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At least 1,063 records · Page 59
All sourcesNASA NTRSDOE OSTI
DOE OSTI2013

Hydrothermal synthesis and metal ions doping effects of single-crystal Mn{sub 3}O{sub 4}

Graphical abstract: - Highlights: • Dopant species and dopant/Mn molar ratio affect the shape of Mn{sub 3}O{sub 4} microcrystal. • For Cu and Ni doped Mn{sub 3}O{sub 4}, the spontaneous valence changes of dopant taken place. • An apparent change in the energy bandgap of Mn{sub 3}O{sub 4} with the metal doping. - Abstract: Synthesis of undoped and transition metal ion doped Mn{sub 3}O{sub 4} microcrystals were achieved through a simple hydrothermal route. The morphologies and structures of the obtained products were characterized using X-ray diffraction, X-ray photoemission spectroscopy and scanning electron microscopy. Results revealed that the low volume percentage of ethanol in the precursor solution limited formation of Mn{sup 2+}, while the introduction of doping ions into the precursor solution caused a direct synthesis of single phase Mn{sub 3}O{sub 4} crystals. For Cu and Ni doping ions, the spontaneous valence changes during the doping process were taken place. The possible doping mechanisms for the formation of single-phase Mn{sub 3}O{sub 4} were discussed briefly. UV–vis spectroscopic studies showed an apparent change in the energy bandgap of Mn{sub 3}O{sub 4} with the metal doping.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Compact Li-doped Gd{sub 2}Ti{sub 2}O{sub 7} prepared with LiO{sub 0.5} self-flux

Graphical abstract: Left figure is the unit cell a-axis of the GLT-2L (■), GLT (●) and GT (▴) as a function of x. Both of the GLT-2L and GLT have the same results so that they have the same chemical formula, (Gd{sub 2−x}Li{sub x})Ti{sub 2}O{sub 7−x}, in which GT has a formula of Gd{sub 2−x}Ti{sub 2}O{sub 7−x}. Relative densities are shown in the right figure. GLT-2L (■) has higher values than GLT (●). 2x moles of LiO{sub 0.5} were added as a self-flux to prepare GLT-2L at 1250 °C, which was about 300 °C lowered than required without the co-existence of the flux and Li ion substitution. - Highlights: • A new series of compounds with a formula as (Gd{sub 2−x}Li{sub x})Ti{sub 2}O{sub 7−x} was synthesized. • Each GLT-2L sample contains (Gd{sub 2−x}Li{sub x})Ti{sub 2}O{sub 7−x} and 2x moles of LiO{sub 0.5} self-flux. • 1250 °C was used to prepare the GLT-2L samples; 300 °C lower than required. • Electrical conductivity of the GLT-2L is not affected by the presence of the flux. • Average valence of the Ti ions is slightly less than 4. - Abstract: A series of (Gd{sub 2−x}Li{sub x})Ti{sub 2}O{sub 7−x} with x = 0.040–0.110 was first reported. Parent compound with 2x moles of LiO{sub 0.5} self-flux was prepared at 1250 °C and abbreviated as GLT-2L. Relative density of all the samples is higher than 92%. By the addition of flux and Li ions substitution, preparation temperature is lowered for more than 300 °C. Comparing the unit cell a-axis, GLT with a formula as (Gd{sub 2−x}Li{sub x})Ti{sub 2}O{sub 7−x} without flux addition has exactly the same as the GLT-2L with the same x. By the addition of flux, materials become more compact and grains grow bigger for the GLT-2L than the corresponding GLT samples. Electrical conductivity of a GLT-2L with x = 0.110 at 700 °C is 3 times larger than the measurements done by other research groups in the Gd{sub 2}Ti{sub 2}O{sub 7}. In order to demonstrate the successful substitution of the Li ion into the Gd site, a series of GT samples was prepared with a formula of Gd{sub 2−x}Ti{sub 2}O{sub 7−1.5x}. With the same amount of x, every GLT samples have a longer unit cell a-axis than that of the GT. The difference is caused by the substitution of the Li ion into the Gd site. Valence of the Ti atoms of the GLT-2L samples is slightly less than 4 studied by the XANES spectroscopy.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Electronic structure, magnetic and structural properties of Ni doped ZnO nanoparticles

