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 130nm, and the grain structure becomes overall more homogeneous. This twinning mechanism and abnormal grain growth occur by 900 degrees C, several hundred degrees lower than reported previously.