Perfect Crystal Structure Research Articles

Localization of propagative phonons in a perfectly crystalline solid.

Perfectly crystalline solids are excellent heat conductors. Prominent counterexamples are intermetallic clathrates, guest-host systems with a high potential for thermoelectric applications due to their ultralow thermal conductivities. Our combined experimental and theoretical investigation of the lattice dynamics of a particularly simple binary representative, Ba(8)Si(46), identifies the mechanism responsible for the reduction of lattice thermal conductivity intrinsic to the perfect crystal structure. Above a critical wave vector, the purely harmonic guest-host interaction leads to a drastic transfer of spectral weight to the guest atoms, corresponding to a localization of the propagative phonons.

Energetic materials composed of coordination polymers: {[Zn(μ-atrz)3](ClO4)2·2H2O}n and {[Cu(μ-atrz)3](NO3)2·2H2O}n

An improved synthetic pathway for trans-4,4′-azo-1,2,4-triazole (atrz) was discovered. Pure atrz was obtained directly without any other separation step in an environment friendly process. Treatment of atrz with zinc perchlorate and cupric nitrate led to the isolation of {[Zn(μ-atrz)3](ClO4)2·2H2O}n and {[Cu(μ-atrz)3](NO3)2·2H2O}n, which were well characterized. Their structures were determined by X-ray crystallographic analysis. The calculation results of the formation of {[Zn(μ-atrz)3](ClO4)2 2H2O}n and {[Cu(μ-atrz)3](NO3)2 2H2O}n indicates that the perfect crystal structures in spite of much resistance. A fundamental understanding of the structure and thermal properties involves factors, such as conjugated system, crystal structure, and inorganic metallic compounds that affect their thermal behavior. The high energy of the coordination compounds consists of chemical, electronic, and potential energy. The potential for the improvement of the coordination polymer opens up a new world for research of energetic materials.

太阳光响应型Ag 2 S/Ag 3 PO 4 复合材料的制备及催化降解水杨酸

以AgNO 3 ，Na 2 HPO 4 和硫粉为原料，采用共沉淀-水热法合成了具有太阳光响应型Ag 2 S/Ag 3 PO 4 复合材料，运用扫描电子显微镜（SEM）、X射线粉末衍射（XRD）、X射线光电子能谱（XPS）和紫外-可见漫反射（UV-VisDR）光谱等方法对样品进行了表征，并在模拟太阳光条件下，考察了Ag 2 S/Ag 3 PO 4 对水杨酸的光催化降解效率.结果表明，与Ag 3 PO 4 相比，Ag 2 S的负载量为1%（质量分数）时Ag 2 S/Ag 3 PO 4 粒径变小，呈立方晶相结构；Ag 2 S/Ag 3 PO 4 复合材料可以有效促进光生电子-空穴分离，使Ag 3 PO 4 禁带宽度降低到2.24eV，并增强了可见光的吸收能力.在Ag 2 S负载量为1%，120℃水热4h条件下，Ag 2 S/Ag 3 PO 4 复合材料具有最佳光催化活性，经模拟太阳光照射60min对10mg/L的水杨酸去除率达到88.2%.

Controlled Synthesis of Phase-Pure InAs Nanowires on Si(111) by Diminishing the Diameter to 10 nm

Here we report the growth of phase-pure InAs nanowires on Si (111) substrates by molecular-beam epitaxy using Ag catalysts. A conventional one-step catalyst annealing process is found to give rise to InAs nanowires with diameters ranging from 4.5 to 81 nm due to the varying sizes of the Ag droplets, which reveal strong diameter dependence of the crystal structure. In contrast, a novel two-step catalyst annealing procedure yields vertical growth of highly uniform InAs nanowires ∼10 nm in diameter. Significantly, these ultrathin nanowires exhibit a perfect wurtzite crystal structure, free of stacking faults and twin defects. Using these high-quality ultrathin InAs nanowires as the channel material of metal-oxide-semiconductor field-effect transistor, we have obtained a high ION/IOFF ratio of ∼10(6), which shows great potential for application in future nanodevices with low power dissipation.

