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All sourcesNASA NTRSDOE OSTI
NASA NTRS2009

Late Reduction Textures in Almahata Sitta Ureilite

The Almahata Sitta ureilite, derived from asteroid 2008 TC3, consists of many individual fragments recovered from the Nubian dessert strewn field [1]. Like most ureilites, it contains abundant carbon and exhibits examples of disequilibrium textures that record a late reduction event accompanied by rapid cooling (tens of degC/h) from high temperatures (1150-1300 C). Variations in Fe/Mg of silicate minerals are accompanied by variations in Fe/Mn, indicating loss of Fe into metal [2]. In coarser-grained fragments of Almahata Sitta, olivine exhibits irregular high mg# rims in contact with networks of interstitial metal 5- 20 microns in typical thickness. This is a common ureilite texture thought to be driven by the reaction of graphite to a CO gas phase and the concurrent reduction of FeO in olivine to Fe metal, with excess silica going primarily into pyroxene (2MgFeSiO4 + C approaches MgSiO4 + MgSiO3 + 2Fe + CO) [3, see also 4,5,6]. Other fragments of Almahata Sitta exhibit anomalous textures such as fine grain size, high porosity, and abundant graphite. Within these fragments pyroxene locally exhibits high-mg# rims in contact with metal and a discreet silica phase, suggesting that the reduction mechanism MgFeSi2O6 + C approaches MgSiO3 + Fe + SiO2 + CO. Metals in Almahata Sitta are particularly unaltered in comparison to ureilite finds. Variations in minor and trace element composition of this metal might partly result from localized dilution as iron is supplied by reduction of silicates.

Herrin, J. S.↗
DOE OSTI2020

Tunable Metal–Organic Frameworks Based on 8-Connected Metal Trimers for High Ethane Uptake

Metal trimers [M 3 (O/OH)](OOCR) 6 are among the most important structural building blocks. From these trimers, a great success has been achieved in the design of 6- or 9-connected framework materials with various topological features and outstanding gas-sorption properties. In comparison, 8-connected trimer-based MOFs are rare. Given multiple competitive pathways for the formation of 6- or 9-connected frameworks, it remains challenging to identify synthetic or structural parameters that can be used to direct self-assembly process towards trimer-based 8-connected materials. Here, we reveal a viable strategy called angle bending modulation for creating a prototypical MOF type based on 8-connected M 3 (OH)(OOCR) 5 (Py-R) 3 trimers, M = Zn, Co, Fe). As a proof of concept, we synthesized six members in this family using three types of ligands (CPM-80, -81, and -82). These materials do not possess open-metal sites and show excellent uptake capacity for various hydrocarbon gas molecules and inverse C 2 H 6 /C 2 H 4 selectivity. CPM-81-Co, made from 2,5-furandicarboxylate and isonicotinate, features selectivity of 1.80 with high uptake capacity for ethane (123 cm 3 /g -1 ) and ethylene (113 cm 3 /g -1 ) at 298 K and 1 bar.

36 MATERIALS SCIENCE↗
DOE OSTI2021

LaNi x Fe 1– x O 3–δ as a Robust Redox Catalyst for CO 2 Splitting and Methane Partial Oxidation

