Specific Crystallographic Planes Research Articles

Effect of Dopant Ordering on the Stability of Ferroelectric Hafnia

Films of the all‐important compound hafnia (HfO2) can be prepared in an orthorhombic ferroelectric (FE) state that is ideal for applications, e.g., in memories or negative‐capacitance field‐effect transistors. The origin of this FE state remains a mystery, though, as none of the proposed mechanisms for its stabilization—from surface and size effects to formation kinetics—is fully convincing. Interestingly, it is known that doping HfO2 with various cations favors the occurrence of the FE polymorph; however, existing first‐principles works suggest that doping by itself is not sufficient to stabilize the polar phase over the usual nonpolar monoclinic ground state. Herein, first‐principles methods are used to re‐examine this question. Two representative isovalent substitutional dopants, Si and Zr, are considered, and their preferred arrangement within the HfO2 lattice is studied. The results reveal that small atoms like Si can adopt very stable configurations (forming layers within specific crystallographic planes) in the FE orthorhombic phase of HfO2 but comparatively less so in the nonpolar monoclinic one. Further, it is found that, at low concentrations, such a dopant ordering yields a FE ground state, the usual paraelectric phase becoming a higher‐energy metastable polymorph. The implications of the findings are discussed.

Nanodeformation in enstatite single crystals: Simulation of micrometeoroid impacts by femtosecond pulsed laser experiments

Space weathering by micrometeoroid bombardment is a cosmic phenomenon on atmosphere-free celestial bodies, a process that is expected to particularly overprint planetesimals and cosmic dust in debris discs. We reproduced micrometeoroid impact craters by femtosecond pulsed laser irradiation on oriented enstatite single crystals (En93Fs7) to investigate the deformation behavior and its orientation dependence. All microcraters show typical bowl shaped morphologies, a glass surface layer with splash like ejecta material and subsurface layering. Although we could reproduce melting and vaporization as typical space weathering effects in the enstatite experiments, there is no formation of agglutinate particles or metallic nanoparticles (npFe0). The shock effects in the deformation layer consist of planar structures like microfractures and cleavages, amorphous lamellae, stacking faults and clinoenstatite lamellae. Their activation and/or orientation depends on the shock direction. In special orientations we observe the activation of glide systems along specific low indexed crystallographic planes. Due to the short timescale and the high strain rates, the most prominent effect is the failure of enstatite by microfracturing along non-rational crystallographic planes. Common deformation mechanisms reported in meteorites like the formation of clinoenstatite lamellae via shearing along [001] (100) occur less frequently. Shear is apparently the dominant mechanism in the formation of the above-mentioned effects and causes also their modification by frictional heating. The wide-spread formation of amorphous lamellae is, for example, interpreted to be the result of this shear heating along planar structures. We interpret this unconventional deformation behavior as a consequence of the small spatial and temporal scale of the experiments, resulting in a short-lived spherical shock wave with high deviatoric stresses in contrast to a long pressure pulse and quasi-hydrostatic compression in large scale impacts that produce typical shock features.

Assessment of hydrogen embrittlement via image-based techniques in Al–Zn–Mg–Cu aluminum alloys

Hydrogen repartitioning and the related embrittlement behavior were characterized by studying Al–Zn–Mg–Cu aluminum alloys with different intermetallic particle contents. Using high-resolution X-ray tomography and related microstructural tracking techniques, hydrogen-induced quasi-cleavage cracks and the related strain localization were observed regardless of the content of the intermetallic particles. The area of quasi-cleavage cracks on the fracture surface increased and the strain localization became more intense with a decrease in the content of intermetallic particles, thereby revealing that trapped hydrogen at intermetallic particles increases the resistance to hydrogen embrittlement. In addition, a quantitative assessment of the hydrogen repartitioning taking into account vacancy production and dislocation multiplication during deformation, was applied to characterize the hydrogen embrittlement behavior. Because of the thermal equilibrium among various hydrogen trap sites, internal hydrogen atoms are mainly repartitioned to vacancies and precipitates in the strain localization region during deformation because of their high trap site densities and high hydrogen trap binding energies. Since the concentration of hydrogen trapped at dislocations is extremely limited, it can be assumed that hydrogen repartitioned to precipitates induces decohesion of precipitates along specific crystallographic planes, where quasi-cleavage cracking may originate.

