  We use the Momentum Average approximation (MA) to study the ground-state
properties of strongly bound bipolarons in the double-well electron-phonon
(el-ph) coupling model, which describes certain intercalated lattices where the
linear term in the el-ph coupling vanishes due tosymmetry. We show that this
model predicts the existence of strongly bound yet lightweight bipolarons in
some regions of the parameter space. This provides a novel mechanism for the
appearance of such bipolarons, in addition to long-range el-ph coupling and
special lattice geometries.
