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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

A broken translational symmetry state in an infinite-layer nickelate

A defining signature of strongly correlated electronic systems is a rich phase diagram, which consists of multiple broken symmetries, such as magnetism, superconductivity, and charge order. In the recently discovered nickelate superconductors a large antiferromagnetic exchange energy has been reported, which implies the existence of strong electronic correlations. However, signatures of a broken-symmetry state other than superconductivity have not yet been observed. Here, we observe charge ordering in infinite-layer nickelates La 1-x Sr x NiO 2 using resonant x-ray scattering. The parent compound orders along the Ni-O bond direction with an incommensurate wave vector, distinct from the stripe order observed in other nickelates that propagates along a direction 45º to the Ni-O bond. The resonance profile we measure indicates that ordering originates from the nickelate layers and induces a parasitic charge modulation of lanthanum electrons. After doping, the charge order diminishes and its wave vector shifts toward commensurate, indicating that strong electronic correlations are likely to be responsible for the ordered state. Our results suggest that the existence of charge order and its potential interplay with antiferromagnetic fluctuations and superconductivity are important themes in nickel-based superconductors.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Origin of Metal-Insulator Transition in Rare-Earth Nickelates

Rare-earth nickelates RNiO3 (R=rare-earth element) undergo coupled structural, metal-insulator, and magnetic changes as temperature is lowered. Because the metal-insulator transition often occurs together with a symmetry-lowering distortion, it is commonly viewed as lattice driven. Here we use QSGW calculations to separate the roles of spin and structure. In the high-symmetry Pbnm phase, imposing spin disproportionation already starts to open an electronic gap, although the undistorted lattice cannot stabilize a full insulating state in both spin channels. In the low-symmetry P21/n phase, removing the spin disproportionation destroys the insulating solution even though the bond disproportionation remains. These tests show that spin disproportionation is the primary electronic driver of gap formation, while structural disproportionation acts as the enabler that allows the inequivalent Ni states to become spatially separated and fully insulating. As an explicit finite-temperature example, machine-learned molecular dynamics for NdNiO3 at 220 K shows that a nominally high-symmetry Pbnm structure dynamically samples local Ni-O bond disproportionation, while retaining Pbnm symmetry on average. This illustrates a general structural channel by which thermal fluctuations in the high-symmetry phase can help stabilize locally spin-disproportionated states.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Nickel B-site substitution in bulk Sr 1-x Ca x FeO 3 perovskite oxygen carriers: Benefits and limitations

We report oxygen (O 2 ) storage materials often rely on the presence of cobalt (Co) to reduce the thermodynamic penalty and increase the kinetics necessary for efficient O 2 storage and release. In this work, we investigate nickel (Ni) as an alternative B-site dopant in Sr 1-x Ca x FeO 3 to identify Co-free carriers that still show improved kinetics at low temperatures. In fact, we show a substantial increase in the reversible O2 release rate through mild Ni B-site substitution (y = 0.06) in select Sr 1-x Ca x Fe 1-y Ni y O 3 systems at 400 to 500 °C, reaching 2.00 wt.% O 2 release up to approximately 75% faster than Ni-free systems. To explain the role of Ni in these systems, we use density functional theory to calculate the O 2 vacancy (V O ) formation energy from separate metal-oxygen (M-O) bonding and relaxation components. We computationally show elongated Ni-O bonds are directly responsible for the decrease in V O upon Ni substitution.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Unusual Role of Point Defects in Perovskite Nickelate Electrocatalysts

Low-cost transition-metal oxide is regarded as a promising electrocatalyst family for an oxygen evolution reaction (OER). The classic design principle for an oxide electrocatalyst believes that point defect engineering, such as oxygen vacancies (VO .. ) or heteroatom doping, offers the opportunities to manipulate the electronic structure of material toward optimal OER activity. Oppositely, in this work, we discover a counterintuitive phenomenon that both VO .. and an aliovalent dopant (i.e., proton (H + )) in perovskite nickelate (i.e., NdNiO 3 (NNO)) have a considerably detrimental effect on intrinsic OER performance. Detailed characterizations unveil that the introduction of these point defects leads to a decrease in the oxidative state of Ni and weakens Ni-O orbital hybridization, which triggers the local electron-electron correlation and a more insulating state. Evidenced by first-principles calculation using the density functional theory (DFT) method, the OER on nickelate electrocatalysts follows the lattice oxygen mechanism (LOM). The incorporation of point defect increases the energy barrier of transformation from OO*(V O ) to OH*(V O ) intermediates, which is regarded as the rate-determining step (RDS). Furthermore, this work offers a new and significant perspective of the role that lattice defects play in the OER process.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Coordination environment tuning of nickel sites by oxyanions to optimize methanol electro-oxidation activity

