X-ray absorption of ${\mathrm{BaBiO}}_3$ and superconducting ${\mathrm{BaBi}}_{0.25}$${\mathrm{Pb}}_{0.75}$${\mathrm{O}}_3$

X-ray-absorption studies of ${\mathrm{BaBiO}}_3$ and superconducting ${\mathrm{BaBi}}_{0.25}$ ${\mathrm{Pb}}_{0.75}$ ${\mathrm{O}}_3$ (Ba-Bi-Pb-O) did not show any structural transformations altering the gross-scale near-neighbor environment or the valence between 4 and 300 K. Two Bi-O distances, separated by about 0.17 Å, could be distinguished for ${\mathrm{BaBiO}}_3$. Also in Ba-Bi-Pb-O, there is a strong indication of two different Bi-O distances, separated by about 0.12 Å, but only one Pb-O distance. The differing Bi-O separations may be connected with local spatial charge fluctuations. The valence states of the two differently located Bi atoms could not be conclusively determined from the absorption-edge shape, but the Bi and Pb near-edge structures for Ba-Bi-Pb-O are nearly identical. The Bi-O and Pb-O vibrational frequencies, considered important for superconductivity, could be estimated from the temperature dependence of the broadening of the nearest-neighbor distributions; they correspond to Einstein temperatures of about 500 K.