  Phase-field simulations demonstrate that the polarization order-parameter
field in the Ginzburg-Landau-Devonshire model of rhombohedral ferroelectric
BaTiO3 allows for an interesting linear defect, stable under simple periodic
boundary conditions. This linear defect, termed here as Ising line, can be
described as about 2 nm thick intrinsic paraelectric nanorod acting as a highly
mobile borderline between finite portions of Bloch-like domain walls of the
opposite helicity. These Ising lines play the role of domain boundaries
associated with the Ising-to-Bloch domain wall phase transition.
