Ab initiostudy of charge order inFe3O4 (original) (raw)

Charge order at magnetite Fe₃O₄(0 0 1): surface and Verwey phase transitions

Journal of physics. Condensed matter : an Institute of Physics journal, 2015

At ambient conditions, the Fe3O4(0 0 1) surface shows a (√2 × √2)R45° reconstruction that has been proposed as the surface analog of the bulk phase below the Verwey transition temperature, T(V). The reconstruction disappears at a high temperature, T(S), through a second order transition. We calculate the temperature evolution of the surface electronic structure based on a reduced bulk unit cell of P2/m symmetry that contains the main features of the bulk charge distribution. We demonstrate that the insulating surface gap arises from the large demand of charge of the surface O, at difference with that of the bulk. Furthermore, it is coupled to a significant restructuration that inhibits the formation of trimerons at the surface. An alternative bipolaronic charge distribution emerges below T(S), introducing a competition between surface and bulk charge orders below T(V).

Electrostatically driven charge-ordering in Fe2OBO3

Nature, 1998

"Charge-ordering is an important phenomenon in conducting metal oxides: it leads to metal-insulator transitions in manganite perovskites (which show `colossal' magnetoresistances), and the Verwey transition in magnetite (in which the material becomes insulating at low temperatures when the conduction electrons freeze into a regular array). Charge-ordered `stripes' are found in some manganites and copper oxide superconductors; in the latter case, dynamic fluctuations of the stripes have been proposed as a mechanism of high-temperature superconductivity. But an important unresolved issue is whether the chargeordering in oxides is driven by electrostatic repulsions between the charges (Wigner crystallization), or by the strains arising from electron-lattice interactions (such as Jahn-Teller distortions) involving different localized electronic states. Here we report measurements on iron oxoborate, Fe2OBO3, that support the electrostatic repulsion charge-ordering mechanism: the system adopts a charge-ordered state below 317 K, in which Fe2+ and Fe3+ ions are equally distributed over structurally distinct Fe sites. In contrast, the isostructural manganese oxoborate, Mn2OBO3, has been previously shown to undergo charge-ordering through Jahn-Teller distortions. We therefore conclude that both mechanisms occur within the same structural arrangement."

Vacancy ordering and electronic structure of gamma-Fe2O3 (maghemite): a theoretical investigation

J Phys Condens Matter, 2010

The crystal structure of the iron oxide γ-Fe2O3 is usually reported in either the cubic system (space group P4332) with partial Fe vacancy disorder or in the tetragonal system (space group P41212) with full site ordering and c/a≈3. Using a supercell of the cubic structure, we obtain the spectrum of energies of all the ordered configurations which contribute to the partially disordered P4332 cubic structure. Our results show that the configuration with space group P41212 is indeed much more stable than the others, and that this stability arises from a favourable electrostatic contribution, as this configuration exhibits the maximum possible homogeneity in the distribution of iron cations and vacancies. Maghemite is therefore expected to be fully ordered in equilibrium, and deviations from this behaviour should be associated with metastable growth, extended anti-site defects and surface effects in the case of small nanoparticles. The confirmation of the ordered tetragonal structure allows us to investigate the electronic structure of the material using density functional theory (DFT) calculations. The inclusion of a Hubbard (DFT + U) correction allows the calculation of a band gap in good agreement with experiment. The value of the gap is dependent on the electron spin, which is the basis for the spin-filtering properties of maghemite.