Joint experimental and theoretical analysis of order-disorder effects in cubic BaZrO3 assembled nanoparticles under decaoctahedral shape

Periodic first-principles calculations based on density functional theory at the B3LYP level has been carried out to investigate the photoluminescence (PL) emission of BaZrO(3) assembled nanoparticles at room temperature. The defect created in the nanocrystals and their resultant electronic features lead to a diversification of electronic recombination within the BaZrO(3) band gap. Its optical phenomena are discussed in the light of photoluminescence emission at the green-yellow region around 570 nm. The theoretical model for displaced atoms and/or angular changes leads to the breaking of the local symmetry, which is based on the refined structure provided by Rietveld methodology. For each situation a band structure, charge mapping, and density of states were built and analyzed. X-ray diffraction (XRD) patterns, UV-vis measurements, and field emission scanning electron microscopy (FE-SEM) images are essential for a full evaluation of the crystal structure and morphology.     

Pressure-Induced Structural Deformations in SeO2

The high-pressure behavior of low-dimensional selenium dioxide SeO₂ (P4₂/mbc, Z=8) is studied with Raman scattering and synchrotron angle-dispersive X-ray powder diffraction in a diamond anvil cell up to 23 GPa at room temperature. Pressure-induced transformations in this material involve a sequence of structural distortions of the chain structure. The transformation occuring above 7.0 GPa is due to symmetry lowering to space group Pbam (Z=8) without major changes of the crystal lattice dimensions and coordination around the Se atoms. Like in the ambient pressure polymorph, the structural unit is a SeO₃E polyhedron, where E is a Se non-bonded electron lone pair, or an irregular tetrahedron with the O atoms and Se lone pair at the vertices. Further structural transitions above 17 GPa are likely to be the result of additional distortions leading to monoclinic symmetry of the crystal structure. All transformations are reversible with little hysteresis.     

Room temperature molecular and lattice structures of a homologous series of anhydrous zinc(II) n-alkanoate

The room temperature structures and lattice arrangements of a homologous series of zinc(II) n-alkanoates from chain length, n(C) = 4-20, inclusive, have been studied using infrared spectroscopy, X-ray diffraction and polarizing light microscopy. Lattice parameters from single crystal and powder diffraction data, for zinc(II) hexanoate, are compared to validate the use of the powder method. Since they are in excellent agreement, the powder data are analyzed by a software programme to determine lattice parameters for all the homologues. These are used, in conjunction with infrared, X-ray, density and molecular model calculations to determine molecular and lattice structures. The compounds are isostructural, in that, each zinc atom is tetrahedrally coordinated to oxygen atoms from four different carboxylate groups and each ligand forms a Z,E-type bidentate bridge with two tetrahedral zinc atoms resulting in a syn-anti arrangement. The hydrocarbon chains are in the fully extended all-trans configuration and are tilted at an average angle of 60 degrees to the zinc basal plane. For the short chain length compounds with n(C) < or =8, a double bilayer in-plane-perpendicular-perpendicular-in-plane arrangement of hydrocarbon chains, with two molecules per unit cell, is indicated. For the others, an interdigitating in-plane-in-plane bilayer with head-to-tail interactions, with one molecule per unit cell, is proposed. A geometric model is presented to account for odd-even chain effects and to explain the differences in melting points and densities between these adducts. All the compounds crystallize in the monoclinic space group with P symmetry and are arranged in a two-dimensional network along the ac plane within the unit cell.     

Melting and Sintering of a Body-Centered Cubic Superlattice of PbSe Nanocrystals Followed by Small Angle X-ray Scattering

The structural evolution of a body-centered cubic (bcc) superlattice of 6.6 nm diameter organic ligand-coated PbSe nanocrystals was studied in situ by small angle X-ray scattering (SAXS) as it was heated in air from room temperature to 350°C. As it was heated above room temperature, the superlattice contracted slightly, but maintained bcc structure up to 110°C. Once the temperature rose above 110°C, the superlattice began to disorder, by first losing long-range translational order and then local positional order. At temperatures exceeding 168°C, the nanocrystals sintered and oxidized, transforming into PbSeO(3) nanorods.     

