Phenomenological theory of a single domain wall in uniaxial trigonal ferroelectrics: Lithium niobate and lithium tantalate

A phenomenological treatment of domain walls based on the Ginzburg-Landau-Devonshire theory is developed for uniaxial trigonal ferroelectrics, lithium niobate and lithium tantalate. The contributions to the domain-wall energy from polarization and strain as a function of orientation are considered. Analytical expressions are developed that are analyzed numerically to determine the minimum polarization, strain, and energy configurations of domain walls. It is found that hexagonal y walls are preferred over x walls in both materials. This agrees well with experimental observation of domain geometries in stoichiometric composition crystals.