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Atomistic simulations with empirical potentials and density-functional theory calculations are used to characterize the structure, energetics, and ferroelectric properties of domain walls in . The two methods yield similar polarization patterns and atomic structures at the domain walls. The structure of the domain wall on the mixed anion-cation planes is very different from that of the domain wall on planes of alternating cations and anions. The breaking of the uniaxial symmetry of the ferroelectric phase by the domain walls leads to nonuniaxial contributions to the polarization in the domain-wall region. In particular, a polarization component parallel to the domain walls leads to a Bloch-type rotation while a polarization component normal to the domain walls leads to Néel-type rotation. The polarization profiles at the domain walls are fitted to Ginzburg-Landau-Devonshire theory. The comparison of energetics at …