Rapid bioenabled formation of ferroelectric BaTiO3 at room temperature from an aqueous salt solution at near neutral pH

A 12-mer peptide, identified through phage display biopanning, has been used for the first time to induce the rapid formation of ferroelectric (tetragonal) nanocrystalline BaTiO3 at room temperature from an aqueous salt precursor solution at near neutral pH. BaTiO3 is widely used in capacitors, thermistors, displays, and sensors owing to its attractive dielectric, ferroelectric, pyroelectric, optical, and electrochemical properties. Two 12-mer peptides (BT1 and BT2) were selected from a phage-displayed peptide library via binding to tetragonal BaTiO3 powder. While these peptides possessed various types of amino acids, 8 of the 12 amino acids were common to both peptides. Each of these peptides induced the formation of faceted nanoparticles (50-100 nm diameter) from an aqueous precursor solution. X-ray diffraction and selected area electron diffraction patterns obtained from these faceted nanoparticles were consistent with the BaTiO3 compound. Rietveld analyses of the X-ray diffraction patterns yielded good fits to tetragonal crystal structures, with the BaTiO3 formed in the presence of the BT2 peptide exhibiting the most tetragonal character. A coating of the latter BaTiO3 nanoparticles exhibited polarization hysteresis (a well-known characteristic of ferroelectric materials) at room temperature and a relative permittivity of 2200. Such rapid, peptide-induced precipitation at room temperature provides new opportunities for direct BaTiO3 formation on low-melting or reactive materials (e.g., plastics, cloths, bio-organics) and the low temperature integration of BaTiO3 into electronic devices (e.g., on silicon or flexible polymer substrates).     

Grain Growth Kinetics of BaTiO3 Nanocrystals During Calcining Process

BaTiO3 nanocrystals were synthesized by sol-gel method using barium acetate (Ba(CH3COO)2) and tetrabutyl titanate (Ti(OC4H9)4) as raw materials. Xerogel precursors and products were characterized by means of thermogravimetric/differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD) and transmission electron microscope (TEM). The influence of the calcination temperature and duration on the lattice constant, the lattice distortion, and the grain size of BaTiO3 nanocrystals was discussed based on the XRD results. The grain growth kinetics of BaTiO3 nanocrystals during the calcination process were simulated with a conventional grain growth model which only takes into account diffusion, and an isothermal model proposed by Qu and Song, which takes into account both diffusion and surface reactions. Using these models, the pre-exponential factor and the activation energy of the rate constant were estimated. The simulation results indicate that the isothermal model is superior to the conventional one in describing the grain growth process, implying that both diffusion and surface reactions play important roles in the grain growth process.     

Flexibility of macromolecules and structure formation in amorphous polymers

The reflection indicating the presence of nanostructures with a coherence length of ~10 nm is found for the first time on the wide-angle X-ray diffraction patterns of amorphous polymers. It is shown that the structural organization in the amorphous polymers is formed much more rapidly and effectively if macromolecules contain regions with increased local flexibility or short spacings with another mechanism of hindered internal rotation around single bonds than that in the chain. Average intermolecular distances in the polymers with nanostructures are substantially smaller than those in the structureless ones. The temperature range of existence of the structural organization in the amorphous polymers above their glass-transition temperature becomes narrower with an increase in the kinetic rigidity of macromolecules.     

Die Kristallstruktur einer triklinen Modifikation von Uranpentachlorid

The Crystal Structure of a Triclinic Modification of Uranium Pentachloride From solution uranium pentachloride crystallizes at room temperature in a triclinic modification belonging to the space group P1 . The unit cell contains one formula unit (UCl₅)₂ and has the dimensions a = 707, b = 965, c = 635 pm and α = 0.495 π, β = 0.652 π, γ = 0.603 π rad. The crystal structure was solved with the aid of X-ray diffraction data and was refined to a reliability index of R = 0.082. The structure consists of (UCl₅)₂ molecules having the point symmetry mmm in which the uranium atoms are linked with one another via two chlorine atoms. The crystal lattice can be derived from a hexagonal closest packing of chlorine atoms in which 1/5 of all octahedral holes are occupied by uranium atoms.     

SiO_2基底中MnFe_2O_4纳米晶的制备与表征

在N,N-二甲基甲酰胺(DMF)存在下,以溶胶-凝胶技术成功地制备了无定形二氧化硅基底中均匀分布的MnFe_2O_4纳米晶.由粉末X射线衍射和电子衍射确证了MnFe_2O_4纯相的生成. 由粉末X射线衍射和红外吸收光谱研究了MnFe_2O_4纳米晶形成过程.尖晶石结构的MnFe_2O_4在800℃时开始形成,900℃时基本完成.磁性质测量表明在烧结到900℃的样品中,MnFe_2O_4纳米晶室温具有超顺磁性,78K时为软磁性.1000℃ 和1100℃下得到的样品室温和78K时都具有软磁性.     

