Atomic Distortion Research Articles

Influence of carbon-ion irradiation on the superconducting critical properties of MgB2 thin films

We investigate the influence of carbon-ion irradiation on the superconducting (SC) critical properties of MgB2 thin films. MgB2 films of two thicknesses, 400 nm (MB400nm) and 800 nm (MB800nm), were irradiated by 350 keV C ions having a wide range of fluence, 1 × 1013–1 × 1015 C atoms cm−2. The mean projected range (R p) of 350 keV C ions in MgB2 is 560 nm, thus the energetic C ions will pass through the MB400nm, whereas the ions will remain into the MB800nm. The SC transition temperature (T c), upper critical field (H c2), c-axis lattice parameter, and corrected residual resistivity (ρ corr) of both the films showed similar trends with the variation of fluence. However, a disparate behavior in the SC phase transition was observed in the MB800nm when the fluence was larger than 1 × 1014 C atoms cm−2 because of the different T cs between the irradiated and non-irradiated parts of the film. Interestingly, the SC critical properties, such as T c, H c2, and critical current density (J c), of the irradiated MgB2 films, as well as the lattice parameter, were almost restored to those in the pristine state after a thermal annealing procedure. These results demonstrate that the atomic lattice distortion induced by C-ion irradiation is the main reason for the change in the SC properties of MgB2 films.

Molecular dynamics simulation and experimental verification for bonding formation of solid-state TiO2 nano-particles induced by high velocity collision

Collision processes of solid-state nano-sized ceramic particles were investigated by molecular dynamics (MD) simulation in order to clarify their bonding mechanisms. Effect of particle temperature on particle bonding formation was examined, and collision behavior of nano-sized TiO2 particle was discussed in terms of particle deformations. Microstructures and bonding qualities of bonded nano-sized TiO2 particles induced by high velocity collision were examined by high resolution transmission electron microscope (HR-TEM) to verify the MD results. Simulation results demonstrate that the bonding formation of nano-sized TiO2 particles can be attributed to the atomic displacement and lattice distortion in localized impact region of particle boundaries. TEM microstructure results prove simulation results and indicate effective chemical bonding formations between nano-particles at low temperature by high velocity collision. Quantitative results show that the high temperature is beneficial to the particle bonding formation. The asperity around nano-sized ceramic particles surface contributes to the displacement and lattice distortion in localized impact region under the high impact compressive pressure. The fact demonstrates a new mechanism of nano-scale ceramic particle bonding formation induced by the localized atomic displacement. The study present opens up a promising prospect of fabricating functional equipment with nano-scale ceramic particles with high velocity collision at ambient temperature.

First-principles study of structural, vibrational, and electronic properties of trigonally bonded II-IV-N2

With the first-principles calculations, we probe into the structural, vibrational, and electronic properties of trigonally bonded planar II-IV-N2 (II = Zn, Cd; IV = Si, Ge, Sn), in contrast to their well-known tetrahedrally bonded counterparts. A detailed analysis is presented for the deformed honeycomb-like structures of planar II-IV-N2 in light of atomic radius and electronegativity of different group-II and group-IV elements. Cohesive energy and phonon property are calculated to investigate the stability of monolayer II-IV-N2. Among the six monolayer II-IV-N2 studied, five of them are dynamically stable except for monolayer CdSnN2 which has imaginary phonon frequencies. The band curvatures of trigonally bonded II-IV-N2 reveal distinct difference comparing to their tetrahedrally bonded counterparts and show connections to atomic transmutation and structural distortions. The different variation trends of band gap from tetrahedrally bonded structure to trigonally bonded structure are explored through density of states analysis. The highest valence band energies of monolayer ZnGeN2 and ZnSnN2 at Γ and X point are very close, resulting in a quasi-direct band gap. Tuning of band gap characteristics by uniaxial strain is investigated for these ternary nitrides and it is shown that a slightly change less than 0.1% on lattice constant b can induce indirect-to-direct band gap transition for monolayer ZnSnN2.

