A charge order-driven ferroelectricity, a novel mechanism completely different from conventional types such as lattice distortion observed in a typical ferroelectric, has been expected for a series of compounds $R{\mathrm{Fe}}_2{\mathrm{O}}_4$ ($R=\mathrm{Ho}\sim \mathrm{Lu}$ and Y, In), but a real problem has been still unsolved whether or not the compounds are truly ferroelectric. Here, we demonstrate that $\mathrm{Tm}{\mathrm{Fe}}_2{\mathrm{O}}_4$, one of the $R{\mathrm{Fe}}_2{\mathrm{O}}_4$ family, is a true ferroelectric and piezoelectric compound with noncentrosymmetric structure (space group: $Cm$) at room temperature by using switching spectroscopy piezoelectric force microscopy, laser interferometry, scanning nonlinear dielectric microscopy, x-ray diffraction, selected-area electron diffraction, nano-beam electron diffraction, convergent-beam electron diffraction, and high-angle annular dark-field scanning transmission electron microscopy for single-crystalline $\mathrm{Tm}{\mathrm{Fe}}_2{\mathrm{O}}_4$. We have also found that $\mathrm{Tm}{\mathrm{Fe}}_2{\mathrm{O}}_4$ exhibits an electric field induced phase transition between ferroelectric and conductive states. We propose that the charge ordering of ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions accompanied by an ordered displacement of ${\mathrm{Tm}}^{3+}$ ions lead to the ferroelectricity and piezoelectricity.

• Received 17 December 2021
• Revised 30 October 2022
• Accepted 30 November 2022

DOI:https://doi.org/10.1103/PhysRevB.108.014105