Lattice dynamics of mixed-phase BiFeO3 films: Insights from micro-Raman scattering

We report an investigation of Raman spectroscopy of the mixed-phase ${\mathrm{BiFeO}}₃$ film grown onto ${\mathrm{LaAlO}}₃$(001) by ${90}^{\ensuremath{\circ}}$ off-axis magnetron sputtering. The polarized Raman spectra unveil different sets of phonon vibrations, which can be assigned to the modes of the tetragonal($T$)-like polymorph, the rhombohedral($R$)-like polymorph, and the bridging phase (${S}_{\mathrm{tilt}}^{\ensuremath{'}}$ and ${T}_{\mathrm{tilt}}^{\ensuremath{'}}$) in between. Furthermore, the unpolarized Raman scattering as function of temperature unravels multiple anomalies in the phonon behavior of the $T$-like phase not only at its multiferroic transition ($\ensuremath{\sim}370\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) but also at the Polomska transition ($\ensuremath{\sim}450\phantom{\rule{0.16em}{0ex}}\mathrm{K}$) and the antiferromagnetic transition of the $R$-like phase ($\ensuremath{\sim}640\phantom{\rule{0.16em}{0ex}}\mathrm{K}$). The unusual phonon behavior indicates the presence of spin-phonon coupling and also long-ranged strain coupling between the $T$-like and $R$-like polymorphs, which provides new insights into hybrid systems at the morphotropic phase boundaries.