The current study reports LaNi 0.5 Fe 0.5 O 3–δ as a robust redox catalyst for CO 2 splitting and methane partial oxidation at relatively low temperatures (~700 °C) in the context of a hybrid redox process. Specifically, perovskite-structured LaNi x Fe 1–x O 3–δ (LNFs) with nine different compositions (x = 0.05–0.5) were prepared and investigated. Among the samples evaluated, LaNi 0.4 Fe 0.6 O 3–δ and LaNi 0.5 Fe 0.5 O 3–δ showed superior redox performance, with ~90% CO 2 and methane conversions and >90% syngas selectivity. The standalone LNFs also demonstrated performance comparable to that of LNF promoted by mixed conductive Ce 0.85 Gd 0.1 Cu 0.05 O 2–δ (CGCO). Long-term testing of LaNi 0.5 Fe 0.5 O 3–δ indicated that the redox catalyst gradually loses its activity over repeated redox cycles, amounting to approximately 0.02% activity loss each cycle, averaged over 500 cycles. This gradual deactivation was found to be reversible by deep oxidation with air. Further characterizations indicated that the loss of activity resulted from a slow accumulation of iron carbide (Fe 3 C and Fe 5 C 2 ) phases, which cannot be effectively removed during the CO 2 splitting step. Reoxidation with air removed the carbide phases, increased the availability of Fe for the redox reactions via solid-state reactions with La 2 O 3 , and decreased the average crystallite size of La 2 O 3 . As a result, reactivating the redox catalyst periodically, e.g., once every 40 cycles, was shown to be highly effective, as confirmed by operating the redox catalyst over 900 cumulative cycles while maintaining satisfactory redox performance.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Interface Design for High‐Performance All‐Solid‐State Lithium Batteries

All‐solid‐state batteries suffer from high interface resistance and lithium dendrite growth leading to low Li plating/stripping Coulombic efficiency (CE) of <90% and low critical current density at high capacity. Here, in this work, both challenges are simultaneously addressed and the Li plating/stripping CE is significantly increased to 99.6% at 0.2 mA cm −2 /0.2 mAh cm −2 , and critical current density (CCD) of > 3.0 mA cm −2 /3.0 mAh cm −2 by inserting a mixed ionic‐electronic conductive (MIEC) and lithiophobic LiF‐C‐Li 3 N‐Bi nanocomposite interlayer between Li 6 PS 5 Cl electrolyte and Li anode. The highly lithiophobic LiF‐C‐Li 3 N‐Bi interlayer with high ionic conductivity (10 −5 S cm −1 ) and low electronic conductivity (3.4×10 −7 S cm −1 ) enables Li to plate on the current collector (CC) surface rather than on Li 6 PS 5 Cl surface avoiding Li 6 PS 5 Cl electrolyte reduction. During initial Li plating on CC, Li penetrates into porous LiF‐C‐Li 3 N‐Bi interlayer and lithiates Bi nanoparticles into Li 3 Bi. The lithiophilic Li 3 Bi and Li 3 N nanoparticles in LiF‐C‐Li 3 N‐Li 3 Bi sub‐interlayer will move to CC along with plated Li, forming LiF‐C/Li 3 N‐Li 3 Bi lithiophobic/lithiophilic sublayer during the following Li stripping. This interlayer enables Co 0.1 Fe 0.9 S 2 /Li 6 PS 5 Cl/Li cell with an areal capacity of 1.4 mAh cm −2 to achieve a cycle life of >850 cycles at 150 mA g −1 . The lithiophobic/lithiophilic interlayer enables solid‐state metal batteries to simultaneously achieve high energy and long cycle life.

25 ENERGY STORAGE↗
DOE OSTI2024

Emerging magnetic materials for electric vehicle drive motors

Abstract Increasing demand for electric vehicles (EVs) is increasing demand for the permanent magnets that drive their motors, as approximately 80% of modern EV drivetrains rely on high-performance permanent magnets to convert electricity into torque. In turn, these high-performance permanent magnets rely on rare earth elements for their magnetic properties. These elements are “critical” (i.e., at risk of limiting the growth of renewable energy technologies such as EVs), which motivates an exploration for alternative materials. In this article, we overview the relevant fundamentals of permanent magnets, describe commercialized and emerging materials, and add perspective on future areas of research. Currently, the leading magnetic material for EV motors is Nd 2 Fe 14 B, with samarium-cobalt compounds (SmCo 5 and Sm 2 Co 17 ) providing the only high-performing commercialized alternative. Emerging materials that address criticality concerns include Sm 2 Fe 17 N 3 , Fe 16 N 2 , and the L1 0 structure of FeNi, which use lower cost elements that produce similar magnetic properties. However, these temperature-sensitive materials are incompatible with current metallurgical processing techniques. We provide perspective on how advances in low-temperature synthesis and processing science could unlock new classes of high-performing magnetic materials for a paradigm shift beyond rare earth-based magnets. In doing so, we explore the question: What magnetic materials will drive future EVs? Graphical abstract

