Effect Of Local Distortion Research Articles

Effects of local distortion on magnetocrystalline anisotropy in α″-Fe16N2: a first-principles study

The effects of lattice distortion on magnetocrystalline anisotropy in α″-Fe16N2 was studied by first-principles calculations. We found that local distortion of the Fe lattice induced by interstitial N atoms is a key factor for realizing uniaxial magnetic anisotropy in α″-Fe16N2 systems. If the local distortion disappears, the uniaxial anisotropy constant K u becomes nearly zero even though there is global distortion with an axial ratio c/a = 1.1. We also discuss one possibility that the random distribution of N atoms suppresses the local distortion effect and the uniaxial magnetic anisotropy in FeN for the α′ phase and martensite phase.

Underwater-image super-resolution via range-dependency learning of multiscale features

Optical imaging instruments have been widely deployed in underwater-engineering systems, playing an important role in underwater-object localization, detection, and recognition. However, underwater images suffer from the adverse effects of local distortion and global haze. Consequently, raw underwater images have limitations when used for display and vision tasks. Super resolution (SR) has been increasingly exploited for perceptual image quality improvement. However, underwater-image SR is a relatively underexplored area, most existing methods being unable to reduce the adverse effects of underwater images. Moreover, in contrast to land-based high-performance platforms, the power supply and computational resources of underwater platforms are limited, making them difficult to use in large-scale models. To solve these problems, this study developed a novel range-dependency learning network to present the short- and long-range dependency of multiscale features. Such a mechanism could provide more detailed and accurate texture information for underwater-image SR, improving underwater-image SR performance. Moreover, a channel-splitting module was designed to generate the channel bands which could extract texture details and global structural information at different scales while reducing the number of parameters, thus accelerating the training speed of the model and maintaining good performance. Our novel network could reach an optimal tradeoff between the underwater-image SR performance and efficiency, which was demonstrated by experimental comparisons and an ablation study.

Robustness of electronic screening effects in electron spectroscopies: Example of V2O5

In bulk and low-dimensional extended systems, the screening of excitations by the electron cloud is a key feature governing spectroscopic properties. Widely used computational approaches, especially in the framework of many-body perturbation theory, such as the GW approximation and the resulting approximate Bethe-Salpeter equation, are explicitly formulated in terms of the screened Coulomb interaction. In the present work we explore the effect of screening in absorption and electron energy loss spectroscopy, concentrating on the effect of local distortions on the screening and elucidating the resulting changes in the various spectra. Using the layered bulk oxide V$_2$O$_5$ as prototype material, we show in which way local distortions affect the screening, and in which way changes in the screening impact electron energy loss and absorption spectra including excitons. We highlight cancellations that make many-body effects in the spectra very robust with respect to structural modifications, while the band structure undergoes significant changes and the nature of the excitations may also be affected. This yields insight concerning the structure-properties relations that are crucial for the use of V$_2$O$_5$ as energy storage material, and more generally, that may be used to optimize the analysis and the calculation of electronic spectra in complex materials.

Investigation of local distortion effects on X-ray absorption of ferroelectric perovskites from first principles simulations.

Understanding the role of ferroelectric polarization in modulating the electronic and structural properties of crystals is critical for advancing these materials for overcoming various technological and scientific challenges. However, due to difficulties in performing experimental methods with the required resolution, or in interpreting the results of methods therein, the nanoscale morphology and response of these surfaces to external electric fields has not been properly elaborated. In this work we investigate the effect of ferroelectric polarization and local distortions in a BaTiO3 perovskite, using two widely used computational approaches which treat the many-body nature of X-ray excitations using different philosophies, namely the many-body, delta-self-consistent-field determinant (mb-ΔSCF) and the Bethe-Salpeter equation (BSE) approaches. We show that in agreement with our experiments, both approaches consistently predict higher excitations of the main peak in the O-K edge for the surface with upward polarization. However, the mb-ΔSCF approach mostly fails to capture the L2,3 separations at the Ti-L edge, due to the absence of spin-orbit coupling in Kohn-Sham density functional theory (KS-DFT) at the generalized gradient approximation level. On the other hand, and most promising, we show that application of the GW/BSE approach successfully reproduces the experimental XAS, both the relative peak intensities as well as the L2,3 separations at the Ti-L edges upon ferroelectric switching. Thus simulated XAS is shown to be a powerful method for capturing the nanoscale structure of complex materials, and we underscore the need for many-body perturbation approaches, with explicit consideration of core-hole and multiplet effects, for capturing the essential physics in these systems.