Highlights: • XRD, and HR-TEM results show the single phase nature of Ni doped ZnO nanoparticles. • dc magnetization results indicate the RT-FM in Ni doped ZnO nanoparticles. • Ni L{sub 3,2} edge NEXAFS spectra infer that Ni ions are in +2 valence state. • O K edge NEXAFS spectra show that O vacancy increases with Ni doping in ZnO. - Abstract: We report structural, magnetic and electronic structural properties of Ni doped ZnO nanoparticles prepared by auto-combustion method. The prepared nanoparticles were characterized by using X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, and dc magnetization measurements. The XRD and HR-TEM results indicate that Ni doped ZnO nanoparticles have single phase nature with wurtzite lattice and exclude the presence of secondary phase. NEXAFS measurements performed at Ni L{sub 3,2}-edges indicates that Ni ions are in +2 valence state and exclude the presence of Ni metal clusters. O K-edge NEXAFS spectra indicate an increase in oxygen vacancies with Ni-doping, while Zn L{sub 3,2}-edge show the absence of Zn-vacancies. The magnetization measurements performed at room temperature shows that pure and Ni doped ZnO exhibits ferromagnetic behavior.

36 MATERIALS SCIENCE↗
DOE OSTI2014

A comparative investigation on structure and multiferroic properties of bismuth ferrite thin films by multielement co-doping

Highlights: • Multielement (Tb, Cr and Mn) co-doped BiFeO{sub 3} films were fabricated by CSD method. • Multielement co-doping induces a structural transition. • It is found effective to stabilize the valence of Fe ions at +3 by the strategy. • The co-doping at A/B-sites gives rise to the superior multiferroic properties. - Abstract: (Tb, Cr and Mn) multielement co-doped BiFeO{sub 3} (BTFCMO) thin films were prepared by the chemical solution deposition method on fluorine doped tin oxide (FTO) substrates. X-ray diffraction, Rietveld refinement and Raman analyses revealed that a phase transition from rhombohedral to triclinic structure occurs in the multielement co-doped BiFeO{sub 3} films. It is found that the doping is conducive to stabilizing the valence of Fe ions and reducing leakage current. In addition, the highly enhanced ferroelectric properties with a huge remanent polarization (2P{sub r}) of 239.6 μC/cm{sup 2} and a low coercive field (2E{sub c}) of 615.6 kV/cm are ascribed to the well film texture, the structure transition and the reduced leakage current by the co-doping. Moreover, the structure transition is the dominant factor resulting in the significant enhancement observed in magnetization (M{sub s} ∼ 10.5 emu/cm{sup 3}), owing to the collapse of the space-modulated spin structure. In this contribution, these results demonstrate that the multielement co-doping is in favor of the enhanced multiferroic properties of the BFO films for possible multifunctional applications.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Structural, magnetic and electronic structure properties of Co doped ZnO nanoparticles

Highlights: • XRD and HR-TEM results show the single phase nature of Co doped ZnO nanoparticles. • XMCD and dc magnetization results indicate the RT-FM in Co doped ZnO nanoparticles. • Co L{sub 3,2} NEXAFS spectra infer that Co ions are in 2+ valence state. • O K edge NEXAFS spectra show that O vacancy increases with Co doping in ZnO. - Abstract: We reported structural, magnetic and electronic structure studies of Co doped ZnO nanoparticles. Doping of Co ions in ZnO host matrix has been studied and confirmed using various methods; such as X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy dispersed X-ray (EDX), high resolution transmission electron microscopy (HR-TEM), Fourier transform infrared spectroscopy (FT-IR), near edge X-ray absorption fine structure (NEXAFS) spectroscopy, magnetic hysteresis loop measurements and X-ray magnetic circular dichroism (XMCD). From the XRD and HR-TEM results, it is observed that Co doped ZnO nanoparticles have single phase nature with wurtzite structure and exclude the possibility of secondary phase formation. FE-SEM and TEM micrographs show that pure and Co doped nanoparticles are nearly spherical in shape. O K edge NEXAFS spectra indicate that O vacancies increase with Co doping. The Co L{sub 3,2} edge NEXAFS spectra revealed that Co ions are in 2+ valence state. DC magnetization hysteresis loops and XMCD results clearly showed the intrinsic origin of temperature ferromagnetism in Co doped ZnO nanoparticles.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2018

