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Charge self-regulation upon changing the oxidation state of transition metals in insulators


Transition-metal atoms embedded in an ionic or semiconducting crystal can exist in various oxidation states that have distinct signatures in X-ray photoemission spectroscopy and ‘ionic radii’ which vary with the oxidation state of the atom. These oxidation states are often tacitly associated with a physical ionization of the transition-metal atoms1,2—that is, a literal transfer of charge to or from the atoms. Physical models have been founded on this charge-transfer paradigm3,4,5,6, but first-principles quantum mechanical calculations show only negligible changes in the local transition-metal charge7,8,9,10,11,12 as the oxidation state is altered. Here we explain this peculiar tendency of transition-metal atoms to maintain a constant local charge under external perturbations in terms of an inherent, homeostasis-like negative feedback. We show that signatures of oxidation states and multivalence—such as X-ray photoemission core-level shifts, ionic radii and variations in local magnetization—that have often been interpreted as literal charge transfer3,4,13,14,15,16 are instead a consequence of the negative-feedback charge regulation.

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Figure 1: Energy-level diagrams for TM–host interaction.
Figure 2: Properties of GaAs:TM systems, as functions of stable system charge q.
Figure 3: Properties of Cu2O:Co and MgO:Cr, as functions of stable system charge q.
Figure 4: Integrated charge inside the TM-centred sphere.
Figure 5: Charge density differences.


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H.R. thanks G. Trimarchi and J. Chan for discussions and for reading the manuscript. A.Z. thanks P. Mahadevan for interest in the early stages of this problem. This work was funded by the US Department of Energy, Office of Science, under NREL Contract No. DE-AC36-99GO10337.

Author Contributions H.R. carried out the calculations, analysed the results and wrote the paper. S.L. and A.Z. contributed to the design of the study, the analysis of results and the writing of the paper.

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Correspondence to Hannes Raebiger or Alex Zunger.

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The file contains Supplementary Discussion 'A' with a detailed account on how electronic configurations are obtained from ab initio calculation, including Supplementary Figures 1 and 2. and Supplementary Discussion 'B' with a discussion how oxidation states can be identified from level occupation and core shifts, including additional references. (PDF 155 kb)

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Raebiger, H., Lany, S. & Zunger, A. Charge self-regulation upon changing the oxidation state of transition metals in insulators. Nature 453, 763–766 (2008).

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