  BaBiO3 is a prototype ``charge ordering system'' forming interpenetrating
sublattices with nominal valence Bi(3+) and Bi(5+). It can also be regarded as
a three-dimensional version of a Peierls insulator, the insulating gap being a
consequence of an ordered distortion of oxygen atoms. When holes are added to
BaBiO3 by doping, it remains insulating until a very large hole concentration
is reached, at which point it becomes superconducting. The mechanism for
insulating behavior of more lightly-doped samples is formation of small
polarons or bipolarons. These are self-organized point defects in the Peierls
order parameter, which trap carriers in bound states inside the Peierls gap. We
calculate properties of the polarons and bipolarons using the Rice-Sneddon
model. Bipolarons are the stable defect; the missing pair of electrons come
from an empty midgap state built from the lower Peierls band. Each bipolaron
distortion also pulls down six localized states below the bottom of the
unoccupied upper Peierls band. The activation energy for bipolaron hopping is
estimated.
