X-ray-absorption studies of the high-$T_c$ superconductors ${\mathrm{La}}_{1.8}$${\mathrm{Sr}}_{0.2}$${\mathrm{CuO}}_4$ and ${\mathrm{La}}_{1.8}$${\mathrm{Ba}}_{0.2}$${\mathrm{CuO}}_4$

X-ray absorption studies of ${\mathrm{La}}_{1.8}$ ${\mathrm{Sr}}_{0.2}$ ${\mathrm{CuO}}_4$ and ${\mathrm{La}}_{1.8}$ ${\mathrm{Ba}}_{0.2}$ ${\mathrm{CuO}}_4$ indicate that no structural transition that alters the valence or the near-neighbor environment by >0.01 Å 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 $T_c$.