Predictions of melting, crystallization, and local atomic arrangements of aluminum clusters using a reactive force field

Abstract

A parametrized reactive force field model for aluminum ReaxFF Al has been developed based on density functional theory ͑DFT͒ data. A comparison has been made between DFT and ReaxFF Al outputs to ascertain whether ReaxFF Al is properly parametrized and to check if the output of the latter has correlation with DFT results. Further checks include comparing the equations of state of condensed phases of Al as calculated from DFT and ReaxFF Al . There is a good match between the two results, again showing that ReaxFF Al is correctly parametrized as per the DFT input. Simulated annealing has been performed on aluminum clusters Al n using ReaxFF Al to find the stable isomers of the clusters. A plot of stability function versus cluster size shows the existence of highly stable clusters ͑magic clusters͒. Quantum mechanically these magic clusters arise due to the complete filling of the orbital shells. However, since force fields do not care about electrons but work on the assumption of validity of Born-Oppenheimer approximation, the magic clusters are therefore correlated with high structural symmetry. There is a rapid decline in surface energy contribution due to the triangulated nature of the surface atoms leading to higher coordination number. The bulk binding energy is computed to be 76.8 kcal/mol. This gives confidence in the suitability of ReaxFF for studying and understanding the underlying dynamics in aluminum clusters. In the quantification of the growth of cluster it is seen that as the size of the clusters increase there is preference for the coexistence of fcc/hcp orders at the expense of simple icosahedral ordering, although there is some contribution from distorted icosahedral ordering. It is found that even for aluminum clusters with 512 atoms distorted icosahedral ordering exists. For clusters with N Ն 256 atoms fcc ordering dominates, which implies that at this point we are already on the threshold of bulklike bonding.

Structure and chemistry of liquid Al–Cu alloys: molecular dynamics study versus thermodynamics-based modelling

“…7b-d, correspondingly. As it can be seen, MEAM-MD data also show high consistency with X-ray diffraction [35] and AIMD simulations data [16,49] with exception of the g CuCu partial. According to our recent observation made for the Al 80 Cu 20 liquid alloy [5], an adequate inconsistency in the position and shape of the high-order peaks in g CuCu (r) partial has also been found.…”

“…As compared to the AIMD data at 1345 K for the Al 60 Cu 40 liquid alloy [49], some similarities have been found concerning abundance and cluster types distinguished in the Al-Cu liquid alloys examined. At first, icosahedral clusters of \ 0 0 12 0 [ indexideal icosahedron are found to be not the most abundant index compared to distorted icosahedra of {\ 0 1 10 2 [ , \ 0 3 6 4 [}.…”

Structures and stabilities of Aln+, Aln, and Aln− (n=13–34) clusters

“…A controversy has arisen regarding the correct global minimum ͑GM͒ for Al 13 : the optimizations based on parametrized potential models 32,34,35,41,42,46,52 invariably predict an icosahedral structure, but these methods do not contain an explicit description of the electronic degrees of freedom and their accuracy is questionable. A majority of the ab initio calculations ͓mostly based on density functional theory ͑DFT͒ and differing in the election of exchangecorrelation functional, aluminum pseudopotential, and basis set͔ predict an icosahedron as the most stable structure, [22][23][24][25]27,28,30,33,37,39,45 but a small number of calculations 26,31,38,48,49 predict a decahedron to be more stable.…”

Thermal Properties of Yttrium Aluminum Garnett From Molecular Dynamics Simulations

“…Nonetheless, the deviation from linearity is due to the coexistence of the liquid and solid state in the material (dynamic coexistence) [21] where the YAG ceases to be a crystal and is instead broken down into mobile clusters. Figure 10.…”