Quasi-cleavage hydrogen-assisted cracking path investigation by fractographic and side surface observations

Despite recent substantial advances in understanding of hydrogen embrittlement (HE), many important aspects of this widespread phenomenon remain a subject of debates. Particularly, remarkably different opinions have been expressed on the nature of hydrogen-assisted cracking (HAC), which produces quasi-cleavage (QC) fracture surfaces in iron and low-carbon steels. Basically, two conflicting groups of theories are distinguished concerning causes the QC phenomenon: brittle cleavage-like models and ductile models exploiting localized ductile micro- or nanovoid concepts at the core. The present study was aimed at gaining a new insight into the QC mechanism through the detailed investigation of the HAC path by the microscopic observations of the side and fracture surfaces of the annealed low-carbon steel, which was tensile tested in the ex- and in-situ cathodic hydrogen charged conditions. Using a combination of the scanning electron microscopy, confocal laser scanning microscopy and electron-backscattering diffraction, it was found that the in-situ charging is more suitable for the investigation of HAC by side surface observations because it provokes QC cracking on the side surface and suppresses normal ductile fracture. As opposes to this, HAC after ex-situ charging occurs mainly internally. In this condition, HAC interferes with the normal ductile fracture on the side surface. It is established that the path of secondary QC cracks in the in-situ charged specimen is determined predominantly by the local stress distributions. This holds even on the scale of individual grain. Large deviations of the quasi-cleavage cracks paths from the specific crystallographic planes are found and explained. Nano-voids are regularly observed at the crack tips. Thus, the nano-voids coalescence is supposed to be an integral part of QC HAC.

Influence of the crystallographic orientation on the electrochemical reactivity measured by Scanning Electrochemical Microscopy on nickel-based alloy 600

The electrochemical reactivity of Alloy 600 in the passive state was examined at the microstructural scale using a local-probe technique, the Scanning Electrochemical Microscopy (SECM). The complementarity of two modes of recording, the current maps and the approach curves, was demonstrated. This study allowed the qualitative determination of high-reactivity zones, but also the quantitative definition of kinetic and thermodynamic parameters, namely the intrinsic kinetic constant and the charge-transfer coefficient. Furthermore, the crystallographic orientation of the studied grains was determined by Electron Backscatter Diffraction (EBSD). The quantitative parameters obtained by SECM were thus assigned to specific crystallographic planes, revealing a relation between the crystallographic orientation of the grains and the reactivity measured on the passive film. The intrinsic kinetic constant increased with the misorientation angle between the grain normal and the 〈111〉 direction, which reveals the passive film anisotropy. These results show that the coupling of these two techniques is promising for the development of quantitative kinetic or corrosion models.

Photoluminescence of SiV centers in CVD diamond particles with specific crystallographic planes**Project supported by the National Natural Science Foundation of China (Grant Nos. 50972129 and 50602039), the International Science Technology Cooperation Program of China (Grant No. 2014DFR51160), the National Key Research and Development Program of China (Grant No. 2016YFE0133200), European Union’s Horizon 2020 Research and Innovation Staff Exchange (RISE) Scheme

We prepared the isolated micrometer-sized diamond particles without seeding on the substrate in hot filament chemical vapor deposition. The diamond particles with specific crystallographic planes and strong silicon-vacancy (SiV) photoluminescence (PL) have been prepared by adjusting the growth pressure. As the growth pressure increases from 2.5 to 3.5 kPa, the diamond particles transit from composite planes of {100} and {111} to only smooth {111} planes. The {111}-faceted diamond particles present better crystal quality and stronger normalized intensity of SiV PL with a narrower bandwidth of 5 nm. Raman depth profiles show that the SiV centers are more likely to be formed on the near-surface areas of the diamond particles, which have poorer crystal quality and greater lattice stress than the inner areas. Complex lattice stress environment in the near-surface areas broadens the bandwidth of SiV PL peak. These results provide a feasible method to prepare diamond particles with specific crystallographic planes and stronger SiV PL.

