Ferroelectric(FE)materials with a high remnant polarization(Pr),a low coercive field(Ec)and a low leakage current are ideal for the fabrication of FE memory devices which use their unique feature of the switchable electric polarization.However,conventional approaches to increase Pr generally enlarge Ec,resulting in a confliction between the signal strength and the reading/writing efficiency.To solve this bottle necking challenge,we report here a novel thermomechanical treatment(TMT)technique,which is able to engineer polycrystalline barium titantate(BTO)thin films at the nanograin level.As a result,an enhanced polarization is achieved together with a reduced coercive field.The enhanced polarization is attributed to the relief of residual tensile stress while the reduced Ec results from the introduced twin structure.With more polarization variants,the twin boundaries serve as nucleation sites and thus reduce the Ec.Phase field simulations verify the mechanism.The mechanism of(111)twinning in BTO is also discussed.As a breakthrough,this work will bring new prospects to perovskite thin films in FE memory applications and open a window for the research on lattice twins as an important and beneficial microstructure for FE properties.