Dynamic 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.

Mechanical Seed Mechanism to Facilitate Homogeneous Li Metal Growth

AbstractAn intriguing mechanical seed (MS) concept that modulates (in)homogeneous Li metal growth is proposed based on an in‐depth understanding of its fundamental mechanism using unified atomistic computations. A large dataset of thermodynamic energies for Li disordered phase decouples the dual‐body interactions into three components: i) crystal‐like, ii) long, and iii) short bonds of Li─Li based on machine learning assisted by density function theory calculations. The contributions of these dual‐body interactions offer a mechanical factor for controlling the disordered‐ordered phase transition during electrochemical deposition. Macroscopic molecular dynamics simulations systematically construct the core–shell sphere and cross‐sectional models to reinforce the MS premise. The former reveals that the lower energy level of disordered phase under the moderate compression causes a slow phase kinetics, whereas the strain‐free mode exhibits a relatively fast transition. In addition, the cross‐sectional model exhibits a smooth surface landscape for the strain‐optimized case. These observations are attributed to the surface area evolutions depending on the MS conditions and elucidate the dynamic atomic displacements near the grain boundary from a local structural perspective. The proposed mechanical design concept facilitates uniform Li growth and is expected to be a global parameter in harnessing the full potential of Li metal batteries.

Mechanochemical synthesis and crystal structure evaluation of Na2ZnSnS4

AbstractPhase‐pure and highly crystalline Na2ZnSnS4 was prepared via a mechanochemical synthesis route. It crystallizes in the kesterite‐type structure. The unusual large Debye‐Waller factors of the sodium atoms were analyzed in detail, respecting also group‐theoretical aspects. The results point to the existence of static disorder, mainly to the presence of various patterns of sodium ordering on a local scale. This is confirmed by quantum‐chemical calculations at hybrid density‐functional theory level. The dynamic atomic displacements due to phonons are much smaller than the observed Debye‐Waller factors.

Dynamic Atomic Displacements and Athermal Dislocation Glide in Crystals

A dislocation glide mechanism at low temperatures is proposed. The mechanism is based on the inclusion of dynamic atomic displacements, i.e., those caused by nonadiabatic transitions of atoms in a crystal with a dislocation under action of an external force. The dynamic displacements initiate the instability of a rectilinear dislocation with respect to low-amplitude displacements during atomic vibrations. The instability development leads to the formation of a double kink and the dislocation displacement by one interatomic distance.

On the Growth and Properties of FeIn2S3.6Se0.4 Single Crystals

FeIn2S3.6Se0.4 single crystals are grown by planar crystallization of the melt (the vertical Bridgman method). The composition and crystal structure of the crystals are determined. It is established that the single crystals crystallize with the formation of the cubic spinel structure. From the transmittance spectra in the region of the fundamental absorption edge, the band gap of the single crystals is determined. Thermal expansion of the FeIn2S3.6Se0.4 single crystals is studied by the dilatometric technique in the temperature range from 80 to 550 K, and the coefficients of thermal expansion are determined. From the coefficients of thermal expansion determined, the Debye temperatures and the root-mean-square (rms) dynamic displacements of atoms are calculated. It is shown that, as temperature is elevated, the Debye temperatures decrease and the rms dynamic displacements of atoms increase. Magnetic studies show that the FeIn2S3.6Se0.4 single crystals are paramagnetic materials at temperatures down to 12.4 K.

Динамические смещения атомов и атермическое скольжение дислокаций в кристаллах

A dislocation glide mechanism at low temperatures is proposed. The mechanism is based on taking into account the dynamic displacements of atoms — displacements caused by nonadiabatic transitions of atoms in a crystal with a dislocation under the action of an external force. Dynamic displacements initiate the instability of a direct dislocation of relatively low-amplitude displacements during atomic vibrations. The development of instability leads to the formation of a double kink and a dislocation shift by one interatomic distance.

RMS Dynamic Displacements of Atoms and Their Complexes from the Equilibrium Position in Crystals

RMS Dynamic Displacements of Atoms and Their Complexes from the Equilibrium Position in Crystals

Выращивание и свойства монокристаллов FeIn-=SUB=-2-=/SUB=-S-=SUB=-3.6-=/SUB=-Se-=SUB=-0.4-=/SUB=-

Abstract FeIn_2S_3.6Se_0.4 single crystals are grown by planar crystallization of the melt (the vertical Bridgman method). The composition and crystal structure of the crystals are determined. It is established that the single crystals crystallize with the formation of the cubic spinel structure. From the transmittance spectra in the region of the fundamental absorption edge, the band gap of the single crystals is determined. Thermal expansion of the FeIn_2S_3.6Se_0.4 single crystals is studied by the dilatometric technique in the temperature range from 80 to 550 K, and the coefficients of thermal expansion are determined. From the coefficients of thermal expansion determined, the Debye temperatures and the root-mean-square (rms) dynamic displacements of atoms are calculated. It is shown that, as temperature is elevated, the Debye temperatures decrease and the rms dynamic displacements of atoms increase. Magnetic studies show that the FeIn_2S_3.6Se_0.4 single crystals are paramagnetic materials at temperatures down to 12.4 K.

