Atomic Order Parameters Research Articles

The effect of graphene oxide nanosheet size and initial temperature on the mechanical and thermal properties of epoxy/graphene oxide structure using molecular dynamics simulation

This study investigated the effect of graphene oxide nanosheet (GON) size, and initial temperature (IT) on the mechanical properties (MP), and thermal properties (TP) of epoxy/GO structure using the molecular dynamics simulation (MDS). The stress-strain curve, Young's modulus (YM), the atomic order parameter (AOP), and the thermal expansion coefficient (length changes) of simulated structures were studied. The results show that increasing GO length improved the simulated structure's MP. As the GO length increased to 20 Å, YM increased from 2.96 to 4.04 GPa. Furthermore, by changing the length of simulated GONs, the amount of van der Waals force applied from each plate to the atoms present in the atomic matrix changes; so, as the size of GONs increased, AOP in the structures increased. Increasing the size of simulated GONs reduced the expansion of structure. This behavior showed higher thermal resistance, and consequently, atomic stability in the samples as the nanosheets became larger. In general, increasing IT led to increasing the atomic fluctuations. Consequently, the attraction energy among the atoms decreased by increasing the atomic oscillations. Therefore, increasing the IT to 350 K led to a decrease in the YM of samples. Therefore, the MP of samples was reduced by increasing IT. Moreover, increasing IT decreased the amount of AOP in the samples. Finally, increasing IT increased the oscillation amplitude, and the distance of atoms from each other, which caused the expansion in the structure under study.

Do Machine-Learning Atomic Descriptors and Order Parameters Tell the Same Story? The Case of Liquid Water.

Machine-learning (ML) has become a key workhorse in molecular simulations. Building an ML model in this context involves encoding the information on chemical environments using local atomic descriptors. In this work, we focus on the Smooth Overlap of Atomic Positions (SOAP) and their application in studying the properties of liquid water both in the bulk and at the hydrophobic air-water interface. By using a statistical test aimed at assessing the relative information content of different distance measures defined on the same data space, we investigate if these descriptors provide the same information as some of the common order parameters that are used to characterize local water structure such as hydrogen bonding, density, or tetrahedrality to name a few. Our analysis suggests that the ML description and the standard order parameters of the local water structure are not equivalent. In particular, a combination of these order parameters probing local water environments can predict SOAP similarity only approximately, and vice versa, the environments that are similar according to SOAP are not necessarily similar according to the standard order parameters. We also elucidate the role of some of the metaparameters in the SOAP definition in encoding chemical information.

Correlation of B2 super-lattice ordering with soft magnetic and mechanical properties of nanocrystalline FeCoNbB HITPERM alloys

The degree of superlattice ordering (S), which may vary from zero (complete disorder) to one (fully ordered) is known to influence the magnetic and other physical properties of Fe-based nanocrystalline alloys. The direct correlation of S with physical properties of these alloys is not yet established. In the present work, we report on the correlation of the atomic order parameter (S) of bcc-FeCo phase, as calculated by anomalous diffraction measurements with the soft magnetic and mechanical properties of (Fe1-xCox)81Nb7B12 (x = 0.25, 0.33, 0.5, 0.66, 0.75) HITPERM alloys. The measured soft magnetic properties such as coercivity and saturation magnetization are found to have better correlation with the degree of atomic order than that of the crystalline volume fraction for the studied partially nanocrystalline alloys. Highest saturation magnetization (147 A.m2/kg) and lowest coercivity (18 A m−1) is recorded for the alloy (x = 0.66) with maximum degree of atomic order (S ∼ 0.82). Ductility test shows better resistance against the embrittlement for the atomically ordered alloys as compared to disordered ones. Micro-hardness values are also high for atomically ordered alloys. The study suggests that apart from grain size, nanocrystalline fraction etc, the degree of atomic order is also an important structural parameter for tailoring the physical properties of such alloys for technological applications.