33 ADVANCED PROPULSION SYSTEMS↗
DOE OSTI2024

Emerging magnetic materials for electric vehicle drive motors [Slides]

Increasing demand for electric vehicles (EVs) is increasing demand for the permanent magnets that drive their motors, as approximately 80% of modern EV drivetrains rely on high-performance permanent magnets to convert electricity into torque. In turn, these high-performance permanent magnets rely on rare earth elements for their magnetic properties. These elements are "critical" (i.e., at risk of limiting the growth of renewable energy technologies such as EVs), which motivates an exploration for alternative materials. In this article, we overview the relevant fundamentals of permanent magnets, describe commercialized and emerging materials, and add perspective on future areas of research. Currently, the leading magnetic material for EV motors is Nd 2 Fe 14 B, with samarium-cobalt compounds (SmCo 5 and Sm 2 Co 17 ) providing the only high-performing commercialized alternative. Emerging materials that address criticality concerns include Sm 2 Fe 17 N 3 , Fe 16 N 2 , and the L10 structure of FeNi, which use lower cost elements that produce similar magnetic properties. However, these temperature-sensitive materials are incompatible with current metallurgical processing techniques. We provide perspective on how advances in low-temperature synthesis and processing science could unlock new classes of high-performing magnetic materials for a paradigm shift beyond rare earth-based magnets. In doing so, we explore the question: What magnetic materials will drive future EVs?

42 ENGINEERING↗
DOE OSTI2020

Friction stir gradient alloying: A high-throughput method to explore the influence of V in enabling HCP to BCC transformation in a γ-FCC dominated high entropy alloy

The compositional possibilities in high entropy alloys (HEAs) is very vast and effective strategies are needed to establish potential alloy chemistries. In this study, friction stir gradient alloying (FSGA), a recently-introduced high-throughput (HT) technique incorporating compositional and microstructural gradients, was used to explore the possibility of introducing a bcc transformation domain in γ-fcc dominated TRIP HEA Fe 38.5 Mn 20 Cr 15 Co 20 Si 5 Cu 1.5 (at.%) by vanadium addition. FSGA has accelerated the process of attaining the composition-microstructure library and the results suggest that vanadium addition of ~1 at.% results in nucleation of α-bcc. Additionally, this is the first observation of ε-hcp to α-bcc transformation in a TRIP HEA and supports the Olson-Cohen model of martensitic transformation (γ-fcc → ε-hcp → α-bcc). ε-hcp is nucleated with a pyramidal orientation by the formation of planar faults on the {111} close-packed planes of γ-fcc, and α-bcc is nucleated in the vicinity of ε-hcp along the {111} γ-fcc trace. There are indications for the formation of basal orientated ε-hcp from intersecting ε-hcp planes orientated for the pyramidal slip. Thus, the study shows the nucleation and growth of the α-bcc phase attributed to the dual effects of chemistry and strain and potential co-existence of all the three phases.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Irradiation-induced shift in the thermodynamic stability of phases and the self-healing effect in transformative high entropy alloys

Here, this work investigated Fe 40 Mn 20 Cr 15 Co 20 Si 5 high entropy alloy (CS-HEA), which exhibits transformation induced plasticity (TRIP) from γ-fcc → ε-hcp, as a probable candidate for nuclear applications. CS-HEA is an extensively-explored, low stacking fault energy alloy with superior strength, ductility, fatigue resistance, and corrosion resistance. This study delved into the effect of irradiation on the shift in thermodynamic stability of the phases and thus radiation tolerance. The evolution of phases, lattice parameters, and transformation volume, V γ→ε , were evaluated from X-ray diffraction experiments along with the mechanical response from nanoindentation. The alloy exhibited a recently-proposed novel self-healing mechanism possible due to the TRIP effect to minimize irradiation damage by restraining the γ-fcc → ε-hcp transformation via thermal aid; this self-healing mechanism was confirmed by transmission electron microscopy. The results were corroborated by a negative change in V γ→ε and a low |V γ→ε |, which is an important criterion for recovery of parent γ-fcc phase. Thus, this alloy was deemed a good radiation-tolerant candidate for nuclear application.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Deformation behavior of a Co-Cr-Fe-Ni-Mo medium-entropy alloy at extremely low temperatures