Piezoelectric La3Ga5.3Ta0.5Al0.2O14 crystal: growth, crystal structure perfection, piezoelectric, and acoustic properties

A five-component crystal of lanthanum–gallium silicate group La3Ga5.3Ta0.5Al0.2O14 (LGTA) was grown by the Czochralski method. The LGTA crystal possesses unique thermal properties and substitution of Al for Ga in the unit cell leads to a substantial increase of electrical resistance at high temperatures. The unit cell parameters of LGTA were determined by powder diffraction. X-ray topography was used to study the crystal structure perfection: the growth banding normal to the growth axis were visualized. The independent piezoelectric constants d 11 and d 14 were measured by X-ray diffraction in the Bragg and Laue geometries. Excitation and propagation of surface acoustic waves were studied by the double-crystal X-ray diffraction at the BESSY II synchrotron radiation source. The analysis of the diffraction spectra of acoustically modulated crystals permitted the determination of the velocity of acoustic wave propagation and the power flow angles in different acoustic cuts of the LGTA crystal.

Measuring three-dimensional positions of atoms to the highest accuracy with electrons

Abstract Recent developments in transmission electron microscopy (TEM) have pushed lateral spatial resolution to well below 1 A. For selected perfect crystal structures, this allows atomic columns to be identified along several crystallographic orientations. Measuring the three-dimensional position of every atom within a TEM specimen, called by some the holy grail of electron microscopy, seems therefore within reach. In this paper, we will discuss recent approaches to this problem and present our own dose-efficient approach that is based on the direct inversion of multiple electron scattering within the sample and that can be applied to various coherent detection schemes, such as high-resolution TEM, confocal scanning TEM, or ptychography. One particular advantage of this approach is that data for only a very limited range of specimen tilt angles is required, and that it can handle the highly dynamical scattering associated with lower electron beam energy.

Synthesis, Characterization and Electrochemistry of Cathode Material Li [Li<sub>0.2</sub>Mn<sub>0.54</sub>Ni<sub>0.13</sub>Co<sub>0.13</sub>]O<sub>2</sub> Nanofibers Prepared by Electrospinning for Lithium Ion Batteries

Nanofibers of Li [Li0.2Mn0.54Ni0.13Co0.13]O2with high crystallinity were easily synthesized by electrospinning. Scanning electron microscopy (SEM) showed that precursor wires were glossy, and the diameters of oxide calcined at 800°C were 50-200 nm. And X-ray diffraction (XRD) exhibited perfect crystal structure. The electrochemical properties of materials with the specific morphology were significantly improved. The first charge/discharge capacity was around 309.3 and 270.2 mAh·g-1at 25 mA·g-1, and the first coulomb efficiency (87.4%) represented a good efficiency for Li2MnO3-type electrodes. After 20 cycles, discharge capacity was over 200 mAh·g-1at 25 mA·g-1, and capacity retention was 79.8% at 250 mA·g-1.Owing to its simplicity and applicability, the electrospinning method is an promising method to fabricate high performance Li [Li0.2Mn0.54Ni0.13Co0.13]O2materials for lithium ion batteries.

Influence of Bragg diffraction on perfect crystal neutron phase shifters and the exact solution of the two-beam case in the dynamical diffraction theory

The phase shift of neutrons passing through a sample is usually determined by the sample's index of refraction based on the coherent neutron scattering length. If the sample has a perfect crystal structure there are, however, additional phase effects due to Bragg diffraction. While Bragg peaks in the diffracted direction are very sharp on the angular scale, the phase of the transmitted beam is influenced on a much wider angular range in the order of degrees around the Bragg condition. The magnitude of this effect is in the order of 10−4 of the refractive phase and clearly visible in interferometry measurements on a perfect silicon sample. In order to calculate the effect, the exact solution is derived for the two-beam case of the dynamical diffraction theory for arbitrary Bragg-plane orientation and arbitrary deviations from the Bragg condition. Even far off any Bragg condition, a residual phase correction remains which is identified as a local field correction.

