Relative Atomic Displacements Research Articles

Lattice Distortion Promotes Incipient Plasticity in Multiprincipal Element Alloys.

Multiprincipal element alloys usually exhibit earlier pop-in events than pure metals and dilute solid solutions during nanoindentation experiments. To understand the origin of this phenomenon, large-scale atomic simulations of nanoindentation were performed on a series of metallic materials to investigate the underlying physics of incipient plasticity at the nanoscale. Statistical result shows that lattice distortion δ and normalized critical pressure pc/Es follow a power-law relationship. Via quantitative analysis on the relative positions of the atoms within the nearest neighbor shell, the physical origin of premature incipient plasticity is revealed as severe lattice distortion induces large relative atomic displacement, so only a small indentation strain is required to meet the critical displacement threshold that triggers incipient plasticity. Therefore, for perfect crystals, lattice distortion is an intrinsic and determinative factor that affects the first pop-in event.

Effect and Regulation Mechanism of Post-deposition Annealing on the Ferroelectric Properties of AlScN Thin Films.

Integrating ferroelectric AlScN with III-N semiconductors to enhance the performance and tunability of nitride devices requires high-quality AlScN films. This work focuses on the effect and regulation mechanism of post-annealing in pure N2 on the crystal quality and ferroelectric properties of AlScN films. It is found that the crystal quality improves with increasing annealing temperatures. Remarkably, the leakage current of AlScN films caused by grain boundaries could be reduced by four orders of magnitude after annealing at 400 °C. The crystal growth dynamics simulations and band structure calculations indicate that the energy supplied by the temperature facilitates the evolution of abnormally oriented grains to have a better c-axis orientation, resulting in the defect states at the Fermi-level disappearing, which is mainly the reason for the leakage current decrease. More interestingly, the reduction of leakage current leads to the previously leaking region exhibiting ferroelectric properties, which is of great significance to improve the ferroelectricity of AlScN and ensure the uniformity of devices. Furthermore, annealing enhances the tensile strain on the film, which flattens the energy landscape of ferroelectric switching and reduces the coercive field. However, the risk of incorporation of oxygen will also be increased if the annealing temperatures are higher than 400 °C, which will not only reduce the relative displacement of metal atoms and nitrogen atoms in AlScN but also enhance the ferroelectric depolarization field, leading to the remnant polarization decreasing dramatically. These discoveries facilitate a deeper understanding of the influencing mechanism on the ferroelectric properties of AlScN films and provide a direction for obtaining high-quality AlScN.

The nanoglasses embedded NiCo2O4 nanocomposite induced by S interstitial doping for supercapacitor electrodes

The nanoglasses embedded NiCo2O4 on Ni foam induced by the S interstitial doping (S-NCO@NF) is achieved in this work. Attributed to the interconnected nanostructure, the generation of nanoglasses as well as the S doping, the obtained S-NCO@NF electrode demonstrates strong OH– adsorption capability and excellent electrochemical performances. Moreover, the derivation of NiCo2O4 nanoglasses was studied by DFT calculations. The insertion of S atoms into NiCo2O4 crystal induces the dramatic relative atomic displacement and thus results in the generation of nanoglasses with many unsaturated atoms and tremendous dangling bonds. Such unique atomic structure and S doping endow the S interstitial doped models with remarkable OH– adsorption capability. Notably, the OH– adsorption calculations reveal that the doped S atoms also take part in redox reactions and perform as active sites, providing a new insight into the nonmetal doped models. The S-NCO@NF electrode demonstrates high capacitance, remarkable rate capacity and excellent cycle stability (84.44% retention after 6000 cycles). This current paper undoubtedly provides a new perspective for describing the enhancement effect of doping atoms on electrode performance.

