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 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. 
David A Scrymgeour, Venkatraman Gopalan, Amit Itagi, Avadh Saxena, and   Pieter J Swart