The {1(1)over-bar1} growth twins in tetragonal barium titanate

The {1 (1) over bar1} twins frequently observed in pressureless-sintered BaTiO3 ceramics have been analyzed by the X-ray diffractometry, scanning and transmission electron microscopy. Both the single twins and double lamellar twins are growth (or annealing) twins. The twins lying in the {1 (1) over bar1} mirror planes, which is not one of the symmetry elements of the (basic) crystal lattice's but that of the superlattice's, are therefore superlattice twins. The {1 (1) over bar1} twins, particularly the double twins, were found more frequently from samples sintered in an Ar atmosphere of lower oxygen partial pressure (pO(2)). Further decreasing of pO(2) using the Ar-5% H-2 mixture has rendered the sintered samples entirely of hexagonal BaTiO3, the 6H-polytype. The formation of such twins is attributed to changing of the corner-sharing TiO6 octahedra to Ti2O9 face-sharing octahedra, which accommodates for local oxygen deficiency in tetragonal BaTiO3. The stacking sequence alters accordingly from c-layer (constituting the 3C-polytype, treating tetragonal pseudo-cubic) to h-layer (as in (chc)(1)(chc)(2) of 6H).