Abstract: <script type='text/javascript'>document.write(unpmarked('Li-rich NMC are considered nowadays as one of the most promising candidates for high energy density cathodes. One significant challenge is nested in adjusting their synthesis conditions to reach optimum electrochemical performance, but no consensus has been reached yet on the ideal synthesis protocol. Herein, we revisited the elaboration of Li-rich NMC electrodes by focusing on the science involved through each synthesis steps using carbonate Ni0.1625Mn0.675Co0.1625CO3 precursor coprecipitation combined with solid state synthesis. We demonstrated the effect of precursors concentration on the kinetics of the precipitation reaction and provided clues to obtain spherically agglomerated NMC carbonates of different sizes. Moreover, we highlighted the strong impact of the Li2CO3/NMC carbonate ratio on the morphology and particles size of Li-rich NMC and subsequently on their electrochemical performance. Ratio of 1.35 was found to reproducibly give the best performance with namely a first discharge capacity of 269 mAh g(-1) and capacity retention of 89.6% after 100 cycles. We hope that our results, which reveal how particle size, morphology, and phase composition affect the materials electrochemical performance, will help in reconciling literature data while providing valuable fundamental information for up scaling approaches.'));

Abstract: <script type='text/javascript'>document.write(unpmarked('By depositing ceria on the surface of yttrium stabilized zirconia (YSZ) nanocrystals and further activation under high-temperature reducing conditions, a 13% mol. CeO2/YSZ catalyst structured as subnanometer thick, pyrochlore-type, ceria-zirconia islands has been prepared. This nanostructured catalyst depicts not only high oxygen storage capacity (OSC) values but, more importantly, an outstandingly stable redox response upon oxidation and reduction treatments at very high temperatures, above 1000 degrees C. This behavior largely improves that observed on conventional ceria-zirconia solid solutions, not only of the same composition but also of those with much higher molar cerium contents. Advanced scanning transmission electron microscopy (STEM-XEDS) studies have revealed as key not only to detect the actual state of the lanthanide in this novel nanocatalyst but also to rationalize its unusual resistance to redox deactivation at very high temperatures. In particular, high-resolution X-ray dispersive energy studies have revealed the presence of unique bilayer ceria islands on top of the surface of YSZ nanocrystals, which remain at surface positions upon oxidation and reduction treatments up to 1000 degrees C. Diffusion of ceria into the bulk of these crystallites upon oxidation at 1100 degrees C irreversibly deteriorates both the reducibility and OSC of this nanostructured catalyst.'));

Abstract: In typical seed-mediated syntheses of metal nanocrystals, the shape of the nanocrystal is determined largely by the seed nucleation environment and subsequent growth environment (where “environment” refers to the chemical environment, including the surfactant and additives). In this approach, crystallinity is typically determined by the seeds, and surfaces are controlled by the environment(s). However, surface energies, and crystallinity, are both influenced by the choice of environment(s). This limits the permutations of crystallinity and surface facets that can be mixed and matched to generate new nanocrystal morphologies. Here, we control post-seed growth to deliberately incorporate twin planes during the growth stage to deliver new final morphologies, including twinned cubes and bipyramids from single-crystal seeds. The nature and number of twin planes, together with surfactant control of facet growth, define the final nanoparticle morphology. Moreover, by breaking symmetry, the twin planes introduce new facet orientations. This additional mechanism opens new routes for the synthesis of different morphologies and facet orientations.