To achieve zero-carbon economy, advanced anode catalysts are desirable for hydrogen production and biomass upgrading powered by renewable energy. Ni-based non-precious electrocatalysts are considered as potential candidates because of intrinsic redox attributes, but in-depth understanding and rational design of Ni site coordination still remain challenging. Here, we perform anodic electrochemical oxidation of Ni-metalloids (NiP x , NiS x , and NiSe x ) to in-situ construct different oxyanion-coordinated amorphous nickel oxyhydroxides (NiOOH-TO x ), among which NiOOH-PO x shows optimal local coordination environment and boosts electrocatalytic activity of Ni sites towards selective oxidation of methanol to formate. Experiments and theoretical results demonstrate that NiOOH-PO x possesses improved adsorption of OH* and methanol, and favors the formation of CH 3 O* intermediates. The coordinated phosphate oxyanions effectively tailor the d band center of Ni sites and increases Ni-O covalency, promoting the catalytic activity. This study provides additional insights into modulation of active-center coordination environment via oxyanions for organic molecules transformation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Probing adsorbates on La1-xSrxNiO3-d surfaces under humid conditions: implications for the oxygen evolution reaction

Earth-abundant nickel-containing perovskite oxides (ANiO3) are highly active materials for the oxygen evolution reaction (OER). The strong nickel-oxygen (Ni-O) covalency, tunable by A-site chemical substitution, contributes to both bulk and surface material properties like the formation of oxygen vacancies (????••) and OER activity. Here we quantify the coverage of OER relevant adsorbates on a series of La1-xSrxNiO3-d (LSNO, 0 = x = 0.5) epitaxial thin films exposed to humid environments by ambient pressure X-ray photoelectron spectroscopy (AP-XPS). While all LSNO film compositions investigated here have comparable hydroxide coverages at the relative humidities (RHs) probed (1.5 x 10-5 – 0.2 %RH), the amount of under-coordinated surface oxygen increases notably with Sr content. We interpret differences in the free energy of adsorption (?Gads,i) of these OER intermediates, inferred from adsorption isotherms, in the context of proposed ????••-mediated OER mechanisms, consistent with the pH-dependent OER activity observed here for LSNO. We find that Sr incorporation enhances the affinity of LSNO surfaces for these under-coordinated oxygen species, in line with calculations in the literature.

Rare-earth nickelates, ambient pressure-X-ray phot↗
DOE OSTI2021

Strong Superexchange in a $d^{9–δ}$ Nickelate Revealed by Resonant Inelastic X-Ray Scattering

The discovery of superconductivity in a $d^{9–δ}$ nickelate has inspired disparate theoretical perspectives regarding the essential physics of this class of materials. A key issue is the magnitude of the magnetic superexchange, which relates to whether cuprate-like high-temperature nickelate superconductivity could be realized. In this work, we address this question using Ni L-edge and O K-edge spectroscopy of the reduced $d^{9–1/3}$ trilayer nickelates R 4 Ni 3 O 8 (where R = La, Pr) and associated theoretical modeling. A magnon energy scale of ~80 meV resulting from a nearest-neighbor magnetic exchange of J = 69 (4) meV is observed, proving that $d^{9–δ}$ nickelates can host a large superexchange. This value, along with that of the Ni-O hybridization estimated from our O K-edge data, implies that trilayer nickelates represent an intermediate case between the infinite-layer nickelates and the cuprates. Layered nickelates thus provide a route to testing the relevance of superexchange to nickelate superconductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Orbital control of metal-to-insulator transition in high-entropy nickelates

