Comment on “Charge order inFe2OBO3: AnLSDA+Ustudy”

Abstract

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 ...

Symmetry and charge order inFe2OBO3studied through polarized resonant x-ray diffraction

“…Although we have been unable to assign an accurate value to the charge segregation, we are able to estimate the disproportionation as being between 0.4e − and 0.8e − . Thus we have demonstrated that despite some previous reports to the contrary, 31 Fe 2 OBO 3 is indeed charge ordered, but that the differences between the 3d electronic configuration of inequivalent sites is noninteger. We have demonstrated that under certain conditions we can observe a nonlinearly polarized component to the scattered beam from linearly polarized incident light, and confirmed this conversion using incident circularly polarized light.…”

Charge Order Superstructure with Integer Iron Valence inFe2OBO3

“…It is natural then to suspect an ordered arrangement of Fe 2+ and Fe 3+ ions (ionic CO), and Fe 2 OBO 3 has been suggested as an example of electrostatically driven CO [6]. However, no experimental evidence of a CO superstructure was found on the available polycrystalline samples, and consequently the occurrence of CO in Fe 2 OBO 3 has been under debate [7].Here, we report the first observation of superstructure reflections in single-crystalline Fe 2 OBO 3 , using X-ray diffraction. Combining structural refinement, Mössbauer spectroscopy, and electronic structure calculations, we establish a diagonal CO configuration.…”

“…It is natural then to suspect an ordered arrangement of Fe 2+ and Fe 3+ ions (ionic CO), and Fe 2 OBO 3 has been suggested as an example of electrostatically driven CO [6]. However, no experimental evidence of a CO superstructure was found on the available polycrystalline samples, and consequently the occurrence of CO in Fe 2 OBO 3 has been under debate [7].…”

Charge ordered state and structural distortions in Fe2OBO3

“…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.…”