Unusual and Highly Tunable Missing-Linker Defects in Zirconium Metal–Organic Framework UiO-66 and Their Important Effects on Gas Adsorption

UiO-66 is a highly important prototypical zirconium metal-organic framework (MOF) compound because of its excellent stabilities not typically found in common porous MOFs. In its perfect crystal structure, each Zr metal center is fully coordinated by 12 organic linkers to form a highly connected framework. Using high-resolution neutron power diffraction technique, we found the first direct structural evidence showing that real UiO-66 material contains significant amount of missing-linker defects, an unusual phenomenon for MOFs. The concentration of the missing-linker defects is surprisingly high, ∼10% in our sample, effectively reducing the framework connection from 12 to ∼11. We show that by varying the concentration of the acetic acid modulator and the synthesis time, the linker vacancies can be tuned systematically, leading to dramatically enhanced porosity. We obtained samples with pore volumes ranging from 0.44 to 1.0 cm(3)/g and Brunauer-Emmett-Teller surface areas ranging from 1000 to 1600 m(2)/g, the largest values of which are ∼150% and ∼60% higher than the theoretical values of defect-free UiO-66 crystal, respectively. The linker vacancies also have profound effects on the gas adsorption behaviors of UiO-66, in particular CO2. Finally, comparing the gas adsorption of hydroxylated and dehydroxylated UiO-66, we found that the former performs systematically better than the latter (particularly for CO2) suggesting the beneficial effect of the -OH groups. This finding is of great importance because hydroxylated UiO-66 is the practically more relevant, non-air-sensitive form of this MOF. The preferred gas adsorption on the metal center was confirmed by neutron diffraction measurements, and the gas binding strength enhancement by the -OH group was further supported by our first-principles calculations.

Molecular dynamics simulation on microstructure evolution during solidification of copper nanoparticles

The effect of cooling rate on the microstructure evolution of liquid Cu nanoparticles during their solidification process is investigated by using a molecular dynamics simulation based on the embedded atom method (EAM) potential developed by Foiles et al.. The potential energy analysis, the pair distribution function and the common neighbor analysis have been used. The results show that the solidification point increases with decreasing cooling rate and that the solidification of the microstructure of Cu nanoparticles varies with the cooling rate. The microstructure consists of fcc, hcp and bcc crystals or mixtures, though the fcc structure dominates, except in the amorphous state. An amorphous structure was obtained when the cooling rate reached 1.0 × 1013 K/s or higher while crystallization degree increased with decreasing cooling rate, and the total content of crystal structures reached to 95% when the cooling rate dropped to 4.0 × 1011 K/s, which was nearly a perfect crystal structure. The results also indicate that a single-crystal nanoparticle will not be obtained by quenching the liquid metal under various cooling rates.

Preparation and Characterization of Rare Earth Composite Catalytic Materials for Conversion of Exhaust from Natural Gas Automobiles

A series of rare-earth composite catalytic material LZ1,LZ2 and LZ3 that contains mixture of La0.9Sr0.1Co0.5Mn0.5O3and Ce0.7Zr0.3O2in different proportion (3:7, 5:5, 7:3) were prepared by a sol-gel method and characterized by X-ray diffraction (XRD), specific surface analysis (BET) , Transmission electron microscopy (TEM) and temperature programmed reduction (TPR). The results indicated that in LZ3, a perfect crystal structure of perovskite and a cubic fluorite structure were formed. Besides LZ3 presented scattered spherical granules with a particle size of about 30 nm and a specific surface area of 20.9870 m2/g. The temperature of reduction peak was lowest (490°C) and peak area was larger, so it had the best catalytic activity for methane combustion.

A molecular dynamics study of helium bubble formation and gas release near titanium surfaces

Helium bubble formation and gas release near titanium surfaces have been studied using molecular dynamics. The results show that the formation of helium bubbles near a titanium surface is accompanied by the production of defects and the drift of defects toward the metal surfaces as well as an increase in the number of surface metal atoms. It is also shown that the helium bubbles burst at the mechanical stability limit of the metal. The metal surface morphology is modified by the bursting of the helium bubbles, with an increase in the surface roughness and the formation of surface pores as well as the re-deposition of the helium atoms. In contrast, the subsurface metal will retain its perfect crystal structure. The size of the resultant surface pore depends on the initial bubble diameter. For a small bubble, the pore can be healed by the thermal evolution of the substrate.

