Crystallographic Axes Research Articles

Morphology and Band Structure of Orthorhombic PbS Nanoplatelets: An Indirect Band Gap Material

PbS quantum dots and nanoplatelets (NPLs) are of enormous interest in the development of optoelectronic devices. However, some important aspects of their nature remain unclear. Recent studies have revealed that colloidal PbS NPLs may depart from the rock-salt crystal structure of bulk and form an orthorhombic (Pnma) modification instead. To gain insight into the implications of such a change over the optoelectronic properties, we have synthesized orthorhombic PbS NPLs and determined the lattice parameters by means of selected area electron diffraction measurements. We have then calculated the associated band structure using density functional theory with Perdew–Burke–Ernzerhof functional for solids and with the GW approximation, including spin–orbit interactions. An indirect band gap is found, which may explain the weak luminescence reported in experiments. We derive effective masses for conduction and valence bands and deduce that quantum confinement along the a crystallographic axis (short axis of the NPL) reinforces the indirect band gap but that along b and c axes favors a direct gap instead. Calculations for colloidal nanoplatelets of 1.8 nm thickness, carried out with k·p theory, show that excitonic effects are strong, with binding energies of about 150 meV.

Synthesis, crystal structure, and electronic structure of Li2PbSiS4: a quaternary thiosilicate with a compressed chalcopyrite-like structure.

The new quaternary thiosilicate, Li2PbSiS4 (dilithium lead silicon tetrasulfide), was prepared in an evacuated fused-silica tube via high-temperature, solid-state synthesis at 800 °C, followed by slow cooling. The crystal structure was solved and refined using single-crystal X-ray diffraction data. By strict definition, the title compound crystallizes in the stannite structure type; however, this type of structure can also be described as a compressed chalcopyrite-like structure. The Li+ cation lies on a crystallographic fourfold rotoinversion axis, while the Pb2+ and Si4+ cations reside at the intersection of the fourfold rotoinversion axis with a twofold axis and a mirror plane. The Li+ and Si4+ cations in this structure are tetrahedrally coordinated, while the larger Pb2+ cation adopts a distorted eight-coordinate dodecahedral coordination. These units join together via corner- and edge-sharing to create a dense, three-dimensional structure. Powder X-ray diffraction indicates that the title compound is the major phase of the reaction product. Electronic structure calculations, performed using the full potential linearized augmented plane wave method within density functional theory (DFT), indicate that Li2PbSiS4 is a semiconductor with an indirect bandgap of 2.22 eV, which compares well with the measured optical bandgap of 2.51 eV. The noncentrosymmetric crystal structure and relatively wide bandgap designate this compound to be of interest for IR nonlinear optics.

Crystal structure, Hirshfeld surface analysis and DFT study of 1-ethyl-3-phenyl-1,2-di-hydro-quinoxalin-2-one.

In the title mol-ecule, C16H14N2O, the di-hydro-quinoxaline moiety is not planar as there is a dihedral angle of 4.51 (5)° between the constituent rings. In the crystal, C-H⋯O hydrogen bonds form helical chains about the crystallographic 21 screw axis in the b-axis direction. Hirshfeld surface analysis indicates that the most important contributions to the crystal packing are from H⋯H (51.7%), H⋯C/C⋯H (26%) and H⋯O/O⋯H (8.5%) inter-actions. The optimized structure calculated using density functional theory (DFT) at the B3LYP/6-311 G(d,p) level is compared with the experimentally determined structure in the solid state. The calculated HOMO-LUMO energy gap is 3.8918 eV.

