Thermodynamics of tin clusters

TL;DR

This study investigates the thermodynamic behavior of Sn20 clusters using density-functional molecular dynamics, revealing temperature-dependent structural changes and specific heat features that differ from bulk tin.

Contribution

First detailed thermodynamic analysis of Sn20 clusters using DFT molecular dynamics, highlighting shape transitions and specific heat features.

Findings

Main melting peak at 1200 K, higher than bulk tin

Prolate to spherical shape transition around 1200 K

Bonding remains covalent across all temperatures

Abstract

We report the results of detailed thermodynamic investigations of the Sn$_{20}$ 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 Sn$_{10}$. 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.