National Renewable Energy Laboratory, Golden, Colorado 80401, USA
Correspondence to: Hannes Raebiger1Alex Zunger1 Correspondence and requests for materials should be addressed to H.R. (Email: hannes_raebiger@nrel.gov) or A.Z. (Email: alex_zunger@nrel.gov).
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.
