Engineering Ferroelectric Phases in BaTiO$_3$ Films via Buffer-Layer Induced Misfit Strain Reversal
Controlling ferroelectric phase symmetry through strain engineering is crucial for tailoring polarization topology and functional properties in oxide-based devices. Here, we show that a thin conductive buffer layer can override substrate-induced strain in BaTiO$_3$ (BTO) films on MgO. Although the lattice parameter of MgO(001) would be expected to induce tensile misfit strain, in 500~nm BaTiO$_3$ films the thermal-expansion mismatch dominates, resulting in effective compressive stress upon cooling. Inserting a 50-nm La$_{0.67}$Sr$_{0.33}$MnO$_3$ (LSMO) buffer layer reverses the effective strain state from compressive to tensile. Using temperature-dependent polarized Raman spectroscopy, X-ray diffraction, and piezoresponse force microscopy, we reveal an unconventional phase sequence upon cooling: paraelectric $\rightarrow$ orthorhombic ($\textit{aa}$-phase, $\sim$407 K) $\rightarrow$ monoclinic ($\textit{r}$-phase, $\sim$300-345 K), characterized in-plane and tilted polarization components. The LSMO layer not only serves as a bottom electrode but also modifies the strain state, while interfacial charge redistribution induces a built-in electric field, evidenced by self-polarization and asymmetric PFM hysteresis. These findings highlight the critical role of conductive buffer layers in controlling ferroelectric symmetry and enabling oxide heterostructures with tailored polarization textures for low-power spintronics, tunable microwave devices, and neuromorphic computing.