In this work, we report the mechanical and microstructural characteristics of a medium-entropy alloy, Co 17.5 Cr 12.5 Fe 55 Ni 10 Mo 5 (atomic percent, at%) at cryogenic temperatures, down to a record low temperature of 0.5 K. The alloy exhibits excellent strength and ductility combined with a high strain-hardening rate in the entire temperature range investigated. Its property profile, including the yield strength, ultimate tensile strength, strain hardening capability, and absorbed mechanical energy, is better than those of most alloys and HEAs used in cryogenics. Within the interval of extremely low temperatures considered (0.5–4.2 K), the alloy exhibits several unusual features, including anomalies of the temperature dependence of the yield strength and tensile ductility, discontinuous plastic deformation (DPF), and a change in the propensity for the deformation-induced martensitic transformation. While the occurrence of these effects in the same temperature interval may be fortuitous, we hypothesize that they are interrelated and provide a tentative explanation of the observed phenomena on this basis.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Additive manufacturing of a metastable high entropy alloy: Metastability engineered microstructural control via process variable driven elemental segregation

Compositional paradigm shift in high entropy alloys (HEAs) provided new opportunities for microstructural engineering, whereas process control during laser powder bed fusion (LPBF) additive manufacturing (AM) enable fine microstructure tailoring. Metastability engineering in transformation induced plasticity (TRIP) HEAs by addition of minor alloying elements is an attractive strategy for fine microstructural tuning. This study explored in detail the microstructural evolution during LPBF AM of a metastable Fe 40 Mn 20 Co 20 Cr 15 Si 5 (at.%) dual phase HEA (CS-HEA). LPBF processing window for CS-HEA was established based on quantitative analysis and experiments. Based on melt pool overlap lack of fusion pores were observed at lower energy densities (J) of $\textit{J}$ ≤ 31.25 J/mm 3 and key-hole formation by melt pool destabilization in case of $\textit{J}$ ≥ 75 J/mm 3 . The microstructure of CS-HEA consists of metastable FCC-γ and HCP-ε phases; LPBF process parameters governed the final phase fraction in the alloy which has been correlated to metastability alteration of the high temperature γ phase. Final microstructural engineering was devised by LPBF process control which enabled cooling rate manipulation to guide Mn and Si segregation at the cell boundaries, thereby controlling the matrix metastability and final phase fraction. Additionally, high resolution transmission electron microscopy (TEM) revealed disparity in stacking fault morphology in CS-HEA with LPBF process variable alterations associated with local variations in chemical composition and stacking fault energy. Further, the phase evolution with process parameters also affected the nanomechanical behavior of the alloy.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Influence of non-magnetic Cu on enhancing the low temperature magnetic properties and Curie temperature of FeCoNiCrCu(x) high entropy alloys

The microstructure and magnetic properties of three face-centered cubic (FCC) FeCoNiCrCu(x) high entropy alloys (HEAs) (x=0, 0.5, 1) are investigated. Interestingly, addition of the nonmagnetic element Cu to FeCoNiCr HEA is found to enhance exchange interactions and low temperature saturation magnetization. The paramagnetic to ferromagnetic Curie transition temperature increases from 85 K for FeCoNiCr to 118 K for FeCoNiCrCu. This is counterintuitive since Cu is nonmagnetic; however, atom probe tomography revealed Cu rich clusters containing 5 at% Ni and 1 at% each of Fe, Co, Cr, within FCC matrix, these clusters altered the matrix composition and consequently its magnetic properties.