Deformation of Nickel Titanium Single Crystal under Compressive Pulse

Nickel titanium single crystal is used to show the importance of the volume compressibility anisotropy in analyzing the elastoplastic deformation processes in metal single crystals with the cubic symmetry. The process of uniform bulk deformation corresponds to the process of nonuniform stress-strain state of metal single crystals of cubic symmetry for several orientations of the theoretical coordinate system relative to crystallographic directions of the main axes. The indicative surface of the volume compressibility (or its reciprocal variable of bulk modulus) has an aspherical shape and is the function of the Euler angles. This is shown for the first time based on the solution of the model problem, viz. determination of the stress-strain state of a spherical body made of nickel titanium single crystal during the transmission of a compressive pulse through the tested material. In general case of orientation of the theoretical coordinate system relative to main crystallographic axes, a spherical body made of nickel titanium single crystal undergoes deformation due to the compressive load and acquires the shape of a two-axial ellipsoid.

INVESTIGATING THE IMPACT OF AN EXTERNAL INFLUENCE ON THE DYNAMIC BEHAVIOR OF DISLOCATIONS IN SILICON CRYSTALS

The results of studying of the influence of a constant magnetic field on the dynamic behavior of dislocations in the crystalline block of an interferometer subsequently subjected to uniaxial mechanical tensile stress along the crystallographic axis of silicon are presented. The periodicity of the dilatation moire image after these influences, the average means free path of a dislocation in a crystal (block), the amount of slip, the average density of dislocations along the slip line, the relative deformation, and Young's modulus are calculated. To study the effect of a stable magnetic field on the dynamic behavior of dislocations in silicon crystals, a special three-block X-ray interferometer was fabricated in one of the blocks, and dislocations were introduced using the technique proposed by us. The experiments were carried out for various values of the magnetic field induction and mechanical stress. It is shown that the sequential action of these external influences leads to an increase in the starting stress, a decrease in the velocity of dislocation movement, a delay in the onset of dislocation movement, and the appearance of a hardening phenomenon. Experiments show that the displacement mobility characteristics are especially sensitive when the modulus of the magnetic induction vector reaches a certain threshold value for a given sample. It was shown that, in a sample containing silicon dislocations, the magnetic memory that appears after storage in a magnetic field is short-lived. It is found that the speed of movement of a dislocation in a magnetic field processed by a magnetic field, "then" subjected to mechanical stress (stretching), changes, but in practice the energy of its activation does not change. A theoretical analysis of the above experimental results is carried out, their scientific substantiation is given, all the factors associated with the phenomena observed in experiments are revealed.

The Phenomenon of Electroohmic Transformation

Peculiarities of electric current distribution in an anisotropic electrically conductive medium are considered and dependences of its longitudinal and transverse components on geometrical factors are established. In the case of a rectangular plate of length a, height b, and width c, the selected crystallographic axes are located in the plane of the side face (a × b), and one of these axes is oriented at an angle α to the edge α. Application to the upper and lower end faces of the plate of some potential difference leads to the appearance of longitudinal and transverse components of the flowing electric current. This leads to the possibility of transforming the electric current magnitude. The methods of optimizing the transformation coefficient magnitude which is determined by both the magnitude of the anisotropy of the electrical conductivity of the plate material and the coefficient of its shape k = a/b. The design variants of anisotropic electrically conductive transformers are proposed. The use of this transformation effect makes it possible to expand the practical use of electroohmic phenomena. This principle of transformation will expand the areas of its use in metrology and measurement technology.

Anisotropy in the hardness of single crystal tungsten before and after neutron irradiation

In this work, the hardness of single crystal tungsten (W) after low, medium and high temperature neutron irradiation, reaching up to 1200°C, is studied. Micro-hardness tests with Vickers pyramid indenter are performed on reference and neutron irradiated samples by varying the orientation of the indenter on the single crystal tungsten contact surface. Due to the availability of multiple slip systems, the anisotropy of the hardness is characterized to allow the construction of uncertainty bands as well as dependence of the absolute value of the hardness on the orientation of the indenter wedges and principal crystallographic axes of the crystal. The applied neutron fluence and flux is representative for the ITER reactor, where tungsten is selected as armour material for plasma facing components. The neutron irradiation is performed in the BR2 reactor with mixed neutron spectrum up to 1 dpa (end of life fluence for ITER) but with appropriate measures to reduce the thermal neutron flux, such that the spurious transmutation into rhenium is reduced. The hardness study is accompanied with a detailed microscale examination to deduce the true indenter surface area. As a result, the true values of the hardness accounting for the pile-up effects are obtained as a function rotation angle of the indenter. The variation of the hardness as a function of indenter orientation before and after irradiation is discussed based on the indenter-slip system geometry. A simple but efficient methodology is proposed to deduce the true hardness and its angle-dependent variation for single crystal tungsten.

