The luminescence properties of Eu2+-activated alkaline-earth silicon-oxynitrides have been studied In the BaO−SiO2−Si3N4 system, a new BaSi2O2N2 compound was obtained having the monoclinic structure with lattice parameters a = 14070(4) A, b = 7276(2) A, c = 13181(3) A, β = 10774(6)° All MSi2O2-δN2+2/3δ:Eu2+ (M = Ca, Sr, Ba) materials can be efficiently excited in the UV to visible region (370−460 nm), making them attractive as conversion phosphors for LED applications A blue-green emission at 490−500 is observed for BaSi2O2N2:Eu2+, yellow emission at 560 nm for CaSi2O2-δN2+2/3δ:Eu2+ (δ ≈ 0), and a green-yellow emission peaking from 530 to 570 nm for SrSi2O2-δN2+2/3δ:Eu2+ (δ ≈ 1), the position depending on the exact value of δ BaSi2O2N2:Eu2+ is the most promising conversion phosphor for white-light LEDs due to its high conversion efficiency for blue light from InGaN-based LEDs related to its very small Stokes shift

Luminescence properties of Eu2+ - activated alkaline-earth silicon-oxynitride MSi2O2-deltaN2+2/3delta (M = Ca, Sr, Ba) : A promising class of novel LED conversion phosphors

Li-ion batteries have contributed to the commercial success of portable electronics, and are now in a position to influence higher-volume applications such as plug-in hybrid electric vehicles. Most commercial Li-ion batteries use positive electrodes based on lithium cobalt oxides. Despite showing a lower voltage than cobalt-based systems (3.45 V versus 4 V) and a lower energy density, LiFePO(4) has emerged as a promising contender owing to the cost sensitivity of higher-volume markets. LiFePO(4) also shows intrinsically low ionic and electronic transport, necessitating nanosizing and/or carbon coating. Clearly, there is a need for inexpensive materials with higher energy densities. Although this could in principle be achieved by introducing fluorine and by replacing phosphate groups with more electron-withdrawing sulphate groups, this avenue has remained unexplored. Herein, we synthesize and show promising electrode performance for LiFeSO(4)F. This material shows a slightly higher voltage (3.6 V versus Li) than LiFePO(4) and suppresses the need for nanosizing or carbon coating while sharing the same cost advantage. This work not only provides a positive-electrode contender to rival LiFePO(4), but also suggests that broad classes of fluoro-oxyanion materials could be discovered.

A 3.6 V lithium-based fluorosulphate insertion positive electrode for lithium-ion batteries.

The crystal structure and photoluminescence properties of undoped and Ce3+-doped CaAlSiN3 as well as the application of white-light LEDs are reported. CaAlSiN3 and CaAlSiN3:Ce3+ have been synthesized, starting from Ca3N2, AlN, Si3N4, and CeN or CeO2 with and without Li3N, by a solid state reaction at 1700 °C for 4 h under high purity nitrogen atmosphere. Instead of an ideal CaAlSiN3, a more appropriate formula is proposed to be CaAl1−4δ/3Si1+δN3 (δ ≈ 0.3−0.4) with an Al/Si ratio of about 1:2 on the basis of the bond valence sum calculations, in which Al/Si is disorderly occupied on the 8b site within Cmc21 space group. Ce3+ can be incorporated into the host lattice of CaAlSiN3, and the estimated maximum solubility of Ce3+ is about x = 0.02 (e.g., 2.0 mol % with respect to Ca) of Ca1−2xCexLixAlSiN3. CaAlSiN3:Ce3+ can be efficiently excited by blue light (450−480 nm) and yields yellow-orange emission with a broadband peaking in the range of 570−603 nm, originating from the 5d1 → 4f1 transition of Ce3+. With...

Yellow-Orange-Emitting CaAlSiN3:Ce3+ Phosphor: Structure, Photoluminescence, and Application in White LEDs

Facile synthesis of nanocrystals of a microporous metal–organic framework by an ultrasonic method and selective sensing of organoamines

We report the first high-pressure single-crystal structures of hybrid perovskites. The crystalline semiconductors (MA)PbX3 (MA = CH3NH3+, X = Br– or I–) afford us the rare opportunity of understanding how compression modulates their structures and thereby their optoelectronic properties. Using atomic coordinates obtained from high-pressure single-crystal X-ray diffraction we track the perovskites’ precise structural evolution upon compression. These structural changes correlate well with pressure-dependent single-crystal photoluminescence (PL) spectra and high-pressure bandgaps derived from density functional theory. We further observe dramatic piezochromism where the solids become lighter in color and then transition to opaque black with compression. Indeed, electronic conductivity measurements of (MA)PbI3 obtained within a diamond-anvil cell show that the material’s resistivity decreases by 3 orders of magnitude between 0 and 51 GPa. The activation energy for conduction at 51 GPa is only 13.2(3) meV, su...