Chaudhary, Varun↗
DOE OSTI2021

Transformative high entropy alloy conquers the strength-ductility paradigm by massive interface strengthening

Recent metastable alloy designs have demonstrated simultaneous attainment of high ultimate tensile strength (UTS) and ductility in high entropy alloys but with low yield strength. In this work, we present new strategy for improving the work hardenability and yield strength (YS) together in Fe 38.5 Mn 20 Co 20 Cr 15 Si 5 Cu 1.5 high entropy alloy (Cu-HEA). Drastic increase in the YS (1.5 GPa) is attributed to the formation of γ/ε, ε/ε (twin type) and ε/ε (plate type) interfaces in the microstructure due to extreme grain refinement whereas high UTS-ductility synergy (2.2 GPa, 15%) is attained by dynamic Hall-Petch hardening across these interfaces (i.e. massive interface strengthening) and transformation induced plasticity in γ phase. Thus, this harmonious combination of YS and UTS-ductility synergy in Cu-HEA outperform all structural materials till date. Therefore, deformation-induced massive interface strengthening is a new, yet cost-effective pathway for synergizing the benefits of advanced steels and high entropy alloys together in a material by conventional processing route.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Intrinsic Kinetic Limitations in Substituted Lithium-Layered Transition-Metal Oxide Electrodes

Substituted Li-layered transition-metal oxide (LTMO) electrodes such as Li x Ni y Mn z Co 1-y-z O 2 (NMC) and Li x Ni y Co 1-y-z Al z O 2 (NCA) show reduced first cycle Coulombic efficiency (90-87% under standard cycling conditions) in comparison with the archetypal Li x CoO 2 (LCO; similar to 98% efficiency). Focusing on Li x Ni 0.8 Co 0.15 Al 0.0.5 O 2 as a model compound, we use operando synchrotron X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy to demonstrate that the apparent first-cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.88, with near full capacity recovered during a potentiostatic hold following the galvanostatic charge- discharge cycle. This kinetic capacity loss, unlike many capacity losses in LTMOs, is independent of the cutoff voltage during delithiation and it is a reversible process. The kinetic limitation manifests not only as the kinetic capacity loss during discharge but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge-discharge voltage hysteresis at x > 0.88. 7 Li NMR measurements indicate that the kinetic limitation reflects limited Li transport at x > 0.86. Electrochemical measurements on a wider range of LTMOs including Li x (Ni,Fe) y Co 1-y O 2 suggest that 5% substitution is sufficient to induce the kinetic limitation and that the effect is not limited to Ni substitution. In this paper, we outline how, in addition to a reduction in the number of Li vacancies and shrinkage of the Li-layer size, the intrinsic charge storage mechanism (two-phase vs solid-solution) and localization of charge give rise to additional kinetic barriers in NCA and nonmetallic LTMOs in general.

25 ENERGY STORAGE↗
DOE OSTI2020

Correlating work hardening with co-activation of stacking fault strengthening and transformation in a high entropy alloy using in-situ neutron diffraction

Transformation induced plasticity (TRIP) leads to enhancements in ductility in low stacking fault energy (SFE) alloys, however to achieve an unconventional increase in strength simultaneously, there must be barriers to dislocation motion. While stacking faults (SFs) contribute to strengthening by impeding dislocation motion, the contribution of SF strengthening to work hardening during deformation is not well understood; as compared to dislocation slip, twinning induced plasticity (TWIP) and TRIP. Thus, we used in-situ neutron diffraction to correlate SF strengthening to work hardening behavior in a low SFE Fe 40 Mn 20 Cr 15 Co 20 Si 5 (at%) high entropy alloy, SFE ~ 6.31 mJ m –2 . Cooperative activation of multiple mechanisms was indicated by increases in SF strengthening and γ-f.c.c. → ε-h.c.p. transformation leading to a simultaneous increase in strength and ductility. The present study demonstrates the application of in-situ, neutron or X-ray, diffraction techniques to correlating SF strengthening to work hardening.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Unraveling the discontinuous plastic flow of a Co-Cr-Fe-Ni-Mo multiprincipal-element alloy at deep cryogenic temperatures

