Confirmation of ferroelectricity, piezoelectricity, and crystal structure of the electronic dielectric <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML"><mml:mrow><mml:mi>Tm</mml:mi><mml:msub><mml:mi>Fe</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">O</mml:mi><mml:mn>4</mml:mn></mml:msub></mml:mrow></mml:math>
Shinya Konishi, Daisuke Urushihara, Tatsuya Hayakawa, Koichiro Fukuda, Toru Asaka, K. Ishii, Noriaki Naoda, Mari Okada, Hirofumi Akamatsu, Hajime Hojo, Masaki Azuma, Katsuhisa Tanaka
Abstract
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.