Kavita Joshi

Why Do Gallium Clusters Have a Higher Melting Point than the Bulk

Density functional molecular dynamical simulations have been performed on Ga17 and Ga13 clusters to understand the recently observed higher-than-bulk melting temperatures in small gallium clusters [Phys. Rev. Lett. 91, 215508 (2003)]]. The specific-heat curve, calculated with the multiple-histogram technique, shows the melting temperature to be well above the bulk melting point of 303 K, viz., around 650 and 1400 K for Ga17 and Ga13, respectively. The higher-than-bulk melting temperatures are attributed mainly to the covalent bonding in these clusters, in contrast with the covalent-metallic bonding in the bulk.

Magic Melters Have Geometrical Origin

Recent experimental reports bring out extreme size sensitivity in the heat capacities of gallium and aluminum clusters. In the present work we report results of our extensive ab initio molecular dynamical simulations on Ga30 and Ga31, the pair which has shown rather dramatic size sensitivity. We trace the origin of this size sensitive heat capacities to the relative order in their respective ground state geometries. Such an effect of nature of the ground state on the characteristics of heat capacity is also seen in case of small gallium and sodium clusters, indicating that the observed size sensitivity is a generic feature of small clusters.

Thermodynamics of tin clusters

We report the results of detailed thermodynamic investigations of the

Electronic and structural investigations of gold clusters doped with copper: Aun−1Cu− (n=13–19)

{\mathrm{Sn}}_{20}

Finite temperature behavior of impurity doped Lithium cluster, Li6Sn

cluster using density-functional molecular dynamics. These simulations have been performed over a temperature range of 150 to 3000 K, with a total simulation time of order 1 ns. The prolate ground state and low-lying isomers consist of two tricapped trigonal prism (TTP) units stacked end to end. The ionic specific heat, calculated via a multihistogram fit, shows a small peak around 500 K and a shoulder around 850 K. The main peak occurs around 1200 K, about 700 K higher than the bulk melting temperature, but significantly lower than that for

Finite-temperature behavior of small silicon and tin clusters: An ab initio molecular dynamics study

{\mathrm{Sn}}_{10}.

Correlation between the variation in observed melting temperatures and structural motifs of the global minima of gallium clusters: An ab initio study

The main peak is accompanied by a sharp change in the prolate shape of the cluster due to the fusion of the two TTP units to form a compact, near spherical structure with a diffusive liquidlike ionic motion. The small peak at 500 K is associated with rearrangement processes within the TTP units, while the shoulder at 850 K corresponds to distortion of at least one TTP unit, preserving the overall prolate shape of the cluster. At all temperatures observed, the bonding remains covalent.

Density functional analysis of the structural evolution of Gan (n=30-55) clusters and its influence on the melting characteristics

We have obtained the ground state and the equilibrium geometries of Au(n) (-) and Au(n-1)Cu(-) in the size range of n=13-19. We have used first principles density functional theory within plane wave and Gaussian basis set methods. For each of the cluster we have obtained at least 100 distinct isomers. The anions of gold clusters undergo two structural transformations, the first one from flat cage to hollow cage and the second one from hollow cage to pyramidal structure. The Cu doped clusters do not show any flat cage structures as the ground state. The copper doped systems evolve from a general 3D structure to hollow cage with Cu trapped inside the cage at n=16 and then to pyramidal structure at n=19. The introduction of copper atom enhances the binding energy per atom as compared to gold cluster anions.

Dopant-induced stabilization of silicon clusters at finite temperature

We have carried out extensive isokinetic ab initio molecular-dynamic simulations to investigate the finite temperature properties of the impurity doped cluster Li6Sn and the host cluster Li7. The data obtained from about 20 temperatures and total simulation time of at least 3 ns is used to extract thermodynamical quantities like canonical specific heat. We observe that, first, Li6Sn becomes liquidlike around 250 K, at much lower temperature than that for Li7 (≈425 K). Second, a weak shoulder around 50 K in the specific heat curve of Li6Sn is observed due to the weakening of Li–Li bonds. The peak in the specific heat of Li7 is very broad and the specific heat curve does not show any premelting features.

Rationalizing the role of structural motif and underlying electronic structure in the finite temperature behavior of atomic clusters

The finite-temperature behavior of small silicon and tin clusters (Si 10 , Si 15 , Si 20 , Sn 10 , and Sn 20 ) is studied using isokinetic Born-Oppenheimer molecular dynamics. We find that the low-lying structures for all the clusters are built upon a highly stable tricapped trigonal prism unit which is seen to play a crucial role in the finite-temperature behavior. The thermodynamics of small tin clusters is revisited in light of the recent experiments on tin clusters of sizes 18-21 [G. A. Breaux et al., Phys. Rev. B, 71, 073410 (2005)]. Our calculated heat capacities for Si 10 , Sn 10 , and Si 15 show main peaks around 2300, 2200, and 1400 K, respectively. The finite-temperature behavior of Si 10 and Sn 10 is dominated by isomerization and it is rather difficult to discern their melting temperatures. On the other hand, Si 15 does show a liquidlike behavior over a short temperature range, which is followed by fragmentation observed around 1800 K. The finite-temperature behavior of Si 20 and Sn 20 shows that these clusters do not melt but fragment around 1200 and 650 K, respectively.