Interplay between size, composition, and phase transition of nanocrystalline Cr(3+)-doped BaTiO3 as a path to multiferroism in perovskite-type oxides

Multiferroics, materials that exhibit coupling between spontaneous magnetic and electric dipole ordering, have significant potential for high-density memory storage and the design of complex multistate memory elements. In this work, we have demonstrated the solvent-controlled synthesis of Cr(3+)-doped BaTiO(3) nanocrystals and investigated the effects of size and doping concentration on their structure and phase transformation using X-ray diffraction and Raman spectroscopy. The magnetic properties of these nanocrystals were studied by magnetic susceptibility, magnetic circular dichroism (MCD), and X-ray magnetic circular dichroism (XMCD) measurements. We observed that a decrease in nanocrystal size and an increase in doping concentration favor the stabilization of the paraelectric cubic phase, although the ferroelectric tetragonal phase is partly retained even in ca. 7 nm nanocrystals having the doping concentration of ca. 5%. The chromium(III) doping was determined to be a dominant factor for destabilization of the tetragonal phase. A combination of magnetic and magneto-optical measurements revealed that nanocrystalline films prepared from as-synthesized paramagnetic Cr(3+)-doped BaTiO(3) nanocrystals exhibit robust ferromagnetic ordering (up to ca. 2 μ(B)/Cr(3+)), similarly to magnetically doped transparent conducting oxides. The observed ferromagnetism increases with decreasing constituent nanocrystal size because of an enhancement in the interfacial defect concentration with increasing surface-to-volume ratio. Element-specific XMCD spectra measured by scanning transmission X-ray microscopy (STXM) confirmed with high spatial resolution that magnetic ordering arises from Cr(3+) dopant exchange interactions. The results of this work suggest an approach to the design and preparation of multiferroic perovskite materials that retain the ferroelectric phase and exhibit long-range magnetic ordering by using doped colloidal nanocrystals with optimized composition and size as functional building blocks.     

The effect of the cation substitution on the structural and vibrational properties of Cs2NaGaxSc(1-x)F6 solid solution

Raman scattering and x-ray diffration were used to characterize the structural and vibrational properties of the Cs2NaGaxSc(1-x)F6 solid solutions, for x ranging from 0.0 to 1.0. The Raman spectra, taken at room and low temperature, allow us to follow the phase evolution in detail and indicate the breaking of the local symmetry since low Ga concentration levels. Five compositions were studied by x-ray diffraction: x = 0.0, 0.2, 0.5, 0.8, and 1.0. A cubic space group, Fm3m, was found to x = 0.0 and x = 0.2 and a trigonal one was found to x = 0.5, 0.8, and 1.0. Details of both phases are presented and the correlation between x-ray diffraction and Raman scattering is discussed.     

Wet-chemical synthesis of crystalline BaTiO3 from stable chelated titanium complex: formation mechanism and dispersibility in organic solvents

Crystalline barium titanate nanoparticles were synthesized in solution at low temperature (70 degrees C) from acetylacetone chelated titanium complex and barium hydroxide. Very fine crystalline solids were characterized to cubic phase of BaTiO(3) by X-ray diffraction studies of the air-dried samples. It was observed that the crystalline barium titanate was formed in solution at Ba/Ti molar ratio > or =2.5. The dependence of the reaction temperature and the Ba(OH)(2) concentration (in terms of Ba/Ti molar ratio) on formation of crystalline BaTiO(3) in solution-phase was studied, and a plausible mechanism toward the formation of crystalline BaTiO(3) was also proposed. Crystallite sizes of the BaTiO(3) were found to be in the range 33-50 nm, while the average particle sizes, measured by dynamic light scattering method were in the range 70-100 nm. The crystalline BaTiO(3) prepared from acetylacetone chelated titanium complex was highly dispersible in organic medium such as N-methyl-2-pyrillidone (NMP) and N,N-dimethyl formamide (DMF).     

Crystal and electronic structure study of AgPd3Se

The AgPd₃Se compound was synthesised from individual elements by solid-state chemical reactions and structurally characterized by powder X-ray diffraction data. AgPd₃Se displays cubic symmetry, space group Pa3?, unit cell parameter a=8.6289(1) Å and Z=8. Double-Friauf polyhedra (DFP), defined by Pd and Se atoms, form the basic structural building block of the AgPd₃Se crystal structure. The Ag atoms occupy the centres of DFPs forming an Ag–Ag dimer (2.792(2) Å). The packing of DFPs forms two kinds of interpenetrating networks that show similar features as three-dimensional Penrose tiles. AgPd₃Se is isostructural with CaAu₃Ga. The electric resistivity as well as the electronic structure calculation suggests metallic behaviour.     

