Charge ordered state and structural distortions in Fe2OBO3

Abstract

Electronic structural features of the charge ordered (CO) state in Fe 2 OBO 3 have been theoretically calculated by using the ab initio method and analyzed in comparison with the experimental results of electron energy-loss spectroscopy (EELS). Structural relaxations using GGA + U reveal that the CO structure has a supercell of 2a × b × c with the CO modulation along the a-axis direction. The theoretical investigations suggest that both lattice distortions and electrostatic repulsion are essentially important for understanding the properties of the CO state in the present system. This conclusion is also supported by the agreement between experimental and theoretical data for the O-K and Fe-L 2,3 edges in the electron energy-loss spectra. Moreover, both bond-valencesum and spectral analysis demonstrate that the CO state adopts a quasi-ionic nature in Fe 2 OBO 3 .Charge ordering phenomenon has been extensively studied in numerous transition metal oxides associated with important physical phenomena, such as the colossal magnetoresistance 1 and the high-temperature superconductivity. 2 Recently it was also found that the charge ordering could give rise to multiferroicity in some transition metal oxides. 3,4 Take magnetite (Fe 3 O 4 ) for example, the CO state on two octahedral Fe sites in the inverted spinel AB 2 O 4 structure is responsible for the remarkable anomaly in the transport properties at about 120 K. 5 However, recent studies [6][7][8][9] show that an ideal ionic model for Fe 3 O 4 is too simple to well explain the experimental results, and further studies suggested that the strong interplay between the Jahn-Teller effect and electrostatic repulsion should be considered. 10 Iron oxyborate (Fe 2 OBO 3 ) is an isostructural compound of magnetite in which the A-site Fe atoms in Fe 3 O 4 are substituted by boron atoms, and Fe atoms are octahedrally coordinated with oxygen atoms. The edge-sharing FeO 6 octahedra form four infinite ribbons along the a-axis direction, which are linked by the trigonal BO 3 planes, as shown in Fig. 1. Fe 2 OBO 3 undergoes a CO transition at T CO = 317 K accompanied by a structural phase transition from orthorhombic Pmcn to monoclinic P 2 1 /c, and an antiferromagnetic phase transition at T N = 155 K. 11 The x-ray diffraction studies indicated that this CO phase has integer valence states for Fe ions. 12 However, certain essential issues for this CO state are still in debate. [13][14][15] In this paper we will report on the electronic structure of the CO state in Fe 2 OBO 3 via the first-principles calculations and electron energy-loss spectroscopy (EELS) measurements. In order to understand the essential properties of the CO state, the effects of lattice distortions and on-site Coulomb repulsion are accounted for, respectively, in our investigations. Based on the theoretical and experimental data, we suggest that both the electron-lattice coupling and electrostatic interaction play important roles for the formation of the CO state.The first-principles calculations were carried out ...

Uniaxial anisotropy and low-temperature antiferromagnetism of Mn2BO4 single crystal

“…The nature of CO in the warwickites is the question of the hot discussion. The temperature dependence of CO in Fe 2 BO 4 was extensively investigated by resistivity and differential scanning calorimetry measurements, Mӧssbauer spectroscopy, synchrotron x-ray scattering, transmission electron microscopy, and electronic-structure calculations [13][14][15][16]. It is supposed that the low-temperature phase (T CCO < 280 K) is a commensurately charge ordered with integer iron valence separation Fe 2+ and Fe 3+ alternating in the a-axis direction.…”

Pressure-induced suppression of charge order and nanosecond valence dynamics in Fe2OBO3

“…It is known that there is a comparatively large onsite Coulomb repulsion U of 4-6 eV, 27,28 which, in conjunction with the intersite repulsion V , is required to stabilize CO in this compound. Pressurizing to 16 GPa results in sufficient alteration of relative values of primarily interchain V and onsite U to disrupt the CO that such Coulomb interactions help to stabilize at low pressure.…”

Electric Field Control of Metal–Insulator Transition in Charge-Ordered Fe2BO4

“…Theoretical investigations based on first-principles calculations have confirmed the roles of electrostatic repulsion U and local structural distortions as crucial factors governing the emergence of CO in Fe 2 BO 4 . 18,19 Additionally, a study in which the local spin density (LSDA) + U approach was applied to the lowtemperature phase of Fe 2 BO 4 has revealed the presence of a charge-ordered insulator with an energy gap of 0.39 eV, 20 further emphasizing the importance of CO in this class of materials.…”