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

Abstract

The crystal and electronic structure of $\mathrm{YMn}{\mathrm{O}}_{3}$ 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 $\ensuremath{\sim}325\phantom{\rule{0.16em}{0ex}}{}^{\ensuremath{\circ}}\mathrm{C}$, we observe a clear transition from the polar $P{6}_{3}cm$ to the centrosymmetric $P{6}_{3}/mmc$ 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 $4d--\mathrm{O} 2p$ hybridization. In particular, a weakening of the Y $4d{}_{z}{}^{2}--\mathrm{O} 2{p}_{z}$ 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 ${\mathrm{K}}_{3}$ phonon mode. Our findings demonstrate the high sensitivity of the O-$K$ near-edge structure to structural distortions in $\mathrm{YMn}{\mathrm{O}}_{3}$ 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.