Identifying structural and energetic trends in isovalent core-shell nanoalloys as a function of composition and size mismatch

Abstract

We locate the putative global minimum structures of Na x Cs 55−x and Li x Cs 55−x nanoalloys through combined empirical potential and density functional theory calculations, and compare them to the structures of 55-atom Li-Na and Na-K nanoalloys obtained in a recent paper [A. Aguado and J. M. López, J. Chem. Phys. 133, 094302 (2010)]. Alkali nanoalloys are representative of isovalent metallic mixtures with a strong tendency towards core-shell segregation, and span a wide range of size mismatches. By comparing the four systems, we analyse how the size mismatch and composition affect the structures and relative stabilities of these mixtures, and identify useful generic trends. The Na-K system is found to possess a nearly optimal size mismatch for the formation of poly-icosahedral (pIh) structures with little strain. In systems with a larger size mismatch (Na-Cs and Li-Cs), frustration of the pIh packing induces for some compositions a reconstruction of the core, which adopts instead a decahedral packing. When the size mismatch is smaller than optimal (Li-Na), frustration leads to a partial amorphization of the structures. The excess energies are negative for all systems except for a few compositions, demonstrating that the four mixtures are reactive. Moreover, we find that Li-Cs and Li-Na mixtures are more reactive (i.e., they have more negative excess energies) than Na-K and Na-Cs mixtures, so the stability trends when comparing the different materials are exactly opposite to the trends observed in the bulk limit: the strongly non-reactive Li-alkali bulk mixtures become the most reactive ones at the nanoscale. For each material, we identify the magic composition x m which minimizes the excess energy. x m is found to increase with the size mismatch due to steric crowding effects, and for Li x Cs 55−x the most stable cluster has almost equiatomic composition. We advance a simple geometric packing rule that suffices to systematize all the observed trends in systems with large size mismatch (Na-K, Na-Cs, and Li-Cs). As the size mismatch is reduced, however, electron shell effects become more and more important and contribute significantly to the stability of the Li-Na system.

Growth analysis of sodium-potassium alloy clusters from 7 to 55 atoms through a genetic algorithm approach

“…This indicates that the formation of nanoalloys is best favored when the percentage of sodium achieves this optimal composition, and such clusters are more likely to be found in experiments. The thorough work of Aguado et al [2,21] in the 55-atom case employing density functional theory (DFT) calculations also confirms this tendency for one specific cluster size. For a visual representation, the alloys with composition that provide the lowest excess energy from 20 to 55 atoms are shown in Fig.…”

“…5, where pentagonal pyramids and bipyramids are interspersed along the surface. For the 55-atom cluster, a thorough study has been carried out by Aguado et al [2,21], where it is shown that the icosahedral shape is assumed by pure alkaline clusters and its alloys at dilute limits. Such results were also achieved within our classical approach for pure sodium and potassium clusters, as well as for the dilute limit on the Na-rich side, where the addition of potassium atoms maintain the essential icosahedral shape up to the Na 45 K 10 , with distortions due to atom size mismatch.…”

“…Experimental and theoretical studies on the NaK alloy [2,7,8,20,21] show a core-shell segregation propensity, where the core tends to be formed by sodium atoms, and that the alloy is miscible, at room temperature, for most compositions [7]. Recently, we have provided benchmark results, for small NaK alloys employing high-level electronic structure methods [8].…”

Equilibrium Cu-Ag nanoalloy structure formation revealed by in situ scanning transmission electron microscopy heating experiments

“…As a result, novel structures, physical properties, and processes may occur with NPs that are not observed for their bulk counterparts. [1][2][3][4] For instance, the melting point of a metallic NP is typically lower than that of the bulk metal. [5][6][7] For the metallic copper (Cu) and silver (Ag) system, it is well known that the solid solution of the binary Ag-Cu alloy is unstable, and upon heating an initially amorphous or nanocrystalline Ag-Cu alloy in bulk form will separate into Ag and Cu phases due to the large miscibility gap and positive enthalpy of mixing for this eutectic system.…”

“…© 2014 Author (s New physical and chemical phenomena related to nanoscale effects are currently the focus of intense research. [1][2][3][4] Nanoparticles (NPs) are characterized by small volumes and a large number of surface atoms, which inherently possess a significantly larger mobility than bulk atoms. As a result, novel structures, physical properties, and processes may occur with NPs that are not observed for their bulk counterparts.…”