The anisotropic quasi-static permittivity of single-crystal β -Ga2O3 measured by terahertz spectroscopy

The quasi-static anisotropic permittivity parameters of electrically insulating beta gallium oxide (β-Ga2O3) were determined by terahertz spectroscopy. Polarization-resolved frequency domain spectroscopy in the spectral range from 200 GHz to 1 THz was carried out on bulk crystals along different orientations. Principal directions for permittivity were determined along crystallographic axes c and b and reciprocal lattice direction a*. No significant frequency dispersion in the real part of dielectric permittivity was observed in the measured spectral range. Our results are in excellent agreement with recent radio frequency capacitance measurements as well as with extrapolations from recent infrared measurements of phonon mode and high-frequency contributions and close the knowledge gap for these parameters in the terahertz spectral range. Our results are important for applications of β-Ga2O3 in high-frequency electronic devices.

Thermal lens astigmatism in glass and in cubic crystals with [001] orientation.

The thermal lens was investigated in CaF2 and Tb3Ga5O12 cubic crystals with [001] crystallographic axes orientation and in magneto-optical glass MOG103 by the method of phase-shifting interferometry. It was demonstrated experimentally that the thermal lens has astigmatism determined by the incident radiation polarization state and by the optical anisotropy parameter ξ. A method of ξ determination by measuring thermal lens astigmatism in cubic crystals with [001] orientation was proposed and verified. It was shown that thermally induced depolarization and the amplitude of phase astigmatism depend on the position of the crystal with [001] orientation in antiphase.

Synthesis, molecular and crystal structure of a heterometallic Cu(II)–Ge(IV) complex with 2-hydroxy-1,3-diaminopropane-N,N,N',N'-tetracetic acid and 2,2'-bipyridine

A novel synthesis method was developed which allows isolating a new coordination compound with polymeric structure {[Ge2(OH)2(3-hpdta)2Cu2(bipy)2]2Н2О}n (І) (where hpdta5– – anion of 2-hydroxy-1,3-diaminopropane-N,N,N',N'-tetracetic acid, bipy – 2,2'-bipyridine) in solid state. Elemental composition, features of thermal decomposition, and molecular and crystalline structure of the synthesized complex were established. According to the data of X-ray diffraction analysis, complex I is a coordination polymer. The polymer chain is formed due to the bridging function of deprotonated ligands hpdta5–, which are simultaneously coordinated with germanium and copper atoms. Ge(1) and Ge(2) atoms have the same coordination environment and distorted octahedral polyhedrons. The coordination polyhedron of Ge(2) is formed due to the coordination of oxygen atoms of two carboxylate and one deprotonated hydroxyl groups of one ligand hpdta5–_1 and carboxylate group of the second ligand hpdta5–_2 in the equatorial direction. In the axial direction, the Ge atom coordinates with the nitrogen atom of the ligand hpdta5–_1 and the hydroxo-ligand. The coordination polyhedrons of Cu(1) and Cu(2) are square pyramids, in which molecules of bipy are coordinated with Cu2+ by two nitrogen atoms. One nitrogen atom and one oxygen atom of the carboxylate group of the hpdta5– ligand are located in the base of a square pyramid. In the apical direction, copper coordinates with deprotonated hydroxyl group of the same hpdta5– ligand. A –-stacking interaction was detected in the crystal between the -systems of bipyridines of two neighboring coordination polymer chains directed along the crystallographic axis a that form double chains with the cavities of 623.04 Å3.