Metallic 1T-MoS2 nanosheets in-situ entrenched on N,P,S-codoped hierarchical carbon microflower as an efficient and robust electro-catalyst for hydrogen evolution

Phosphorus contained metallic 1T-MoS2 produced from in-situ ammonium intercalation induced surface sulfur atom distortion of MoS2 rooted in N,P,S-codoped hierarchical flowerlike carbon (HCMF) hybrids were successfully prepared via a dopamine self-polymerization together with molybdate process followed by a hydrothermal reduction route. On account of the coupling effect of metallic phase, defect-rich character, heteroatomic dopants and highly conductive carbon support, the P-MoS2@HCMF demonstrates prominent performances with an overpotential of merely 86 mV to deliver a current density of 10 mA cm–2 and a small Tafel slope of 42.35 mV dec–1 for hydrogen evolution reaction (HER) in 0.5 M H2SO4, which are among the best results for molybdenum disulfide based HER catalysts. Strikingly, benefiting from S vacancies and P dopant functioning as electron donors, as well as strain arisen from the tensile of rigid carbon microflower scaffold to MoS2 nanosheets to overcome agglomeration barriers of nanosheets, P-MoS2@HCMF remained well the pristine 1 T phase and exhibited superior cycling stability with indistinguishable overpotential decay over 5000 sweeps and extraordinary HER durability during the 100 h long-term operation with negligible current deterioration. These findings highlight the prospective potential of P-MoS2@HCMF as a highly efficient and stable noble metal-free electrocatalyst towards HER.

Non-monotonic thickness dependence of Curie temperature and ferroelectricity in two-dimensional SnTe film

Recently, the observation of atomic thin film SnTe with a Curie temperature (Tc) higher than that of the bulk [Chang et al., Science 353, 274 (2016)] has boosted the research on two-dimensional (2D) ferroic materials tremendously. However, the origin of such a phenomenon is yet to be thoroughly investigated, which hinders the understanding and design of materials with ferroic orders at the 2D limit. By using the density functional theory, we investigated the structural and ferroelectrical properties of 2D SnTe to reveal the thickness dependence. The calculated results demonstrate that 2D SnTe automatically transforms into a periodical bilayer structure, resulting from the surface effect. Moreover, based on the double-well potential and atomic distortion analysis, we found that the Tc of 2D SnTe is higher than that of the bulk counterpart, and more surprisingly, Tc exhibits a non-monotonous dependence of thickness, featuring a pronounced atomic distortion and Curie temperature maximum at 8 atomic-layers (4 unit cells). In addition, this non-monotonous dependence is sensitive to the external strain and it can be easily tuned by the external compressive strain.

Learning from Correlations Based on Local Structure: Rare-Earth Nickelates Revisited

Statistical analysis of local atomic distortions in crystalline materials is a powerful tool for understanding coupled electronic and structural phase transitions in transition metal compounds. The analyses of such complex materials, however, often require significant domain knowledge to recognize limitations in the available data, whether it be experimentally reported crystal structures, property measurements, or computed quantities, and to understand when additional experiments or simulations may be necessary. Here we show how additional descriptive statistics and computational experiments can help researchers explicitly recognize these limitations and fill in missing gaps by constructing amplitude ( a) and normalized-amplitude ( n) distortion-mode property correlation-coefficient heat maps, aCCHMs and nCCHMs, respectively. We demonstrate this utility within the rare-earth nickelate perovskites RNiO3 (R = rare earth ≠ La), which exhibit antiferromagnetic and metal-insulator transitions with crystallographic symmetry breaking, and analyze the CCHMs obtained from experimental and first-principles derived symmetry modes. In contrast with the crystallographic trends gleaned from the reported experimental structures, the equilibrium structures obtained from density functional theory indicate that the Jahn-Teller distortion mode plays a negligible role in affecting the Néel temperature. We explain this discrepancy and discuss how different researchers might draw disparate conclusions from the same evidence, in particular from aCCHMs and nCCHMs. Last, we propose a general method for utilizing CCHMs for screening large databases of structures.