Tuning of carrier type, enhancement of Linear magnetoresistance and inducing ferromagnetism at room temperature with Cu doping in Bi2Te3 Topological Insulators

Highlights: • With Cu doping in Bi{sub 2}Te{sub 3}, resistivity increases as Fermi level is shifted into valence band with extra scattering centers. • Cu doping tunes the carrier from n to p type, which is attributed to the presence of Te{sub Bi} and Bi{sub Te} antisites. • The observed Linear Magneto resistance is believed to be associated with gapless linear energy spectrum of surface Dirac Fermions. • Cu doping induces room temperature ferromagnetism. - Abstract: Structural, resistivity, thermoelectric power, magneto-transport and magnetic properties of Cu doped Bi{sub 2}Te{sub 3} topological insulators have been investigated. The tuning of charge carriers from n to p type by Cu doping at Te sites of Bi{sub 2}Te{sub 3} is observed both from Hall effect and thermoelectric power measurements. Carrier mobility decreases with the doping of Cu which provides evidence of the movement of Fermi level from bulk conduction band to the bulk valence band. Thermoelectric power also increases with doping of Cu. Moreover, linear magnetoresistance (LMR) is observed at high magnetic field in pure Bi{sub 2}Te{sub 3} which is associated to the gapless topological surface states protected by time reversal symmetry (TRS). Furthermore, doping of non-magnetic Cu induces ferromagnetism at room temperature and also enhances the magnetoresistance.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Preparation and electrochemical properties of high-entropy oxide Li x (CrMnCoNiZn) 3-x O 4

High-entropy oxides (HEOs) have gained significant attention as anode materials for lithium-ion batteries (LIBs) due to their high theoretical specific capacity, synergistic effects of constituent elements, and enhanced structural stability induced by high entropy. Here, this study investigates the electrochemical properties of a novel series of HEOs, Li x (CrMnCoNiZn) 3-x O 4 , synthesized via the sol-gel method. As lithium content increases, the electrochemical lithium storage performance of the HEOs improves, attributed to the rise in high-valence states and oxygen vacancies. The high valence state of elements facilitates greater electron transfer, enhancing the specific capacity during lithium insertion. Additionally, a higher concentration of oxygen vacancies boosts ionic conductivity and lithium ion diffusion kinetics, leading to superior electrochemical performance. Among the synthesized materials, Li 0.11 (CrMnCoNiZn) 2.89 O 4 exhibited the best performance, delivering a discharge capacity of 365 mAh·g −1 after 100 cycles at 100 mA·g −1 , 257 mAh·g −1 after 1000 cycles at 1 A·g −1 , and a rate capacity of 196 mAh·g −1 at 2 A·g −1 . The Li + diffusion coefficient reached 2.78 × 10 −15 cm 2 ·s −1 . This work not only deepens the understanding of HEOs' electrochemical behavior in LIBs but also provides insights into the impact of lithium ion substitution on their performance.

25 ENERGY STORAGE↗
DOE OSTI2022

Unveiling the two-proton halo character of 17 Ne: Exclusive measurement of quasi-free proton-knockout reactions

The proton drip-line nucleus 17 Ne is investigated experimentally in order to determine its two-proton halo character. A fully exclusive measurement of the 17 Ne (p, 2p) 16 F* 15 O+p quasi-free one-proton knockout reaction has been performed at GSI at around 500 MeV/nucleon beam energy. All particles resulting from the scattering process have been detected. The relevant reconstructed quantities are the angles of the two protons scattered in quasi-elastic kinematics, the decay of 16 F into 15 O (including γ decays from excited states) and a proton, as well as the 15 O+p relative-energy spectrum and the 16 F momentum distributions. The latter two quantities allow an independent and consistent determination of the fractions of l = 0 and l = 2 motion of the valence protons in 17 Ne. With a resulting relatively small l = 0 component of only around 35(3)%, it is concluded that 17 Ne exhibits a rather modest halo character only. The quantitative agreement of the two values deduced from the energy spectrum and the momentum distributions supports the theoretical treatment of the calculation of momentum distributions after quasi-free knockout reactions at high energies by taking into account distortions based on the Glauber theory. Moreover, the experimental data allow the separation of valence-proton knockout and knockout from the 15O core. The latter process contributes with 11.8(3.1) mb around 40% to the total proton-knockout cross section of 30.3(2.3) mb, which explains previously reported contradicting conclusions derived from inclusive cross sections.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2024