How stacking disorder can conceal the actual structure of micas: the case of phengites

The present study deals with how stochastic stackings of tetrahedral/octahedral phengitic sheets bearing diverse cation distributions affect diffraction signals and the structural inferences therefrom derived. The interest for such minerals is dictated by that the stability of phengite polytypes, their cation distributions and P/T conditions of crystallization are related to each other. We focus our attention on layers’ probabilistic sequences that preserve the topology of the polytypes 2M1(SG: C2/c) and 3T(SG: P3112). Neutron diffraction intensities are modelled by a Monte Carlo approach and then used as artificial experimental data for conventional structure refinements that yield the occupancy factors in the fourfold (Si, Al) and sixfold (Al, Mg) coordination sites of 2M1 and 3T. The cation ordering from structure refinement tallies with the one of the “average structure” of a stochastic stacking, but it can significantly differ from those of the individual tetrahedral/octahedral sheets. For instance, sheets having ordered cation arrangements can lead to a stochastic structure which is supposed to bear a fully disordered cation partitioning according to structure refinement. This affects the configuration entropy contributions: the values obtained by conventional refinements can deviate from the correct ones up to 30 %. The analysis of the equivalent reflection intensities brings to light the anomalies hinting at the occurrence of such stacking disorder (using modelled reflections, the mean ratio between standard deviation and average intensity of symmetry equivalent reflections is ideally 0 for perfect crystal structures, but it can amount up to 6 in stochastically disordered phengites). However, taking into account the instrumental uncertainties and the deviations from ideality of actual crystals, such phenomena are very difficult to be detected experimentally.

Trends in formation of the nanocrystalline structure and cationic ordering in the Dy2O3-HfO2 (1: 1) system

Evolution of the nanocrystalline structure of the complex oxide Dy2HfO5 in the course of thermal annealing at temperatures to 1600°C has been studied by a combination of X-ray and synchrotron methods, including traditional and anomalous X-ray diffraction, PDF, EXAFS, and SAXS. The changes in crystallite size upon annealing of the as-synthesized amorphous precursor have been analyzed in detail. The systematic distortions of a fluorite-type perfect crystal structure (space group \(Fm\bar 3m\)) related to the nonequivalence of the local environment of the Dy and Hf cations but not resulting in formation of a pyrochlore-type cationordered structure in this system have been examined.

The role of reducing agent in perylene tetracarboxylic acid coating on graphene sheets enhances Pd nanoparticles-electrocalytic ethanol oxidation

New catalysts, consisting of perylene tetracarboxylic acid functionalized graphene sheets support-enhanced electrocatalytic Pd nanoparticles (Pd/PTCA–GS), were fabricated using different reducing agents, including H2, NaBH4 and ethylene glycol (EG). The graphene sheets (GS) were functionalized via π–π stacking and hydrophobic forces. The information of the morphologies, sizes, and dispersion of Pd nanoparticles (NPs) for the as-prepared catalysts was verified by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), Raman spectra and X-ray diffraction (XRD). As the ethanol electro-oxidation anode catalysts, the new catalysts exhibited better kinetics, higher electrocatalytic activity, better tolerance and better electrochemical stability than the Pd/GS and Pd/C, which illustrated that the new catalysts had potential applications in direct ethanol alkaline fuel cells (DEAFCs). Most attractively, the role of the chemical reduction methods (the NaBH4, EG and H2 as reducing agents) were studied systematically for the ethanol electro-oxidation anode catalysts in DEAFCs. As expected, the chemical reduction method remarkably affected the electrochemical behavior. Among all the Pd/PTCA–GS catalysts tested, Pd/PTCA–GS(NaBH4) exhibited the highest catalytic activity and stability, which may be due to the Pd NPs for Pd/PTCA–GS(NaBH4) having a narrow size distribution, uniform distribution and more perfect crystal structure than that of other as-prepared nanocomposites. These Pd/PTCA–GS are promising catalysts for developing a highly efficient direct ethanol alkaline fuel cells system for power applications.