Thermally Induced Anomaly in the Shear Behavior of Magnetite at High Pressure

Understanding the crystal structure, phase stability, and elasticity of magnetite (Fe${}_{3}$O${}_{4}$) at high pressure and temperature will explain the behavior of the Fe-O system deep within the Earth, and promote applications and engineering of Fe-O under extreme conditions. Using ultrasonic interferometry plus $i\phantom{\rule{0}{0ex}}n$ $s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}u$ x-ray techniques, the authors discover a temperature-driven anomaly in magnetite's shear behavior and a pressure-induced softening of its shear modulus, due to local atomic distortions and charge ordering of Fe cations at the octahedral sites in the inverse-spinel structure. This work will interest researchers in physics, chemistry, materials science, and geoscience.

Effects of Charge, Size, and Shape of Transition States, Bound Intermediates, and Confining Voids in Reactions of Alkenes on Solid Acids

AbstractSolid acids with sites varying in acid strength and in confining environments are used here to examine, through kinetic experiments and theory, how size and shape of organic guests and inorganic hosts at ion‐pair transition states (TS) influence reactivity. Reactions of C2−C4 alkene mixtures serve as the illustrative example; they involve kinetically‐relevant C−C coupling between alkenes and bound alkoxides that saturate the surface on microporous (TON) and mesoporous (SiAl) aluminosilicates of similar acid strength and on stronger heteropolytungstic acids (HPW). Rate constants (per H+) increase with increasing TS size (number of C‐atoms) because of the combined effects of size/substitution on proton affinities of coupling products and on van der Waals (vdW) contacts between carbocations and voids. When the voids and organic moieties are similar in dimension, these contacts require framework distortions, leading to dispersive interactions that are compensated in part by enthalpic penalties associated with the distortions, which are quantified here with DFT‐derived atomic displacements and distortion energies. DFT‐derived proton affinities of the coupling products and vdW interaction energies between the TS and voids represent the most accurate and complete descriptors of reactivity, as the size and shape of TS structures vary in C−C coupling steps and more generally in acid‐catalyzed transformations, and can be used in designing voids for stabilizing specific host‐guest pairs.

Effects of Annealing on the Residual Stress in γ-TiAl Alloy by Molecular Dynamics Simulation.

In this paper, molecular dynamics simulations are performed to study the annealing process of γ-TiAl alloy with different parameters after introducing residual stress into prepressing. By mainly focusing on the dynamic evolution process of microdefects during annealing and the distribution of residual stress, the relationship between microstructure and residual stress is investigated. The results show that there is no phase transition during annealing, but atom distortion occurs with the change of temperature, and the average grain size slightly increases after annealing. There are some atom clusters in the grains, with a few point defects, and the point defect concentration increases with the rise in temperature, and vice versa; the higher the annealing temperature, the fewer the point defects in the grain after annealing. Due to the grain boundary volume shrinkage and and an increase in the plastic deformation of the grain boundaries during cooling, stress is released, and the average residual stress along Y and Z directions after annealing is less than the average residual stress after prepressing.

Shifting the Shear Paradigm in the Crystallographic Models of Displacive Transformations in Metals and Alloys

Deformation twinning and martensitic transformations are characterized by the collective displacements of atoms, an orientation relationship, and specific morphologies. The current crystallographic models are based on the 150-year-old concept of shear. Simple shear is a deformation mode at constant volume, relevant for deformation twinning. For martensitic transformations, a generalized version called invariant plane strain is used; it is associated with one or two simple shears in the phenomenological theory of martensitic crystallography. As simple shears would involve unrealistic stresses, dislocation/disconnection-mediated versions of the usual models have been developed over the last decades. However, a fundamental question remains unsolved: how do the atoms move? The aim of this paper is to return to a crystallographic approach introduced a few years ago; the approach is based on a hard-sphere assumption and linear algebra. The atomic trajectories, lattice distortion, and shuffling (if required) are expressed as analytical functions of a unique angular parameter; the habit planes are calculated with the simple “untilted plane” criterion; non-Schmid behaviors associated with some twinning modes are also predicted. Examples of steel and magnesium alloys are taken from recent publications. The possibilities offered in mechanics and thermodynamics are briefly discussed.

