Absence of charge ordering below the Verwey transition temperature in magnetite

García, José Rodríguez; Subı́as, G.; Proietti, M. G.; Blasco, J.; Renevier, H.; Hodeau, Jean-Louis; Joly, Yves

doi:10.1103/physrevb.63.054110

Cited by 108 publications

(86 citation statements)

References 28 publications

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“…Our complete resonant x-ray scattering ͑XRS͒ studies in magnetite demonstrate the lack of CO at the octahedral Fe sites. [8][9][10] In particular, it resolves definitely the controversy on the possible CO with ͓001͔ and ͓001/ 2͔ periodicities. 10 If it exists, the charge disproportionation must be very small ͓lower than 0.1 e− and 0.05 e− for the ͑001͒ and ͑001/ 2͒ periodicities, respectively͔.…”

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confidence: 74%

“…10 If it exists, the charge disproportionation must be very small ͓lower than 0.1 e− and 0.05 e− for the ͑001͒ and ͑001/ 2͒ periodicities, respectively͔. We only pay attention to two important conclusions of the XRS experiments in magnetite regarding the description of its electronic structure: ͑i͒ the interaction time is extremely short, about 10 −15 sec, so the electron is shared among different octahedral Fe atoms in times lower than this one and ͑ii͒ the observed trigonal anisotropy of the octahedral sites in both, magnetite 8,9 and spinel ferrites. 11 Some resonant x-ray diffraction experiments on magnetite have also remarked on charge segregation along the c axis but their analysis is far from convincing, the anomalous scattering factor was treated as a scalar instead of a tensor.…”

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confidence: 99%

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Comment on “Charge order inFe2OBO3: AnLSDA+Ustudy”

Garcı́a

Subı́as

2006

Phys. Rev. B

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In a recently published paper ͓Phys. Rev. B 72, 014407 ͑2005͔͒, Leonov et al. present an LSDA+ U study of the low temperature monoclinic structure of the iron oxoborate ͑Fe 2 OBO 3 ͒. They report on Fe 2+ -Fe 3+ charge ordering without taking into account recent resonant x-ray scattering experiments, which demonstrate the lack of charge ordering in a closely related oxide such as Fe 3 O 4 . They propose that the charge ordering occurs between equivalent crystallographic sites. First, this result, apart from surprising, is at odds with basic concepts in condensed matter. Second, we argue on the reliability of this theoretical approach, showing that a strong discrepancy is obtained for the calculated total and d-projected charges at Fe atoms with formally the same valence state and local environment between two closed related compounds, Fe 3 O 4 and Fe 2 OBO 3 , using the same theoretical method. Finally, we reconsider the reported theoretical calculations on the basis of a rigorous definition of the concepts of charge and orbital ordering and we show that Fe 2 OBO 3 does not show charge ordering. DOI: 10.1103/PhysRevB.74.176401 PACS number͑s͒: 71.20.Ϫb, 71.28.ϩd, 71.30.ϩh Electronic localization in transition metal ͑TM͒ oxides has continuously been a matter of debate from the beginning of modern investigation. The most widely accepted theory to explain the breakdown of the Bloch band theory, giving rise to conduction in TM oxides derived from Mott's ideas. [1][2][3] This model supports two basic ingredients in the description of the electronic properties of TM oxides; ͑i͒ d electrons are considered as localized at the TM atoms and ͑ii͒ due to this strong localization, the intra-atomic Coulomb repulsion ͑U͒ is considered the relevant parameter. However, these ideas enter in conflict with formally mixed valence TM oxides where two different d n , d n+1 valence states occur. The formal valence of the TM atom has a noninteger value in these oxides so its electronic configuration should be defined as d noninteger . This fractional occupation of the d orbital is in clear contradiction with Mott's ideas of strong d-localization and intra-atomic Coulomb repulsion. In order to overcome this incongruity maintaining Mott's theory, there are two possible solutions either a temporal or a spatial separation. Within the first, the electronic configuration of the TM atom is described as fluctuating between two integer d-electronic configurations, in such a way that the "mobile" electron expends nearly all the time in one configuration, the hopping time from this configuration to the other being long. This is called a fluctuating mixed valence compound. The other possibility is that the two integer d-electronic configurations freeze in the lattice. In such a way, the mixed valence compound is described as a mixture of two ions with different formal valence states, i.e., inhomogeneous mixed-valence state. When the two different valence states localize in an ordered way in the lattice, we call it a charge ordered ͑CO͒ phase. 4 ...