Abstract: The creation and ordering of A-cation vacancies and the effect of cation substitutions in the scheelite-type framework are investigated as a factor for controlling the scheelite-type structure and luminescence properties. AgxEu3+(2−x)/3□(1−2x)/3WO4 and AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4 (x = 0.5−0) scheelite-type phases were synthesized by a solid state method, and their structures were investigated using a combination of transmission electron microscopy techniques and powder synchrotron X-ray diffraction. Transmission electron microscopy also revealed the (3 + 1)D incommensurately modulated character of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.286, 0.2) phases. The crystal structures of the scheelite-based AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286, 0.2) red phosphors have been refined from high resolution synchrotron powder X-ray diffraction data. The luminescence properties of all phases under near-ultraviolet (n-UV) light have been investigated. The excitation spectra of AgxEu3+(2−x)/3□(1−2x)/3WO4 (x = 0.5, 0.286,0.2) phosphors show the strongest absorption at 395 nm, which matches well with the commercially available n-UV-emitting GaN-based LED chip. The excitation spectra of the Eu2/3□1/3WO4 and Gd0.367Eu0.30□1/3WO4 phases exhibit the highest contribution of the charge transfer band at 250 nm and thus the most efficient energy transfer mechanism between the host and the luminescent ion as compared to direct excitation. The emission spectra of all samples indicate an intense red emission due to the 5D0 → 7F2 transition of Eu3+. Concentration dependence of the 5D0 → 7F2 emission for AgxEu(2−x)/3□(1−2x)/3WO4 samples differs from the same dependence for the earlier studied NaxEu3+(2−x)/3□(1−2x)/3MoO4 (0 ≤ x ≤ 0.5) phases. The intensity of the 5D0 → 7F2 emission is reduced almost 7 times with decreasing x from 0.5 to 0, but it practically does not change in the range from x = 0.286 to x = 0.200. The emission spectra of Gd-containing samples show a completely different trend as compared to only Eu-containing samples. The Eu3+ emission under excitation of Eu3+(5L6) level (λex = 395 nm) increases more than 2.5 times with the increasing Gd3+ concentration from 0.2 (x = 0.5) to 0.3 (x = 0.2) in the AgxGd(2−x)/3−0.3Eu3+0.3□(1−2x)/3WO4, after which it remains almost constant for higher Gd3+ concentrations.

Abstract: The recent revival of the Na-ion battery concept has prompted intense activities in the search for new Na-based layered oxide positive electrodes. The largest capacity to date was obtained for a Na-deficient layered oxide that relies on cationic redox processes only. To go beyond this limit, we decided to chemically manipulate these Na-based layered compounds in a way to trigger the participation of the anionic network. We herein report the electrochemical properties of a Na-rich phase Na2IrO3, which can reversibly cycle 1.5 Na+ per formula unit while not suffering from oxygen release nor cationic migrations. Such large capacities, as deduced by complementary XPS, X-ray/neutron diffraction and transmission electron microscopy measurements, arise from cumulative cationic and anionic redox processes occurring simultaneously at potentials as low as 3.0 V. The inability to remove more than 1.5 Na+ is rooted in the formation of an O1-type phase having highly stabilized Na sites as confirmed by DFT calculations, which could rationalize as well the competing metal/oxygen redox processes in Na2IrO3. This work will help to define the most fertile directions in the search for novel high energy Na-rich materials based on more sustainable elements than Ir.

Abstract: The R-2(MoO4)(3) (R = rare earth elements) molybdates doped with Eu3+ cations are interesting red-emitting materials for display and solid-state lighting applications. The structure and luminescent properties of the R2-xEux(MoO4)(3) (R = Gd, Sm) solid solutions have been investigated as a function of chemical composition and preparation conditions. Monoclinic (alpha) and orthorhombic (beta') R2-xEux(MoO4)(3) (R = Gd, Sm; 0 <= x <= 2) modifications were prepared by solid-state reaction, and their structures were investigated using synchrotron powder X-ray diffraction and transmission electron microscopy. The pure orthorhombic beta'-phases could be synthesized only by quenching from high temperature to room temperature for Gd2-xEux(MoO4)(3) in the Eu3+-rich part (x > 1) and for all Sm2-xEux(MoO4)(3) solid solutions. The transformation from the alpha-phase to the beta'-phase results in a notable increase (similar to 24%) of the unit cell volume for all R2-xEux(MoO4)(3) (R = Sm, Gd) solid solutions. The luminescent properties of all R2-xEux(MoO4)(3) (R = Gd, Sm; 0 <= x <= 2) solid solutions were measured, and their optical properties were related to their structural properties. All R2-xEux(MoO4)(3) (R = Gd, Sm; 0 <= x <= 2) phosphors emit intense red light dominated by the D-5(0)-> F-7(2) transition at similar to 616 nm. However, a change in the multiplet splitting is observed when switching from the monoclinic to the orthorhombic structure, as a consequence of the change in coordination polyhedron of the luminescent ion from RO8 to RO7 for the alpha- and beta'-modification, respectively. The Gd2-xEux(MoO4)(3) solid solutions are the most efficient emitters in the range of 0 < x < 1.5, but their emission intensity is comparable to or even significantly lower than that of Sm2-xEux(MoO4)(3) for higher Eu3+ concentrations (1.5 <= x <= 1.75). Electron energy loss spectroscopy (EELS) measurements revealed the influence of the structure and element content on the number and positions of bands in the ultraviolet-visible-infrared regions of the EELS spectrum.