High-entropy materials have emerged as a promising platform for exploring unique electronic and structural properties. In this study, we investigated the orbital control of the metal-to-insulator transition in thin films of high-entropy nickelates, which are composed of five rare-earth elements in equi-atomic concentrations. By manipulating the levels of misfit strain through substrate choice and film thickness, we modulated the electronic properties and examined the intricate interplay between strain, orbital interactions, and the metal-to-insulator transition. Compared to other nickelates thin films like NdNi O 3 and SmNi O 3 , thin films of high-entropy nickelates exhibited remarkable resilience in maintaining control over their transport properties and orbital polarization, even in the presence of considerable A-site disorder. Further, this finding suggests that the electronic properties of A-site disordered high-entropy nickelates are still governed by the electronic bandwidth, primarily influenced by the Ni-O bonding geometry. This study provides valuable insights into the complex interplay among composition, structure, and electronic properties in perovskite oxides. These insights have the potential to guide the development of perovskite oxide materials with tailored electronic properties for a wide range of applications.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Electronic Layer Decoupling Driven by Density-Wave Order in La 4⁢ Ni 3 ⁢O 10

We probe the density-wave transition of the trilayer nickelate La 4⁢ Ni 3 ⁢O 10 with polarization-resolved infrared spectroscopy. The low-energy electrodynamics is strongly anisotropic, with metallic in-plane and insulating out-of-plane character. In the ordered phase, the anisotropy grows more than an order of magnitude as the out-of-plane conductivity is sharply suppressed. We interpret this enhancement as an effective electronic decoupling of the Ni-O layers driven by a spin-density-wave-induced redistribution of Ni−𝑑 𝑧 2 occupation within the trilayers. This electronic response is accompanied by clearly shifting and splitting out-of-plane phonons, compatible with a density-wave instability of electronic origin.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRSDate not supplied

Scalable Variable Charge Molecular Dynamics Simulations of Metal-Oxide Systems

Interfaces between metals and oxides are an important feature in many technologically relevant materials, e.g., oxidation of metal surfaces, oxide-dispersion strengthened (ODS) alloys, dielectric components, and thermal barrier coatings among others. Experimental studies of such interfaces are challenging since the majority are buried within the bulk, making computational modeling an attractive alternative. Molecular dynamics (MD) simulations operate at the length scales relevant to many interface-mediated mechanisms, but the requisite interatomic potentials for metal-oxide systems require computationally expensive variable charge schemes to account for the disparate bonding types, thus often limiting their effectiveness. Here we introduce several improvements to the charge transfer interatomic potential (CTIP) model which enable greater computational efficiency for large scale MD simulations. Then, using a new CTIP parametrization for the Ni-O system, we demonstrate its capabilities to capture critical atomic scale mechanisms associated with metal-oxide interfaces. Long time scale simulations (>10 ns) are used to investigate high temperature oxidation and oxide precipitation from the melt, and large length scale simulations (> 1 million atoms) are used to study the interaction of dislocations with oxide particles. We have implemented the new CTIP model in the widely used, open-source MD code LAMMPS.

Gabriel Plummer↗
DOE OSTI2020

Materials Data on NiO2 by Materials Project

NiO2 is trigonal omega-like structured and crystallizes in the trigonal R-3m space group. The structure is two-dimensional and consists of three NiO2 sheets oriented in the (0, 0, 1) direction. Ni4+ is bonded to six equivalent O2- atoms to form edge-sharing NiO6 octahedra. All Ni–O bond lengths are 1.88 Å. O2- is bonded in a distorted T-shaped geometry to three equivalent Ni4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ni3O4 by Materials Project

Ni3O4 crystallizes in the orthorhombic Cmmm space group. The structure is three-dimensional. there are two inequivalent Ni+2.67+ sites. In the first Ni+2.67+ site, Ni+2.67+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing NiO6 octahedra. The corner-sharing octahedra tilt angles range from 9–14°. There are two shorter (2.06 Å) and four longer (2.07 Å) Ni–O bond lengths. In the second Ni+2.67+ site, Ni+2.67+ is bonded to six O2- atoms to form edge-sharing NiO6 octahedra. There is two shorter (1.88 Å) and four longer (1.89 Å) Ni–O bond length. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded to five Ni+2.67+ atoms to form a mixture of edge and corner-sharing ONi5 square pyramids. In the second O2- site, O2- is bonded in a see-saw-like geometry to four Ni+2.67+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NiO2 by Materials Project