Beam energy dependence of net-hyperon yield and its implication on baryon transport mechanism

In the constituent quark model, each quark inside a baryon carries 1/3 unit of the baryon number. An alternative picture exists where the center of a Y-shaped topology of gluon fields, called the baryon junction, carries a unit baryon number. Studying baryon transport over a large rapidity gap (δy) in nuclear collisions provides a possible tool to distinguish these two pictures. A recent analysis of global data on net-proton yield at mid-rapidity in Au+Au collisions showed an exponential dependence on δy and the exponential slope does not vary with event centrality, favoring the baryon junction picture. Since junctions are flavor blind, hyperons – baryons containing valence strange quarks – are expected to exhibit a similar behavior as the proton. This study aims to test this prediction by analyzing hyperon yields in Au+Au collisions at various energies. We observe that net-hyperon yields, after correcting for the strangeness production suppression, adhere to the expected exponential form. The extracted slope parameters for net-Λ, net-$Ξ$ and net-Ω are consistent with each other and with those of net-proton within uncertainties, and exhibit no centrality dependence. Various implementations of the PYTHIA event generator, primarily based on valence quarks for baryon transport, are unable to simultaneously describe the slope parameters for all baryons.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Isomer spectroscopy of neutron-rich 168 Tb 103

In-flight fission of a 345 MeV per nucleon 238U primary beam on a 2 mm thick 9Be target has been used to produce and study the decays of a range of neutron-rich nuclei centred around the doubly mid-shell nucleus 170Dy at the RIBF Facility, RIKEN, Japan. The produced secondary fragments of interest were identified event-by-event using the BigRIPS separator. The fragments were implanted into the WAS3ABI position sensitive silicon active stopper which allowed pixelated correlations between implants and their subsequent β-decay. Discrete γ-ray transitions emitted following decays from either metastable states or excited states populated following beta decay were identified using the 84 coaxial high-purity germanium (HPGe) detectors of the EURICA spectrometer, which was complemented by 18 additional cerium-doped lanthanum bromide (LaBr3) fast-timing scintillation detectors from the FATIMA collaboration. This paper presents the internal decay of a metastable isomeric excited state in the odd-odd nucleus 168Tb, which corresponds to a single proton-neutron hole configuration in the valence maximum nucleus 170Dy. These data represent the first information on excited states in this nucleus, which is the most neutron-rich odd-odd isotope of terbium (Z=65) studied to date. Nilsson configurations associated with an axially symmetric, prolate-deformed nucleus are proposed for the 168Tb ground state the observed isomeric state by comparison with Blocked BCS-Nilsson calculations.

Gamma-ray spectroscopy and nuclear isomerism↗
DOE OSTI2020

Hydroxypyridinone-based stabilization of Np(IV) enabling efficient U/Np/Pu separations in the Adapted PUREX process

