Epitaxial strain induced phase transitions in La-doped BiFeO$_{3}$ thin films on Si substrates

Epitaxial strain is a powerful pathway to trigger phase transitions with emergent phenomena in oxide thin films, e.g., strain induced ferroelectric to ferroelectric (PE-PE) phase transition from tetragonal-like to rhombohedral-like phase in Pb(Zr$_{\mathrm{x}}$Ti$_{\mathrm{1-x}})$O$_{3}$ and BiFeO$_{3}$ films. In this study, we report a strain driven antiferroelectric to ferroelectric (AFE-FE) phase transition from orthorhombic (O) to rhombohedral (R) phase in La$_{\mathrm{x}}$Bi$_{\mathrm{1-x}}$FeO$_{3}$ (LBFO) thin film on Si substrates. The ground state of La$_{\mathrm{x}}$Bi$_{\mathrm{1-x}}$FeO$_{3}$ bulk is antiferroelectric PbZrO$_{3}$ type orthorhombic phase. We show that epitaxial strain from Si substrates can stabilize a rhombohedral structure of LBFO in 20 nm films and intermediate strains position LBFO into a nanoscale mixture of rhombohedral and orthorhombic phases in 30-100 nm films and then strain relaxation in 125nm films leads to the orthorhombic phase. Transmission electron microscopy (TEM) shows atomically sharp O/R morphotropic phase boundary (MPB) with O phase domains larger than 10 nm in width. In summary, our findings open a new path to drive AFE-FE phase transition in LBFO and provide a route to study O/R MPB.