Molecular properties of a bent-core nematic liquid crystal A131 by multi-level theory simulations

ABSTRACTMulti-level theory simulations have been performed to model a number of important molecular properties of a bent-core nematic liquid crystal (LC) A131. These important properties include molecular conformations, molecular Raman spectra, differential polarisability ratios, molecular crystals packing, atomic LC structures, order parameters, and Raman depolarisation spectra. The simulations contain four theory levels, involving molecular quantum chemistry, molecular crystal packing, super cell density functional based tight binding optimisation, and super cell molecular dynamics calculations. To heat initial optimised super cell structures, molecular dynamics simulations reveal phase transitions to uniaxial and biaxial nematic phases from molecular crystals. LC atomic structures result in direct calculations on order parameters, which can be further applied to computations on Raman depolarisation spectra with differential polarisability ratios, obtained in the molecular quantum chemistry theory level. The good agreement of simulated Raman depolarisation spectra with the experiment provides a detailed analysis on the unusually low values of experimental uniaxial order parameters.

Impact of the Number of dpa on the Superconducting Properties in HiLumi-LHC and FCC Accelerators

The aim of this paper is to estimate the change of the superconducting properties T c, J c, and B c2 in the quadrupoles of the future accelerators HiLumi-LHC and FCC, submitted to irradiation by multiple high-energy sources. The estimations are performed based on the number of displacements per atom ( dpa ), calculated by means of the FLUKA code. It is found that the dpa can be used as a single parameter for describing the radiation induced change of T c, the degree of atomic order parameter S , and the lattice parameter a in the case of multiple energy sources. The mechanism leading to the enhancement of $J_{\text{c}}$ after proton and neutron irradiation is analyzed based on a possible correlation with the concentration of Frenkel defects produced by the irradiation. Experimental data on J c/ J co versus dpa at 4.2 K allow estimating J c in the quadrupoles of HiLumi-LHC and FCC/Run1, where the values of dpa for lifetime are 2.5 × 10−4 and 5 × 10−4, respectively. The dpa in FCC/Run2 is much higher, 3 × 10 −3, thus reaching well beyond the maximum of J c versus dpa. An accurate estimation of the variation of T c, J c, and B c2 is performed based on earlier irradiation data with 14-MeV neutrons.

Order parameters for symmetry-breaking structural transitions: The tetragonal-monoclinic transition in ZrO2

Group/subgroup structural phase transitions are exploited in a wide variety of technologies, including those that rely on shape-memory behavior and on transformation toughening. Here, we introduce an approach to identify symmetry-adapted strain and shuffle order parameters for any group/subgroup structural transition between a high-symmetry parent phase and its symmetrically equivalent low-symmetry product phases. We show that symmetry-adapted atomic shuffle order parameters can be determined by the diagonalization of an orbital covariance matrix, formed by taking the covariance among the atomic displacement vectors of all symmetrically equivalent product phase variants. We use this approach to analyze the technologically important tetragonal to monoclinic structural phase transformation of ${\mathrm{ZrO}}_{2}$. We explore the energy landscapes, as calculated with density functional theory, along distinct paths that connect $m{\mathrm{ZrO}}_{2}$ to $t{\mathrm{ZrO}}_{2}$ and to other $m{\mathrm{ZrO}}_{2}$ variants. The calculations indicate favorable pairs of variants and reveal intermediate structures likely to exist at coherent twin boundaries and having relatively low deformation energy. We identify crystallographic features of the monoclinic ${\mathrm{ZrO}}_{2}$ variant that make it very sensitive to shape changing strains.

Deformation-Induced Atomic Disordering and Reordering in Alloys with L12 Structure

The paper presents a mathematical model of thermal and strain hardening of alloys having L12 superstructure which allows calculating the atomic long-range order (LRO) parameter depending on the deformation degree under various temperature and loading conditions. The observed non-monotonic change in the atomic LRO parameter during plastic deformation occurs due to the two types of competitive processes. These processes are caused by the motion and accumulation of the deformation defects and their healing due to the migration of point defects of different nature. The competitiveness between these two types of processes leads to the periodical destruction and reconstruction of the atomic LRO parameter, while the equilibrium between them can stabilize it after which it continues to be stable despite the deformation.

