In situ investigation of the ferroelectric phase transition in improper ferroelectric YMnO3 thin films by electron energy loss spectroscopy

The crystal and electronic structure of YMnO3 thin films at the paraelectric-ferroelectric phase transition are monitored by scanning transmission electron microscopy and electron energy loss spectroscopy (EELS) during in situ heating experiments. At the phase transition of a 14-unit-cell-thick film at & SIM;325 & DEG;C, we observe a clear transition from the polar P63cm to the centrosymmetric P63/mmc space group, manifested by the disappearance of the unit-cell tripling. Further, we reveal significant changes in the EELS data across the phase transition, and interpret them with the help of real-space multiple-scattering calculations. Upon crossing the Curie temperature, striking changes in the O -K near-edge structure are identified, which are attributed to a modification in the Y 4d-O 2p hybridization. In particular, a weakening of the Y 4dz2-O 2pz hybridization is detected for the paraelectric phase compared to the ferroelectric phase. Upon cooling below the Curie temperature, the ferroelectric phase is recovered and a stronger hybridization along the c-axis polarization direction is restored, stabilizing the zone-boundary K3 phonon mode. Our findings demonstrate the high sensitivity of the O -K near-edge structure to structural distortions in YMnO3 polymorphs and lay the groundwork for future studies on polymorphic phase transformations in improper ferroelectrics whose ferroelectricity emerges through coupling to a nonpolar lattice distortion.