铝掺杂锆酸钕纳米晶的制备以及对甲基橙的催化降解

以硝酸锆、硝酸铝和硝酸钕作为原材料，甘氨酸水溶液和无机盐作为溶剂和分散剂，通过盐助甘氨酸溶液燃烧法(salt-assistant gly combustion method, SGCM)制备了具有立方晶系的烧绿石型铝掺杂锆酸钕纳米晶。产物经X-射线粉末衍射、透射电镜、高分辨电子显微镜、傅里叶变换红外光谱以及拉曼光谱表征。结果表明：钕离子被铝离子部分替代而保持原有烧绿石结构，制得的纳米粒子具有较完美的晶体结构，分散性较好，热处理温度在700℃时单一相的Nd2−xAlxZr2O7纳米晶已完全形成；所制得的纳米晶由平均粒径为10~30 nm的球状粒子组成，根据Scherrer公式计算出的平均粒径为14.5 nm；从透射电镜照片的(222)面的晶面距为0.305 nm，与理论计算值一致。以光催化降解甲基橙为探针，考察复合产物的催化活性。检测到相对于Nd2Zr2O7而言，Al3+的掺杂可以明显的提高它的催化性能，当nAl:nNd = 1:19时其催化活性最强。 Cubic pyrochlore type aluminium ion doped neodymium zirconate nanocrystals were prepared by salt- assistant gly combustion method (SGCM) with zirconium nitrate, aluminum nitrate and neodymium nitrate as raw materials, glycine as the incendiary agent and inorganic salt as solvent and dispersant. The products were characterized by XRD, TEM, HRTEM, High-resolution electron microscopy, FT-IR and Raman spectrum. The results showed that neodymium ions were partially substituted by aluminium ions, while maintaining the original pyrochlore structure. The obtained nano particles had a perfect crystal structure and good dispersion. Single phase Nd2−xAlxZr2O7 nanocrystals al-ready fully formed with the heat treatment temperature in 700°C. The obtained nanocrystals were consisted of spheroidal particles with the average grain size of 10 - 30 nm. According to the Scherrer formula, the calculated mean grain size is 14.5 nm. The crystal plane distance from the (222) plane in the photo of transmission electron microscopy is 0.305 nm, which is consistent with the theoretical calculated value. The catalytic activity of the composite product is determined by the reaction speed of photocatalytic degradation of methyl orange. It is detected that compared to Nd2Zr2O7, the doping of Al3+ can obviously improve its catalytic performance. When nAl:nNd = 1:19, it has the strongest catalytic activity.

Synthesis of single-crystal dendritic iron hydroxyl phosphate as a Fenton catalyst

In this work, the single-crystal dendritic iron hydroxyl phosphate is synthesized via a facile and well-controlled hydrothermal process without any additional surfactant or template. The morphology of the product at various scales is characterized by scanning electron microscopy and transmission electron microscopy. The phase and crystal structure is determined and carefully confirmed by using X-ray powder diffraction, high resolution transmission electron microscopy and selected area electron diffraction combined with thermal analysis comprehensively. A reasonable self-template route, as well as the precipitation–dissolution–recrystallization process is revealed according to the time dependence of morphology, crystal structure, zeta potential and precipitate amount. Finally, the single-crystal dendrite is tested as a Fenton catalyst for the degradation of phenol, a typical advanced oxidation process (AOP). Attributed to the perfect crystal structure and the hierarchical architecture, this novel catalyst presents numerous advantages including remarkable catalytic activity over a wide pH range (e.g., pH = 3–8), outstanding cycle performance (no observation of activity loss in five runs), and extremely low leaching iron concentration (below 0.5 mg L−1).

