Formal Valence,3d-Electron Occupation, and Charge-Order Transitions

Abstract

While the formal valence and charge state concepts have been tremendously important in materials physics and chemistry, their very loose connection to actual charge leads to uncertainties in modeling behavior and interpreting data. We point out, taking several transition metal oxides (La2VCuO6, YNiO3, CaFeO3, AgNiO2, V4O7) as examples, that while dividing the crystal charge into atomic contributions is an ill-posed activity, the 3d occupation of a cation (and more particularly, differences) is readily available in first principles calculations. We discuss these examples, which include distinct charge states and charge-order (or disproportionation) systems, where different "charge states" of cations have identical 3d orbital occupation. Implications for theoretical modeling of such charge states and charge-ordering mechanisms are discussed.Spin ordering, and often orbital ordering, is normally unambiguous, as these properties are subject to direct observation by magnetic and spectroscopic measurements, respectively. Charge ordering (CO) and the actual charge of an ion is rarely measured directly, and the formal charge of an ion in the solid state can be a point of confusion and contention. Valence, oxidation number, and formal charge are concepts borrowed from chemistry, where it is emphasized they do not represent actual charge [1,2] and have even been labeled hypothetical.[1] As the interplay between spin, charge, orbital, and lattice degrees of freedom become more closely watched [3] and acknowledged to be a complex phenomenon, disproportionation and CO have become entrenched as the explanation of several high profile metalinsulator transitions (MIT). The possibility that CO in the charge transfer regime is associated with the oxygen sublattice, with negligible participation of the metal, has been raised[4] and considered as an alternative. [5] Charge density is a physical observable of condensed matter, and the desire to assign charge to atoms has evident pedagogical value, so theoretical approaches have been devised to share it amongst constituent nuclei. Mulliken charge population, which socializes shared charge (divides it evenly between overlapping orbitals) is notoriously sensitive to the local orbital basis set that is required to specify it. Born effective charges are dynamical properties and are often quite different from any conceivable formal charge or actual charge. Integrations over various volumes have been used a great deal, but dividing the static crystal charge density into atomic contributions is, undeniably, an ill-defined activity.A possibility that has not been utilized is that, taking 3d oxides as an example, there is a directly relevant metric that is well defined: the d occupation n d . This quantity is in fact what the physical picture of formal charge or oxidation state brings to mind. 3d cations, in their various environments and charge states, have maxima in their spherically averaged radial densityρ(r) in the range 0.6-0.9 a o . At this short distance from the nucleus, the only oth...

Charge ordering inNi1+/Ni2+nickelates:La4Botana1, Pardo2, Pickett3 et al. 2016Phys. Rev. BSelf Cite554461View full textAdd to dashboardCite

“…2 in the Supplementary Material). This invariance of the actual d electron occupation (i.e., the charge) in many charge-ordered oxide systems has been discussed in the past 22,23 . The formal charge of a cation involves the environment of the cation, including the distance to neighboring oxygen ions and the Madelung potentials from the structure (note that the energy difference of the Ni-2s core levels for Ni 1+ and Ni 2+ ions is 0.2 eV).…”

Complete phase diagram of rare-earth nickelates from first-principles

“…3b). Although atomic charges are not uniquely defined in DFT calculations and there are no unambiguous ways to extract them, 40,41 sphere integrations around the Ni cations can provide some insight into the possible charge ordering, keeping in mind that they might not reflect the real ionization state 40 (We emphasize that the number of electrons extracted from the sphere integrations is not reminiscent of the real site occupancy. We have performed calculations on BaNiO 3 -a non magnetic insulator adopting a P6 3 cm hexagonal structure and in which the Ni cations are formally in a 4+ oxidation state-and La 2 TiNiO 6 -an insulator adopting a double perovskite P2 1 /n phase and in which Ni cations are formally in a 2+ oxidation state.…”

Confinement-driven transitions between topological and Mott phases in(LaNiO3)N/(LaAlDoennig1, Pickett2, Pentcheva3 2014Phys. Rev. BSelf Cite686870View full textAdd to dashboardCite

“…a temperature-driven metal-to-insulator transition (MIT), related to the strongly distorted perovskite structure and the size of rare earth ion R 1,2 . The origin of MIT is strongly debated: instead of the Jahn-Teller (JT) distortion that one may expect of an e 1 g ion, charge order 3 , a site-selective Mott transition 4 or a prosaic order-disorder origin 5 have been discussed. Recently, LaNiO 3 (LNO), the only RNO representative that remains metallic at all temperatures 6 , has been in the spotlight of research, due to the proposal that a cuprate-like behavior can be stabilized when confined in a superlattice (SL) with a band insulator, e.g.…”