Dynamic Spatial and Temporal Structures on the Surface of Solids under Load

A model and a method are proposed to explain the origin and main specific features of the dynamic patterns of different types, which were previously observed on the free surface of solids under load in experiments. It is shown that the pattern is formed due to the dynamic instability of the flat surface of the solid under load. This instability develops at the relaxation mechanism caused by the dynamic atomic displacements due to a change in the interatomic interaction during the electron-density redistribution.

Динамические пространственно-временные структуры на поверхности нагруженных твердых тел

AbstractA model and a method are proposed to explain the origin and main specific features of the dynamic patterns of different types, which were previously observed on the free surface of solids under load in experiments. It is shown that the pattern is formed due to the dynamic instability of the flat surface of the solid under load. This instability develops at the relaxation mechanism caused by the dynamic atomic displacements due to a change in the interatomic interaction during the electron-density redistribution.

Dynamic Instability of Solid Surfaces under Load

The mechanism of dynamic instability of a solid surface under a load, determined by perturbations of the electron density and the change in the interatomic interaction, is considered. This instability is manifested upon the excitation of dynamic displacements of atoms in the surface layer, which can lead to formation of finite-lifetime structures in the form of waves with a large amplitude. Such structures were earlier observed experimentally on a stretched germanium (111) surface.

Динамическая неустойчивость поверхности твердых тел под нагрузкой

AbstractThe mechanism of dynamic instability of a solid surface under a load, determined by perturbations of the electron density and the change in the interatomic interaction, is considered. This instability is manifested upon the excitation of dynamic displacements of atoms in the surface layer, which can lead to formation of finite-lifetime structures in the form of waves with a large amplitude. Such structures were earlier observed experimentally on a stretched germanium (111) surface.

Mn0.1Ag0.9In4.7S7.6 single crystals: Crystal structure, band gap, and thermal expansion

Mn0.1Ag0.9In4.7S7.6 single crystals are for the first time grown by the Bridgman method (vertical variant). The single crystals crystallize in the cubic spinel structure. The band gap of the single crystals is determined from the transmittance spectra in the region of the fundamental absorption edge at temperatures of T = 295 and 80 K. Thermal expansion is studied by the dilatometric method in the temperature range 80–500 K. The coefficients of thermal expansion, the Debye temperatures, and the rms (root mean square) dynamic displacements of atoms are calculated.

UV absorption and effects of local atomic disordering in the nickel oxide nanoparticles

The results of the investigation of optical absorption temperature behaviour for the NiO nanoparticles with sizes 25 and 10nm are first presented. The spectral dependences of the NiO absorption coefficient in the near UV region are found to obey the Urbach rule. On the basis on independence and additivity of the static and dynamic disorders the information about energy and vibrational structures of nanoparticles is obtained. The fundamental parameters such as energy gap, optical vibrational modes, Debye temperature, deformation potential constant, strength of the exciton-phonon coupling, mean-square atomic displacement of the nanoscaled nickel oxide are determined in terms of the Cody's and Wasim's models. The role of the structural disorder and the size effect on the optical properties of nanoparticles is discussed.Based on the described approach it was established that the dynamic atomic displacements is dominant in the common structural disorder, however the decreasing of size leads to the increasing of static component. In addition there is a simultaneous growth of the energy gap for direct optical transitions and strength of the electron-phonon coupling. It was found that the characteristic feature of small NiO nanoparticles is participation of surface optical vibrational modes in the formation of optical absorption edge.

On the band gap and thermal expansion of MnIn5.0S8.5 single crystals

MnIn5.0S8.5 single crystals are grown by the Bridgman method. The width and length of the crystals are ∼14 and ∼40 mm, respectively. The composition, structure and unit-cell parameter of the crystals are established. From the transmittance spectra recorded in the temperature range 20–300 K, the band gap Eg and its temperature dependence Eg(T) are determined. The dependence Eg(T) is adequately described by the corresponding theoretical expression. The specific expansion (Δl/l0) versus temperature is measured, and the coefficient of linear thermal expansion (αL) is determined. From the data on αL, the Debye temperature (ΘD) and the rms dynamic atomic displacements \((\sqrt {\bar u^2 } )\) are calculated. It is established that the Debye temperature ΘD decreases with increasing temperature, whereas the rms atomic displacements increase.