Series of 2D multilayered perovskites constructed by slicing the 3D [(CH3NH3)PbI3] with 4-fluorobenzylamine

Due to its unique optical properties, two-dimensional (2D) organic-inorganic hybrid perovskite attracts wide attention. In this paper, series of 2D perovskites with the formula [(4-FBA)2(MA)n-1PbnI3n + 1] (4-FBA = 4-FC6H4CH2NH3, MA = CH3NH3, n = 1, 2, 3, n represents the number of 2D perovskite inorganic layers) were obtained by controlling the starting material ratios among 4-fluorobenzylamine, methylammonium iodide and lead iodide. The multilayered perovskite structures were formed by slicing the 3D [MAPbI3] with 4-fluorobenzylamine along the specific crystallographic plane. According to the UV−vis absorption, the bandgaps of the multilayered compounds decreased with the increase of inorganic perovskite layers (Eg = 2.23 eV for n = 1; Eg = 2.01 eV for n = 2 and Eg = 1.78 eV for n = 3), thus leading to their potential application in solar cell.

Controlled Surface Structure and Chemistry with Transition Metal Enrichment for Cathode Materials

Transition metals enrichment on the surface of layered nickel-cobalt-manganese oxide cathodes leads to the structural changes, hindering the facile lithium-ion transportation at the surface of the particles. These inhomogeneous metal distributions and structural evolutions are progressively proceeded with electrochemical cycling from the surface to the bulk, which causes to severe material degradation and short cycle life. In this study, we develop the new surface structure and chemistry on the layered cathode materials to enhance the battery performances by controlling the local concentration of transition metal. Our strategy is to improve the surface instabilities by either suppressing unwanted transition metal segregation or creating an ion conductive oxide phase or both on the specific crystallographic planes for the layered host materials. This is conducted by adding electrochemically inactive transition metals together with the active metals during the synthesis because that the transition metal elements are individually and preferentially enriched to the particular facets such as (20–2), (200), and (001) of the layered bulk. As a result, the unique surface layers are spontaneously formed along the surface planes within a few nanometer. The electrochemical characteristics shows that the newly created surface phases strongly influence on the high rate performance and long cycle life. This work is supported by the National Research Foundation of Korea (NRF-2011-C1AAA001-0030538).

Engineering Fractal MTW Zeolite Mesocrystal: Particle-Based Dendritic Growth via Twinning-Plane Induced Crystallization

Constructing superstructured crystalline materials by crystal engineering is an attractive objective for miscellaneous fields of researchers spanning biomimetics to catalytic materials. Zeolite is a kind of important crystalline catalyst, and superstructured zeolite has great potential for widespread applications. However, the ambiguous crystallization mechanisms hamper the effective and scientific fabrication of superstructured zeolite with exceptional properties. Herein, a fractal superstructured MTW zeolite with mesocrystal side branches is prepared via a nanoparticle-based nonclassical pathway with twinning-plane induced crystallization, which is distinct from the formation of general mesocrystal via crystal–crystal oriented attachment. Deformed atomic connection at a specific crystallographic plane contributes to the production of side branches. Moreover, this intriguing morphology could be regulated merely via adjusting the crystallization kinetics based on the unequivocal nonclassical crystallizati...

Growth pattern and orientation selection of magnesium alloy dendrite: From 3-D experimental characterization to theoretical atomistic simulation

Based on synchrotron X-ray tomography and EBSD techniques, it was found that magnesium alloy dendrite exhibits a typical 3-D growth pattern with eighteen-primary branches, of which six grow along 〈112¯0〉 in the basal plane and the other twelve along 〈112¯3〉 in non-basal planes. The underlying mechanism behind such growth pattern was investigated by determining the surface energy related anisotropy along different crystallographic orientations via atomistic simulation in light of density functional theory and hcp lattice structure. Results showed that the surface energy exhibits an orientation-dependent behavior, and these specific crystallographic planes perpendicular to which lay the preferred growth directions of α-Mg dendrite always exhibit higher surface energy. Accordingly, the orientation selection of α-Mg dendrite growth was confirmed to be 〈112¯0〉 and 〈112¯3〉, which agreed well with the experimental findings. A geometrical model was developed to illustrate the 3-D growth pattern of the α-Mg dendrite based on its outer contour shape at equilibrium energy state.