NiO2 crystallizes in the orthorhombic Immm space group. The structure is one-dimensional and consists of two NiO2 ribbons oriented in the (1, 0, 0) direction. Ni4+ is bonded in a distorted square co-planar geometry to four equivalent O2- atoms. All Ni–O bond lengths are 1.86 Å. O2- is bonded in a distorted water-like geometry to two equivalent Ni4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ni15O16 by Materials Project

Ni15O16 crystallizes in the cubic Im-3m space group. The structure is three-dimensional. there are two inequivalent Ni+2.13+ sites. In the first Ni+2.13+ site, Ni+2.13+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing NiO6 octahedra. The corner-sharing octahedra tilt angles range from 0–11°. There are a spread of Ni–O bond distances ranging from 2.08–2.14 Å. In the second Ni+2.13+ site, Ni+2.13+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles are 0°. There are two shorter (1.94 Å) and four longer (2.10 Å) Ni–O bond lengths. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded to six Ni+2.13+ atoms to form ONi6 octahedra that share corners with two equivalent ONi6 octahedra, corners with four equivalent ONi5 square pyramids, edges with eight ONi6 octahedra, and edges with four equivalent ONi5 square pyramids. The corner-sharing octahedral tilt angles are 0°. In the second O2- site, O2- is bonded to five Ni+2.13+ atoms to form ONi5 square pyramids that share corners with four equivalent ONi6 octahedra, corners with five equivalent ONi5 square pyramids, and edges with eight ONi6 octahedra. The corner-sharing octahedral tilt angles are 6°. In the third O2- site, O2- is bonded to six equivalent Ni+2.13+ atoms to form ONi6 octahedra that share corners with six equivalent ONi6 octahedra, edges with six equivalent ONi6 octahedra, and edges with six equivalent ONi5 square pyramids. The corner-sharing octahedral tilt angles are 3°.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NiO2 by Materials Project

NiO2 is Rutile-like structured and crystallizes in the monoclinic Cm space group. The structure is three-dimensional. there are four inequivalent Ni4+ sites. In the first Ni4+ site, Ni4+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles are 51°. There are a spread of Ni–O bond distances ranging from 1.86–1.89 Å. In the second Ni4+ site, Ni4+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles are 51°. There are a spread of Ni–O bond distances ranging from 1.85–1.88 Å. In the third Ni4+ site, Ni4+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles are 51°. There are a spread of Ni–O bond distances ranging from 1.85–1.88 Å. In the fourth Ni4+ site, Ni4+ is bonded to six O2- atoms to form a mixture of corner and edge-sharing NiO6 octahedra. The corner-sharing octahedral tilt angles are 51°. There are a spread of Ni–O bond distances ranging from 1.85–1.89 Å. There are eight inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ni4+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ni4+ atoms. In the third O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ni4+ atoms. In the fourth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ni4+ atoms. In the fifth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the sixth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the seventh O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the eighth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NiO2 by Materials Project

NiO2 crystallizes in the trigonal R3m space group. The structure is three-dimensional. there are two inequivalent Ni4+ sites. In the first Ni4+ site, Ni4+ is bonded to four O2- atoms to form corner-sharing NiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 57–58°. There is three shorter (1.85 Å) and one longer (1.90 Å) Ni–O bond length. In the second Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share corners with three equivalent NiO4 tetrahedra and edges with four equivalent NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.85–1.99 Å. There are four inequivalent O2- sites. In the first O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ni4+ atoms. In the second O2- site, O2- is bonded in a distorted trigonal pyramidal geometry to four Ni4+ atoms. In the third O2- site, O2- is bonded in a distorted T-shaped geometry to three equivalent Ni4+ atoms. In the fourth O2- site, O2- is bonded in a water-like geometry to two equivalent Ni4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on NiO2 by Materials Project

