Mono-atomic Metallic Glasses

Deep in the meta-equilibrium of the under-cooled liquid, the local microscopic structural fluctuations associated with the equilibrium liquid (and those necessary for crystal nucleation) become increasingly rare as the temperature drops. Rather, local liquid-like structural motifs dominate, which ultimately "freeze-in", as the temperature passes through the glass transition regime to produce the amorphous solid. For those interested in the details, the seminal paper by Kauzman makes for a good read [W. Kauzmann, Chem. Rev. 43, 216 (1948)]. What these low energy motifs are, and how they may be arranged though space, is a system specific problem. Frank, in a short note [F. C. Frank, Proc. R. Soc. A 215, 43 (1952)], discussed this problem for hard spheres, by posing the question "In how many different ways can one put twelve billiard balls in simultaneous contact with one, counting as different the arrangements which cannot be transformed into each other without breaking contact with the centre ball?". It turns out there are three distinct configurations, the FCC, HCP and Icosahedral arrangements. Although the original question involved hard spheres, the same can be asked for a soft inter-atomic interactions, since there will be a potential barrier when transforming between these three different configurations. Indeed, Frank went onto show that for a simple pair-wise interaction, the icosahedral arrangement had the lowest energy.

As Frank rightly went on to say, the five-fold symmetry axis of the icosahedron is "abhorrent" to crystal symmetry, and that the higher energy FCC and HCP motifs only become "economical when extended over considerable volume". Thus, unlike that of the FCC and HCP local motifs, icosahedra cannot be packed in a space filling way without introducing defected icosahedra. The corresponding low energy defects are well known: the over-coordinated Frank-Kasper motifs [F. C. Frank and J. S. Kasper, Acta Crystallogr. 11, 184 (1958)] and the under-coordinated Bernal structures [J. D. Bernal, Proc. R. Soc. London Ser. A 280, 299 (1964)]. In an icosahedron, any neighbour of the central atom shares five common neighbours with the central atom, forming a "bond" referred to as 5-fold bond. Defects of icosahedron are characterized as structures containing 4-fold (Bernal structures) and 6-fold (Frank-Kasper structures) bonds. David Nelson unified these structural concepts introducing new defects containing both 4-fold and 6-fold bonds - developing a representation in which the amorphous structure is described by a network of connected 4-fold and 6-fold defect bonds. In my own work I have applied these descriptors to model binary alloy systems, developing an analysis approach within the OVITO atomistic visualization platform. This approach was the basis for a previous post, where we compared a binary colloidal glass to that of simulated model glass, via a comparison of such bonding networks. The above ideas where developed for mono-atomic systems, however depending on the degree of bonding chemistry, this picture can be extended to binary and even ternary systems.

Whilst, it is rather routine to produce multi-component metallic glasses, it turns out to be much more difficult to produce mono-atomic metallic glasses. From a theoretical perspective, such simple glasses would be ideal to experimentally investigate the above concepts of glass structure, especially under load. There has been much effort in trying to produce mono-atomic glasses, the key to which is to inhibit heterogeneous nucleation of the crystalline phase - a fast process for such compositionally trivial metals. In a recent article further progress was made in this direction by using laser ablation of bulk metallic targets immersed in a liquid medium. Here the liquid medium removes heat from the ablated metallic droplets sufficiently fast so that crystallization does not occur. The authors were able to produce high-purity mono-atomic metallic glass nano-particles for a range of BCC, HCP and FCC metals. For more details see Breaking the vitrification limitation of monatomic metals, Tong, X., Zhang, YE., Shang, BS. et al. Nat. Mater. 23, 1193 (2024).