Low temperature structural phase transition of Ba3NaIr2O9

Single crystal X-ray and synchrotron X-ray powder diffraction have been used to probe the structure of Ba₃NaIr₂O₉ from 300 K down to 20 K. Ba₃NaIr₂O₉ is found to undergo a structural transition from hexagonal symmetry, P6₃/mmc, at ambient temperature to monoclinic symmetry, C2/c, at low temperature. The evolution of the unit cell volume upon cooling is indicative of a higher order structural transition, and the symmetry breaking becomes apparent as the temperature is decreased. The low temperature monoclinic structure of Ba₃NaIr₂O₉ contains strongly distorted [NaO₆] and [IrO₆] octahedra in comparison to the room temperature hexagonal structure.     

Influence of structural disorder on the photoluminescence emission of PZT powders

Intense and broad visible photoluminescent (PL) band was observed at room temperature in disordered Pb(Zr0.53Ti0.47)O3 powders. Structural order-disorder was evaluated by different methods. XANES results pointed to the presence of different coordination modes of disordered Ti powders, and in the ordered sample the local structure around titanium atoms is characteristic of the structurally ordered PZT with only TiO6 units. Only samples containing simultaneous structural order and disorder in their network present the intense visible PL emission at room temperature.     

Synthesis of single-crystalline perovskite nanorods composed of barium titanate and strontium titanate

We report the solution-based synthesis of single-crystalline nanorods composed of barium titanate (BaTiO3) and strontium titanate (SrTiO3), which yields well-isolated nanorods with diameters ranging from 5 to 60 nm and lengths reaching up to >10 mum. Electron microscopy and diffraction measurements show that these nanorods are composed of single-crystalline cubic perovskite BaTiO3 and SrTiO3 with a principal axis of the unit cell preferentially aligned along the wire length. These BaTiO3 and SrTiO3 nanorods should provide promising materials for fundamental investigations on nanoscale ferroelectricity, piezoelectricity, and paraelectricity.     

cis—Mo(DMF)_3Br_3异构体的晶体和分子结构

制得了cis—Mo(DMF)_3Br_3(DMF指N,N—二甲基甲酰胺)的一种异构体并用X射线三维衍射数据进行了晶体结构测定。晶体的空间群属Pca2_1。晶胞大小为a=20.157(4),b=13.950(4),c=13.142(4)。晶体的测量密度为2.02g/cm~3,与Z=8相符。结构用直接法-Fourier合成法解得,最终R因子为0.060。结构分析表明,晶体仍为cis-Mo(DMF)_3Br_3,分子中Mo周围的三个Br原子和三个DMF配位基的O原子仍按顺式排列成近八面体,但DMF的取向不规则。分子不再具有对称性。     

Single crystal X-Ray structure of BeF2: α-quartz

We report for the first time, the synthesis and X-ray diffraction studies of single crystals of BeF(2). The crystals were obtained during the sublimation of amorphous BeF(2) under static reduced pressure. BeF(2) crystallizes in the chiral trigonal space group P3(1)21. A single-crystal X-ray diffraction study on these crystals shows that each of the Be atoms is bonded to four F atoms, and each of the F atoms is bonded to two Be atoms with associated Be-F bond distances of 1.5420(13) and 1.5471(13) ?, showing an almost regular tetrahedron. The infrared spectrum of these crystals recorded at room temperature shows distinct peaks around 770 and 410 cm(-1).     

棒状LaF3∶Eu3+纳米晶的制备与发光性能

采用一种简单的液相反应法在室温下合成了棒状的LaF3∶Eu3+纳米晶, 对其结构和发光性能进行了表征. XRD分析结果表明, 室温下即可得到结晶良好的六方晶相的LaF3, 灼烧之后样品的衍射峰增强, 没有杂相产生. TEM照片表明, 棒状LaF3∶Eu3+纳米材料的直径为8 nm左右, 长度达到50 nm. 荧光光谱表明, 室温下合成的棒状LaF3∶Eu3+纳米晶的最强发射峰位于589 nm, 对应于Eu3+的5D0-7F1跃迁发射, 说明Eu3+占据LaF3基质中La3+晶格点的C2对称格位上. 同时Eu3+的猝灭摩尔分数为5%, 荧光寿命随着灼烧温度的升高而延长.