NiO2 crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are twelve inequivalent Ni4+ sites. In the first Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share a cornercorner with one NiO4 tetrahedra and edges with five NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.84–1.92 Å. In the second Ni4+ site, Ni4+ is bonded to six O2- atoms to form edge-sharing NiO6 octahedra. All Ni–O bond lengths are 1.88 Å. In the third Ni4+ site, Ni4+ is bonded to six O2- atoms to form edge-sharing NiO6 octahedra. There is five shorter (1.88 Å) and one longer (1.89 Å) Ni–O bond length. In the fourth Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share a cornercorner with one NiO4 tetrahedra and edges with five NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.84–1.92 Å. In the fifth Ni4+ site, Ni4+ is bonded to six O2- atoms to form edge-sharing NiO6 octahedra. All Ni–O bond lengths are 1.88 Å. In the sixth Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share a cornercorner with one NiO4 tetrahedra and edges with five NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.84–1.92 Å. In the seventh Ni4+ site, Ni4+ is bonded to six O2- atoms to form edge-sharing NiO6 octahedra. All Ni–O bond lengths are 1.88 Å. In the eighth Ni4+ site, Ni4+ is bonded to six O2- atoms to form edge-sharing NiO6 octahedra. All Ni–O bond lengths are 1.88 Å. In the ninth Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share corners with two equivalent NiO4 tetrahedra and edges with five NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.86–1.89 Å. In the tenth Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share corners with two equivalent NiO4 tetrahedra and edges with five NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.86–1.89 Å. In the eleventh Ni4+ site, Ni4+ is bonded to six O2- atoms to form NiO6 octahedra that share corners with two equivalent NiO4 tetrahedra and edges with five NiO6 octahedra. There are a spread of Ni–O bond distances ranging from 1.86–1.89 Å. In the twelfth Ni4+ site, Ni4+ is bonded to four O2- atoms to form corner-sharing NiO4 tetrahedra. The corner-sharing octahedra tilt angles range from 57–59°. There is three shorter (1.87 Å) and one longer (2.01 Å) Ni–O bond length. There are twenty-four inequivalent O2- sites. In the first O2- site, O2- is bonded in a water-like geometry to two Ni4+ atoms. In the second O2- site, O2- is bonded in a water-like geometry to two Ni4+ atoms. In the third O2- site, O2- is bonded in a distorted T-shaped geometry to three Ni4+ atoms. In the fourth O2- site, O2- is bonded in a water-like geometry to two Ni4+ atoms. In the fifth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the sixth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the seventh O2- site, O2- is bonded in a distorted trigonal pyramidal geometry to four Ni4+ atoms. In the eighth O2- site, O2- is bonded in a distorted T-shaped geometry to three Ni4+ atoms. In the ninth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the tenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the eleventh O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the twelfth O2- site, O2- is bonded in a distorted T-shaped geometry to three Ni4+ atoms. In the thirteenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the fourteenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the fifteenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the sixteenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the seventeenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the eighteenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the nineteenth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the twentieth O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the twenty-first O2- site, O2- is bonded in a trigonal planar geometry to three Ni4+ atoms. In the twenty-second O2- site, O2- is bonded in a trigonal planar geometry to three Ni4+ atoms. In the twenty-third O2- site, O2- is bonded in a 3-coordinate geometry to three Ni4+ atoms. In the twenty-fourth O2- site, O2- is bonded in a trigonal planar geometry to three Ni4+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ni5O4 by Materials Project

Ni5O4 crystallizes in the cubic P-43m space group. The structure is three-dimensional. there are two inequivalent Ni+1.60+ sites. In the first Ni+1.60+ site, Ni+1.60+ is bonded to six equivalent O2- atoms to form distorted NiO6 octahedra that share corners with six equivalent NiO6 octahedra, corners with three equivalent NiO4 tetrahedra, edges with twelve equivalent NiO6 octahedra, and a faceface with one NiO4 tetrahedra. The corner-sharing octahedral tilt angles are 3°. There are three shorter (2.03 Å) and three longer (2.53 Å) Ni–O bond lengths. In the second Ni+1.60+ site, Ni+1.60+ is bonded to four equivalent O2- atoms to form a mixture of corner and face-sharing NiO4 tetrahedra. The corner-sharing octahedral tilt angles are 53°. All Ni–O bond lengths are 2.15 Å. O2- is bonded in a 7-coordinate geometry to seven Ni+1.60+ atoms.

36 MATERIALS SCIENCE↗