Materials Science and Engineering, A178 (1994) 19-22 19 Structure of diatomic clusters Marco Ronchetti and Stefano Cozzini Dipartimento di Fisica, Universitd di Trento, 38050 Povo (TN) (Italy) Abstract We investigate the structure of 13-particle clusters in binary alloys for various size ratios and concentrations to predict which systemsare likely to form local icosahedral structures when rapidly supercooledfrom the melt. 1. Introduction Metallic glasses would not exist in a two-dimen- sional world. In fact, by minimizing the volume of small clusters of equal disks one gets hexagonal structures. Two-dimensional crystals also exhibit extended hexag- onal order; therefore a natural, easy path exists and leads from small structures to extended ones. The picture is quite different in the three-dimensional world: small clusters of equal balls are dominated by icosahedral ordering, but the closest packing of extended structures exhibits cubic symmetry (FCC). Growing from small structures which optimize space filling on a small scale to structures which do so on a large scale requires large rearrangements. These geo- metric considerations are true also from an energetic point of view: the energy of a 13-atom icosahedral cluster is significantly lower than the energy of corre- spondent FCC or HCP clusters; for a Lennard-Jones potential (LJ) the difference is as much as 8%, and a similar preference for local icosahedral order is shown by purely repulsive r -n potentials. The competition between the preferred local structure and the optimal extended one gives rise to frustration, as discussed in depth by Nelson and Spaepen [1]. Steinhardt et al. [2, 3] suggested the importance of (extended) icosahedral order in monoatomic LJ super- cooled liquids. Honeycutt and Andersen [4] showed that the lowest energy structures in LJ clusters are ico- sahedral up to a size of 5000 atoms. Nos~ and Yonezawa [5] reported that there is an increase in local icosahedral structure during the quenching process on LJ liquids. Similar results were obtained by Tanaka [6] on a rubidium system and by Tsumuraya and Watanabe [7] on a metallic system (Na) with a potential which shows Friedel oscillations. Recently, Dzugutov [8] reported enhanced icosahedral local order in mono- atomic samples by using a potential which resembles an effective metallic potential and makes the formation of FCC more difficult. Rather surprisingly, such a potential leads the same system to form a dodecagonal quasicrystal [9]. On the other hand, for glasses of a binary system the scenario is not unique. Nelson and Spaepen [1] suggest that a small concentration of large particles should favour the onset icosahedral order. Jonsson and Andersen [10] reported a large amount of icosahedrai clusters in their simulations, while Dasgupta et al. [11 ] and Ernst et al. [12] did not find such structures. In the present work we study the structure of 13-atom binary clusters in order to contribute to the understanding of these contrasting results. Our objec- tive is twofold: we wish to derive a detailed map of the potential energy minima with respect to the concentra- tion of small atoms in the clusters, and we intend to explore these minimal energy configurations from a structural point of view. 2. Simulation method Our simulation method was the molecular dynamics technique (see ref. 13 for an introduction to this method). We used an LJ potential with parameters for the large atoms (oll and e11) suited for argon. The potential for the second atomic specie was defined by parameters and ce22 = Cell and 022 = 1711 , where ct is the radii-ratio. The interaction between unlike particles was defined by Cel2 = Ce22 = Cell and 012=(01t + 022)/2. The mass is not a relevant parameter when only struc- tural properties are of interest, and we therefore used equal masses for the two species. We studied 13-atom clusters for three values of a (1.6, 1.4, 1.25), and for all possible values of concentration ~//13. (The limiting cases r/= 0 and r/= 13 are equivalent and well known, since they correspond to a monoatomic system.) 0921-5093/94/$7.00 © 1994 - Elsevier Sequoia. All rights reserved SSDI 0921-5093(93)04504-B