Exploring the dependence of phonon modes on polarization in thin film ferroelectrics

Phonons determine thermal transport and heat transfer, and play a major role in localization and chirality in thin film ferroelectric materials such as PbTiO₃ and BaTiO₃. We can therefore expect that ferroelectric polarization affects the behavior of phonon modes at the atomic level in these systems. Here, we use differential momentum resolved vibrational Electron Energy Loss Spectroscopy (diff q-EELS) technique in a NION USTEM to explore the dependence of phonon modes in thin film PbTiO₃ on its c and a polarization. We perform the experiments at 60 keV, with a convergence angle of 10 mrad , a collection angle of 19 mrad and a diffraction tilt of 40 mrad at room temperature. We find a clear difference between the phonon mode intensity observed in the opposite Friedel pair disks at the polar axis, especially near 100 meV. However, for the nonpolar axis, change in phonon mode intensity was minimal. When the same experiment was performed for paraelectric SrTiO₃ with no spontaneous polarization at room temperature, no change in the intensity of phonon modes at the opposite Friedel pair disks was observed. This suggests that the phonon modes in PbTiO₃ are affected by polarization axis, with implications for the characterization of ferroelectric domains using q-EELS. The methods employed could be extended to other materials systems beyond PbTiO₃ to oxide membranes, relaxors, two-dimensional ferroelectrics, improper ferroelectrics and metastable ferroelectrics.