Confirmation of ferroelectricity, piezoelectricity, and crystal structure of the electronic dielectric TmFe2O4

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 $R{\mathrm{Fe}}₂{\mathrm{O}}₄$ ($R=\mathrm{Ho}\ensuremath{\sim}\mathrm{Lu}$ 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.