The classical process to recover uranium (U) and plutonium (Pu) from used nuclear fuel using tributyl phosphate (TBP), namely the Plutonium Uranium Redox EXtraction (PUREX) process, is complicated by the persistent presence of neptunium (Np) and thus requires extra purification steps. The concept of Adapted PUREX seeks to achieve Np recovery by adjusting the valence of the metal more effectively, thereby controlling its behavior more precisely. This study introduces the use of an aqueous hydroxypyridinone chelator, 3,4,3-LI(1,2-HOPO) (abbreviated as HOPO), to dictate the behavior of Np for recovery and meanwhile simplify cumbersome reprocessing steps. The interactions between Np and HOPO were probed mechanistically by way of absorption spectrophotometry, in conjunction with cyclic voltammetry. UV–Vis-NIR spectra illustrated the reduction of NpO 2 2+ to Np 4+ , with a fast reaction rate. Cyclic voltammetry revealed quasi-reversible processes between the oxidized and reduced forms of the ligand and its Np complexes. The corresponding heterogeneous rate constants (k 0 ) were estimated from the peak-to-peak separation potentials (ΔE p ), at ~ 4 – 35 μm/s for both HOPO and NpHOPO, with scan rates of 0.01 – 0.4 V/s. Meanwhile, the electromotive force (E MF ) as well as the change of Gibbs free energy (ΔG) were assessed from the half-wave potential (E 1/2 ), demonstrating the completeness of NpO 2 2+ reduction to Np 4+ by HOPO. The cumulative formation constant of the resulting NpHOPO complex (logβ 101 ) was determined by metal competition titration to be 42.0 ± 0.6, corroborating the extraordinarily high affinity of HOPO to tetravalent metal ions. Here, the prowess of valence control by HOPO and the high stability of the formed complex resulted in enhanced separations of Np from U and of Pu from U, with a maximum separation factor of ~7000 for both, nearly 90- and 10300-fold higher, respectively, than the values obtained using conventional PUREX formulae.

38 RADIATION CHEMISTRY, RADIOCHEMISTRY, AND NUCLEA↗
DOE OSTI2016

Stabilization of MgAl 2 O 4 spinel surfaces via doping

Here, the surface structure of complex oxides plays a vital role in processes such as sintering, thin film growth, and catalysis, as well as being a critical factor determining the stability of nanoparticles. We report atomistic calculations of the low-index stoichiometric magnesium aluminate spinel (MgAl 2 O 4 ) surfaces, each with two different chemical terminations. High temperature annealing was used to explore the potential energy landscape and provide more stable surface structures. We find that the lowest energy surface is {100} while the highest energy surface is {111}. The surfaces were subsequently doped with three trivalent dopants (Y 3+ , Gd 3+ , La 3+ ) and one tetravalent dopant (Zr 4+ ) and both the surface segregation energies of the dopants and surface energies of the doped surface were determined. All of the dopants reduce the surface energy of spinel, though this reduction in energy depends on both the size and valence of the dopant. Dopants with larger ionic radius tend to segregate to the surface more strongly and reduce the surface energy to a greater extent. Furthermore, the ionic valence of the dopants seems to have a stronger influence on the segregation than does ionic size. For both undoped and doped spinel, the predicted crystal shape is dominated by {100} surfaces, but the relative fraction of the various surfaces changes with doping due to the unequal changes in energy, which has implications on equilibrium nanoparticle shapes and therefore on applications sensitive to surface properties.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Temperature Dependence Discontinuity in the Stability of Manganese doped Ceria Nanocrystals

CeO 2 has strong potential for chemical-looping water splitting. It has been shown that manganese doping decreases interface energies of CeO 2 , allowing increased stability of high surface areas in this oxygen carrier oxide. The phenomenon is related to the segregation of Mn3+ at interfaces, which causes a measurable decrease in excess energy. Here in the present work, it is shown that, despite the stability of nanocrystals of manganese-doped CeO 2 with relation to undoped CeO 2 , the effect is strongly dependent on the oxidation state of manganese, i.e., on the temperature. At temperatures below 800 °C, Mn is in the 3+ valence state, and coarsening is hindered by the reduced interface energetics, showing smaller crystal sizes with increasing Mn content. At temperatures above 800 °C, Mn is reduced to its 2+ valence state, and coarsening is enhanced with increasing Mn content. Atomistic simulations show the segregation of Mn to grain boundaries is relatively insensitive to the charge state of the dopant. However, point defect modeling finds that the reduced state causes a decrease in cation vacancy concentration and an increase in cation interstitials, reducing drag forces for grain boundary mobility and increasing growth rates.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Reducing Surface Recombination Velocity of Methylammonium-Free Mixed-Cation Mixed-Halide Perovskites via Surface Passivation

