Metal clusters with hidden ground states: Melting and structural transitions in Al115+, Al116+, and Al117+

Abstract

Heat capacities measured as a function of temperature for Al(115)(+), Al(116)(+), and Al(117)(+) show two well-resolved peaks, at around 450 and 600 K. After being annealed to 523 K (a temperature between the two peaks) or to 773 K (well above both peaks), the high temperature peak remains unchanged but the low temperature peak disappears. After considering the possible explanations, the low temperature peak is attributed to a structural transition and the high temperature peak to the melting of the higher enthalpy structure generated by the structural transition. The annealing results show that the liquid clusters freeze exclusively into the higher enthalpy structure and that the lower enthalpy structure is not accessible from the higher enthalpy one on the timescale of the experiments. We suggest that the low enthalpy structure observed before annealing results from epitaxy, where the smaller clusters act as a nucleus and follow a growth pattern that provides access to the low enthalpy structure. The solid-to-solid transition that leads to the low temperature peak in the heat capacity does not occur under equilibrium but requires a superheated solid.

Effect of disorder on the structure of small aluminum clusters during heat treatment

“…In 40% of the cases, the energy of the clusters increased, but the initial fcc structure, despite some violation of the order of stacking of atomic layers with the formation of icosahedral fragments, persisted up to a temperature T = 370 K, and only further heating led to the forma tion of the icosahedral structure. In this connection, it is necessary to consider the data of the experimental study [29] on the temperature dependence of the heat capacity for the Al 115 , Al 116 , and Al 117 clusters. In [29], it was found that the heat capacity exhibits two well defined peaks at approxi mately 450 and 600 K. However, it was noted that, after heating of the clusters to a temperature above both peaks (773 K) followed by cooling to room tem perature, the repeated measurements of the heat capacity demonstrated the presence of only the high temperature peak of the heat capacity, while the lower temperature peak disappeared.…”

Reactions of liquid and solid aluminum clusters with N2: The role of structure and phase in Al114+, Al115+, and Al117+

“…39 For annealed clusters, the abrupt changes in the kinetic energy thresholds at around 450 K are expected to disappear and the kinetic energy thresholds below 450 K are expected to be similar to the values above 450 K. However, this has not yet been confirmed experimentally.…”

“…There are several possible explanations for the existence of two peaks. 39 They could result from different parts of the clusters melting at different temperatures. For example, surface premelting is a well-known phenomenon that occurs for some metal surfaces.…”

“…But even more compelling is the observation that the low temperature peak disappears when the clusters are annealed. 39 This indicates that the purported liquid layer does not refreeze, which is difficult to explain because a solid nucleus already exists. Thus the premelting explanation appears to be ruled out.…”

“…[31][32][33][34][35][36][37][38] Cationic clusters with 115, 116, and 117 atoms, on the other hand, show two clearly resolved peaks. 39 In these cases, the lower temperature peak was attributed to a structural transition and the higher temperature peak to the melting transition. In this paper we present kinetic energy thresholds measured for the chemisorption of N 2 onto Al 114 + , Al 115 + , and Al 117 + , as a function of temperature (we did not perform threshold measurements for Al 116 + ).…”