Lattice distortion and electronic structure of magnesium-doped nickel oxide epitaxial thin films

Magnesium-doped nickel oxide (${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$) thin films are transparent over a wide ultraviolet-visible spectral range and over a wide Mg content range. However, the influence of the Mg dopant on the structure and properties of NiO films is poorly understood. In this work, the lattice distortion and the electronic structure of ${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ ($0\ensuremath{\le}x\ensuremath{\le}0.52$) thin films deposited on ultrasmooth sapphire substrates were investigated using synchrotron x rays. Films with higher Mg content had lower values of Debye temperature and atomic order parameter. The nearest Ni-O distance and the in-plane nearest Ni-Ni distance both expanded with increasing Mg content. The Ni $2p$ core-level spectra and the valence band spectra of the ${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ thin films showed complex multiplet structures that were caused by the strong electron correlation in the Ni $3d$ states, where the spectral features are strongly dependent on both the distortion of the ${\mathrm{NiO}}_{6}$ octahedra and the Mg content. We found that the electronic structures are mainly a result of hybridization of Ni $3d$ and O $2p$ in the ${\mathrm{NiO}}_{6}$ octahedra and the reduction of the Zhang-Rice bound state following Mg doping. Finally, the flexibility of the band gap tuning in ${\mathrm{Mg}}_{x}{\mathrm{Ni}}_{1\ensuremath{-}x}\mathrm{O}$ thin films is explained.

Variation of Tc, lattice parameter and atomic ordering in Nb3Sn platelets irradiated with 12 MeV protons: correlation with the number of induced Frenkel defects

Nb3Sn platelets with thicknesses between 0.12 and 0.20 mm produced by a high isostatic pressure process at 1250 °C were irradiated at 300 K with 12 MeV protons. The effects of irradiation on the lattice parameter a, the atomic order parameter S and the transition temperature Tc were measured as a function of proton fluence. In view of the presence of multiple energy radiation sources in future accelerators, the present proton data are compared with neutron irradiation data from the literature. The fluences for both types of radiation were replaced by the dpa number, the ‘displacements per atom’, calculated using the FLUKA code, which is proportional to the number of radiation induced Frenkel defects. It was found that the variation of both a and S for Nb3Sn after proton and neutron irradiation as a function of dpa fall almost on the same curve, in analogy to the recently reported correlation between Tc and the dpa number. By a simultaneous irradiation of two adjacent thin Nb3Sn platelets, we have shown that this correlation is not only valid for the state of ‘steady energy loss’ (protons traveling through the first platelet) but also for the state of higher damage at the Bragg peak (second platelet). It follows that the number of radiation induced Frenkel defects in the A15 grains, calculated via the dpa number, can be considered as a ‘universal’ parameter, allowing the calculation of the variation of Tc, a and S of Nb3Sn under the effect of multiple high energy radiation sources, as in future superconducting accelerators.

Temperature dependence of the short-range order parameter for Fe0.90Cr0.10 and Fe0.88Cr0.12 alloys

Abstract The 57Fe Mössbauer spectra for the iron-based solid solutions Fe0.90Cr0.10 and Fe0.88Cr0.12 were measured at different temperatures ranging from 300 K to 900 K. Analysis of the obtained spectra shows that the distribution of impurity atoms in the two first coordination shells of 57Fe nuclei is not random and it cannot be described by the binomial distribution. Quantitatively, the effects were described in terms of the atomic short-range order (SRO) parameters and the pair-wise interaction energy with the help of a quasi-chemical type formulation introduced by Cohen and Fine. The obtained results reveal strong clustering-type correlations in the studied samples (a predominance of Fe-Fe and Cr-Cr bonds). Moreover, the changes in SRO values observed during thermal processing suggest that the distribution of Cr atoms in an α-iron matrix is strongly temperature dependent.