We control surface recombination in the mixed-cation, mixed-halide perovskite, FA0.83Cs0.17Pb(I0.85Br0.15)3, by passivating nonradiative defects with the polymerizable Lewis base (3-aminopropyl)trimethoxysilane (APTMS). We demonstrate average minority carrier lifetimes >4 µs, nearly single exponential monomolecular photoluminescence decays, and high external photoluminescence quantum efficiencies (>20%, corresponding to ~97% of the maximum theoretical quasi-Fermi-level splitting) at low excitation fluence. We confirm both the composition and valence band edge position of the FA0.83Cs0.17Pb(I0.85Br0.15)3 perovskite using multi-institutional, cross-validated, X-ray photoelectron spectroscopy and UV photoelectron spectroscopy measurements. We extend the APTMS surface passivation to higher bandgap double-cation (FA and Cs) compositions (1.7, 1.75, and 1.8 eV) as well as the widely used triple-cation (FA, MA, and Cs) composition. Finally, we demonstrate that the average surface recombination velocity decreases from ~1000 to ~10 cm/s post APTMS passivation for FA0.83Cs0.17Pb(I0.85Br0.15)3. Our results demonstrate that surface-mediated recombination is the primary nonradiative loss pathway in many methylammonium (MA)-free mixed-cation mixed-halide films with a range of different bandgaps, which is a problem observed for a wide range of perovskite active layers and reactive electrical contacts. Our study also provides insights to develop passivating molecules that help reduce surface recombination in MA-free mixed-cation mixed-halide films and indicates that surface passivation and contact engineering will enable near-theoretical device efficiencies with these materials.

14 SOLAR ENERGY↗
DOE OSTI2015

Origin of high Li⁺ conduction in doped Li₇La₃Zr₂O₁₂ garnets

Substitution of a native ion in the crystals with a foreign ion that differs in valence ( aliovalent doping ) has been widely attempted to upgrade solid-state ionic conductors for various charge carriers including O²⁻, H⁺, Li⁺, Na⁺, etc. The doping helps promote the high-conductive framework and dredge the tunnel for fast ion transport. The garnet-type Li₇La₃Zr₂O₁₂ (LLZO) is a fast Li⁺ solid conductor, which received much attention as an electrolyte candidate for all-solid-state lithium ion batteries, showing great potential to offer high energy density and minimize battery safety concerns to meet extensive applications in large energy storage systems such as those for electric vehicles and aerospace. In the Li-stuffed garnet framework of LLZO, the 3D pathway formed by the incompletely occupied tetrahedral sites bridged by a single octahedron enables the superior Li⁺ conductivity. For optimal performance, many aliovalent-doping efforts have been made throughout metal elements (Al³⁺, Ta⁵⁺) and metalloid elements (Ga³⁺, Te⁶⁺) in the periodic table with various valences to stabilize the high-conductive phase and increase the Li vacancy concentration.

36 MATERIALS SCIENCE↗
DOE OSTI2015

Crystal and Electronic Structures of Complex Bismuth Iodides A 3 Bi 2 I 9 ( A = K, Rb, Cs) Related to Perovskite: Aiding the Rational Design of Photovoltaics

Ternary bismuth halides form an interesting functional materials class in the context of the closely related Pb halide perovskite photovoltaics, especially given the significantly reduced toxicity of Bi when compared with Pb. The compounds A 3 Bi 2 I 9 (A = K, Rb, Cs) examined here crystallize in two different structure types: the layered defect-perovskite K 3 Bi 2 I 9 type, and the Cs 3 Cr 2 Cl 9 type. The latter structure type features isolated Bi 2 I 9 3– anions. Here, the crystal structures of the ternary iodides are redetermined and a corrected structural model for Rb 3 Bi 2 I 9 , as established by single crystal X-ray diffraction and solid state 87Rb NMR spectroscopy and supported by density functional theory (DFT) calculations is presented. A variety of facile preparation techniques for single crystals, bulk materials, as well as solution-processed thin films are described. The optical properties and electronic structures are investigated experimentally by optical absorption and ultraviolet photoemission spectroscopy and computationally by DFT calculations. Absolute band positions of the valence and conduction bands of these semiconductors, with excellent agreement of experimental and calculated values, are reported, constituting a useful input for the rational interface design of efficient electronic and optoelectronic devices. The different structural connectivity in the two different structure types, somewhat surprisingly, appears to not impact band positions or band gaps in a significant manner. Computed dielectric properties, including the finding of anomalously large Born effective charge tensors on Bi 3+ , suggest proximal structural instabilities arising from the Bi 3+ 6s 2 lone pair. These anomalous Born effective charges are promising for defect screening and effective charge carrier transport. The structural, electronic, and optical properties of the complex bismuth iodides are to some extent similar to the related lead iodide perovskites. In conclusion, the deeper valence band positions in the complex bismuth iodides point to the need for different choices of hole transport materials for Bi-iodide based solar cell architectures.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2015

