Transformation-induced twinning:: The 90° and 180° ferroelectric domains in tetragonal barium titanate

Ferroelectric twin domains resulting from the cubic (c) to tetragonal (t) phase transformation at the Curie point T-C approximate to 130 degrees C in pressureless-sintered BaTiO3 ceramics, using TiO2-excess powder, have been investigated using scanning and transmission electron microscopy. Both 90 degrees and 180 degrees domains were identified by spot splitting along characteristic crystallographic directions in the selected-area diffraction patterns and/or from the shape of domain boundaries. Lamellar domains were found predominantly with the 90 degrees types. The 180 degrees domain boundaries mostly appeared wavy in shape, while the 90 degrees ones, having sharpened ends, attained a dagger shape. Failure of Friedel's law in the non-centrosymmetric t-BaTiO3 was adopted to validate the existence of the 180 degrees domains. The 90 degrees domains with boundaries lying in (0 (1) over bar1)(t) are reflection-inversion twins, and the 180 degrees domains lying in {100)(t) and {220)(t) are inversion twins. Convergent-beam electron diffraction was performed to ensure that changing of the polar direction [001](t) across the 90 degrees and the 180 degrees domain boundaries was consistent with the domain type. It was also used to confirm whether the 180 degrees-type walls are inversion domain boundaries produced by the loss of an inversion center when the cubic phase transforms into tetragonal symmetry. The formation of such ferroelectric domains is discussed with reference to the crystal symmetry reduction from Pm (3) over barm (c-phase) to P4mm (t-phase) with a loss of mirror plane (m) and roto-inversion axis ((3) over bar) upon c -> t phase transformation.