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Leonov, I.; Yaresko, A. N.; Antonov, V. N.; Korotin, M. A.; Анисимов, В. И.

doi:10.1103/physrevlett.93.146404

Cited by 223 publications

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“…Above T CO , the outer electron is jumping between different crystallographic sites and below T CO , this electron is localized in the lattice in such a way that a periodic ordering of two valence states ͑i.e., Fe 2+ /Fe 3+ ͒ is obtained. 3,6 This description enters in conflict with the implicit theoretical approach used by Leonov et al 19,20 in their calculations. In those papers, the electronic structure of Fe 2 OBO 3 is described in a band structure approach despite the Fe d states are considered localized.…”

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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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“…3,4 Recently, the local spin density approximation ͑LSDA͒ + U method [5][6][7] has been used for investigation of the low-temperature monoclinic structure 8,9 of Fe 3 O 4 . 10 The charge and orbitally ordered insulating ground state with two different order parameters: small for the charge and large for the orbital order was obtained self-consistently. While the first order parameter was introduced as the total 3d charge separation between Fe 2+ and Fe 3+ cations, the latter one corresponds to the charge difference between occupancies of occupied t 2g↓ orbital of Fe 2+ and unoccupied t 2g↓ orbital of Fe 3+ cations.…”

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confidence: 99%

Charge order inFe2OBO3: AnLSDA+Ustudy

et al. 2005

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Charge ordering in the low-temperature monoclinic structure of iron oxoborate ͑Fe 2 OBO 3 ͒ is investigated using the local spin density approximation ͑LSDA͒ + U method. While the difference between t 2g minority occupancies of Fe 2+ and Fe 3+ cations is large and gives direct evidence for charge ordering, the static "screening" is so effective that the total 3d charge separation is rather small. The occupied Fe 2+ and Fe 3+ cations are ordered alternately within the chain which is infinite along the a direction. The charge order obtained by LSDA+ U is consistent with observed enlargement of the ␤ angle. An analysis of the exchange interaction parameters demonstrates the predominance of the interribbon exchange interactions which determine the whole L-type ferrimagnetic spin structure.

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“…Also, the importance of the orbital degrees of freedom has recently been highlighted. 5 Magnetite crystallizes in a cubic high-temperature structure and exhibits a monoclinic distortion below the Verwey transition. In spite of enormous progress in resolving the low-temperature monoclinic structure of magnetite, [6][7][8] full details could not be completely resolved so far.…”

Section: Introductionmentioning

confidence: 99%

Terahertz conductivity at the Verwey transition in magnetite

et al. 2005

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The complex conductivity at the (Verwey) metal-insulator transition in Fe3O4 has been investigated at THz and infrared frequencies. In the insulating state, both the dynamic conductivity and the dielectric constant reveal a power-law frequency dependence, the characteristic feature of hopping conduction of localized charge carriers. The hopping process is limited to low frequencies only, and a cutoff frequency ν1 ≃ 8 meV must be introduced for a self-consistent description. On heating through the Verwey transition the low-frequency dielectric constant abruptly decreases and becomes negative. Together with the conductivity spectra this indicates a formation of a narrow Drude-peak with a characteristic scattering rate of about 5 meV containing only a small fraction of the available charge carriers. The spectra can be explained assuming the transformation of the spectral weight from the hopping process to the free-carrier conductivity. These results support an interpretation of Verwey transition in magnetite as an insulator-semiconductor transition with structure-induced changes in activation energy.

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Charge and Orbital Order inFe3O4

Cited by 223 publications

References 29 publications

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Charge order inFe2OBO3: AnLSDA+Ustudy

Terahertz conductivity at the Verwey transition in magnetite

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