We report an analysis of the discontinuous plastic flow of a multiprincipal-element alloy, Co 17.5 Cr 12.5 Fe 55 Ni 10 Mo 5 (atomic percent, at. %), in the temperature range of 0.5–4.2 K showing serrated deformation curves. Using the analytical techniques, we studied the statistics of the stress drops associated with the unstable plastic flow. The analysis showed that the complexity and heterogeneity of a discontinuous plastic flow were reduced when the temperature was lowered. This behavior was associated with the effects of dynamic recovery and adiabatic heating on the dislocation-density evolution.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Mn2FeCoP2 by Materials Project

Mn2FeCoP2 crystallizes in the orthorhombic Pmc2_1 space group. The structure is three-dimensional. there are two inequivalent Mn sites. In the first Mn site, Mn is bonded in a 5-coordinate geometry to five P atoms. There are a spread of Mn–P bond distances ranging from 2.40–2.51 Å. In the second Mn site, Mn is bonded in a 5-coordinate geometry to two equivalent Fe, four equivalent Co, and five P atoms. There are one shorter (2.70 Å) and one longer (2.71 Å) Mn–Fe bond lengths. There are two shorter (2.66 Å) and two longer (2.68 Å) Mn–Co bond lengths. There are a spread of Mn–P bond distances ranging from 2.42–2.52 Å. Fe is bonded to two equivalent Mn and four P atoms to form distorted corner-sharing FeMn2P4 tetrahedra. There are three shorter (2.20 Å) and one longer (2.23 Å) Fe–P bond lengths. Co is bonded in a 4-coordinate geometry to four equivalent Mn and four P atoms. There are a spread of Co–P bond distances ranging from 2.19–2.24 Å. There are two inequivalent P sites. In the first P site, P is bonded in a 9-coordinate geometry to five Mn, two equivalent Fe, and two equivalent Co atoms. In the second P site, P is bonded in a 9-coordinate geometry to five Mn, two equivalent Fe, and two equivalent Co atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Radical SAM enzymes: Nature's choice for radical reactions

Enzymes that use a [4Fe-4S] 1+ cluster plus S-adenosyl-L-methionine (SAM) to initiate radical reactions (radical SAM) form the largest enzyme superfamily, with over half a million members across the tree of life. This review summarizes recent work revealing the radical SAM reaction pathway, which ultimately liberates the 5´-deoxyadenosyl (5´-dAdo•) radical to perform extremely diverse, highly regio- and stereo-specific, transformations. Most surprising was the discovery of an organometallic intermediate Ω exhibiting an Fe-C5´-adenosyl bond. Ω liberates 5´-dAdo• through homolysis of the Fe-C5´ bond, in analogy to Co-C5´ bond homolysis in B12, previously viewed as biology’s paradigmatic radical generator. The 5´-dAdo• has been trapped and characterized in radical SAM enzymes via a recently-discovered photoreactivity of the [4Fe- 4S] + /SAM complex, and has been confirmed as a catalytically active intermediate in enzyme catalysis. The regioselective SAM S-C bond cleavage to produce 5´-dAdo• originates in the JahnTeller effect. The simplicity of SAM as a radical precursor, and the exquisite control of 5´-dAdo• reactivity in radical SAM enzymes, may be why radical SAM enzymes pervade the tree of life, while B12 enzymes are only a few.

