Molecular dynamics simulations of the meltinglike transition in<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Li</mml:mi><mml:mn>13</mml:mn></mml:msub><mml:msub><mml:mi mathvariant="normal">Na</mml:mi><mml:mn>42</mml:mn></mml:msub></mml:mrow></mml:math>and<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mi mathvariant="normal">Na</mml:mi><mml:mn>13</mml:mn></mml:msub><mml:msub><mml:

The Journal of Chemical Physics

López

2010

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The structure of 55-atom Li-Na and Na-K nanoalloys is determined through combined empirical potential ͑EP͒ and density functional theory ͑DFT͒ calculations. The potential energy surface generated by the EP model is extensively sampled by using the basin hopping technique, and a wide diversity of structural motifs is reoptimized at the DFT level. A composition comparison technique is applied at the DFT level in order to make a final refinement of the global minimum structures. For dilute concentrations of one of the alkali atoms, the structure of the pure metal cluster, namely, a perfect Mackay icosahedron, remains stable, with the minority component atoms entering the host cluster as substitutional impurities. At intermediate concentrations, the nanoalloys adopt instead a core-shell polyicosahedral ͑p-Ih͒ packing, where the element with smaller atomic size and larger cohesive energy segregates to the cluster core. The p-Ih structures show a marked prolate deformation, in agreement with the predictions of jelliumlike models. The electronic preference for a prolate cluster shape, which is frustrated in the 55-atom pure clusters due to the icosahedral geometrical shell closing, is therefore realized only in the 55-atom nanoalloys. An analysis of the electronic densities of states suggests that photoelectron spectroscopy would be a sufficiently sensitive technique to assess the structures of nanoalloys with fixed size and varying compositions.

Section: Introductionsupporting

confidence: 89%

“…For x = 19, the Li part is a double icosahedron formed by two interpenetrating 13-atom icosahedra. For x = 23 and x = 26, there are respectively three and four interpenetrated Li 13 icosahedra.…”

Section: A Model Potential Structuresmentioning

confidence: 99%

Structure determination in 55-atom Li–Na and Na–K nanoalloys

The Journal of Chemical Physics

López

2010

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“…For some compositions, we indeed observe poly-decahedral packing, obtained by interpenetrating several 13-atom decahedra. For x = 20, the lithium subsystem is formed by two interpenetrated Li 13 decahedra, while three decahedra form the core of the highsymmetry structure observed for x = 25. Close to the stability limit of the decahedral family, we observe a close competition of poly-decahedral and pIh core motifs.…”

Section: B Structures Of LI X Cs 55−xmentioning

confidence: 99%

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

The Journal of Chemical Physics

López²

2011

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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.

“…The same preference for polyicosahedral and/or polytetrahedral packing has been independently obtained by Calvo and Yurtsever 28 and Doye and Meyer 29 for mixed LJ clusters and by Aguado and López for binary and ternary mixtures of alkali metals. 35,36 Anti-Mackay overlayers are not specially stable for homogeneous clusters because of the associated bond strain, but they are very stable in those mixtures which present ͑a͒ a sufficiently large size mismatch and ͑b͒ a lower surface tension of the atomic species with larger radius. In this case, both core ͑Ni/ Na/ Ar͒ and surface ͑Ag/ Cs/ Xe͒ atoms may have interatomic distances close to optimal in the polyicosahedral structure, which explains its stabilization.…”

Section: Introductionmentioning

confidence: 99%

Structural and thermal behavior of compact core-shell nanoparticles: Core instabilities and dynamic contributions to surface thermal stability

López

2005

Phys. Rev. B

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An orbital-free density-functional-theory molecular dynamics technique is applied to investigate the minimum-energy structure and meltinglike transition of Cs 55 , Li 13 Na 32 Cs 42 , and Li 55 Cs 42 nanoparticles. Icosahedral packing is found to be optimal for homogeneous Cs 55 , as expected. Heterogeneous particles show a complete segregation of Cs atoms to the cluster surface, and form perfect core-shell structures, that is, structures where each atomic species occupies and completes a different concentric atomic shell. For Li 13 Na 32 Cs 42 , the size mismatch between atomic species forming different shells leads to polyicosahedral packing. For Li 55 Cs 42 , however, the size mismatch is huge and perfect polyicosahedral ordering is frustrated, resulting in more complex structural behavior. The three clusters investigated share the same surface shell, formed by 42 Cs atoms, and comparison of their melting behaviors helps to rationalize the increased thermal stability of the cluster surface upon alloying. Cs 55 melts homogeneously at approximately 85 K. Both Li 13 Na 32 Cs 42 and Li 55 Cs 42 show a substantial thermal stability, compared to Cs 55 and other alloy compositions where a perfect core-shell structure does not appear. We demonstrate that an important contribution to this increased thermal stability in the nanoalloys comes from the large difference in the atomic masses of the constituent particles, which results in a poor coupling of atomic vibrations along the radial direction. We also give arguments to show that the meltinglike transition in these clusters is triggered by the thermal instability of interior rather than surface atoms. Segregation of Cs atoms to the cluster surface is fully maintained in the liquid state, so that core and surface shells form two inmiscible liquid layers.