Static Atomic Displacements Research Articles

Static and dynamic atom displacements in nanocrystalline powders from diffraction analysis at finite temperature

We evaluate the accuracy of x-ray diffraction analysis performed by Rietveld refinement in extracting the static and dynamic atomic displacement parameters of spherical gold nanocrystals with sizes of 30 nm and below and within a temperature interval of 10–300 K. A self-consistent computational workflow composed of real- and reciprocal- space analyses is designed for this purpose. We obtain average lattice parameters, static and thermal atomic displacements as well as their temperature and particle size dependence in an ideally monodispersed and realistic nanocrystalline powder generated by molecular dynamics simulations. Then, we compare crystallographic parameters obtained from real- and reciprocal- space analyses to evaluate the accuracy of diffraction analysis. We also extend the comparison to local variations of crystallographic parameters in the nanocrystals and show how Rietveld refinement relates to the average parameters of the core and surface regions separately. We show that 1) average lattice parameters obtained from Rietveld refinement are greater than the mean lattice parameter of small nanocrystals and converge to the core lattice parameter with increasing particle size, 2) Rietveld refinement significantly underestimates static atom displacements of surface atoms in small nanocrystals, 3) dynamic atomic displacements in nanocrystalline gold powders are reliably obtained within 5 % error by Rietveld refinement, provided that thermal diffuse scattering is adequately removed from the diffraction data, 4) the discrepancy between real- and reciprocal-space analysis increases for the smallest gold nanocrystal.

Quantitative composition determination by ADF-STEM at a low-angular regime: a combination of EFSTEM and 4DSTEM

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is a valuable method for composition determination of nanomaterials. However, light elements do not scatter efficiently into the scattering angles employed for HAADF-STEM which hinders the composition determination of material systems containing light elements by HAADF-STEM. This makes the usage of lower scattering angles favourable. Moreover, static atomic displacements (SADs) caused by the small covalent radius of the substituting light elements in semiconductor alloys increase the scattering intensity at low angles. Nevertheless, at low angles, a quantitative match between complementary image simulations and experiments is not straight forward, since e.g. inelastic scattering and correlated phonon movement is often neglected in simulations.In this study, we establish a method to quantify material systems containing light elements at low angles by resolving the remaining sources of discrepancy. An outstanding agreement between simulations and experiments is achieved by using a combination of an in-column energy filter and a fast pixelated detector. By applying this method to GaNxAs1-x quantum wells, a good agreement of the TEM results with results from high-resolution x-ray diffraction is obtained. The method developed enables the nanoscale analysis of functional materials containing light elements, especially in the presence of SADs.

Short Range Order and Static Displacements of Atoms in Polycrystalline Solid Solution of Fe-5 at. %Re

Intensity of X-ray diffuse scattering depends onscattering vector q...

Static Displacements and Short-Range Order in Ni-14 at. % Ir Alloy

In this paper, we are proposing a novel method to estimate static displacements of atoms caused by size effects in fcc substitutional binary polycrystalline solid solutions. Fourier transforms of static displacements of the atoms on every considered shell were calculated using the equations that include dynamical matrix and Fourier transform of interatomic forces. Short-range order parameters on the first seven shells of Ni-14 at. % Ir alloy have been identified from X-ray diffuse scattering intensity by accounting microscopic static displacements of atoms on a particular shell. Pairwise interatomic potentials on the considered shells and critical temperature of disorder-order phase transition were calculated using values of short-range order parameters.

Calculation of Pairwise Effective Potentials in the Disordered Ni-22.5at.%Fe Alloy Using Model Potential Method with Account of Size Effect

Pairwise effective potentials in first seventeen shells of the Ni-22.5at.%Fe alloy are calculated using model potential method with account of the linear size effect. Using obtained values of pairwise effective potentials, the short range order parameters on the first seventeen shells of alloy are calculated by Krivoglaz-Clapp-Moss method. The calculated values ​​of the short-range order parameters were fitted to the experimental values ​​by varying the parameters of static atomic displacements. Reliable value of critical temperature of order-disorder phase transition in Ni-22.5at.%Fe alloy was calculated using obtained meanings of pairwise effective potentials.