Atomic Interchange Potentials Calculation of Ni-Al-Cr Alloy Based on Microscopic Phase Field Method

The effects of increasing atomic interchange potentials to the precipitation process and microstructure of Ni-Al-Cr alloy have been simulated based on the microscopic phase field theory. The first nearest neighbour atomic interchange potentials of Ni-Al-Cr alloys for L12 and D022 phase was calculated out according to the formula which were referenced on the relation equation between atomic interchange potentials and long range order parameters by Khachaturyan. The results indicated that Ni-Al (WNi-Al) and Ni-Cr (WNi-Cr) s first nearest neighbor atomic interaction potentials will increase linearly while the temperatures rose. Moreover WNi-Al increased but WNi-Cr decreased roughly linearly if Al atoms concentration rose, and conversely inversed. In addition, these atomic interchange potentials changing with temperature and concentration were in good agreement with earlier study.

ChemInform Abstract: Influence of Long‐Range Atomic Order on the Structural and Magnetic Properties of Ni—Mn—Ga Ferromagnetic Shape Memory Alloys

AbstractReview: 67 refs.

Thermodynamics of f.c.c.-Ni–Fe Alloys in a Static Applied Magnetic Field

Within the scope of the self-consistent field and mean (‘‘molecular’’) self-consistent field approximations, applying the static concentration wave method, the thermodynamics of f.c.c.-Ni–Fe alloys undergoing the static applied magnetic field effects is studied in detail. Under such conditions, the analytical corrections to expressions for the configuration-dependent part of free energy of macroscopically ferromagneticL12-Ni3Fe-type orL10-NiFe-type ordering phases are taken into account. The obtained results for thermodynamically equilibrium states are compared with the refined phase diagram for f.c.c.-Ni–Fe alloys calculated recently without taking into account the applied magnetic field effects. Considering the specific character of microscopic structure of the magnetic and atomic orders in f.c.c.-Ni–Fe alloys, the changes of shape (and in arrangement) of order-disorder phase-transformation curves (Kurnakov points) are thoroughly analysed. A special attention is addressed to the investigation of the concentration, temperature, and magnetic-field induction-dependent atomic and magnetic long-range order parameters, especially, near their critical points. As revealed unambiguously, influence of a static applied magnetic field promotes the elevation of Kurnakov points for all the atomically ordering phases that is in an overall agreement with reliable experimental data. On the base of revealed phenomenon, the magneto external field analog-to-digital converter of the monochromatic radiations (X-rays or thermal neutrons) is hypothesized as a claim.

Low-stability pre-transitional states, orderdisorder phase transitions, and B2–A1 structure transformations in the Cu – 40 at.% Pd alloys

Low-stability states and peculiarities of structural changes in the region of phase transitions are investigated using the Cu–Pd ~40 at.% Pd alloys as an example. To this end, the crystal lattice and long-range atomic order parameters and the Debye-Waller factors were determined in situ directly in the temperature range of the B2–A1 phases and the order – disorder phase transition using X-ray structural analysis. Based on the data obtained, peculiarities of the B2 and A1 phases in the CuPd alloys were analyzed near the structure-phase transitions and the behavior pattern of atomic vibrations in the crystal lattice was discussed. It is shown that in the CuPd alloys with ~40 at.% Pd, low-stability states are observed, which exhibit a number of anomalous phenomena (such as anisotropy of atomic displacements, splitting, heterophase fluctuations, nonlinearity of the lattice parameter and long-range order parameter dependences, etc.), which precede transformation of the alloy.

A comparative study between MEAM and Tersoff potentials on the characteristics of melting and solidification of carborundum

Molecular dynamic simulations of bulk melting, surface melting and crystal growth of SiC are carried out. The atomic interactions in SiC are calculated by MEAM and Tersoff potentials separately. The results show that the bulk melting of SiC with MEAM potential exhibits its relations to temperature similar to that with Tersoff potential, while can be indicated by the mean atomic energy, Lindemann index and structure order parameter. The difference between them is the bulk melt point: MEAM is 4250 K, while Tersoff is 4750 K. At the same superheat degree, the velocities of surface melting of SiC separately, with MEAM and Tersoff potentials are in substantial agreement. But at the same absolute temperature, the surface melting of SiC with MEAM potential is faster than that which the Tersoff potential, which is due to the difference in thermodynamic melting point. The Measured value of the thermodynamic melting point of MEAM is 3338 K compared with 3430 K of Tersoff. On the crystal growth side, the crystal growth velocity of SiC with MEAM potential is related to the undercooling. The fastest velocity corresponds to the undercooling of 400 K. However, the crystal of SiC with Tersoff potential cannot grow in the undercooling of 0 K1000 K. Overall, the MEAM potential is better than Tersoff potential in the sense of describing the melting and solidification of carborundum.

