BISMUTH AS AN ELEMENTAL LIQUID CRYSTAL

Following observations of a first-order liquid-liquid phase transition in Bi, at 1010K and ambient pressure, an interpretation suggesting that elemental Bi forms liquid-crystal phase(s), i.e., exhibits long-range bond-orientational order in the melt, has been offered [Argaman (2010)]. This was motivated by the bilayered structure of solid Bi, and the fact that the 3 covalent bonds of each Bi atom define a "bonding" hemisphere, opposing a "non-bonding" hemisphere, in analogy with the hydrophobic and hydrophilic parts of amphiphilic molecules, which are known to form smectic phases. If the covalently bonded bilayers which form upon cooling a disordered liquid state have a spherical or tubular topology, rather than a planar one, they will not easily rearrange into a neatly stacked smectic phase upon further cooling, explaining the irreversibility.

Recent observations provide substantial evidence for this. Bi samples quenched from high temperatures at ~2GPa were observed (using HRTEM backed by DFT computations) to have "warped" bilayers, with a different stacking of the bilayers in nearby regions: the underlying A7 rombohedral phase has an abc stacking; alternatives such as an abab stacking were found [Shu et al. (2016a)]. Furthermore, these samples contain structural defects which survive repeated "melting" and "solidification" cycles, and the samples do not flow when "melted" [Shu et al. (2016b)]. Topological defects, such as disclinations in the smectic order, if present in sufficient density and entangled with each other, could explain this behavior.

Experimentalists are encouraged to confirm (or falsify) the existence of long-range order in "liquid" Bi, and to identify the particular symmetries involved.