Change of tension state of electrolytic nickel under exposure to plasma

A study of the effect of cold air plasma on cathode nickel grade N-1 has been carried out. The changes in the microstructure, macro- and microstresses, as well as static displacements of atoms from equilibrium positions are investigated. By the method of aim metallography it was shown that the microstructure does not change under plasma exposure. X-ray structural studies have established that the level of microstresses does not change when exposed to plasma. A decrease in the magnitude of macrostresses is shown when exposed to plasma for 5 and 10 min. Macrostresses in this case vary from tensile stresses in the initial state to compressive stresses after exposure to plasma. The change in the static displacements of atoms from the equilibrium positions and their decrease when exposed to plasma have been established.

Exchange coupling constants at finite temperature

An approach to account for the effect of thermal lattice vibrations when calculating exchange coupling parameters is presented on the basis of the Korringa-Kohn-Rostoker Green function method making use of the alloy analogy model. Using several representative systems, it is shown that depending on the material the effect of thermal lattice vibrations can have a significant impact on the isotropic exchange as well as the Dzyaloshinskii-Moriya interactions (DMI). This should lead in turn to an additional contribution to the temperature dependence of the magnetic properties of solids, which cannot be neglected in the general case. As an example, we discuss such an influence on the critical temperature of various magnetic phase transitions. In particular, in the case of skyrmion hosting materials, a strong impact of lattice vibrations on the DMI is an additional source for the temperature dependent skyrmion size and stability, which should be taken into consideration. The present approach gives also access to features of magnetic properties that are associated with static atomic displacements as, for example, in random high-entropy alloys.

On the structural stratification of iron nanoparticles

It is shown that the presence of dispersion of particles with nano-sizes can lead to the appearance of a local phase transition on the surface layer of a sub-granule or a local bcc / fcc phase transition in pure Fe can occur at the intergranular boundary. In the presence of a large size factor, for example, as in Fe- (Mo, W) systems, the magnitude of the static displacements of Fe atoms located at 2–3 coordination spheres relative to the impurity atom, and simultaneously located in the surface / surface layer of the grain, when the critical value of the atomic displacement is exceeded Fe will lead to a violation of the local mechanical stability of the bcc lattice of Fe.

Angular dependence of fast-electron scattering from materials

Angular resolved scanning transmission electron microscopy is an important tool for investigating the properties of materials. However, several recent studies have observed appreciable discrepancies in the angular scattering distribution between experiment and theory. In this paper we discuss a general approach to low-loss inelastic scattering which, when incorporated in the simulations, resolves this problem and also closely reproduces experimental data taken over an extended angular range. We also explore the role of ionic bonding, temperature factors, amorphous layers on the surfaces of the specimen, and static displacements of atoms on the angular scattering distribution. The incorporation of low-loss inelastic scattering in simulations will improve the quantitative usefulness of techniques such as low-angle annular dark-field imaging and position-averaged convergent beam electron diffraction, especially for thicker specimens.

The modulated low-temperature structure of malayaite, CaSnOSiO4.

The crystal structure of the mineral malayaite has been studied by single-crystal X-ray diffraction at a temperature of 20 K and by calculation of its phonon dispersion using density functional perturbation theory. The X-ray diffraction data show first-order satellite diffraction maxima at positions q = 0.2606 (8)b*, that are absent at room temperature. The computed phonon dispersion indicates unstable modes associated with dynamic displacements of the Ca atoms. The largest-frequency modulus of these phonon instabilities is located close to a wavevector of q = 0.3b*. These results indicate that the malayaite crystal structure is incommensurately modulated by static displacement of the Ca atoms at low temperatures, caused by the softening of an optic phonon with Bg symmetry.

Study of interfacial random strain fields in core-shell ZnO nanowires by scanning transmission electron microscopy

Imaging strain fields at the nanoscale is crucial for understanding the physical properties as well as the performance of oxide heterostructures and electronic devices. Based on scanning transmission electron microscopy (STEM) techniques, we successfully imaged the random strain field at the interface of core-shell ZnO nanowires. Combining experimental observations and image simulations, we find that the strain contrast originates from dechanneling of electrons and increased diffuse scattering induced by static atomic displacements. For a thin sample with a random strain field, a positive strain contrast appears in the low-angle annular dark-field (LAADF) image and a negative contrast in the high-angle annular dark-field (HAADF) image, but for a thick sample (> 120 nm), the positive contrast always occurs in both the LAADF and HAADF images. Through the analysis of the relationship between strain contrast and various parameters, we also discuss the optimum experimental condition for imaging random strain fields.