Influence of Long-Range Atomic Order on the Structural and Magnetic Properties of Ni-Mn-Ga Ferromagnetic Shape Memory Alloys

This chapter presents a review of the most recent and systematic works performed on the study of the effect of atomic order on the structural and magnetic properties of Ni-Mn-Ga ferromagnetic shape memory alloys. It is shown that a correlation between long-range atomic order and the martensitic and Curie temperatures can be established from the analysis of its evolution under high and low temperature thermal treatments. In particular, it is demonstrated that, irrespectively of the thermal treatment, both transformation temperatures increase with the increasing L21 atomic order degree, showing practically the same linear dependence on the nextnearest- neighbors atomic order parameter, in such a way that the effect of the atomic order on the relative stability between austenite and martensite can be quantitatively determined. In this respect, it is shown that the effect of atomic order on the martensitic (and also the premartensitic) transformation is directly related to the variation of the magnetic exchange coupling.

Surface Domain Structures and Mesoscopic Phase Transition in Relaxor Ferroelectrics

AbstractRelaxor ferroelectrics are a prototypical example of ferroic systems in which interplay between atomic disorder and order parameters gives rise to emergence of unusual properties, including non‐exponential relaxations, memory effects, polarization rotations, and broad spectrum of bias‐ and temperature‐induced phase transitions. Despite more than 40 years of extensive research following the original discovery of ferroelectric relaxors by the Smolensky group, the most basic aspect of these materials – the existence and nature of order parameter – has not been understood thoroughly. Using extensive imaging and spectroscopic studies by variable‐temperature and time resolved piezoresponse force microscopy, we find that the observed mesoscopic behavior is consistent with the presence of two effective order parameters describing dynamic and static parts of polarization, respectively. The static component gives rise to rich spatially ordered systems on the ∼100 nm length scales, and are only weakly responsive to electric field. The surface of relaxors undergoes a mesoscopic symmetry breaking leading to the freezing of polarization fluctuations and shift of corresponding transition temperature.

Peculiarities of structural–phase states in ternary alloys Ni3(Mn, Ti)

The results of investigation of structural and physical properties of ternary alloys Ni3(Mn, Ti) are presented. A correlation between their physical and structural properties is revealed. The presence of a minimum is established in the curve for the long-range atomic order parameter in the superlattice L12 in the concentration dependence plot. It is assumed that an introduction of titanium would give rise to degradation of antiferromagnetic manganese–manganese interaction.

Green function decoupling scheme for spin-spin correlations in Ising model: Example of partly ordered binary alloy

In this paper we present Green function technique applied to calculations of spin-spin correlations in systems governed by Ising Hamiltonian. This offers approximate yet reasonably accurate analytical results, as an alternative approach to direct computer simulation. Local spin operators are represented in terms of particle operators for fermions. Chain of Green functions for local magnetization on given site involves nearest neighbors and it is decoupled at the next-nearest-neighbors level. Therefore, still accounts at the lowest level for magnetic correlations between nearest neighbors. This technique was applied to binary alloy A x B 1-x with given atomic short range order. Calculations for stoichiometric x=1/2 composition show how temperature dependence of magnetization and spin-spin correlations are modified when atomic order parameter changes. We scanned this parameter from the case of ordered binary alloy, via random alloy, to clusterlike structure. In model calculations we (a) confirm characteristic λ -like shape of the magnetic specific heat near Curie temperature and (b) recover essential results of exact, two-dimensional Ising model. This supports our claim of usefulness of such analytical approach, which is easy and flexible to describe different geometries (clusters, thin films) and conditions imposed on spins located on specific atoms (spins partly pinned on the surface atoms).

Reinvestigation of the Magnetic Structure in L10-Type MnPt Alloy Powder Samples

Reinvestigation of the Magnetic Structure in L1₀-Type MnPt Alloy Powder Samples