The structure and formation of nanotwins in BaTiO₃ thin films

Abstract

The structure and configuration of {111} twins in polycrystalline BaTiO₃ films prepared by laser ablation deposition are investigated by means of high-resolution transmission electron microscopy. The number density of the twins in these films is high and the thickness of the twin lamellae is in the range of nanometres. In some grains, ordering of these nanotwins gives rise to a local structure which resembles that of the hexagonal phase. In most cases these twin lamellae extend to penetrate the whole grain. Nevertheless, twin lamellae which terminate in the grain are found. Some of them are very small (less than 1 nm in thickness) and are accompanied by partial dislocations with a Burgers vector of (a/3)〈111〉. These small twins can be considered to represent an early stage of twin formation. Based on the structure of these small twins the crystallography and models for twin formation in the films are discussed. It turns out that the nucleation of a {111} twin from dissociation of an a〈111〉 edge dislocation is one of the mechanisms of twin formation in polycrystalline BaTiO₃ films.