Abstract: Indium phosphide colloidal quantum dots (QDs) are emerging as an efficient cadmium-free alternative for optoelectronic applications. Recently, syntheses based on easy-to-implement aminophosphine precursors have been developed. We show by solid-state nuclear magnetic resonance spectroscopy that this new approach allows oxide-free indium phosphide core or core/shell quantum dots to be made. Importantly, the oxide-free core/shell interface does not help in achieving higher luminescence efficiencies. We demonstrate that in the case of InP/ZnS and InP/ZnSe QDs, a more pronounced oxidation concurs with a higher photoluminescence efficiency. This study suggests that a II-VI shell on a III-V core generates an interface prone to defects. The most efficient InP/ZnS or InP/ZnSe QDs are therefore made with an oxide buffer layer between the core and the shell: it passivates these interface defects but also results in a somewhat broader emission line width.

Abstract: The recent infatuation for LiFePO4 as positive electrode material in Li-ion batteries has prompted a renewed interest in olivine-type structures, with a view to enhance their conduction proper-ties. We show that the dual substitution of Li for Fe and of P for Si in the olivine LiFePO4 phase leads to a complete solid solution Li1-xFe1+xP1-xSixO4 as deduced from combined X-ray diffraction, Mossbauer, and NMR experiments. Our findings challenge the common belief that the anionic network cannot be substituted. Moreover. it is found that such a substitution promotes Li intersite mixing between the olivine M1 and M2 sites. Such mixing, together with the worsening of the conducting properties of the dually substituted samples, is believed to be responsible for the poor electrochemical performances of the member's series. Beyond x = 0.20, the samples were electrochemically inactive. While the current materials are disappointing application-wise, such a study provides clues to the rich chemistry remaining to be unveiled with olivine-type structures in particular and polyanionic compounds in general.

Abstract: Synthesis protocols for colloidal nanocrystals (NCs) with narrow size and shape distributions are of particular interest for the successful implementation of these nanocrystals into devices. Moreover, the preparation of NCs with well-defined crystal phases is of key importance. In this work, we show that Sn(IV)-thiolate complexes formed in situ strongly influence the nucleation and growth rates of colloidal Cu2-x S polyhedral NCs, thereby dictating their final size, shape, and crystal structure. This allowed us to successfully synthesize hexagonal bifrustums and hexagonal bipyramid NCs with low-chalcocite crystal structure, and hexagonal nanoplatelets with various thicknesses and aspect ratios with the djurleite crystal structure, by solely varying the concentration of Sn(IV)-additives (namely, SnBr4) in the reaction medium. Solution and solid-state 119Sn NMR measurements show that SnBr4 is converted in situ to Sn(IV)-thiolate complexes, which increase the Cu2-x S nucleation barrier without affecting the precursor conversion rates. This influences both the nucleation and growth rates in a concentration-dependent fashion and leads to a better separation between nucleation and growth. Our approach of tuning the nucleation and growth rates with in situ-generated Sn-thiolate complexes might have a more general impact due to the availability of various metal-thiolate complexes, possibly resulting in polyhedral NCs of a wide variety of metal-sulfide compositions.

