Possible charge disproportionation in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:mn>3</mml:mn><mml:mi>R</mml:mi><mml:msub><mml:mrow><mml:mtext>-AgNiO</mml:mtext></mml:mrow><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:math>studied by neutron powder diffraction

Chung, Jaiho; Lim, Ji-Young; Shin, Yu-Ju; Kang, J. S.; Jaiswal‐Nagar, D.; Kim, Kee Hoon

doi:10.1103/physrevb.78.214417

Cited by 9 publications

(7 citation statements)

References 19 publications

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“…These data suggest that fluorination promotes the formation of Ni 3+ during Ni oxidation, changing the oxidation mechanism from direct Ni 2+ → Ni 4+ oxidation in the pure oxide case to Ni 2+ → Ni 3+ → Ni 4+ in the oxyfluoride case. Equivalently, fluorination appears to suppress the 2Ni 3+ → Ni 2+ + Ni 4+ disproportionation reaction, which has been previously reported at low temperature in Ni 3+ -containing oxides both computationally and experimentally. − …”

Section: Resultssupporting

confidence: 55%

“…4,8 We note that even upon further delithiation beyond 4.6 V, the Equivalently, fluorination appears to suppress the 2Ni 3+ -> Ni 2+ + Ni 4+ disproportionation reaction, which has been previously reported at low-temperature in Ni 3+ -containing oxides both computationally and experimentally. [45][46][47] Figure 5. Distribution of F environments in the disordered-rocksalt Li1.166-xNi0.333Ti0.5O1.833F0.166 system as a function of voltage.…”

Section: Electrochemical Properties and Local Environments Formed On mentioning

confidence: 99%

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Short-Range Order and Unusual Modes of Nickel Redox in a Fluorine-Substituted Disordered Rocksalt Oxide Lithium-Ion Cathode

et al. 2018

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Fluorine substitution for oxygen in cation-disordered lithium-excess transition metal oxides (Li1+xTM1-xO2) used as lithium-ion cathodes was recently demonstrated to improve the reversibility of the processes taking place on charge and discharge by reducing the amount of oxygen loss on charge and preventing major structural rearrangements at high voltage. Yet, little is understood about how fluorine incorporates the oxide structure and impacts its electrochemical properties. Here, we use a combination of experimental (solid-state Nuclear Magnetic Resonance (NMR) spectroscopy) and theoretical techniques (density functional theory (DFT) calculations and Monte Carlo simulations) to investigate the evolution of the local structure around fluorine and lithium, and the oxidation state of redox-active nickel during charge and discharge of the Li1.15Ni0.45Ti0.3Mo0.1O1.85F0.15 (LNF15) cathode. We show that fluorine doping introduces short-range order in as-synthesized LNF15, by incorporating in lithium-rich sites with five or six lithium nearest-neighbors. We observe the emergence of new signals in the ex situ 19 F NMR spectra taken at high states of charge, which we tentatively assign to undercoordinated, diamagnetic fluorine environments seen in our computed models. Our theoretical results also suggest that octahedral nickel ions directly bonded to fluorine follow a different oxidation mechanism than those surrounded by six oxygens, forming Ni 3+ intermediates instead of oxidizing from Ni 2+ directly to Ni 4+. While the oxidation of Ni 2+ towards Ni 4+ is incomplete in oxides, due to overlap between the oxygen and nickel valence states, this result suggests that fluorination may be an efficient strategy to utilize the Ni 2+ /Ni 4+ redox reservoir to a greater extent.

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Section: Resultssupporting

confidence: 55%

Section: Electrochemical Properties and Local Environments Formed On mentioning

confidence: 99%

Short-Range Order and Unusual Modes of Nickel Redox in a Fluorine-Substituted Disordered Rocksalt Oxide Lithium-Ion Cathode

et al. 2018

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“…X-ray and neutron scattering experiments do indicate substantial 3-fold charge disproportionation [10][11][12] , compatible with the emergence of large magnetic moments on the charge-rich sites 9,10 . In our scenario, an AF coupling between itinerant and localized species 26 leads to the screening of the pin local moments giving way to Fermi liquid behavior at low temperatures as in heavy fermions.…”

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confidence: 82%

Pinball liquid phase from Hund's coupling in frustrated transition-metal oxides

Ralko

Merino²,

Fratini³

2015

Phys. Rev. B

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The interplay of nonlocal Coulomb repulsion and Hund's coupling in the d-orbital manifold in frustrated triangular lattices is analyzed by a multiband extended Hubbard model. We find a rich phase diagram with several competing phases, including a robust pinball liquid phase, which is an unconventional metal characterized by threefold charge order, bad metallic behavior, and the emergence of high-spin local moments. Our results naturally explain the anomalous charge-ordered metallic state observed in the triangular layered compound AgNiO 2 . The potential relevance to other triangular transition-metal oxides is discussed.

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“…AgNiO 2 , a triangular, magnetically frustrated lattice compound with nominal Ni 3+ ions, undergoes a structural transition at 365 K although remaining metallic. [27][28][29][30] Three inequivalent Ni sites arise, with a high spin Ni1 ion in an enlarged octahedron and two low spin Ni2, Ni3 = Ni2,3 ions in small octahedra. Based on the structural changes (which were quantified in terms of bond valence sums), the magnetic moments, and resonant x-ray scattering that confirms a calculated ∼1 eV difference in core level energies between Ni1 and Ni2,3, this transition has been welcomed as the first realization of such a highly unusual 3e 1 g → e 2 g + 2e 0.5 g type of CO.…”

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confidence: 99%

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

2012

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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...

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Possible charge disproportionation in3R-AgNiO2studied by neutron powder diffraction

Cited by 9 publications

References 19 publications

Short-Range Order and Unusual Modes of Nickel Redox in a Fluorine-Substituted Disordered Rocksalt Oxide Lithium-Ion Cathode

Short-Range Order and Unusual Modes of Nickel Redox in a Fluorine-Substituted Disordered Rocksalt Oxide Lithium-Ion Cathode

Pinball liquid phase from Hund's coupling in frustrated transition-metal oxides

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

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