Theoretical investigation of Fe–Rh binary nanoalloys: Chemical ordering and magnetic behavior

Gupta and Density Functional Theory (DFT) calculations were performed to investigate of structural and magnetic behaviors of 19 atom FenRh[Formula: see text] ([Formula: see text]–19) nanoalloys. A double icosahedron structure was considered for FenRh[Formula: see text] ([Formula: see text]–19) nanoalloys. Significantly, the effects of Fe atom addition on the chemical ordering, stability and total magnetic moments of the nanoalloys were investigated. Local optimization results at the Gupta level show that the Fe atoms are located in the center of the double icosahedron structure and finally in the equatorial region on the surface. The mixing energy analysis obtained that Fe[Formula: see text]Rh7 and Fe4Rh[Formula: see text] nanoalloys are the most stable compositions at Gupta and DFT levels, respectively. It was found that FenRh[Formula: see text] ([Formula: see text]–19) nanoalloys are energetically suitable for mixing at both Gupta and DFT levels. Also, the bond order parameter result is compatible with the mixing energy analysis result. The total magnetic moments of the FenRh[Formula: see text] ([Formula: see text]–19) nanoalloys increase with the addition of the Fe atom, which is a ferromagnetic metal.

Investigation of the chemical ordering and thermal properties of the Rh–Ag–Au nanoalloys

In this paper, the melting behaviors of Rh–Ag–Au nanoalloys are investigated with MD simulation. For Rh–Ag–Au nanoalloys, icosahedron structure was considered. The local optimizations of Rh–Ag–Au nanoalloys were carried out with the BH algorithm. The interatomic interactions were modeled with the Gupta potential. The local optimization results of Rh–Ag–Au nanoalloys show that Au and Ag atoms prefer to locate on the surface, and Rh atoms prefer to locate in the inner shells. The bond order parameter result is compatible with the excess energy analysis. It is noted that structures with more Ag–Au bonds are more energetically stable. Caloric curve, heat capacity, Lindemann index, and RMSD methods were used for estimating the melting temperatures of Rh–Ag–Au nanoalloys. According to the simulation results, melting temperatures depend on the composition. Also, it is discovered that nanoalloys are generally melting in two stages. Surface melting of the third shell is occupied by Ag and Au atoms, and then homogeneous melting of the inner shells is occupied by Rh atoms. It is found that the difference between surface melting temperatures and homogeneous melting temperatures in Ag-poor compositions is more significant than that of Ag-rich nanoalloys. In addition, the melting temperatures of the nanoalloys are found to be increased as the size of nanoalloys increases.

Structural relaxation in Ag-Ni nanoparticles: Atomistic modeling away from equilibrium

The out-of-equilibrium structural relaxation of Ag-Ni nanoparticles containing about 1000--3000 atoms was investigated computationally by means of molecular dynamics trajectories in which the temperature is decreased gradually over hundreds of nanoseconds. At low silver concentration of 10--30\%, the evolution of chemical ordering in Ni$_{\rm core}$Ag$_{\rm shell}$ nanoparticles with different surface arrangements is found to proceed spontaneously and induce some rounding of the nickel core and its partial recristallization. Fast cooling of an initially hot metal vapor mixture was also considered, and it is shown to disfavor silver aggregation at the surface. Silver impurities are also occasionally produced but remain rare events under the conditions of our simulations.