Effect of Local Structural Distortions on Antiferroelectric-Ferroelectric Phase Transition in Dilute Solid Solutions of KxNa1-xNbO3.

The fundamental principles that govern antiferroelectric (AFE)-ferroelectric (FE) transitions are not well understood for many solid solutions of perovskite compounds. For example, crystal chemical considerations based on the average Goldschmidt tolerance factor or ionic polarizability do not precisely predict the boundary between the AFE and FE phases in dilute solid solutions of alkali niobates, such as KxNa1-xNbO3 (x ≤ 0.02). Here, based on detailed structural analysis from neutron total scattering experiments, we provide insights about how the relative local distortions around the A- and B-sites of the ABO3 perovskite structure affect the AFE/FE order of the average crystallographic phases in KxNa1-xNbO3. We show that a higher (lower) ratio of B-site-centered distortions over A-site-centered distortions drives transition toward a long-range FE (AFE) phase, which is based on a competition between the long-range polarizing field of the Nb-O dipoles and the disordering effect of local distortions around the A-site. Our study provides a predictive tool for designing complex solid-solution perovskites with tunable (anti)ferroelectric polarization properties, which can be of interest for various energy-related applications such as high-density energy storage and solid-state cooling.

The effect of different energy portions on the 2D/3D stability swapping for 13-atom metal clusters.

The complexity of Cu13, Ag13, and Au13 coinage-metal clusters was investigated through their energy contributions via a density functional theory study, considering improvements in the PBE functional, such as van der Waals (vdW) corrections, spin-orbit coupling (SOC), Hubbard term (+U), and their combinations. Investigating two-dimensional (planar 2D) and three-dimensional (distorted 3D, CUB - cuboctahedral, and ICO - icosahedral) configurations, we found that vdW corrections are dominant in modulating the stability swapping between 2D and ICO (3D) for Ag13 (Au13), whereas for Cu13 its role is increasing the relative stability between 2D (least stable) and 3D (most stable), setting ICO as the reference. Among the energy portions that constitute the relative total energy, the dimensionality difference correlates with the magnitude of the relative dispersion energy (large for 2D/ICO and small for 3D/ICO) as the causal factor responsible for an eventual stability swapping. For instance, empirical vdW corrections may favor Ag13 as ICO, while semi empirical ones tend to swap the stability by favoring 2D. The same tendency is observed for Au13, except when SOC is included, which enlarges the stability of 3D over 2D. Energy decomposition analysis combined with the natural orbitals for the chemical valence approach confirmed the correlations between the dimensionality difference and the magnitude of the relative dispersion energies. Our structural analysis protocol was able to capture the local distortion effects (or even their absence) through the quantification of the Hausdorff chirality measure. Here, ICO, CUB, and 2D are achiral configurations for all coinage-metal clusters, whereas Cu13 as 3D presents a slight chirality when vdW correction based on many body dispersion is used, at the same time Ag13 as 3D turned out to be chiral for all calculation protocols as evidence of the role of the chemical composition.

Exchange interactions, Jahn-Teller coupling, and multipole orders in pseudospin one-half 5d2 Mott insulators

We develop a microscopic theory of multipole interactions and orderings in $5{d}^{2}$ transition metal ion compounds. In a cubic environment, the ground state of $5{d}^{2}$ ions is a non-Kramers ${E}_{g}$ doublet, which is nonmagnetic but hosts quadrupole and octupole moments. We derive low-energy pseudospin one-half Hamiltonians describing various spin-orbital exchange processes between these ions. Direct overlap of the ${t}_{2g}$ orbitals results in bond-dependent pseudospin interactions similar to those for ${e}_{g}$ orbitals in manganites, except for different orientations of the pseudospin easy axes. On the other hand, the superexchange process, where two different ${t}_{2g}$ orbitals communicate via oxygen ions, generates new types of pairwise interactions. In perovskites with ${180}^{\ensuremath{\circ}}$ bonding, we find nearly equal mixture of Heisenberg and ${e}_{g}$ orbital compass-type couplings. The ${90}^{\ensuremath{\circ}}$ superexchange in compounds with edge-shared octahedra is most unusual: Despite highly anisotropic shapes of the ${E}_{g}$ wave functions, the pseudospin interactions have no bond dependence. We consider the ${E}_{g}$ pseudospin models on various lattices and obtain their ground state properties using analytical and numerical methods. On the honeycomb lattice, we observe a duality with the extended Kitaev model, and use it to uncover a critical point where the quadrupole and octupole states are exactly degenerate. On the triangular lattice, an exotic pseudospin state, corresponding to the coherent superposition of vortex-type quadrupole and ferri-type octupole orders, is realized due to geometrical frustration. This state breaks both spatial and time-reversal symmetries, but possesses no dipolar magnetism. We also consider Jahn-Teller coupling effects and lattice mediated interactions between ${E}_{g}$ pseudospins, and find that they support quadrupole order. Possible implications of the results for recent experiments on double perovskite osmates are discussed, including effects of local distortions on the pseudospin wave functions and interactions.

