packing scheme preferring fivefold symmetry is in fact intrinsically associated

Packing scheme preferring fivefold symmetry is in

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packing scheme preferring fivefold symmetry is in fact intrinsically associated with the dense packing in amorphous materials, and is not uncommon in the literature for the packing of small clusters or particles 36,39–41 . We stress that the ab initio calculated structures produce partial RRDFs in very good agreement with the experimental ones: see the excellent match of the RRDFs, Supplementary Fig. S7. Note also in Supplementary Fig. S7 that the splitting B–B first correlation peak seen in the experimental RRDF of Ni–B (and solute–solute corre- lation in some other TM–metalloid systems as well) was not reproduced by the previously proposed model assuming f.c.c. pack- ing of overlapping equal-sized clusters 9 . This strongly suggests that the icosahedral fivefold packing is a more realistic ordering pattern of cluster–cluster connection in MGs. Our solute–solute–solute bond-angle analysis corroborates this conclusion. The cluster- connection diagrams in Fig. 5b–d for the several MGs illustrate the specific packing/connection schemes of the quasi-equivalent clusters, through the sharing of edges, faces and vertices. In fact, these different sharing schemes explain the different solute–solute dis- tances that lead to the splitting in the peaks in the respective solute– solute partial RRDFs (Supplementary Fig. S7). Similar schemes for the communication among clusters were predicted in Gaskell’s model for the connection of TTP clusters 7 , but in a random chain configuration. Cavities. As suggested in the previously proposed model of f.c.c. packing of stoichiometric clusters 9 , the packing of quasi-equivalent clusters may leave behind some ‘cavities’. They can be identified by locating spaces with relatively low electron charge densities. We found that the larger cavities are associated with multiple clusters, and have a correlation with the cluster arrangement (See Supplemen- tary Fig. S8). The location of the larger cavities resembles Bernal’s canonical holes 2 , if regarding the clusters as hard spheres. The analysis of the size and distribution of the cavities would have implications for glass-forming ability, especially when additional solute species with different sizes are introduced to fill such cavities to form bulk MGs 42 , as well as in the understanding of transport mechanism, glass transition, shear transformation in plastic flow, and the structure factors in scattering experiments (for example, the pre-peaks observed in some systems). But this topic is beyond the scope of this paper. Strings and networks The icosahedral ordering of single-solute-centred quasi-equivalent clusters is an efficient packing scheme, but is not the only type of MRO. When the concentration of the solute species increases to beyond the level at which each of all the solute atoms can be encaged in a single-solute-centred cluster, neighbour contact between solute atoms becomes inevitable. This produces direct bonding between like atoms. It is tempting to surmise that the excess solute atoms might be
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  • Summer '19
  • Amorphous metal

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