Low-flux scanning electron diffraction reveals substructures inside the ordered membrane domain

Ordered/disordered phase separation occurring in bio-membranes has piqued researchers’ interest because these ordered domains, called lipid rafts, regulate important biological functions. The structure of the ordered domain has been examined with artificial membranes, which undergo macroscopic ordered/disordered phase separation. However, owing to technical difficulties, the local structure inside ordered domains remains unknown. In this study, we employed electron diffraction to examine the packing structure of the lipid carbon chains in the ordered domain. First, we prepared dehydrated monolayer samples using a rapid-freezing and sublimation protocol, which attenuates the shrinkage of the chain-packing lattice in the dehydration process. Then, we optimised the electron flux to minimise beam damage to the monolayer sample. Finally, we developed low-flux scanning electron diffraction and assessed the chain packing structure inside the ordered domain formed in a distearoylphosphatidylcholine/dioleoylphosphatidylcholine binary monolayer. Consequently, we discovered that the ordered domain contains multiple subdomains with different crystallographic axes. Moreover, the size of the subdomain is larger in the domain centre than that near the phase boundary. To our knowledge, this is the first study to reveal the chain packing structures inside an ordered domain.

Anisotropic Magnetoabsorption of Light in Cobalt Ferrite and Its Correlation with Magnetostriction

The absorption spectra of a CoFe2O4 ferrite spinal single crystal, which has a giant magnetostriction, demonstrate an absorption edge at 1.18 eV and a fine structure of impurity absorption bands in the IR region. In the Voight geometry, the crystal is shown to exhibit magnetoabsorption, which is related to the field-induced changes in the fundamental absorption edge and impurity absorption bands. The magnetoabsorption (magnetotransmission and magnetoreflection of light) is anisotropic and depends on the magnetic field direction with respect to the crystallographic axes of the crystal. The light magnetoabsorption is found to be related to the magnetostriction of the crystal. The magnetostriction of CoFe2O4 is shown to significantly contribute to its magnetic anisotropy constant, which is accompanied by changes in the electronic spectrum and optical properties when a magnetic field is applied. The high magnetoabsorption in CoFe2O4 in a relatively low magnetic field makes it possible to use this magnetic material for the development of a new trend in spintronics, namely, strain-magnetooptics.

A Molecular Crystal Shows Multiple Correlated Magnetic and Ferroelectric Switchings

Simultaneous control of the magnetic and electric properties of materials is crucial for their application in next-generation memory and sensor devices. Herein, we report a single-crystal Co(II) co...

Revealing Solvent Effects on Ion Insertion within Battery Primary Particles Using Operando Scanning Transmission X-Ray Microscopy

LiFePO4 (LFP), the attractive commercial cathode materials, exhibits the phase separation between lithium-poor and lithium-rich phase within individual particles due to the wide miscibility gap. The boundaries between such phases develop large stress, lead to cracks within individual particles, and decay the cycle life. There have been lots of interest in investigating the origin of lithium heterogeneity and manipulating the phase boundaries within the particles during cycling. Previously, we revealed that insertion kinetics is dependent on the local lithium composition from which the hysteresis of lithium spatiodynamics originate. [ref.1] In addition, uniformity of ion insertion kinetics at electrolyte-electrode interface governs lithium heterogeneity and spatiodynamics in ether-based organic electrolyte. [ref.1] However, it remains still unclear how the chemical interaction of lithium ions, electrolyte molecules, and solid electrode controls nanoscale ion insertion kinetics. By developing operando scanning transmission x-ray microscopy (STXM) platform which maps lithium composition within individual LFP particles at 50 nm resolution during cycling, we investigated the interfacial lithium transport in various electrolytes: water, carbonates, and ethers. We synthesized LFP particles with platelet morphology and [010] crystallographic axis, which is fast ion diffusion direction, lied parallel to the monochromatic X-ray allowing to visualize lithium transport in the particles. Herein, we discover that aqueous electrolyte greatly accelerates lithium insertion kinetics, completely redirecting lithium heterogenesity and spatiodynamics compared with organic-based electrolyte. Aqueous environment kinetics show higher exchange current density (~700 mAm-2) than organic EC/DMC (~200 mAm-2) and also higher surface diffusion rate which account for the higher degree of intraparticle phase separation within the particles. By extracting kinetics parameters from our result, we experimentally demonstrate that heterogeneity and spatiodynamics in the single particle kinetics is determined by the strong interaction between the electrolyte molecules and the surface of electrode particles.Ref. 1 J. Lim et al. Science (2016) 353 p566 Figure 1