Semiconducting SiGeSn high-entropy alloy: A density functional theory study

High-entropy alloys (HEAs), which have been intensely studied due to their excellent mechanical properties, generally refer to alloys with multiple equimolar or nearly equimolar elements. According to this definition, Si-Ge-Sn alloys with equal or comparable concentrations of the three group IV elements belong to the category of HEAs. As a result, the equimolar elements of Si-Ge-Sn alloys likely cause their atomic structures to exhibit the same core effects of metallic HEAs such as lattice distortion. Here, we apply density functional theory (DFT) calculations to show that the SiGeSn HEA indeed exhibits a large local distortion effect. Unlike metallic HEAs, our Monte Carlo and DFT calculations show that the SiGeSn HEA exhibits no chemical short-range order due to the similar electronegativity of the constituent elements, thereby increasing the configurational entropy of the SiGeSn HEA. Hybrid density functional calculations show that the SiGeSn HEA remains semiconducting with a bandgap of 0.38 eV, promising for economical and compatible midinfrared optoelectronic applications. We then study the energetics of neutral single Si, Ge, and Sn vacancies and (expectedly) find wide distributions of vacancy formation energies, similar to those found in metallic HEAs. However, we also find anomalously small lower bounds (e.g., 0.04 eV for a Si vacancy) in the energy distributions, which arise from the bond reformation near the vacancy. Such small vacancy formation energies and their associated bond reformations retain the semiconducting behavior of the SiGeSn HEA, which may be a signature feature of a semiconducting HEA that differentiates from metallic HEAs.

Effects of local distortion on the electrical properties of lead free perovskite-type electro-ceramics Ba1-xCaxTi0.9Zr0.1O3

The electro-ceramic industry has focused their attention on replacing lead-based materials due to their toxic effects and unfavourable environment impact. Barium titanate doped with calcium and zirconium denoted in this work as BCZT is a well-known perovskite-type solid solution. In particular, Ba1-xCaxTi0.9Zr0.1O3 (where x = 0, 0.1 and 0.15) compositions are located near the so called morphotropic phase boundary (MPB): coexistence of ferroelectric phases. This work shows the relationship between the crystal and electronic structure with the dielectric and piezo-ferroelectric properties. The high-resolution X-ray diffraction (HR-XRD) results suggest the coexistence of tetragonal and rhombohedral phases over the obtained compositions. The dielectric analysis indicates a relaxor behaviour with a Curie temperature in a range of 100-120°C. Piezoelectric and ferroelectric evaluation complement the electrical characterization for these compositions. Micro–X-ray absorption near edge spectroscopy (μ-XANES) was performed to monitor possible changes of electronic structure profile due to the local distortions as Ca2+ increases.

Monoclinic Cc-phase stabilization in magnetically diluted lead free Na1/2Bi1/2TiO3—Evolution of spin glass like behavior with enhanced ferroelectric and dielectric properties

The effect of magnetic cation substitution on the phase stabilization, ferroelectric, dielectric and magnetic properties of a lead free Na0.5Bi0.5TiO3 (NBT) system prepared by O2 atmosphere solid state sintering were studied extensively. Cobalt (Co) was chosen as the magnetic cation to substitute at the Ti-site of NBT with optimized 2.5 mol%. Rietveld analysis of x-ray diffraction data favours the monoclinic Cc phase stabilization strongly rather than the parent R3c phase. FE-SEM micrograph supports the single phase characteristics without phase segregation at the grain boundaries. The stabilized Cc space group was explained based on the collective local distortion effects due to spin–orbit stabilization at Co3+ and Co2+ functional centres. The phonon mode changes as observed in the TiO6 octahedral modes also support the Cc phase stabilization. The major Co3+-ion presence was revealed from corresponding crystal field transitions observed through solid state diffuse reflectance spectroscopy. The enhanced spontaneous polarization (Ps) from ≅38 μC cm−2 to 45 μC cm−2 could be due to the easy rotation of polarization vector along the in Cc phase. An increase in static dielectric response (ϵ) from ϵ ≅ 42 to 60 along with enhanced diffusivity from γ ≅ 1.53 to 1.75 was observed. Magneto-thermal irreversibility and their magnetic field dependent ZFC/FC curves suggest the possibility of a spin glass like behaviour below 50 K. The monoclinic Cc phase stabilization as confirmed from structural studies was well correlated with the observed ferroic properties in magnetically diluted NBT.

