Shape of ferroelectric domains in LiNbO3 and LiTaO3 from defect/domain-wall interactions

The observed orientations and shapes of ferroelectric domains in stoichiometric and congruent LiTaO3 and LiNbO3 are explained in terms of the differences in the energetics of domain walls, and the energetics of defect/domain-wall interactions. Density functional theory calculations show that the energy of a Y-wall, parallel to (112¯0), is lower than that of the X-walls, parallel to (101¯0), in both stoichiometric LiNbO3 and LiTaO3. As a result the domains are hexagonal and delineated by Y-walls. In congruent LiTaO3, the interaction of antisite defects with domains walls can reverse this order of stability, leading to triangular domains delineated by one type of X-wall. In congruent LiNbO3, the interactions of the antisite defects with the domain walls are too weak to reverse the order of stability of the domains walls; as a result the domains remain hexagonal.