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Comment on “Charge order inFe2OBO3: AnLSDA+Ustudy”

Garcı́a

Subı́as

2006

Phys. Rev. B

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“…[4] A resonant X-ray diffraction c EDP Sciences study reported that if a charge disproportionation takes place it should be below the sensitivity limit of 25% of the experiment. [5] Recently it was noticed that the structural distortions lead to significantly different mean Fe−O distances for the octahedrally coordinated iron sites. [2] When these distances are used to calculate bond valence sums a charge modulation along [001] is found and the octahedral iron sites split into two groups with a charge disproportion of about 0.2 electron.…”

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confidence: 99%

Charge order in magnetite. An LDA+ U study

2005

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-Metal-insulator transitions and other electronic transitions. PACS. 71.15.Mb -Density functional theory, local density approximation, gradient and other corrections.Abstract. -The electronic structure of the monoclinic structure of Fe3O4 is studied using both the local density approximation (LDA) and the LDA+U . The LDA gives only a small charge disproportionation, thus excluding that the structural distortion should be sufficient to give a charge order. The LDA+U results in a charge disproportion along the c-axis in good agreement with the experiment. We also show how the effective U can be calculated within the augmented plane wave methods.

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“…26 We obtain the pre-edge spectra as a single peak as a function of the photon energy, in agreement with the experiment. 21,22 Furthermore, we calculate explicitly the dependence on the direction of local magnetic moment in the atomic amplitude. The depending parts are found about an order of magnitude smaller than non-dependent ones.…”

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confidence: 99%

Parity and Time-Reversal Breaking Effects on Resonant X-ray Scattering at the Fe Pre-K-Edge in Magnetite

Igarashi

Nagao

2008

J. Phys. Soc. Jpn.

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We study noncentrosymmetric effects on resonant x-ray scattering (RXS) in magnetite.The noncentrosymmetry at A sites in spinel structure makes the 4p states strongly hybridize with the 3d states through neighboring oxygen 2p states, giving rise to the non-vanishing contribution of the dipole-quadrupole (E1-E2) process in the RXS spectra. We substantiate this observation by introducing a microscopic model of a FeO 4 cluster with multiplets and the 4p band. We show that the hybridization changes its sign between two kinds of A sites and accordingly the local amplitude from the E1-E2 process changes its sign, resulting in non-vanishing RXS intensities at forbidden spots (002) and (006) in the pre-edge region in agreement with the experiment. A large dependence of the pre-edge intensity on the direction of the applied magnetic field is predicted as a consequence of breaking both centrosymmetry and time-reversal symmetry. Furthermore we analyze the intensity difference between two opposite directions of the applied magnetic field at the allowed spot (222) in connection with the experiment. We obtain the intensity difference of a "dispersion" form, which resembles the observed spectra at the Mn pre-K-edge in MnCr 2 O 4 but is quite different from the observed one in magnetite. Although the observed spectra are claimed to arise from "magnetoelectric" amplitude, we argue that this claim has no ground.

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Absence of charge ordering below the Verwey transition temperature in magnetite

Cited by 108 publications

References 28 publications

Comment on “Charge order inFe2OBO3: AnLSDA+Ustudy”

Comment on “Charge order inFe2OBO3: AnLSDA+Ustudy”

Charge order in magnetite. An LDA+ U study

Parity and Time-Reversal Breaking Effects on Resonant X-ray Scattering at the Fe Pre-K-Edge in Magnetite

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