First-principles study of 75As NQR in arsenic-chalcogenide compounds

We present a theoretical study of the nuclear quadrupole interaction,νQ, of 75As in crystalline and amorphous materials containing sulfur and selenium, and compare themwith experiment. We studied a combination of hydrogen-terminated molecular clusters andperiodic cells at various levels of quantum chemical theory. The results show clearly thatthe standard density functional theory (DFT) approximations, LDA and GGA,underestimate the nuclear quadrupole (NQR) interaction systematically, whileHartree–Fock theory overestimates it to an even greater degree. However, various levels ofconfiguration interaction and the B3LYP hybrid exchange–correlation functional, whichincludes some exact exchange, give very good quantitative agreement for As bonded only tothe chalcogen species. As–As bonds require highly converged basis sets. We have performeda systematic study of the effect of local distortions around an arsenic atom onνQ andη. Usinga simple, semiclassical model, we have combined our total energy results with our NQR calculations topredict νQ lineshapes for bond angle and bond length distortions. Our predictions for lineshape, includingfirst and second moments, are in excellent agreement with the results of Su et al fora-As2S3,a-As2Se3 anda-AsSe. We offer new insight into the distortions that led to this inhomogeneous broadening.Our results show clearly that, for trivalent arsenic atoms with zero or one arsenicnearest neighbor, symmetric bond stretching is the predominant contributor to theνQ linewidth. However, in the presence of two arsenic nearest neighbors, distortions of theAs–As–As apex angle dominates and, in fact, leads to a much larger second moment, inagreement with experiment.

Quantum diffusion: effects of local distortion of phonons

The diffusion of vacancies and self-interstices in crystals is accompanied by a strong change of local springs. We propose a theory, which takes into account this change. It is shown that the local softening of the lattice by a vacancy due to the breaking of bonds produce an enhancement of the contribution of low- frequency phonons, while the local hardening of the lattice by an interstitial atom leads to the opposite effect. As a consequence, at low temperatures the diffusion coefficient of vacancies is much smaller than that of interstitial atoms; the temperature dependences of these coefficients are also different. The obtained results are compared with experimental data for the diffusion of vacancies in helium crystals. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The effect of local distortions on time‐domain electromagnetic measurements

Based on the time‐domain integral equation, we derive expressions for the effect of an anomalous body close to the receiver or close to the transmitter on transient electromagnetic measurements. Similar to magnetotellurics, the distortion of electric fields at late times can be described by a constant distortion tensor relating the secondary electric field to the primary field components that would be obtained in the absence of the body. The distortion of a single electric field transient is a static shift only for particular configurations over a layered half‐space. In the general case, the perturbation is time dependent because the direction of the total electric field vector varies with time. The theory nicely explains spatial variations in electric field transients measured during a high‐redundancy long‐offset transient electromagnetics (LOTEM) survey over an underground gas storage site. An inversion example with synthetic data illustrates how distortion can be corrected. The elements of the distortion tensor are determined simultaneously with the model parameters. Ambiguity is reduced by a regularization of the distortion parameters. In the example, the background model is recovered well, even for the difficult case where only one transmitter is used. The distortion of the magnetic field time derivatives caused by bodies close to the receiver is proportional to the time derivative of the primary electric step response. The distortion is generally not limited to early times and cannot be neglected in general. Transmitter overprint effects resulting in static shifts of vertical magnetic field time derivatives may also be understood from the theory.

Effect of doped ions on soft mode and phase transitions in Bi layered compound

Raman spectra of Bi2-xAxTi4-yByO11(A = Gd, Yb, La, Nd, Sb, Fe; B = Te, Zr, Sn) have been systematically investigated. The results show that these doped ions have different effects on the lowest frequency phonon mode (soft mode) and the phase transition in Bi2Ti4O11. The phase transitions in Bi2-xAxTi4O11 system is directly associated with softening of the lowest frequency phonon mode, which results from the changes in the short-range forces. Temperature, pressure and doped ion concentration - induced phase transition is separately correlated with an unstable displacement of Bi ions with a charge transfer effect in Bi-O-Ti system. The softening and normal behavior observed can be explained as due to the effects of local distortion on Bi ion displacement. The non-zero soft mode frequency at the phase transition may come from a coupling between the soft mode and an additional phonon mode.

