Bloch-type Domain Walls in Rhombohedral BaTiO3

Ferroelectric domain walls (FDWs) are usually considered to be of Ising type, but there have been suggestions in recent years that Bloch-type FDWs are also possible in some cases, e.g., in the rhombohedral phase of BaTiO3. The mechanically compatible and electrically neutral FDWs in rhombohedral BaTiO3 are of 71-degree, 109-degree, and 180-degree type. We have investigated these FDWs based both on first-principles calculations and on a Ginzburg-Landau-Devonshire (GLD) model [P. Marton, I. Rychetsky, and J. Hlinka, Phys. Rev. B 81, 144125 (2010)]. The results from both approaches confirm the Ising nature of the 71-degree FDW and the Bloch nature of the 180-degree FDW, and predict both Ising-type and Bloch-type FDWs are possible for the 109-degree case. Considering the relatively small rhombohedral strain in BaTiO3, the competition between the energies of Bloch and Ising FDWs can be discussed in terms of a picture in which a Bloch wall is regarded as being composed of a pair of smaller-angle Ising ones. A reduction by 40% in the parameters describing the gradient term in the GLD model brings it into better agreement with the first-principles results for detailed properties such as the energies and widths of the FDWs.