ANALYTICAL RESEARCH METHODS OF HOT DEFORMATION OF METALS

Hot processing of metals by pressure has a number of positive qualities: the constituent parts of the metal are distributed more evenly than before processing; grain sizes decrease, which leads to improved mechanical properties; the metal becomes denser. Pressure-treated metals have a fibrous structure. The hot working process is one of the types of metal forming processes and it takes place at temperatures higher than the recrystallization temperature. During hot deformation, the metal is also strengthened (hot defamation), but it is completely removed in the process of recrystallization. With it, the plasticity of the metal is higher, and the resistance to deformation is approximately 10 times lower than during cold deformation. Most metals have a recrystallization temperature that is half or one-third of the melting temperature of the metal. Therefore, we can say that when the metal is plastically deformed above the recrystallization temperature, but below the burning point, this is a hot process of metal deformation. Elastic and then plastic deformation occur under the influence of external forces on the material. Pressure treatment of metals is possible due to their plasticity. A sign of elastic deformation is its reversibility, that is, its disappearance after removal of the load. The physical essence of elastic deformation is explained by the slight relative displacement of atoms in the crystal lattice, which return to their previous positions after the load is removed. There are such methods of hot processing as: hot rolling, hot forging, hot stamping, hot drawing.

Topological analysis of chemical bonding in the layered FePSe3 upon pressure-induced phase transitions.

Two pressure-induced phase transitions have been theoretically studied in the layered iron phosphorus triselenide (FePSe3 ). Topological analysis of chemical bonding in FePSe3 has been performed based on the results of first-principles calculations within the periodic linear combination of atomic orbitals (LCAO) method with hybrid Hartree-Fock-DFT B3LYP functional. The first transition at about 6 GPa is accompanied by the symmetry change from to C2/m, whereas the semiconductor-to-metal transition (SMT) occurs at about 13 GPa leading to the symmetry change from C2/m to . We found that the collapse of the band gap at about 13 GPa occurs due to changes in the electronic structure of FePSe3 induced by relative displacements of phosphorus or selenium atoms along the c-axis direction under pressure. The results of the topological analysis of the electron density and its Laplacian demonstrate that the pressure changes not only the interatomic distances but also the bond nature between the intralayer and interlayer phosphorus atoms. The interlayer P-P interactions are absent in two non-metallic FePSe3 phases while after SMT the intralayer P-P interactions weaken and the interlayer P-P interactions appear.

Origin of pressure-induced insulator-to-metal transition in the van der Waals compound FePS3 from first-principles calculations.

Pressure-induced insulator-to-metal transition (IMT) has been studied in the van der Waals compound iron thiophosphate (FePS3 ) using first-principles calculations within the periodic linear combination of atomic orbitals method with hybrid Hartree-Fock-DFT B3LYP functional. Our calculations reproduce correctly the IMT at ∼15 GPa, which is accompanied by a reduction of the unit cell volume and of the vdW gap. We found from the detailed analysis of the projected density of states that the 3p states of phosphorus atoms contribute significantly at the bottom of the conduction band. As a result, the collapse of the band gap occurs due to changes in the electronic structure of FePS3 induced by relative displacements of phosphorus or sulfur atoms along the c-axis direction under pressure.

First-order Raman scattering of rare-earth containingi-MAX single crystals(Mo2/3RE1/3)2AlC (RE=Nd,Gd,Dy,Ho,Er)

Herein, we report on the growth of single crystals of various ${\left({\text{Mo}}_{2/3}{\mathrm{RE}}_{1/3}\right)}_{2}\text{Al}\text{C} (\mathrm{RE}=\mathrm{Nd},\phantom{\rule{0.16em}{0ex}}\mathrm{Gd},\phantom{\rule{0.16em}{0ex}}\mathrm{Dy},\phantom{\rule{0.16em}{0ex}}\mathrm{Ho},\phantom{\rule{0.16em}{0ex}}\mathrm{Er}) i$-MAX phases and their Raman characterization. Using first principles, the wave numbers of the various phonon modes and their relative atomic displacements are calculated and compared to experimental results. Twelve high-intensity Raman peaks are identified as the fingerprint of this new family of rare-earth containing $i$-MAX phases, thus being a useful tool to investigate their corresponding composition and structural properties. Indeed, while a redshift is observed in the low-wave-number range due to an increase of the rare-earth atomic mass when moving from left to right on the lanthanide row, a blueshift is observed for most of the high-wave-number modes due to a strengthening of the bonds. A complete classification of bond stiffnesses is achieved based on the direct dependence of a phonon mode wave number with respect to the bond stiffness. Finally, STEM images are used to confirm the crystal structure.

