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

Abstract

The finite-temperature behavior of small silicon and tin clusters (Si₁₀, Si₁₅, Si₂₀, Sn₁₀, and Sn₂₀) 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 Si₁₀, Sn₁₀, and Si₁₅ show main peaks around 2300, 2200, and 1400 K, respectively. The finite-temperature behavior of Si₁₀ and Sn₁₀ is dominated by isomerization and it is rather difficult to discern their melting temperatures. On the other hand, Si₁₅ does show a liquidlike behavior over a short temperature range, which is followed by fragmentation observed around 1800 K. The finite-temperature behavior of Si₂₀ and Sn₂₀ shows that these clusters do not melt but fragment around 1200 and 650 K, respectively.