Overall and local distortion effects on the metal–nonmetal transitions of mixed valent perovskite type manganese oxides

The metal–nonmetal (MN) transition data of mixed valent manganese oxide perovskites A1−xAx′MnO3 (A: trivalent lanthanide metals, A′: divalent metals, 0.2<x<0.5) are analyzed in terms of the tolerance factor (t), and the standard deviation (σ) of the A-site cation sizes. The interplay of these two parameters that measure the overall and atomic scale distortions, respectively, determine whether a compound can show a MN transition or not and its transition temperature (TMN). In order to show a MN transition, a compound should have t>0.906 and σ≲σ=232t−202 (pm). Large t and small σ are favorable for high TMN.TMN data of A0.7Ba0.15Sr0.15MnO3 (A=La, Pr, Nd, Sm), A0.7Ba0.15Ca0.15MnO3 (A=La, Pr, Nd, Sm), (La0.7Ca0.3)x(Gd0.7Ba0.3)1−xMnO3 (x=0.1, 0.3, 0.5, 0.7, 0.9), and (Nd0.7Ca0.3)x(Gd0.7Ba0.3)1−xMnO3(x=0.25, 0.3, 0.5, 0.55, 0.7, 0.75) newly synthesized are reported.

Crystallography of a two-dimensional binary crystal with local distortions

We have used computer simulations to study the effects of local distortions with given low symmetries on the structure factors and pair distribution functions of a two-dimensional binary crystal. The disorder leads to specific modulations of the diffuse scattering. In the case of clusters of such defects the diffuse scattering is distributed around the Bragg peaks. The length scales are found to be most directly identified by analysis of direct-space pair distribution functions. {copyright} {ital 1997} {ital The American Physical Society}

The magnetotelluric response over 2D media with resistivity frequency dispersion1

AbstractWe investigate the magnetotelluric response of two‐dimensional bodies, characterized by the presence of low‐frequency dispersion phenomena of the electrical parameters. The Cole‐Cole dispersion model is assumed to represent the frequency dependence of the ‘impedivity’ complex function, defined as the inverse of Stoyer's ‘admittivity’ complex parameter. To simulate real geological situations, we consider three structural models, representing a sedimentary basin, a geothermal system and a magma chamber, assumed to be partially or totally dispersive. From a detailed study of the frequency and space behaviours of the magnetotelluric parameters, taking known non‐dispersive results as reference, we outline the main peculiarities of the local distortion effects, caused by the presence of dispersion in the target media. Finally, we discuss the interpretative errors which can be made by neglecting the dispersion phenomena. The apparent dispersion function, which was defined in a previous paper to describe similar effects in the one‐dimensional case, is again used as a reliable indicator of location, shape and spatial extent of the dispersive bodies. The general result of this study is a marked improvement in the resolution power of the magnetotelluric method.

The effect of local distortions in the intermolecular hydrogen bond network on the lifetime of aramid yarns

The effect of local distortions in the intermolecular hydrogen bond network on the lifetime of aramid yarns

Dynamical properties of Ga1−x InxAs solid solutions: Influence of local distortion effects

Abstract Polarization measurements on Raman spectra of Ga 1−x In x As solid solutions are performed over a wide energy range (10 cm −1 - 600 cm −1 ) for several concentrations ( x = 0, 0.09, 0.20, 0.40, 0.53, 1). They show that this system follows the so-called “two-mode behaviour”. The predominance of the GaAs band in the Raman spectra and the large splitting of the two longwavelength optical GaAs modes over a large range of composition are correlated to the enhancement of the coupling between the two types of oscillators by local distortion effects.

Properties of One Oxygen Hole Motion in a Quantum Antiferromagnet11The project supported by the Fund of the National Center for R & D on Superconductivity of China and the Fund of the Chinese Academy of Sciences through the Grant No. LWTZ-1298.

By making use of the equation of motion of the Green's function method and the self-consistent nondiagonal Fock approximation technique, we present a systematic analysis of the ground state properties of one-hole doping in the cupric oxides for the two-band model. The quant urn Bogoliubov-de Gennes formalism is developed to study the renormalized hole motion with the effects of local distortion of spin configurations. The ground state wave function, the ground state energy and the effective mass are derived. The calculations are carried out with the variational approach.