Living lamellar crystal initiating polymerization and brittleness mechanism investigations based on crystallization during the ring-opening of cyclic butylene terephthalate oligomers

The brittleness of polymerized cyclic butylene terephthalate (pCBT) is generally ascribed to its high crystallization degree and perfect crystal structure. Herein, a simple and effective method for improving the brittleness of pCBT is presented. Unlike other published toughening methods, this method did not decrease or destroy any of the natural advantages of cyclic butylene terephthalate oligomers (CBT) and its ring-opening polymerization (ROP). The melting behaviour and crystallization of CBT demonstrated that some CBT crystals were unable to melt completely during ROP. These un-melted crystals served as self-compatible nucleators and reduced the crystallization induction time of pCBT during ROP, they also plant many stress concentration points which result in the brittleness of pCBT, according to mechanical tests and crystallization kinetics. Finally, the “living lamellar crystal” initiating ROP and new brittleness mechanisms were proposed.

Study of Degradation of Photocatalytic Methyl Orange on Nano Nd<sub>x</sub>Co<sub>2-x</sub>Zr<sub>2</sub>O<sub>7</sub>

Cubic pyrochlore type NdxCo2-xZr2O7 nano-crystals were prepared by salt-assistant glycine solution combustion method (SGCM), with neodymium nitrate, cobalt nitrate and zirconium nitrate as raw materials, glycine as the incendiary agent, and KCl as a reaction inert salt. The NdxCo2-xZr2O7 nano-crystals were characterized by means of XRD (X-ray powder diffraction), FT-IR (Fourier trans-form-infrared spectroscopy), Raman spectroscopy, TEM (Transmission electron microscope) and HRTEM (High resolution transmission electron microscopy). The results showed that neodymium ions were partially substituted by cobalt ions, while maintaining the original pyrochlore structure. The nano particles obtained had a perfect crystal structure, good dispersion, and the size was about 31nm. For Nd1.9Co0.1Zr2O7 nanocrystals, the four strong diffraction peaks were at 2θ=29.18°, 33.80°, 48.49° and 57.53°. The corresponding crystal plane distances calculated by Bragg equation λ = 2dSinθ were 0.306, 0.265, 0.188 and 0.160 nm. With the study of the photocatalytic activity of the product, it is found that compared with the undoped crystals, photocatalytic activity of dopped products had been significantly improved and different doping amount led to different levels of catalytic activity. When nNd:nCo = 9:1, the 2 hours net degradation amount was as high as 83%.

Study of Preparation of Nano Nd<sub>x</sub>Co<sub>2-x</sub>Zr<sub>2</sub>O<sub>7</sub> and its Catalytic Properties on Ammonium Perchlorate Thermal Decomposition

Cubic pyrochlore type NdxCo2-xZr2O7nano-crystals were prepared by salt-assistant glycine solution combustion method (SGCM), with neodymium nitrate, cobalt nitrate and zirconium nitrate as raw materials, glycine as the incendiary agent, and KCl as a reaction inert salt. The NdxCo2-xZr2O7nano-crystals were characterized by means of XRD (X-ray powder diffraction), FT-IR (Fourier trans- form-infrared spectroscopy), Raman spectroscopy, TEM (Transmission electron microscope) and HRTEM (High resolution transmission electron microscopy). The results showed that neodymium ions were partially substituted by cobalt ions, while maintaining the original pyrochlore structure. The nano particles obtained had a perfect crystal structure, good dispersion, and the size was about 31nm. For Nd1.9Co0.1Zr2O7nanocrystals, the four strong diffraction peaks were at 2θ=29.18°, 33.80°, 48.49° and 57.53°. The corresponding crystal plane distances calculated by Bragg equation λ=2dSinθ were 0.306, 0.265, 0.188 and 0.160 nm. Study the catalyst effect of NdxCo2-xZr2O7on the thermal decomposition of ammonium perchlorate (AP) using DSC (Differential scanning calorimetry). The results showed that nano NdxCo2-xZr2O7had high catalytic activity during on the thermal decomposition of ammonium perchlorate. With 2% more nano NdxCo2-xZr2O7, the peak temperature of AP thermal decomposition reaction dropped by nearly 88°C. The apparent decomposition reaction heat increased from 655J•g-1to 1073J•g-1. The results showed that the catalytic effect of thermal decomposition of AP with nano cobalt-doped zirconium acid neodymium is better than the single component of nano-metal oxides and undoped zirconate neodymium nanocrystals.