Influence of dispersity on local physical, structural, magnetic and thermodynamic properties of bcc Fe and its binary alloys

The model was proposed for iron nano-particles (and solid solutions based on bcc-Fe) containing a core and a surface layer coherent with each other in the form of a two-phase stratifying parts (“surface” and bulk) system with different local physical properties. Using the results of measurements of the experimental temperature dependence of the heat capacity and the application of the Debye models for both the “core”-phase and the surface layer (“surface”-phase) as two-phase heat capacity components for Fe nanoparticles, it was shown by calculations that the local elastic modulus for the surface layer is nano-particles are approximately 2 times smaller than for the “core”-phase of the nano-particle iron. The purpose of the research is to establish the relationship between local physical properties (bulk modulus of elasticity, static displacements of iron atoms surrounding an impurity atom, magnetic moments) for the core part and the surface layer of nanoparticles. From the condition of local mechanical equilibrium it was shown that static displacements of Fe atoms around an impurity atom in various coordination spheres on the surface of nanoparticles of Fe-rich binary BCC solutions of Fe - (Cr, V, Mo, W) with nano-particles increase significantly (compared to static displacements of Fe atoms “core” of nano-particles).

Argentite–Acanthite Transition in Silver Sulfide as a Two-Sublattice Ordering

We propose a model of phase transformation cubic argentite–monoclinic acanthite in silver sulfide Ag2S (AgS0.5) as ordering in two argentite sublattices. We have determined the channel of the disorder–order transition including four nonequivalent superstructure vectors of stars {k9} and {k4}. For monoclinic acanthite α-Ag2S, we have calculated the distribution function for silver atoms occupying b positions in argentite, as well as the distribution function for sulfur atoms. Ordering in both sublattices is complicated by static atomic displacements. The displacement of S atoms distort the body-centered cubic (bcc) nonmetallic argentite sublattice, forming a monoclinic lattice in which silver atoms are at large distances from one another and occupy their crystallographic positions with unit probability. We have determined the range of admissible values of long-range order parameters η9 and η4 for the model monoclinic ordered α-Ag2S phase.

Fluctuations as a source of new physical properties of ultra-small CdSe nanoparticles

The presence the static mean-square displacement of atoms from the equilibrium position caused by the structural defects can lead to the order-disorder transition in the ultra-small nanoparticles (USNs). It is shown that after the appropriate choose of the order parameter, the Landau mean-field theory can be used to describe this transition in USNs. In present article, the fluctuations of the order parameter are considered by statistical methods. The order-disorder transition in USNs is accompanied by large fluctuations of the order parameter. The order parameter changes continuously around its equilibrium value. This dynamic state of USNs can be considered as a new state of matter at the nanoscale, which is characterized by a continuous change of the order parameter. For the first time, the Landau theory is applied to describe the white-light emitting of USNs. It is shown that the fluctuations nearby the order-disorder transition are responsible for the white-light emission and that the emission spectrum has a crossover between the monochromatic and white-light emission at the critical nanoparticle diameter. Methods for controlling USNs properties by changing the order parameter are presented for the first time. The present theory gives a new approach to explain the white-light emission by CdSe quantum dots. It also follows from the present theory that the crossover can be suppressed (or shifted to a region of smaller nanoparticles) by an artificial change of the order parameter. The developed theory gives a new approach to predict the physical properties of USNs.

Displacive structural phase transitions and the magnetic ground state of quadruple perovskite YMn7O12

We present an experimental study of the structural and magnetic phase transitions of the quadruple perovskite ${\mathrm{YMn}}_{7}{\mathrm{O}}_{12}$. On cooling below 640 K a displacive structural phase transition occurs, lowering the crystal symmetry from cubic $(Im\overline{3})$ to monoclinic $(I2/m)$, driven by long range B site ${\mathrm{Mn}}^{3+}$ orbital order. A second, unconventional structural phase transition takes place on further cooling below 200 K, which is characterized by static atomic displacements $(P{2}_{1}/n)$ as well as a dipolar-glass-like state of yttrium. This low temperature structural transition is driven by the underbonding of yttrium, which is present due to the rigid framework of octahedra tilts unique to the quadruple perovskites that primarily accommodates A site manganese ions. A ground state ferrimagnetic structure was also determined, similar to that found in other $R{\mathrm{Mn}}_{7}{\mathrm{O}}_{12}$ compounds that do not present a low temperature structural phase transition.