Room-temperature ferroelectricity in MoTe2 down to the atomic monolayer limit

Ferroelectrics allow for a wide range of intriguing applications. However, maintaining ferroelectricity has been hampered by intrinsic depolarization effects. Here, by combining first-principles calculations and experimental studies, we report on the discovery of robust room-temperature out-of-plane ferroelectricity which is realized in the thinnest monolayer MoTe2 with unexploited distorted 1T (d1T) phase. The origin of the ferroelectricity in d1T-MoTe2 results from the spontaneous symmetry breaking due to the relative atomic displacements of Mo atoms and Te atoms. Furthermore, a large ON/OFF resistance ratio is achieved in ferroelectric devices composed of MoTe2-based van der Waals heterostructure. Our work demonstrates that ferroelectricity can exist in two-dimensional layered material down to the atomic monolayer limit, which can result in new functionalities and achieve unexpected applications in atomic-scale electronic devices.

High critical transition temperature of lead-based perovskite ferroelectric crystals: A machine learning study

Ferroelectrics undergoes a reversible structural phase from the ferroelectric phase to the paraelectric phase when its temperature exceeds the critical temperature namely Curie temperature <i>T</i><sub>c</sub>. As ferro-paraelectric phase transition is always accompanied by heat-flow, dielectric and pyroelectric anomaly, the value of <i>T</i><sub>c</sub> is extremely important for ferroelectrics. In this paper, the Curie temperature of lead-based perovskite ferroelectric solid solution is studied by machine learning methods including kernel ridge regression (KRR), support vector regression (SVR) and extremely randomized trees regression (ETR). We collect the <i>T</i><sub>c</sub> values of 205 different lead-based perovskites from published experimental papers, both simple perovskites with only one type of <i>B</i> site ion and complex perovskites with up to 5 kinds of ions in <i>B</i> position such as PMN-PFN-PZT are gathered. The diversity of our dataset is guaranteed for the good generalization of our model in perovskite solid solution of different complexity. The features are constructed from the physical and chemical properties of the <i>B</i> site elements in corresponding materials. The weighted-average and variance of the elemental properties are calculated and fed to machine learning models. We use the 5 runs of ten fold cross-validation method to evaluate the machine learning models. The hyperparameters are also chosen carefully with the cross-validation to avoid over fitting. The radial basis function kernel is used in both KRR and SVR. The insensitive error in the SVR is set to be 4 which is comparable to the random error in experiment. From our cross-validation, we find that the mean average errors (MAEs) between the predicted and experimental values of the machine learning methods are 14.4 K, 14.7 K, and 16.1 K, respectively. And the root-mean-square errors (RMSEs) are 22.5 K, 23.4 K, 23.8 K, respectively. After the optimization and the evaluation, our three machine learning models are stacked together by averaging the output of each regression model and thus building an ensemble model. The MAE of the ensemble model is 13.9 K. The RMSE of the ensemble model is 21.4 K. The predicted values keep a correlation coefficient of 0.97 with the experimental values. From the variance reduction in ETR, we derive the importance of our features when determining the Curie temperatures. The five most important factors in our ETR model are " weighted-average thermal conductivity”, " weighted-average conductivity”, " variance of specific heat capacity”, " weighted-average element number”, and " weighted-average relative atomic displacement”. We predict the Curie temperatures higher than those of 200000 types of lead-based perovskites after being trained. Now, we provide two ferroelectric materials that may have high Curie temperatures: 0.02PbMn<sub>1/2</sub>Nb<sub>1/2</sub>O<sub>3</sub>-0.98PbTiO<sub>3</sub> (0.02PMN-0.98PT) and 0.02PbGa<sub>1/2</sub>Nb<sub>1/2</sub>-0.02PbMn<sub>1/2</sub>Nb<sub>1/2</sub>O<sub>3</sub>-0.96PbTiO<sub>3</sub> (0.02PGN-0.02PMN-0.96PT). The predicted Curie temperatures of them are 481 ℃ and 466 ℃, respectively.

