  Superconducting cuprates and pnictides composed of CuO2 or AsFe planes
respectively with intercalated insulating layers, are at the crossroads of
three families of crystalline solids: metals, doped Mott insulators, and
ferroelectrics. In the latter atomic displacements play a key role. Both the
metallic and the doped insulator approaches to high temperature
superconductivity are essentially electronic ones and do not directly involve
the lattice. By contrast, in a recently proposed Bond Contraction Pairing (BCP)
model, contraction of in-plane Cu-O (or As-Fe) bonds plays an essential role in
the pairing mechanism. Here we apply it to low angle grain boundaries and show
that their reduced critical current is due to tensile deformation generated by
dislocations. The model also explains why interface misfit dislocations, which
can result in a dead layer in the case of ferro-electrics, may improve vortex
pinning in the cuprates.