Electrochemical polishing of monocrystalline silicon with specific crystallographic planes

In this study, an electrolytic polishing experimental system was developed to obtain a uniform, flat-surfaced monocrystalline silicon with specific crystallographic planes. Several key factors reflecting specific electrolytic polishing on monocrystalline silicon with specific crystallographic planes were summarized. These factors, including electrolyte, conduction mode, Schottky barrier, semiconductor body resistance, and unidirectional conductivity, were analyzed comprehensively through energy spectrum analysis, theoretical modeling, and potential simulation. The effects of electrolytic polishing process were obtained, and corresponding solutions were proposed. Finally, the electrolytic polishing experiment for monocrystalline silicon with specific crystallographic planes was conducted. A uniform, flat-surfaced monocrystalline silicon with no metamorphic layer was then obtained. The flatness error of the center area was less than 0.201µm. Furthermore, the crystallographic planes of monocrystalline silicon wafers showed no change.

Dynamic diffraction effects and coherent breathing oscillations in ultrafast electron diffraction in layered 1T-TaSeTe.

Anisotropic lattice movements due to the difference between intralayer and interlayer bonding are observed in the layered transition-metal dichalcogenide 1T-TaSeTe following femtosecond laser pulse excitation. Our ultrafast electron diffraction investigations using 4D-transmission electron microscopy (4D-TEM) clearly reveal that the intensity of Bragg reflection spots often changes remarkably due to the dynamic diffraction effects and anisotropic lattice movement. Importantly, the temporal diffracted intensity from a specific crystallographic plane depends on the deviation parameter s, which is commonly used in the theoretical study of diffraction intensity. Herein, we report on lattice thermalization and structural oscillations in layered 1T-TaSeTe, analyzed by dynamic diffraction theory. Ultrafast alterations of satellite spots arising from the charge density wave in the present system are also briefly discussed.

Machining damage of monocrystalline silicon by specific crystallographic plane cutting of wire electrical discharge machining

The machining damage on the certain crystal face of single crystal silicon, the manufacture of which are under diverse parameters of wire electrical discharge machining (WEDM), is tested by means of the micro-observation and X-ray diffraction rocking curve method. In the process of monocrystalline silicon machined by WEDM, when the pulse width is small, the basic methods of material removal are melting and gasification, which are also called normal removal methods. When the pulse width increases to a certain degree beyond the normal removal, thermal spalling removal also occurs, which is considered a compound removal method. The depth of machining damage is difficult to control. The structure of machining damage under two conditions is divided into normal removal and compound removal in this study. To make the depth of machining damage easy to control, compound removal should be avoided when processing the single crystal silicon by certain crystal face cutting of WEDM. Such an approach can provide the premise and guarantee for the subsequent processing of single crystal silicon with certain crystal face in the future.

Anisotropy in geometrically rough structure of ice prismatic plane interface during growth: Development of a modified six-site model of H2O and a molecular dynamics simulation.

This paper presents a modified version of the six-site model of H2O [H. Nada and J. P. J. M. van der Eerden, J. Chem. Phys. 118, 7401 (2003)]. Although the original six-site model was optimized by assuming the cut-off of the Coulomb interaction at an intermolecular distance of 10 Å, the modified model is optimized by using the Ewald method for estimating the Coulomb interaction. Molecular dynamics (MD) simulations of an ice-water interface suggest that the melting point of ice at 1 atm in the modified model is approximately 274.5 K, in good agreement with the real melting point of 273.15 K. MD simulations of bulk ice and water suggest that the modified model reproduces not only the structures and density curves of ice and water, but also the diffusion coefficient of water molecules in water near the melting point at 1 atm. Using the modified model, a large-scale MD simulation of the growth at an ice-water interface of the prismatic plane is performed to elucidate the anisotropy in the interface structure during growth. Simulation results indicate that the geometrical roughness of the ice growth front at the interface is greater in the c-axis direction than in the direction normal to the c-axis when it is analyzed along the axes parallel to the prismatic plane. In addition, during the growth at the interface, the transient appearance of specific crystallographic planes, such as a {202¯1} pyramidal plane, occurs preferentially at the ice growth front. The effect of different ensembles with different simulation systems on the anisotropy in the interface structure is also investigated.

