The {111} Growth Twins in Tetragonal Barium Titanate

Abstract

The {1 1 1} twins frequently observed in pressureless‐sintered BaTiO 3 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 1} 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 1} 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 BaTiO 3 , the 6H‐polytype. The formation of such twins is attributed to changing of the corner‐sharing TiO 6 octahedra to Ti 2 O 9 face‐sharing octahedra, which accommodates for local oxygen deficiency in tetragonal BaTiO 3 . 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).