deoxyadenosyl radical↗
DOE OSTI2020

Materials Data on Fe25O32 by Materials Project

Fe25O32 crystallizes in the orthorhombic Amm2 space group. The structure is three-dimensional. there are fifteen inequivalent Fe+2.56+ sites. In the first Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of distorted edge, corner, and face-sharing FeO6 pentagonal pyramids. The corner-sharing octahedra tilt angles range from 1–9°. There are two shorter (2.12 Å) and four longer (2.20 Å) Fe–O bond lengths. In the second Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 0°. There are a spread of Fe–O bond distances ranging from 2.02–2.10 Å. In the third Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 4°. There are a spread of Fe–O bond distances ranging from 2.03–2.07 Å. In the fourth Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing FeO6 octahedra. The corner-sharing octahedra tilt angles range from 0–1°. There are two shorter (2.03 Å) and four longer (2.05 Å) Fe–O bond lengths. In the fifth Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing FeO6 octahedra. The corner-sharing octahedra tilt angles range from 2–4°. There are a spread of Fe–O bond distances ranging from 2.03–2.06 Å. In the sixth Fe+2.56+ site, Fe+2.56+ is bonded in a 5-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.00–2.75 Å. In the seventh Fe+2.56+ site, Fe+2.56+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.05–2.41 Å. In the eighth Fe+2.56+ site, Fe+2.56+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.13–2.32 Å. In the ninth Fe+2.56+ site, Fe+2.56+ is bonded in a 6-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.12–2.66 Å. In the tenth Fe+2.56+ site, Fe+2.56+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.10–2.45 Å. In the eleventh Fe+2.56+ site, Fe+2.56+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.19–2.70 Å. In the twelfth Fe+2.56+ site, Fe+2.56+ is bonded in a 6-coordinate geometry to six O2- atoms. There are two shorter (2.20 Å) and four longer (2.22 Å) Fe–O bond lengths. In the thirteenth Fe+2.56+ site, Fe+2.56+ is bonded in a 7-coordinate geometry to seven O2- atoms. There are a spread of Fe–O bond distances ranging from 2.20–2.65 Å. In the fourteenth Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 2°. There are a spread of Fe–O bond distances ranging from 1.95–2.07 Å. In the fifteenth Fe+2.56+ site, Fe+2.56+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing FeO6 octahedra. The corner-sharing octahedral tilt angles are 1°. There are a spread of Fe–O bond distances ranging from 1.95–2.10 Å. There are eighteen inequivalent O2- sites. In the first O2- site, O2- is bonded to six Fe+2.56+ atoms to form distorted edge-sharing OFe6 octahedra. In the second O2- site, O2- is bonded in a 5-coordinate geometry to six Fe+2.56+ atoms. In the third O2- site, O2- is bonded in a square co-planar geometry to four Fe+2.56+ atoms. In the fourth O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Fe+2.56+ atoms. In the fifth O2- site, O2- is bonded in a distorted rectangular see-saw-like geometry to six Fe+2.56+ atoms. In the sixth O2- site, O2- is bonded in a distorted square co-planar geometry to five Fe+2.56+ atoms. In the seventh O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the eighth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the ninth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the tenth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the eleventh O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the twelfth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the thirteenth O2- site, O2- is bonded to six Fe+2.56+ atoms to form distorted OFe6 pentagonal pyramids that share corners with four OFe5 trigonal bipyramids, edges with two OFe5 trigonal bipyramids, and faces with two equivalent OFe6 pentagonal pyramids. In the fourteenth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the fifteenth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the sixteenth O2- site, O2- is bonded to five Fe+2.56+ atoms to form distorted OFe5 trigonal bipyramids that share corners with two equivalent OFe6 pentagonal pyramids, an edgeedge with one OFe6 pentagonal pyramid, and edges with two equivalent OFe5 trigonal bipyramids. In the seventeenth O2- site, O2- is bonded in a 5-coordinate geometry to five Fe+2.56+ atoms. In the eighteenth O2- site, O2- is bonded to five Fe+2.56+ atoms to form distorted OFe5 trigonal bipyramids that share corners with two equivalent OFe6 pentagonal pyramids, a cornercorner with one OFe5 trigonal bipyramid, an edgeedge with one OFe6 pentagonal pyramid, and edges with two equivalent OFe5 trigonal bipyramids.

36 MATERIALS SCIENCE↗