Electronic effects on the melting of small gallium clusters

Abstract

Motivated by experimental reports of higher-than-bulk melting temperatures in small gallium clusters, we perform first-principles molecular dynamics simulations of Ga(20) and Ga(20)(+) using parallel tempering in the microcanonical ensemble. The respective specific heat (C(V)) curves, obtained using the multiple histogram method, exhibit a broad peak centered at approximately 740 and 610 K--well above the melting temperature of bulk gallium (303 K) and in reasonable agreement with experimental data for Ga(20)(+). Assessment of atomic mobility confirms the transition from solid-like to liquid-like states near the C(V) peak temperature. Parallel tempering molecular dynamics simulations yield low-energy isomers that are ~0.1 eV lower in energy than previously reported ground state structures, indicative of an energy landscape with multiple, competing low-energy morphologies. Electronic structure analysis shows no evidence of covalent bonding, yet both the neutral and charged clusters exhibit greater-than-bulk melting temperatures.

Cluster melting: new, limiting, and liminal phenomena

“…The use of DFT based BOMD for these small gallium clusters has demonstrated the ability of these calculations to replicate the experimental findings, both with respect to the melting temperature variations and the nature of melting in those cases where multiple transitions are observed [51,52,83,84]. As expected, the complex phase diagram of gallium leads to many interesting features in the electronic structure as a function of temperature, and these have been isolated through statistical analysis of the dynamic structures [84] to provide evidence of the relative stability of different solid phases -related to the various low temperature polymorphs of gallium (β, δ, or γ ), but never to the dimeric standard (α) phase -at finite temperature [51].…”

Characterizing the Greater-Than-Bulk Melting Behavior of Ga–Al Nanoalloys

“…19 different temperatures between 250 K and 1150 K at 50 K intervals were chosen to capture the solid-liquid-like phase transition. Relative convergences of the canonical and microcanonical specific heat curves obtained using the multiple histogram (MH) method [14,50], ascertained using the sliding window analysis [50], were achieved after a total run time of 160 ps (step size 2 fs) for Ga 11 Al 9 + and 46.2 ps (step size 0.8 fs) for Ga 3 Al 17 + . A smaller step size was used in Ga 3 Al 17 + to counter the increase in energy during microcanonical runs.…”

Quantum Size Effects in the Size–Temperature Phase Diagram of Gallium: Structural Characterization of Shape‐Shifting Clusters

“…We obtained specific heat curves by histogram reweighting technique of the multiple histogram method 18. Previous work summarizes the comparison between our simulated specific heat curves11, 13 and the experimental curves 10…”