Abstract: A new Tl-doped strontium ferrite Fe2(Sr2-Tl)Sr3Fe4O14.65, with an original structure, has been synthesized and structurally characterized by powder X-ray diffraction and transmission electron microscopy. The TGA and Mssbauer studies evidence a mixed valence of iron. The structure exhibits a commensurate modulation, with a F-type subcell a ≈ b ≈ 5.4 Å (≈ ap√2), c ≈ 42 Å with a modulation vector q = αa* with α = 0.4. The supercell parameters have been refined as a= 27.1101(8) Å, b= 5.5187(2) Å and c= 42.0513(9) Å, in the space group Fmmm. The electron diffraction and electron microscopy data of this novel ferrite show that it can be described as a FeTl-2234-type structure corresponding to the intergrowth of a quadruple perovskite slice [(SrFeO2.8)4], with a complex rock salt related slice [Fe2(Sr2-Tl)O3.4]∞, built up of one double iron layer [Fe2O2.4] sandwiched between two [SrO] layers. The HRTEM images show that the oxygen atoms and vacancies are randomly distributed in the perovskite layers while the HAADF STEM images evidence the absence of Tl segregation in the matrix. Fe2(Sr2-Tl)Sr3Fe4O14.65 exhibits a very large value of χ (11emu/mol) at 5 K, which remains large at 400 K; the M(H) loop presents a shape characteristic of ferrimagnetism, with a large coercive field of 0.3 T. The value of magnetization saturates at 400 K at 0.68 μB/Fe. At 10 K, the value of magnetization reaches a maximum of 2 μB/Fe. The resistivity presents a semiconducting-like behavior, with ρ 800 Ω·cm at 300 K.

Abstract: The investigation of the system SmBaFe-O in air has allowed an oxygen deficient perovskite Sm2-epsilon Ba3+epsilon Fe5O15-delta (delta = 0.75, epsilon = 0.125) to be synthesized. In contrast to the XRPD pattern which gives a cubic symmetry (a(p) = 3.934 angstrom), the combined HREM/EELS study shows that this phase is nanoscale ordered with a quintuple tetragonal cell, a(p) X a(p) X 5(ap). The nanodomains exhibit a unique stacking sequence of the A-site cationic layers along the crystallographic c-axis, namely SmBaBa/SmBa/SmBaSm, and are chemically twinned in the three crystallographic directions. The nanoscale ordering of this perovskite explains its peculiar magnetic properties on the basis of antiferromagnetic interactions with spin blockade at the boundary between the nanodomains. The variation of electrical conductivity and oxygen content of this oxide versus temperature suggest potential SOFC applications. They may be related to the particular distribution of oxygen vacancies in the lattice and to the 3d(5)(L) under bar configuration of iron.

Abstract: Solar energy plays an important role in solving serious environmental
problems and meeting the high energy demand. However, the lack of suitable
materials hinders further progress of this technology. Here, we present the largest
inorganic solar cell material search till date using density functional theory (DFT) and
machine-learning approaches. We calculated the spectroscopic limited maximum
efficiency (SLME) using the Tran−Blaha-modified Becke−Johnson potential for 5097
nonmetallic materials and identified 1997 candidates with an SLME higher than 10%,
including 934 candidates with a suitable convex-hull stability and an effective carrier
mass. Screening for two-dimensional-layered cases, we found 58 potential materials
and performed G0W0 calculations on a subset to estimate the prediction uncertainty. As the above DFT methods are still computationally expensive, we developed a high accuracy machine-learning model to prescreen efficient materials and applied it to over a million materials. Our results provide a general framework and universal strategy for the design of high-efficiency solar
cell materials. The data and tools are publicly distributed at: https://www.ctcms.nist.gov/~knc6/JVASP.html, https://www.
ctcms.nist.gov/jarvisml/, https://jarvis.nist.gov/, and https://github.com/usnistgov/jarvis.