Correlated Debye model for atomic motions in metal nanocrystals

The Correlated Debye model for the mean square relative displacement of atoms in near-neighbour coordination shells has been extended to include the effect of finite crystal size. This correctly explains the increase in Debye–Waller coefficient observed for metal nanocrystals. A good match with Molecular Dynamics simulations of Pd nanocrystals is obtained if, in addition to the phonon confinement effect of the finite domain size, proper consideration is also given to the static disorder component caused by the undercoordination of surface atoms. The new model, which addresses the analysis of the Pair Distribution Function and powder diffraction data collected at different temperatures, was preliminarily tested on recently published experimental data on nanocrystalline Pt powders.

Novel two-dimensional ferroelectric PbTe under tension: A first-principles prediction

Enhanced ferroelectricity in two-dimensional (2D) SnTe exhibiting a higher transition temperature (Tc) than its bulk counterpart was recently discovered [Chang et al., Science 353(6296), 274–278 (2016)]. Herein, we report that nonferroelectric PbTe can be transformed into a ferroelectric phase by downsizing to two dimensions with suitable equi-biaxial tension. The crystal structure of the ferroelectric phase of 2D PbTe was determined using evolutionary algorithms and density functional theory. The dynamic stabilities of the predicted new phases were investigated using phonon calculations. To validate our results obtained using PbTe, we have also studied the ferroelectricity in GeTe and SnTe at the 2D level and compared them with the literature. The unequal lattice constants and the relative atomic displacements are found to be responsible for ferroelectricity in 2D GeTe, SnTe, and strained PbTe. This study facilitates the development of new 2D ferroelectrics via strain engineering and promotes the integration of ferroelectric devices.

Crystallization of amorphous materials and deformation mechanism of nanocrystalline materials under cutting loads: A molecular dynamics simulation approach

The crystallization of amorphous and cutting process of nanocrystalline copper was investigated by the use of a molecular dynamics (MD) simulation. Stress-induced crystallization of amorphous in cutting process was observed. By the common neighbor analysis (CNA), it was found that nuclei were first formed in the larger stress region around the rake face and tip of the tool and then they grew quickly under stress. The growth and consolidation process of grains was analyzed, using relative atomic displacement vector method. When grains grew in contact with each other, a merger occurred. Grains were combined firstly in a local area, and rotated to discharge the excess free volume out of boundaries before the merging process completed. Through calculating the shear strain of atoms in the cutting process, the serrated chips and four shear bands were observed clearly. The radius distribution function (RDF) curve of the shear band shows that the cutting of crystalline was carried out in the amorphous state. The shear angle of less than 45° was observed in the simulation, which is consistent with the experiments reported and validates the simulation results in this paper.

Plastic deformation of a model glass induced by a local shear transformation.

The effect of a local shear transformation on plastic deformation of a three-dimensional amorphous solid is studied using molecular dynamics simulations. We consider a spherical inclusion, which is gradually transformed into an ellipsoid of the same volume and then converted back into the sphere. It is shown that at sufficiently large strain amplitudes, the deformation of the material involves localized plastic events that are identified based on the relative displacement of atoms before and after the shear transformation. We find that the density profiles of cage jumps decay away from the inclusion, which correlates well with the radial dependence of the local deformation of the material. At the same strain amplitude, the plastic deformation becomes more pronounced in the cases of weakly damped dynamics or large time scales of the shear transformation. We show that the density profiles can be characterized by the universal function of the radial distance multiplied by a dimensionless factor that depends on the friction coefficient and the time scale of the shear event.

