  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 which 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.