Low-Temperature Synthesis, Structural Characterization, and Electrochemistry of Ni-Rich Spinel-like LiNi 2– y Mn y O 4 (0.4 ≤ y ≤ 1)

The thermal conversion of chemically delithiated layered Li 0.5 Ni 1-y Mn y O 2 (0.2 ≤ y ≤ 0.5) into spinel-like LiNi 2-y Mn y O 4 (0.4 ≤ y ≤ 1) has been systematically investigated. The formed spinel-like phases are metastable and cannot be accessed by a conventional high-temperature solid-state method. The layered-to-spinel transformation mechanism has been studied by the Rietveld refinement of in situ neutron diffraction as a function of temperature (25–300 °C). In particular, the ionic diffusion of Li and M ions is quantified at different temperatures. Electrochemistry of the metastable spinel-like phases obtained has been studied in lithium-ion cells. A bond valence sum map has been performed to understand the ionic diffusion of lithium ions in the Ni-rich layered, spinel, and rock-salt structures. The study can aid the understanding of the possible phases that could be formed during the cycling of Ni-rich layered oxide cathodes.

36 MATERIALS SCIENCE↗
DOE OSTI2016

Flux Synthesis, Optical and Photocatalytic Properties of n -type Sn 2 TiO 4 : Hydrogen and Oxygen Evolution under Visible Light

Here, the n -type Sn 2 TiO 4 phase was synthesized using flux methods and found to have one of the smallest visible-light bandgap sizes known that also maintains suitable conduction and valence band energies for driving photocatalytic water-splitting reactions. The Sn 2 TiO 4 phase was synthesized using either a SnCl 2 flux or a SnCl 2 /SnF 2 peritectic flux in a 2:1 flux-to-precursor ratio heated at 600 and 400 °C for 24 h, respectively. The two types of salt fluxes resulted in large rod-shaped particles at 600 °C and smaller tetragonal prism-shaped particles at 400 °C. Surface photovoltage spectroscopy measurements produced a negative photovoltage under illumination >1.50 eV, which confirmed electrons as the majority charge carriers and ~1.50 eV as the effective band gap. Mott–Schottky measurements at pH 9.0 showed the conduction (–0.54 V vs NHE) and valence band (+1.01 V vs NHE) positions meet the critical thermodynamic requirements for total water splitting. The Sn 2 TiO 4 particles were deposited and annealed as polycrystalline films on FTO slides, and exhibited photoanodic currents in aqueous solutions under visible-light irradiation. The Sn 2 TiO 4 particles were also suspended in aqueous methanol solutions and irradiated with visible and ultraviolet light. The larger rod-shaped Sn 2 TiO 4 particles had the higher rates of photocatalytic hydrogen production (~11.6 μmol H 2 h –1 ) in comparison to the smaller tetragonal prism-shaped Sn 2 TiO 4 particles (~3.4 μmol H 2 h –1 ). Conversely, for photocatalytic oxygen production, the rates for the smaller tetragonal prism-shaped particles in aqueous AgNO 3 solution were slightly higher (~16.3 μmol O 2 h –1 ) than for the larger rod-shaped particles (~11.9 μmol O 2 h –1 ). Apparent quantum yields of 0.995% and 0.0098% were measured for O 2 and H 2 production, respectively, under 435 nm light.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