Local vibrational dynamics of hematite (α-Fe₂O₃) studied by extended x-ray absorption fine structure and molecular dynamics.

The local vibrational dynamics of hematite (α-Fe2O3) has been investigated by temperature-dependent extended x-ray absorption fine structure spectroscopy and molecular dynamics simulations. The local dynamics of both the short and long nearest-neighbor Fe-O distances has been singled out, i.e., their local thermal expansion and the parallel and perpendicular mean-square relative atomic displacements have been determined, obtaining a partial agreement with molecular dynamics. No evidence of the Morin transition has been observed. More importantly, the strong anisotropy of relative thermal vibrations found for the short Fe-O distance has been related to its negative thermal expansion. The differences between the local dynamics of short and long Fe-O distances are discussed in terms of projection and correlation of atomic motion. As a result, we can conclude that the short Fe-O bond is stiffer to stretching and softer to bending than the long Fe-O bond.

Ab initiolattice dynamical studies of silicon clathrate frameworks and their negative thermal expansion

The thermal and lattice dynamical properties of seven silicon clathrate framework structures are investigated with ab initio density functional methods (frameworks I, II, IV, V, VII, VIII, and H). The negative thermal expansion (NTE) phenomenon is investigated by means of quasiharmonic approximation and applying it to equal time displacement correlation functions. The thermal properties of the studied clathrate frameworks, excluding the VII framework, resemble those of the crystalline silicon diamond structure. The clathrate framework VII was found to have anomalous NTE temperature range up to 300 K and it is suitable for further studies of the mechanisms of NTE. Investigation of the displacement correlation functions revealed that in NTE, the volume derivatives of the mean square displacements and mean square relative displacements of atoms behave similarly to the vibrational entropy volume derivatives and consequently to the coefficients of thermal expansion as a function of temperature. All studied clathrate frameworks, excluding the VII framework, possess a phonon band gap or even two in the case of the framework V.

Structural Transformation Relationship for Hafnia Ferroelectric Phase

The ferroelectricity of hafnia-based thin films with a dominant phase of orthorhombic Pca21 has been reported. However, the relationship of structural transformations between the orthorhombic Pca21 and other hafnia structures remains unclear. In this work, all the structures have been optimized. Then, the fluorite-related structures have been used to analyze the structural relationship. Calculations of the lattice energies and the relative atomic displacements between the structures suggest that the Pca21 phase may originate from the P42/nmc or Pbca phases.

Comparison of all-atom and coarse-grained normal mode analysis in the elastic network model

Elastic network-based normal mode analyses (EN-NMA) of four pairs of open-closed proteins (Lactoferrin, Maltodextrin-binding protein, LAO-binding protein, and Adenylate kinase) were conducted using both all-atom and coarse-grained models. The results indicated that the performance of the all-atom model was similar to that of the coarse-grained model in terms of predicting the conformational changes of backbones. Moreover, dynamic behavior was examined by studying relative atomic displacements and shapes of the dominant mode. For instance, for Maltodextrin-binding protein, the results from the all-atom model differed from those of the coarse-grained model, especially for residues that are biologically relevant. The coarse-grained model has better computational efficiency than the allatom model. However, the former may misrepresent the key dynamics of a protein related to biological functions as a consequence of excessive coarse approximation. Considering that the current power even in a high-end personal computer is sufficient to handle most of protein structures with up to 1,000 residues in a reasonable manner, which can only be used with supercomputers a few decades ago, an all-atom-based EN-NMA may deserve more attention as a reliable and powerful computational tool for protein dynamics study over the conventional coarse-graining approach.