Probing Multiscale Disorder in Pyrochlore and Related Complex Oxides in the Transmission Electron Microscope: A Review

Transmission electron microscopy (TEM), and its counterpart, scanning TEM (STEM), are powerful materials characterization tools capable of probing crystal structure, composition, charge distribution, electronic structure, and bonding down to the atomic scale. Recent (S)TEM instrumentation developments such as electron beam aberration-correction as well as faster and more efficient signal detection systems have given rise to new and more powerful experimental methods, some of which (e.g., 4D-STEM, spectrum-imaging, in situ/operando (S)TEM)) facilitate the capture of high-dimensional datasets that contain spatially-resolved structural, spectroscopic, time- and/or stimulus-dependent information across the sub-angstrom to several micrometer length scale. Thus, through the variety of analysis methods available in the modern (S)TEM and its continual development towards high-dimensional data capture, it is well-suited to the challenge of characterizing isometric mixed-metal oxides such as pyrochlores, fluorites, and other complex oxides that reside on a continuum of chemical and spatial ordering. In this review, we present a suite of imaging and diffraction (S)TEM techniques that are uniquely suited to probe the many types, length-scales, and degrees of disorder in complex oxides, with a focus on disorder common to pyrochlores, fluorites and the expansive library of intermediate structures they may adopt. The application of these techniques to various complex oxides will be reviewed to demonstrate their capabilities and limitations in resolving the continuum of structural and chemical ordering in these systems.

Atomic scale chemical ordering in franckeite—a natural van der Waals superlattice

The mineral franckeite is a naturally occurring van der Waals superlattice which has recently attracted attention for future applications in optoelectronics, biosensors and beyond. Furthermore, its stacking of incommensurately modulated 2D layers, the pseudo tetragonal Q-layer and the pseudo hexagonal H-layer, is an experimentally accessible prototype for the development of synthetic van der Waals materials and of advanced characterization methods to reveal new insights in their structure and chemistry at the atomic scale that is crucial for deep understanding of its properties. While some experimental studies have been undertaken in the past, much is still unknown on the correlation between local atomic structure and chemical composition within the layers. Here we present an investigation of the atomic structure of franckeite using state-of-the-art high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) and atom probe tomography (APT). With atomic-number image contrast in HAADF STEM direct information about both the geometric structure and its chemistry is provided. By imaging samples under different zone axes within the van der Waals plane, we propose refinements to the structure of the Q-layer and H-layer, including several chemical ordering effects that are expected to impact electronic structure calculations. Additionally, we observe and characterize stacking faults which are possible sources of differences between experimentally determined properties and calculations. Furthermore, we demonstrate advantages and discuss current limitations and perspectives of combining TEM and APT for the atomic scale characterization of incommensurately modulated von der Waals materials.

First Principles Study of Ga₍₂₀₋x₎Alx⁺ Nanoalloys: Structure, Thermodynamics and Phase Diagram

<p>Nanoalloys (a finite framework of two or more metal atoms) represent a rapidly growing field owing to the possibilities of tuning its properties as desired for various applications. Their properties are size, shape, composition, chemical ordering, and temperature dependent, thereby offering a large playground for varied research motivations. This thesis documents the investigations on how the addition of aluminium affects the cationic gallium clusters, both in terms of geometric & electronic structure and thermodynamics, which have been observed to show greater-than-bulk melting behaviour for small sizes. A specific cluster size of 20 atoms is selected, Ga₍₂₀₋x₎Alx⁺, with the overall intention of creating a phase diagram which is the most reliable way to predict the phase changes in the system. All the first principles (density functional theory) based Born-Oppenheimer molecular dynamics calculations have been performed in the microcanonical ensemble. Melting behaviour is first studied in the pure Al₂₀⁺ clusters and then in three representative clusters of Ga₍₂₀₋x₎Alx⁺ series: Ga₁₉Al⁺, Ga₁₁Al₉⁺ and Ga₃Al₁₇⁺ clusters. We observe that all the three nanoalloy compositions show greater-than-bulk melting behaviour behaviour as well and in Ga₁₉Al⁺, specifically, Al prefers the internal sites, contrary to the previous arguments. We go on to complete the solid-liquid-like melting phase diagram using the calculated information and further propose a model of these greater-than-bulk melting clusters to be components of the corresponding bulk phases, whether metals or alloys, with additional size-dependent contributions added to it.</p>

First Principles Study of Ga₍₂₀₋x₎Alx⁺ Nanoalloys: Structure, Thermodynamics and Phase Diagram

