

X-ray absorption studies of La1.8Sr0.2CuO4 and La1.8Ba0.2CuO4 indicate that no structural transition that alters the valence or the near-neighbor environment by >0.01 A occurs between 4 and 300 K. The doping with Sr and Ba primarily changes the Cu-O distance and width along the c axis. The larger width of the superconducting transition region observed in the Ba-doped material may result from its larger static disorder. Estimates of the Cu-O vibrational frequencies suggest that a phonon-coupling mechanism is just compatible with the observed Tc.