Noble-transition metal nanoparticle breathing in a reactive gas atmosphere

In situ high-energy X-ray diffraction coupled to atomic pair distribution function analysis is used to obtain fundamental insight into the effect of the reactive gas environment on the atomic-scale structure of metallic particles less than 10 nm in size. To substantiate our recent discovery we investigate a wide range of noble-transition metal nanoparticles and confirm that they expand and contract radially when treated in oxidizing (O2) and reducing (H2) atmospheres, respectively. The results are confirmed by supplementary XAFS experiments. Using computer simulations guided by the experimental diffraction data we quantify the effect in terms of both relative lattice strain and absolute atomic displacements. In particular, we show that the effect leads to a small percent of extra surface strain corresponding to several tenths of Ångström displacements of the atoms at the outmost layer of the particles. The effect then gradually decays to zero within 4 atomic layers inside the particles. We also show that, reminiscent of a breathing type structural transformation, the effect is reproducible and reversible. We argue that because of its significance and widespread occurrence the effect should be taken into account in nanoparticle research.

A comprehensive understanding of melting temperature of nanowire, nanotube and bulk counterpart

Surface energies of nanostructures are of considerable interest, and thermodynamic methods have provided valuable insight into the physics and chemistry of these systems. Although the effect of surface energy on melting behaviors of nanostructures has been widely investigated in theoretical calculations and simulations, from the thermodynamics at the nanometer scale point of view, the comprehensive understanding of the fundamental physical and chemical issues involved in nanostructures' melting is still lacking. For instance, nanostructures with negative curvature, such as nanotubes, show different melting behaviors compared with the nanostructures with positive curvature such as nanowires, and both nanotubes and nanowires exhibit abnormal melting temperature compared with that of the bulk counterparts. Herein, we put forward a general model to elucidate the melting temperature of the nanostructures with positive and negative curvatures based on the surface energy at the nanometer. Further, the surface mean square relative atomic displacement (MSRD) of these nanostructures has been studied from the perspective of the size-dependent cohesive energy consideration, which can provide the atomic understanding of the nanostructures' melting. Theoretical analyses indicate that both melting temperatures of the nanostructures with the positive and negative curvatures decrease with decreasing dimensionality, and the surface MSRDs show different size effects in the systems with the positive and negative curvatures, respectively. The melting temperature of the surface with the negative curvature is higher than that of the surface with the positive curvature, and both melting temperatures are smaller than that of the bulk counterpart when the size of nanostructures is less than a threshold value. The unique melting behaviors of nanostructures are attributed to the size- and curvature-dependent surface energy of nanostructures. These results provide new insight into the fundamental understanding of the melting temperature of nanostructures.

Bond thermal expansion and effective pair potential in crystals: the case of cadmiumselenide

The local dynamics of cadmium selenide (CdSe) with wurtzite structure has beeninvestigated by molecular dynamics simulations, using a many-body Tersoff potential. Theradial distribution functions (i.e., the effective pair potentials) of the first sevencoordination shells have been determined as a function of temperature, as well as theirparallel and perpendicular mean-square relative atomic displacements. The bond thermalexpansion of the first coordination shell is mainly due to the asymmetry of theeffective pair potential. In contrast, the bond thermal expansion of the outer shells ismostly due to a rigid shift of the effective pair potential. This behavior, recentlyobserved also in simple cubic monoatomic crystals, can be generalized and related tothe correlation of atomic motion. Finally, a shift toward lower values of the firstSe–Cd effective pair potential has been observed when increasing the temperature,confirming previous findings by extended x-ray absorption fine-structure measurements.Differently from superionic conductors like AgI and CuBr, in which this anomalousnegative shift was tentatively explained by cluster distortion and cation diffusion,the negative shift of CdSe is related to the peculiar properties of the crystallinepotential.