Speaker
Description
We may expect lithium to be the simplest metal as it has only a single $2s$ valence electron. Surprisingly, lithium's crystal structure at low temperature and ambient pressure has long been a matter of debate. In 1984, A. W. Overhauser proposed a rhombohedral $9R$ structure. Subsequent neutron experiments by Schwarz et al. in 1990 favour a disordered polytope. More recently, in 2017, Elatresh et al. argued against the $9R$ structure while Ackland et al. found fcc ordering. In this work, we seek to understand the physical principles that could lead to such conflicting findings. We describe metallic bonding in an arbitrary close-packed structure within the tight-binding approximation. Close-packed structures, also called Barlow stackings, are infinite in number. They can be codified by stacking sequence (e.g. fcc $\leftrightarrow ABC$) or by a Hagg code (e.g. fcc $\leftrightarrow +++$). From the point of view of an atomic orbital, all close-packed structures offer similar local environments with the same number of nearest neighbours. When hoppings are short-ranged, the tight binding description shows a surprising gauge-like symmetry. As a result, the electronic spectrum is precisely the same for every close-packed structure. This results in competition across a large class of structures that all have the same binding energy.
A preference for one ordering pattern can only emerge from (a) long-ranged (third-neighbour and further) hoppings or (b) phonon free energies at finite temperatures. Our results could explain the observed fcc structure in lithium under high pressure.
| Keyword-1 | tight-binding |
|---|---|
| Keyword-2 | crystal-structure |
| Keyword-3 | lithium |
