Low-Temperature Control of Twins and Abnormal Grain Growth in BaTiO₃

The microstructure of polycrystalline barium titanate (BaTiO₃) thin films processed with a liquid-phase can be controlled by the crystallographic orientation of the underlying sapphire substrate. During postdeposition crystallization, the tendency for {111} twin nucleation, which drives subsequent abnormal grain growth, depends upon the specific sapphire facet. Specifically, tilting away from the close-packed c-plane modifies the orientation, morphology, and relative amount of an interfacial BaAl₂O₄ second phase. These factors control the density of twin formation, and thus overall grain size of the crystallized BaTiO₃. As the substrate orientation transitions from c-plane, to r-plane, to a-plane, the twin density is reduced, the average grain size decreases systematically from 270 to 130 nm, and the grain structure becomes overall more homogeneous. This twinning mechanism and abnormal grain growth occur by 900 C, several hundred degrees lower than reported previously.