Ferroelectricity from Electron Ordering
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0966-T01-03
Ferroelectricity from Electron Ordering Naoahi Ikeda1, Shigeo Mori2, Kenji Yoshii3, and Yoji Matsuo4 1 Physics, Okayama University, 3-1-1 tsushimanaka, Okayama, 700-8530, Japan 2 Department of Physics, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, 599-8531, Japan 3 Synchrotron Radiation Research Center, Japan Atomic Energy Research Institute, Sayo, Sayo, 679-5148, Japan 4 Department of Physics, Osaka Prefecture University, 1-1 Gakuen-cho, Sakai, Osaka, Sakai, Osaka, 599-8531, Japan
ABSTRACT We report ferroelectricity in the triangular mixed valence material RFe2O4 (R=Y, Dy, Ho, Er, Tm, Yb, Lu). This ferroelectricity originates from the polar ordering of Fe3+ and Fe2+ or the polar ordering of electrons on Fe3+. The electric polarization arising from such electron ordering may not couple with the lattice modulation. The ordering process of Fe2+ and Fe3+ are explained by the frustrated interaction of charges in a triangular lattice. The ferroelectric property of this material is present at room temperature. INTRODUCTION The degrees of freedom of electrons are utilized in many modern electric devices. For example spin ordering is the key concept in magnetic devices. The overlap of electron wave functions is fundamental to the semiconductor industry. Ferroelectricity is also one of the most important phenomena in the electronic capacitor, a key component in modern electronic devices. The origin of ferroelectricity is the displacement of ion. Recent developments in first principal calculations have revealed that this displacement originates from the creation of a covalent bond between the cation and anion on the basis of quantum dynamics[1]. We found ferroelectricity in triangular mixed valence material only where electric polarization arises from the electron density modulation of cation Fe3+. It is considered that the polarized electron distribution originates from the competing interaction of charges on frustrated geometry. The RFe2O4 family (R=Y or Dy to Lu) has a hexagonal structure with spacegroup of R3m (166) consisting of an alternating stacking of triangular lattices[2]. As shown in figure 1 the relative position of the triangular plane shows a slight change following the stacking sequence as displayed A, B and C in the figure. The scheme is an alternating stacking of a double iron triangular layer (W-layer) and a single rare-earth triangular layer ( T-layer ). The iron ion has fivefold oxygen coordination in the W-layer.
B B
A C
B
C A
B A
C B
B
B
R Fe O
W
Figure 1; Crystal structure of RFe2O4. Hatched, black and white circles represent rare-earth, iron and oxygen ions, respectively. Double iron layer and rare-earth layer are illustrated as W- and T-layer, respectively. Charge ordering Equal numbers of Fe2+ and Fe3+ ions are situated in the W-layer. Considering the chemical formula of the iron valence 2.5+, Fe2+ and Fe3+ act as excess and vacant charges (e/2 and –e/2), respectively, where e is the electron charge. So, Fe2+ and Fe3+ are considered as a negative charge and a positive
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