Evidence from Fermi surface analysis for the low-temperature structure of lithium

Abstract

The low-temperature crystal structure of elemental lithium, the prototypical simple metal, is a several-decades-old problem. At 1 atm pressure and 298 K, Li forms a body-centered cubic lattice, which is common to all alkali metals. However, a low-temperature phase transition was experimentally detected to a structure initially identified as having the 9R stacking. This structure, proposed by Overhauser in 1984, has been questioned repeatedly but has not been confirmed. Here we present a theoretical analysis of the Fermi surface of lithium in several relevant structures. We demonstrate that experimental measurements of the Fermi surface based on the de Haas-van Alphen effect can be used as a diagnostic method to investigate the low-temperature phase diagram of lithium. This approach may overcome the limitations of X-ray and neutron diffraction techniques and makes possible, in principle, the determination of the lithium low-temperature structure (and that of other metals) at both ambient and high pressure. The theoretical results are compared with existing low-temperature ambient pressure experimental data, which are shown to be inconsistent with a 9R phase for the low-temperature structure of lithium.lithium | Fermi surface | de Haas-van Alphen effect | low temperature | crystal structure T he behavior of the alkali metals under pressure has been a subject of considerable interest because of the emergence of unexpected physical properties (1-9). Lithium presents the simplest electronic structure of a metal under ambient conditions-a model for a nearly free electron crystal, with a simple and highly symmetric body-centered cubic (bcc) structure. Under application of external pressure, lithium undergoes a series of structural transitions to complex low-symmetry phases (3,8,10). These structural transformations are coupled with changes of its electronic properties, leading to a deviation from simple metallic behavior, including a complex phase diagram, a dramatic change in the superconducting Tc, metal-semiconductor phase transitions, as well as an anomalous melting curve (1,4,5,(10)(11)(12)(13)(14). Despite its apparent simplicity and numerous studies, there are still many open questions regarding the properties of lithium, even at P = 1 atm.Among the outstanding questions is the structure of Li at low temperature and pressure. At 1 atm and 298 K lithium crystallizes in the bcc phase. However, upon cooling it undergoes a martensitic transformation that commences at ∼80 K. Identifying the phases involved in the martensitic transition of lithium has been a challenge since its initial discovery (15). This has been due to several factors, including relatively poor response of lithium to both X-rays and neutrons; incomplete transition to the lowest measured temperature; and dependence of the transition temperature on multiple factors such as grain size, defects, and strain. The transition was first reported by C. S. Barrett (15,16). Initial neutron scattering data by McCarthy et al. (17) identified the posttransition ...

Emergence of topological electronic phases in elemental lithium under pressure

“…The closed loop in the Fermi surfaces, would lead to characteristic quantum oscillations in de Haas-van Alphen measurements for the Pbca, Cmca, and P42/mbc structures (shown in the SI). Given the small lithium mass, nuclear motion associated with zero-point and finite temperature effects would be expected to alter the structural phase diagram (53)(54)(55); such effects may shift the zero-temperature structural phase transition pressures predicted in (18,19) and used here. Future calculations, for example including anharmonic effects in calculations of structural energetics, as has recently been demonstrated for hydrogen (56), would be desirable.…”

Reactivity of lithium and platinum at elevated densities

“…Being the simplest and lightest metallic element, dense lithium (Li) is of fundamental importance due to the strong influence of quantum effects on its behavior [1][2][3][4][5][6][7][8]. As other alkali metals, Li shows a pronounced minimum in the melting temperature with pressure, as low as 190 K, leading to counterintuitive cold melting above 40 GPa at room temperature [7,9,10].…”

Theory of Confined High Tc Superconductivity in Monovalent Metals

“…Indeed reconstruction of fcc lattice to 9R, a quasi two dimensional structure has been seen in experiment [24] for Ag and Cu. Further, theoretical studies of 9R structure for Li [25] a monovalent metal, shows a quasi 2d Fermi surface. An emergent local 2d Fermi surface will also support superconductivity.…”