Strain-magneto-optics in CoFe2O4: Magneto-absorption in Voight geometry

The infrared optical, magneto-optical properties of CoFe2O4 single crystals are considered. The magneto-absorption of natural light in the infrared spectral range in CoFe2O4 spinel is studied in the Voight experimental geometry. This magneto-optical effect is very high and associates with a change in the fundamental absorption edge and impurity absorption bands under a magnetic field. The magneto-absorption strongly depending on both the magnitude and the orientation of the magnetic field relative to the crystallographic axes of the crystal is presented in this paper. The contribution of magnetostriction to the magnetic anisotropy constant of the CoFe2O4 crystal is shown to be abnormally large. The clear connection between magneto-absorption of light in the infrared spectral range and magnetostriction of the CoFe2O4 spinel is established.

Dehydration process in 1D ammonium lead halide and mixing of organic cations in hybrid perovskites through mechanosynthesis

Organic-lead halide perovskites have attracted much attention as a promising material for optoelectronic and photovoltaic applications. However, the broad commercial use of such materials is hindered by their chemical instability. The detrimental processes of degradation often involve the occurrence of hydrated compounds. However, the knowledge of some aspects related to the thermal stability of hydrated lead halides compounds is still very limited. In this work, we report the structural study dealing with the formation of NH4PbI3 obtained by removing crystallization water from NH4PbI3 (H2O)2 with thermal treatment. The hydrated compound is prepared by solvent-free grinding applied on a mixture of NH4I and PbI2 powders. Upon heating, the structural evolution of the de-hydration process, monitored by powder x-ray diffraction, consists in the rearrangement of the 1D chains of octahedral PbI6 units throughout rotations around a specific crystallographic axis. Besides, the fabrication of the solid solution (CH3NH3)1-x(NH4)xPbI3 with x = 0, 0.05, 0.10, 0.20 is attempted with different conditions of mechanosynthesis. The experimental results confirmed the limited solubility of the NH4 + group in the methylammonium lead iodate perovskite with a maximal substitution limit of 5%.

Reversible structural transition in monocrystalline Ni2FeGa microwires for shape-memory applications

We report on the production and characterization of Heusler-based Ni2FeGa microwires exhibiting two – way shape memory effect. The microwires are characterized by a monocrystalline structure with a strong preferred crystallographic orientation that shows [1 1 1] axis parallel to the wire’s axis for high-temperature L21 austenite phase, while the [0 1 7] axis is preferred for low-temperature monoclinic phase. Variation of crystallographic axis (and corresponding easy magnetization axis) leads to 1600% variation of magnetic permeability due to a 2% strain in axial direction. Such straining is reversible immediately after production without the necessity of further thermal treatment. These properties give the microwire function of very sensitive SMART actuators that can be easily produced in a large amount.

