  We analyze the hypothetical link between octahedral straightening and
increased conductivity inside the domain walls of BiFeO3. Our calculations for
109 degree walls predict a lattice parameter expansion of c.a. 1 percent in the
direction perpendicular to the wall, and an associated straightening of the
octahedral rotation angle of 4 degrees, which is comparable to that observed in
the high temperature metallic phase of BiFeO3. On the other hand, in the
closely related family of rare-earth orthoferrites, straighter octahedra do not
correlate with increased bandgap, which suggests that the correlation between
octahedral straightening and bandgap reduction in BiFeO3 is perhaps fortuitous
and not necessarily the cause of increased conductivity at the walls.