First-principles study of lithium-ion diffusion in β-Li3PS4 for solid-state electrolytes

Understanding the role of partially occupied sites in Li-based superionic conductors is key to improving performance of solid-state electrolyte materials. We study the optimized structure of crystalline β-Li3PS4 and the Li-ion diffusion using first-principles calculations and the nudged elastic band method. Considering diffusion paths through both interstitial and vacancy exchanges, we calculate the migration energies of Li ions. We find that the phonon-mode softening and concurrent inversion symmetry breaking leads to a more stable structure with low symmetry. Atomic distortion from the phonon softening provides diffusion paths for Li ions with less migration energies than the ones in high-symmetry structures. Our results show that diffusion of Li ion is highly anisotropic through the armchair- or zigzag-shaped channels along the b-axis that contain Li-ion sites with fractional occupation.

First-principles surface interaction studies of aluminum-copper and aluminum-copper-magnesium secondary phases in aluminum alloys

First-principles density functional theory-based calculations were performed to study θ-phase Al2Cu, S-phase Al2CuMg surface stability, as well as their interactions with water molecules and chloride (Cl−) ions. These secondary phases are commonly found in aluminum-based alloys and are initiation points for localized corrosion. Density functional theory (DFT)-based simulations provide insight into the origins of localized (pitting) corrosion processes of aluminum-based alloys. For both phases studied, Cl− ions cause atomic distortions on the surface layers. The nature of the distortions could be a factor to weaken the interlayer bonds in the Al2Cu and Al2CuMg secondary phases, facilitating the corrosion process. Electronic structure calculations revealed not only electron charge transfer from Cl− ions to alloy surface but also electron sharing, suggesting ionic and covalent bonding features, respectively. The S-phase Al2CuMg structure has a more active surface than the θ-phase Al2Cu. We also found a higher tendency of formation of new species, such as Al3+, Al(OH)2+, HCl, AlCl2+, Al(OH)Cl+, and Cl2 on the S-phase Al2CuMg surface. Surface chemical reactions and resultant species present contribute to establishment of local surface chemistry that influences the corrosion behavior of aluminum alloys.

Impacts of atomic scale lattice distortion on dislocation activity in high-entropy alloys

The concept of high-entropy alloys (HEAs) introduces a new path of developing advanced materials with unique properties, among which excellent mechanical properties have attracted intensive interests. Lattice distortion was believed to contribute to the excellent mechanical properties of HEAs. However, it is not known that how atomic scale lattice distortion impacts dislocation behavior in HEAs. By performing large-scale molecular dynamic simulations, we demonstrate that local lattice distortion leads to considerable variation of dislocation dissociation width and introduces energy barriers strongly against dislocation motion, which significantly affect dislocation activity in HEAs.

The Enabling Electronic Motif for Topological Insulation in ABO3 Perovskites

Stable oxide topological insulators (TIs) have been sought for years, but none have been found; whereas heavier (selenides, tellurides) chalcogenides can be TIs. The basic contradiction between topological insulation and thermodynamic stability is pointed out, offering a narrow window of opportunity. The electronic motif is first identified and can achieve topological band inversion in ABO3 as a lone‐pair, electron‐rich B atom (e.g., Te, I, Bi) at the octahedral site. Then, twelve ABO3 compounds are designed in the assumed cubic perovskite structure, which satisfy this electronic motif and are indeed found by density function theory calculations to be TIs. Next, it is illustrated that poorly screened ionic oxides with large inversion energies undergo energy‐lowering atomic distortions that destabilize the cubic TI phase and remove band inversion. The coexistence windows of topological band inversion and structure stability can nevertheless be expanded under moderate pressures (15 and 35 GPa, respectively, for BaTeO3 and RbIO3). This study traces the principles needed to design stable oxide topological insulators at ambient pressures as a) a search for oxides with small inversion energies; b) design of large inversion‐energy oxide TIs that can be stabilized by pressure; and c) a search for covalent oxides where TI‐removing atomic displacements can be effectively screened out.