<p>Nanoalloys (a finite framework of two or more metal atoms) represent a rapidly growing field owing to the possibilities of tuning its properties as desired for various applications. Their properties are size, shape, composition, chemical ordering, and temperature dependent, thereby offering a large playground for varied research motivations. This thesis documents the investigations on how the addition of aluminium affects the cationic gallium clusters, both in terms of geometric & electronic structure and thermodynamics, which have been observed to show greater-than-bulk melting behaviour for small sizes. A specific cluster size of 20 atoms is selected, Ga₍₂₀₋x₎Alx⁺, with the overall intention of creating a phase diagram which is the most reliable way to predict the phase changes in the system. All the first principles (density functional theory) based Born-Oppenheimer molecular dynamics calculations have been performed in the microcanonical ensemble. Melting behaviour is first studied in the pure Al₂₀⁺ clusters and then in three representative clusters of Ga₍₂₀₋x₎Alx⁺ series: Ga₁₉Al⁺, Ga₁₁Al₉⁺ and Ga₃Al₁₇⁺ clusters. We observe that all the three nanoalloy compositions show greater-than-bulk melting behaviour behaviour as well and in Ga₁₉Al⁺, specifically, Al prefers the internal sites, contrary to the previous arguments. We go on to complete the solid-liquid-like melting phase diagram using the calculated information and further propose a model of these greater-than-bulk melting clusters to be components of the corresponding bulk phases, whether metals or alloys, with additional size-dependent contributions added to it.</p>

Coupled Strengthening Effects by Lattice Distortion, Local Chemical Ordering, and Nanoprecipitates in Medium-Entropy Alloys

Extraordinary mechanical properties can be achieved in high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) with nanoprecipitates. In the present study, the extra coupled strengthening effects by lattice distortion, local chemical ordering, and nanoprecipitates in the HEAs and MEAs with nanoprecipitates have been systematically investigated by large-scale molecular dynamics simulations. The moving of the dislocation can be slowed down, and the dislocation line shows a wavy configuration due to lattice distortion and local chemical ordering, resulting in strengthening. The degree of the wavy configuration increases and the sliding velocity of the dislocation decreases with increasing degrees of local chemical ordering. It is clearly indicated that the dislocation moves via nanoscale segment detrapping mechanism due to the effects of lattice distortion and local chemical ordering, resulting in roughened dislocation pathways for strengthening. The activated nanoscale segments are observed to be easier to detrap from the regions with stronger Co-Cr local chemical ordering and then propagate into the regions without such chemical ordering. These moving characteristics of the dislocation can delay the unpinning process from nanoprecipitates; thus, extra coupled strengthening effect has been revealed in the HEAs and MEAs with nanoprecipitates compared to pure Orowan’s strengthening.

Alloying nanoparticles by discharges in liquids: a quest for metastability

Resorting to ultrafast processes to synthesize alloy nanoparticles far from thermodynamic equilibrium is subjected to phase transformations that keep particles at a given temperature for periods of time that are usually long with respect to the process pulse durations. Then, reaching non-equilibrium conditions is not straightforwardly associated with the process, as fast as it can be, but rather to heat transfer mechanisms during phase transformations. This latter aspect is dependent on nanoparticle size. Furthermore, other important phenomena, like chemical ordering, are essential to explain the final structure adopted by an alloy nanoparticle. In this work, a specific attention is paid to suspensions submitted either to electrical discharges or to ultrashort laser excitations. After discussing thermodynamic considerations that give the frame beyond which non-equilibrium alloys form, a description of the heating processes at stake is provided. This leads to maximum temperature reached for particles with nanometric sizes and specific conditions to fulfil practically during the quenching step. The way solidification must be processed in that purpose is discussed next. The example of the Cu-Ag system is finally considered to illustrate the advantage of better controlling processes that are currently used to create homogeneously-alloyed nanoparticles made of immiscible elements, but also to show the actual limitations of these approaches.