Nitrogen mapping from ADF imaging analysis in quaternary dilute nitride superlattices

A method for the determination of N distribution in dilute nitride GaAsSbN superlattices (SLs) by using different STEM imaging settings is proposed. The method combines the simultaneous acquisition under Low Angle (LA-) and High Angle (HA-) Annular Dark Field (ADF) conditions by exploiting two different dedicated angle intervals. On one hand, HAADF technique gives information that principally depends on the atomic number (Sb sensitive) and on the other hand, N atoms produce high static atomic displacements affecting the image intensity especially under LAADF conditions. However, the simultaneous presence of Sb and N supposes an important handicap to differentiate both elements. N distribution maps could be obtained from suitable normalization and the separation of the intensity ratios in regions with/without Sb. Semi-quantitative maps are also available by combination of the results from high-resolution X-ray diffraction and energy dispersive X-Ray spectroscopy techniques. Type-I (GaAsSbN/GaAs) and type-II (GaAsSb/GaAsN) SL structures are evaluated using the proposed methodology. Differences in the N distribution between both samples such as inhomogeneities and cluster formation are discussed. Specifically, we have found a greater number of N-rich regions in type-I structure as compared to their type-II counterparts, which could have an influence on the optical response of each design.

Spin-disorder resistivity of random fcc-NiFe alloys

The spin-disorder resistivity (SDR) of a disordered fcc-(${\mathrm{Ni}}_{1\ensuremath{-}x},{\mathrm{Fe}}_{x}$) alloy is determined from first principles. We identify the SDR at and above the critical temperature with the residual resistivity of the corresponding paramagnetic state evaluated in the framework of the disordered local moment (DLM) model. The underlying electronic structure is determined by means of the tight-binding linear muffin-tin orbital method, which employs the coherent potential approximation (CPA) to describe both the DLM state and the chemical disorder in alloys. An extension of the DLM fixed-spin moment method for two independent magnetic moments is used and combined with the paramagnetic lattice gas entropy to determine local moments by minimizing the corresponding free energy. The effect of phonon scattering is included through the mapping of static atomic displacements into a multicomponent random alloy which is then treated in the CPA. Finally, the Kubo-Greenwood-CPA approach is employed to estimate the SDR. We also address the problem of the validity of the Matthiessen rule at the Curie point. Good agreement of calculated and measured SDR is obtained over the whole studied concentration range; the results point to the importance of nonzero Ni magnetic moments in the limit of pure nickel.

Short-Range Order and Static Displacements in Polycrystalline Ni-13.1at. %W Alloy

A methodology accounting for the contributions of static displacements of atoms to X-ray diffuse scattering by binary solid solutions with a face-centered cubic lattice was proposed. By microscopic accounting of static displacements of the atoms on the particular shell, we identified short- range order parameters on the first eight shells of Ni-13.1at. %W alloy from the intensity of X-ray diffuse scattering. The Fourier transform of static displacement of the atoms on every considered shell was calculated with the De Launay model using expressions of elements of dynamical matrix and Fourier transforms of quasi elastic forces, which are calculated by the pseudopotential method.

Composition determination of multinary III/V semiconductors via STEM HAADF multislice simulations

Knowledge of the microscopic elemental composition of multinary III/V semiconductor materials is crucial to the development of functionalized opto-electronic devices. Well-proven composition analysis methods, such as high resolution X-ray diffraction (HRXRD), fail to determine the elemental composition when more than three atomic species are involved. In this work we propose a procedure for the composition analysis of multinary III/V semiconductors at atomic resolution using high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) image simulations. Our method exploits the dependence of HAADF-STEM image intensities on the atomic number and static atomic displacements (SAD) at different detector inner angles. Here, we describe the proposed method in detail using Ga(NAsP) as an example multinary material.

Effect of nonstoichiometry on the lattice constant of cubic vanadium carbide VC y

The effect of nonstoichiometry on the lattice constant of cubic vanadium carbide VC y (0.65 < y < 0.875) is studied. It is found that the ordering of vanadium carbide VC y with the formation of superstructures V6C5 and V8C7 leads to an increase in the base lattice constant in comparison with disordered carbide. Taking into account the change in the lattice constant, the direction of the static displacements of atoms near the vacancy is discussed.