Chemisorption of hydrogen on graphene: insights from atomistic simulations

The properties of graphene can be chemically altered by changing its local binding configurations. In the present work, we investigate fundamentals of chemisorption of atomic hydrogen on graphene and its influence on mechanical properties of as-hydrogenated graphene by means of molecular dynamics simulations. Our simulation results indicate that there are diversiform hydrogen-graphene configurations formed in the chemisorption process. Especially, energetically favorable hydrogen pairs result in less even no atomic distortion of graphene than sp3 hybridization. The hydrogenation-induced deterioration of mechanical properties of graphene shows a strong dependence on its chirality. The evolution of bond structures in uniaxial tension along armchair direction is more sensitive to local failure of graphene than zigzag direction, leading to a more pronounced decrease in both fracture stress and fracture strain. It is indicated that the chemisorption of hydrogen on graphene can be strongly affected by operating temperature primarily due to the temperature dependent graphene morphology. These findings advance our understanding of chemical vapor deposition of graphene synthesis and hydrogenation of graphene.

Amino Acid Functionalization of Doped Single-Walled Carbon Nanotubes: Effects of Dopants and Side Chains as Well as Zwitterionic Stabilizations.

Functionalization of single-walled carbon nanotubes (SWCNTs) is necessitated in a number of conditions such as drug delivery, and here amino acid functionalization of SWCNTs is conducted within the framework of density functional theory. Functionalization efficiencies of Gly are largely determined by dopants, as a combined effect of atomic radius, electronic configuration, and distortion to SWCNTs. Different functionalization sites in Gly have divergent interaction strengths with M/SWCNTs that decline as Ob > N > Oa, and this trend seems almost independent of the identity of metallic dopants. B/SWCNT behaves distinctly and prefers to the N site. Dopants affect principally interaction strengths, while amino acids regulate significantly both functionalization configurations and interaction energies. Then focus is given to stabilization of zwitterionic amino acids due to enhanced interactions with the widely used zwitterionic drugs. All metallic dopants render zwitterionic Gly to be the most stable, and side chains in amino acids rather than dopants in M/SWCNTs cause more pronounced effects to zwitterionic stabilizations. Charge transfers between amino acids and M/SWCNTs are closely associated with zwitterionic stabilization effects, and different charge transfer mechanisms between M/SWCNTs and metal ions are interpreted. Thus, this work provides a comprehensive understanding of amino acid functionalization of M/SWCNTs.

Tuning electrochemical catalytic activity of defective 2D terrace MoSe2 heterogeneous catalyst via cobalt doping

Defective 2D terrace MoSe2/CoMoSe lateral and vertical heterostructures nanolayers electrocatalyst via metal cobalt doping displays ameliorative HER activity.

Scale invariance of structural transformations in plastically deformed nanostructured solids

The scale-invariant mechanical behavior of a nanostructured solid is associated with plastic distortion as a major mechanism of nano- and microscale structural transformations. Active grain boundary sliding in a deformed material (microscale) within its highly developed planar subsystem (nanograin boundaries) causes a progressive increase in lattice curvature and plastic distortion of atoms which produces nonequilibrium vacant sites in the nanostructure. The motion of nonequilibrium point defects in nanostructure curvature zones provides conditions for noncrystallographic plastic flow, dissolution or dispersion of initial phases, and formation of nonequilibrium phases in a deformed material. The possibility of reversible structural phase transformations in the presence of high lattice curvature opens the way to greatly increase the fatigue life of surface nanostructured polycrystalline materials.

Binding Isotope Effects for Interrogating Enzyme-Substrate Interactions.

Equilibrium binding isotope effects (BIEs) report on the bond vibrational status of enzyme substrates in the Michaelis complex prior to the transition state and how they differ from the solution state. Accordingly, BIEs provide an experimental means of interrogating enzyme-substrate interactions and inform on the influence of enzyme-mediated atomic distortions in modulating substrate reactivity. In this chapter, we outline a rapid equilibrium dialysis method that our lab has used to measure BIEs for several enzyme systems. Implementation of the rapid equilibrium dialysis approach is described in the context of our recent studies on the substrate bonding environment for the human protein lysine N-methyltransferase NSD2. A summary of BIE effects provides context for the range of experimental values.

Atomic Resolution Distortion Analysis of Yttrium-Doped Barium Zirconate

Atomic Resolution Distortion Analysis of Yttrium-Doped Barium Zirconate