Structure of solvation water around the active and inactive regions of a type III antifreeze protein and its mutants of lowered activity.

Water molecules from the solvation shell of the ice-binding surface are considered important for the antifreeze proteins to perform their function properly. Herein, we discuss the problem whether the extent of changes of the mean properties of solvation water can be connected with the antifreeze activity of the protein. To this aim, the structure of solvation water of a type III antifreeze protein from Macrozoarces americanus (eel pout) is investigated. A wild type of the protein is used, along with its three mutants, with antifreeze activities equal to 54% or 10% of the activity of the native form. The solvation water of the ice-binding surface and the rest of the protein are analyzed separately. To characterize the structure of solvation shell, parameters describing radial and angular characteristics of the mutual arrangement of the molecules were employed. They take into account short-distance (first hydration shell) or long-distance (two solvation shells) effects. The obtained results and the comparison with the results obtained previously for a hyperactive antifreeze protein from Choristoneura fumiferana lead to the conclusion that the structure and amino acid composition of the active region of the protein evolved to achieve two goals. The first one is the modification of the properties of the solvation water. The second one is the geometrical adjustment of the protein surface to the specific crystallographic plane of ice. Both of these goals have to be achieved simultaneously in order for the protein to perform its function properly. However, they seem to be independent from one another in a sense that very small antifreeze activity does not imply that properties of water become different from the ones observed for the wild type. The proteins with significantly lower activity still modify the mean properties of solvation water in a right direction, in spite of the fact that the accuracy of the geometrical match with the ice lattice is lost because of the mutations. Therefore, we do not observe any correlation between the antifreeze activity and the extent of modification of the properties of solvation water.

A numerical procedure enabling accurate descriptions of strain rate-sensitive flow of polycrystals within crystal visco-plasticity theory

The plastic deformation of polycrystalline metals is carried by the motion of dislocations on specific crystallographic glide planes. According to the thermodynamics theory of slip, in the regime of strain rates, roughly from 10−5/s to 105/s, dislocation motion is thermally activated. Dislocations must overcome barriers in order to move, and this concept defines critical activation stresses τcs on a slip system s that evolve as a function of strain rate and temperature. The fundamental flow rule in crystal visco-plasticity theory that involves τcs in order to activate slip has a power-law form: γ̇s=γ̇0(|τs|τcs)nsign(τs). This form is desirable because it provides uniqueness of solution for the active slip systems that accommodate an imposed strain rate; however, it also introduces a strain rate dependence, which in order to represent the actual behavior of polycrystalline materials deforming in relevant conditions of temperature and strain-rate usually needs to be described by a high value of the exponent n. However, since until now the highest value of n was limited by numerical tractability, the use of the power-law flow rule frequently introduced an artificially high rate-sensitivity. All prior efforts to correct this extraneous rate sensitivity have only lessened its effect and unfortunately also at the expense of substantial increases in computation time. To this day, a solution for the power-law exponent reflecting true material behavior is still sought. This article provides a novel method enabling the use of realistic material strain rate-sensitivity exponents to be used within the crystal visco-plasticity theory without increasing computation time involved in polycrystal simulations. Calculations are performed for polycrystalline pure Cu and excellent agreement with experimental measurement is demonstrated.

Thickness measurement of deterioration layer of monocrystalline silicon by specific crystallographic plane cutting of wire electrical discharge machining

The thickness of the deterioration layer in the specific crystallographic plane of monocrystalline silicon, which was processed under different conditions of wire-cut electrical discharge machining (WEDM), was measured through X-ray diffraction rocking curve method in this work. Results showed that the deterioration layer thickness differed for the same crystallographic planes of monocrystalline silicon, which was processed under a different pulse width. The thickness of the deterioration layer increased with the increase of pulse width. Under the same electrical parameters of WEDM, the various crystallographic planes of the monocrystalline silicon correspond to the different deterioration layer thicknesses. The thickness also decreases with the increase of bond density between adjacent crystals, indicating that anisotropy exists in monocrystalline silicon processing of WEDM. Hence, the technology based on the X-ray diffraction rocking curve method, which can detect the deterioration layer thickness of the monocrystalline silicon, is proposed in this paper. Furthermore, this research will provide a basis for future studies on the deterioration layer of monocrystalline silicon.