Thermal expansion of CuIn5S8 single crystals and the temperature dependence of their band gap

Single crystals of the CuIn5S8 ternary compound are grown by planar crystallization of the melt (the vertical Bridgman method). The composition and structure of the crystals are established. The specific expansion is measured by the dilatometric technique, and the coefficients of thermal expansion are calculated. From the data, the Debye temperatures (ΘD) and the root-mean-square dynamic displacements of atoms $$\left( {\sqrt {\bar u^2 } } \right)$$ in the CuIn5S8 compound are calculated. From the transmittance spectra recorded in the region of the fundamental absorption edge in the temperature range 20 to 300 K, the band gap is determined and its temperature dependence is constructed.

A new type of antiferrodistortive structure in a double perovskite with Pb

We have found a new structural transition in Pb(2)MnReO(6) at 410 K. Above this temperature, Pb(2)MnReO(6) is cubic with disordered and dynamic atomic displacements manifested in the large thermal parameters of Pb and O atoms. Below 410 K, the antiferrodistortive shift of 2/3 of Pb(2+) cations away from the high-symmetry cubic site produces a new type of monoclinic cell. The unit cell expands at the transition and the heat capacity shows a peak with thermal hysteresis. These features agree with a first order transition. The entropy content of the transition is quite low indicating that the structural disorder has not been completely removed in the low temperature phase. The monoclinic phase of Pb(2)MnReO(6) shows thermally activated conductivity which does not vary when an external magnetic field is applied. A change in the slope of the resistivity curve, observed at the structural phase transition temperature, is related to a slight difference in the activation energy between both phases. It suggests that the condensation of the distortions likely affects the conduction mechanism. The isothermal magnetization measurements reveal the presence of ferromagnetic contributions below 85 K. The ac magnetic susceptibility shows a dynamic peak at 50 K and, in addition, zero-field-cooled and field-cooled magnetization curves diverge strongly below 80 K. These features might be signature of magnetic inhomogeneity. Magnetic loops, obtained at 5 K, do not show saturation in fields up to 9 T. Furthermore, the measured coercivity increases sharply at low temperature indicating an abrupt change in the magnetic anisotropy. We show that all these magnetic properties point out to a ferrimagnetic ordering of Mn and Re atoms in an intermediate valence state.

Multiscale studies and modeling of SPD materials

Abstract Recent results of experimental studies and computer modeling of bulk pure metals, subjected to severe plastic deformation (SPD) are presented. Experimental investigations and computer modeling were conducted on different scale levels. Influence of the processing parameters on the flow character of the deformed material and elastic stresses’ fields appearing in the die-set is studied on the macrolevel. Conformities of grain and subgrain size evolution, as well as the texture development at SPD were revealed on the mesolevel. On the microlevel as a result of computer simulation there were evaluated densities of the extrinsic grain boundary dislocations; there was analyzed the influence of SPD on the values of static and dynamic atomic displacements and the Debye temperature in the materials under investigation. The obtained results make it possible to explain novel properties, such as simultaneous appearance of high strength and ductility (the SPD paradox), enhanced diffusion properties, low-temperature and/or high-rate superplasticity of bulk SPD nanomaterials.

Thermal expansion and thermal conductivity of CuGa1−x InxTe2 solid solutions

The thermal expansion coefficients and the thermal conductivity of Bridgman-grown crystals of CuGa1−x InxTe2 solid solutions are investigated. It is found that the thermal expansion coefficient varies with x linearly, while the thermal conductivity is minimal when x=0.5. The Debye temperature and the rms dynamic atomic displacements are calculated from experimental data. It is shown that the Debye temperature decreases and the rms displacements in the crystal lattice sharply increase as the In content in the solid solutions grows.

The role of volume expansion in the formation of metallic glasses

Amorphisation and melting processes are generally regarded as topological order–disorder phase transitions characterised by basically different thermodynamic and kinetic features. Experimental and theoretical findings suggest, however, the existence of intimate connections between them. In particular, it seems that both the static and dynamic atomic displacements from the equilibrium positions characteristic of the ideal crystalline lattice play a fundamental role in driving the necessary instabilities for the loss of crystalline order. If the melting of a pure element and the amorphisation of a binary mixture with solute atoms larger than the matrix atoms are considered, the static and dynamic atomic displacements are simply related to the volume expansion. In this study, the volume expansions associated with the occurrence of amorphisation and melting are compared. The critical volume required in a binary system for the metallic glass to form is found to be approximately coincident with the volume reached by one of the two metals at the melting temperature. Theoretical estimates of the critical volume are obtained through the application of the criterion of Egami and Waseda and compared with experimental values. Significant connections are established between the so-called Born's shear instability and the loss of crystalline order.