The interfacial lattice origin of anatase titanium dioxide by hydrothermal architecture

The interfacial microstructure and defects formed during the growth of anatase TiO2 nanocrystalline by oriented attachment (OA) in the process of hydrothermal synthesis have been studied by using high-resolution transmission electron microscope (HRTEM) combined with theoretical simulation. In the {101} crystal plane of anatase TiO2 nanoparticles, it is easy to observe that the nanocrystal grains are attached to form large single crystals or twin structure through high resolution energy filtered images. At the initial stage of synthesis, obvious changes of interplanar spacing between crystal planes, lattice distortions or dislocations could be observed at the interfaces of those unstable grains. The analyses on the Burgers vector loop of dislocations conclude that the Burgers vectors are 1/2〈101〉. Furthermore, the simulated images and the atomic structure models based on the captured images are in perfect agreement with the original ones. Meanwhile, the OA of anatase TiO2 nanoparticles on {112} crystal plane is also observed and twin crystals are formed along 〈311〉 crystallographic zone axis including dislocations at the interfaces. However, due to the damages of electron irradiation, few high-resolution images of these grains have been obtained. Analysis of these interfacial structures provide a new idea to understand the formation and transformation of the microstructure of anatase TiO2 nanoparticles during hydrothermal synthesis.

Extrusion kinematics of UHP terrane in a collisional orogen: EBSD and microstructure-based approach from the Tso Morari Crystallines (Ladakh Himalaya)

Several extrusion mechanisms have been proposed to explain the occurrences of ultra-high pressure (UHP) rocks at collisional margins. Here we assess the suitability of the channel flow model of extrusion for the Tso Morari Crystallines (TMC) by investigating the syn-extrusion deformation kinematics of the UHP eclogite-bearing gneiss that dominates the TMC. Samples collected from the two transects viz. Sumdo-Karzok and Sumdo-Debring, have been examined. The channel flow model predicts high simple shear component at the channel margins during extrusion. Gneisses near the upper margin of the subduction channel – also the northern margin of the TMC on map – support this claim and exhibit closely spaced shear planes, and elevated fluid activity as revealed by X-ray diffraction results. Quartz grain parameters e.g. Feret diameter, aspect ratio, and vector mean strength measured from oriented thin-sections of the gneiss do not exhibit distinct pattern of variation of deformation intensity across the TMC. EBSD-based low-angle intragranular 'neighbor to neighbor' misorientation analysis and micro-textural evidences of incipient partial melting in the gneiss reflect temperatures >600 °C. We propose that return flow and buoyancy of the subducted continental crust triggered the extrusion, which was then facilitated by the Indo-Eurasia collision. The mechanism could thus be a combination of both channel flow and ductile wedge extrusion. Continued convergence possibly promoted non-planar triclinic transpression – also evident from the crystallographic vorticity axis (CVA) analysis – such that the extrusion direction varied spatially, producing the diverse shear senses observed at both meso- and micro-scales.

Orientation effects and links between hydrostatic parameters in piezo-active 2–0–2 composites

The paper reports results on the large hydrostatic parameters of novel three-component 2–2-type composites based on [011]-poled relaxor-ferroelectric single crystals. The role of the rotation of the main crystallographic axes X and Y in forming the considerable hydrostatic piezoelectric response is studied. The composite consists of the single-crystal and 0–3 corundum ceramic/polyethylene layers that are regularly arranged along the non-polar axis. 0.72Pb(Mg1/3Nb2/3)O3–0.28PbTiO3 and 0.93Pb(Zn1/3Nb2/3)O3–0.07PbTiO3 single crystals poled along [011] of the perovskite unit cell are considered as main piezoelectric components of the composites with 2–0–2 connectivity. Links between the hydrostatic parameters (piezoelectric coefficients and squared figure of merit and electromechanical coupling factor ) are analysed by taking into account specifics of the microgeometry and elastic anisotropy of the 0–3 layer. Due to the large values of the aforementioned hydrostatic parameters, the studied 2–0–2 composites are of interest for hydroacoustic and piezoelectric energy-harvesting applications.