WIP: A Web‐based program for indexing planar features in quartz grains and its usage

AbstractPlanar deformation features (PDFs) in quartz are the most important diagnostic features that allow the unambiguous identification of impact structures on Earth. In order to confirm that these features (that are characterized by planar character and form along specific crystallographic planes) are indeed PDFs, they need to be properly investigated and indexed. Following universal‐stage measurements, the process of indexing is usually performed manually, using a Wulff stereonet and following a strict procedure, which is time consuming and error prone. In this article, we present WIP, a new Web‐based program for indexing planar deformation features in quartz. The correctness of our program is shown by its application to measurements that had previously been indexed manually. The observed minor differences, especially in the absolute frequency percentage of PDFs, are negligible and not significant enough to influence the estimation of shock pressure that could be calculated from the indexed results. Usability of this program is shown using the spatial relationships between a statistically significant number of 278 quartz grains with 409 sets of PDFs analyzed within the area (~35 mm2) of a single thin section of a meta‐greywacke from the Bosumtwi impact structure. Our program is not only more accurate and faster than the manual (graphical) method but also removes the human error from the plotting process and allows control of several parameters, such as the value of estimated measurement error used in the indexing calculation or method of aggregated error handling. The program also provides information about the angles between the planes of the measured PDF sets present in a grain, which allows determination of the angles between (for example) indexed {} and {} sets.

Bimetallic PtxCoy nanoparticles with curved faces for highly efficient hydrogenation of cinnamaldehyde.

The control of the curved structure of bimetallic nanocrystals is a challenge, due to the rate differential for atom deposition and surface diffusion of alien atomic species on specific crystallographic planes of seeds. Herein, we report how to tune the degree of concavity of bimetallic PtxCoy concave nanoparticles using carboxylic acids as surfactants with an oleylamine system, leading to the specific crystallographic planes being exposed. The terminal carboxylic acids with a bridge ring or a benzene ring serving as structure regulators could direct the formation of curved faces with exposed high-index facets, and long-chain saturated fatty acids favored the production of curved faces with exposed low-index facets, while long-chain olefin acids alone benefited the formation of a flat surface with exposed low-index planes. Furthermore, these PtxCoy particles with curved faces displayed superior catalytic behaviour to cinnamaldehyde hydrogenation when compared with PtxCoy with flat faces. PtxCoy nanoparticles with curved faces exhibited over 6-fold increase in catalytic activity compared to PtxNiy nanoparticles with curved faces, and near 40-fold activity increase was observed in comparison with PtxFey nanoparticles with curved faces.

Assessing the Bonding Properties of Individual Molecular Orbitals.

Molecular orbitals (MOs), while one of the most widely used representations of the electronic structure of a system, are often too complex to intuit properties. Aside from the simplest of cases, it is not necessarily possible to visually tell which orbitals are bonding or antibonding along particular directions, especially in cases of highly delocalized and nontrivial bonding like metal clusters or solids. We propose a method for easily assessing and comparing the relative bonding contributions of MOs, by calculating their response to stress (e.g., compression). We find that this approach accurately describes relative bonding or antibonding character in both the simplest cases and provides new insight in more complex cases. We test the approach on four systems: H2, Am2, benzene, and the Pt4 cluster. In exploring this methodology, a scheme became elucidated, for predicting changes in the ground electronic configuration upon compression, including changes in bonding order, angular momenta of occupied MOs, and trends in MO ordering. We note that the applications of this work go beyond simple molecules and could be straightforwardly extended to, for example, solids and their response to stress along the specific crystallographic plane. Additionally, predictions of structures and properties of chemical systems under stress could result from the emerging intuition about changes in the electronic structure.