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High-Pressure Single-Crystal Structures of 3D Lead-Halide Hybrid Perovskites and Pressure Effects on their Electronic and Optical Properties

The efficiency of the successive dichotomy method for powder diffraction pattern indexing [Louer & Louer (1972). J. Appl. Cryst. 5, 271–275] has been proved over more than 30 years of usage. Features implemented in the new version of the computer program DICVOL04 include (i) a tolerance to the presence of impurity (or inaccurately measured) diffraction lines, (ii) a refinement of the `zero-point' position, (iii) a reviewing of all input lines from the solution found from, generally, the first 20 lines, (iv) a cell analysis, based on the concept of the reduced cell, to identify equivalent monoclinic and triclinic solutions, and (v) an optional analysis of input powder data to detect the presence of a significant `zero-point' offset. New search strategies have also been introduced, e.g. each crystal system is scanned separately, within the input volume limits, to limit the risk of missing a solution characterized by a metric lattice singularity. The default values in the input file have been extended to 25 A for the linear parameters and 2500 A3 for the cell volume. The search is carried out exhaustively within the input parameter limits and the absolute error on peak position measurements. Many tests with data from the literature and from powder data of pharmaceutical materials, collected with the capillary technique and laboratory monochromatic X-rays, have been performed with a high success rate, covering all crystal symmetries from cubic to triclinic. Some examples reported as `difficult' cases are also discussed. Additionally, a few recommendations for the correct practice of powder pattern indexing are reported.

Powder pattern indexing with the dichotomy method

Some improvements have been introduced in the current computer program for powder diffraction pattern indexing using the dichotomy algorithm. The resulting version, DICVOL14, includes optimizations and extension of scanning limits for triclinic cases, a detailed review of the input data from the indexing solutions, cell centering tests and a new approach for zero-point offset evaluation. The performance of the new version is illustrated with many examples, such as triclinic cases with long axes and dominant zones. Some important parameters in pattern indexing based on the dichotomy algorithm are commented upon, e.g. the precision of data and spurious lines.

Some further considerations in powder diffraction pattern indexing with the dichotomy method

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0885715600015621

X-Ray Powder Diffraction Analysis of Barium Titanyl Oxalate Tetrahydrate

Ab initio structure determination from powder diffraction data of compounds resulting from solid state transformations is still rather limited. Two major factors influence the stages of the structure elucidation of these solids, i.e. the lower precision in peak position for indexing and the strong line overlap for extracting integrated intensities. The present study deals with the consideration of these two problems for solving the crystal structure of two thermal-decomposition inorganic products, Nd(NO 3 ) 3 .4 H 2 O and Pb 3 O 2 (NO 3 ) 2 . The new features of DICVOL04, as zero shift refinement, a priori zero search and tolerance of spurious lines have been used for indexing several phases obtained during the thermal treatment of neodymium nitrate hexahydrate. The crystal structures of Nd(NO 3 ) 3 .4 H 2 O and Pb 3 O 2 (NO 3 ) 2 have been solved with the direct methods and the structure model of Pb 3 O 2 (NO 3 ) 2 has been completed using a global optimisation approach. The monoclinic structure of neodymium nitrate tetrahydrate [a = 10.1744(4) A, b = 8.9716(5) A, c = 11.7295(5) A, β = 97.384(4)°, V = 1061.80 A 3 , S.G. P2 1 /c] is built from isolated ten-fold coordinated Nd polyhedra. The orthorhombic structure of lead oxide nitrate [a = 7.6034(4) A, b = 5.7691(4) A, c = 18.5817(9) A, V = 815.09 A 3 , S.G. Pnma] is built from double chains of edge-sharing OPb 4 tetrahedra connected by nitrate groups. In order to estimate the chance of solving crystal structure of nanocrystalline powdered compounds, a comparison of the efficiency of direct methods and direct-space approaches is made from powder diffraction patterns simulated for various crystallite sizes from the crystal structure of the decomposition product γ-Zn 2 P 2 O 7 .

Ali Boultif

Papers

Powder pattern indexing with the dichotomy method

Some further considerations in powder diffraction pattern indexing with the dichotomy method

X-Ray Powder Diffraction Analysis of Barium Titanyl Oxalate Tetrahydrate

Structure determination of thermal decomposition products from laboratory X-ray powder diffraction