Abstract
Physical properties of lithium under extreme pressures continuously reveal unexpected features. These include a sequence of structural transitions to lower symmetry phases, metal-insulator-metal transition, superconductivity with one of the highest elemental transition temperatures, and a maximum followed by a minimum in its melting line. The instability of the bcc structure of lithium is well established by the presence of a temperature-driven martensitic phase transition. The boundaries of this phase, however, have not been previously explored above 3 GPa. All higher pressure phase boundaries are either extrapolations or inferred based on indirect evidence. Here we explore the pressure dependence of the martensitic transition of lithium up to 7 GPa using a combination of neutron and X-ray scattering. We find a rather unexpected deviation from the extrapolated boundaries of the hR3 phase of lithium. Furthermore, there is evidence that, above ∼3 GPa, once in fcc phase, lithium does not undergo a martensitic transition.
Highlights
Physical properties of lithium under extreme pressures continuously reveal unexpected features
All structures were mapped by neutron scattering and X-ray angle dispersive diffraction using the SNAP instrument at the Spallation Neutron Source and by the High Pressure Collaborative Access Team (HPCAT) at the Advanced Photon Source (APS), respectively (Methods section)
Above 3.5 GPa would show the presence of hR3 phase, which is not supported by the current experiments (Figs 2 and 3)
Summary
Physical properties of lithium under extreme pressures continuously reveal unexpected features. These include a sequence of structural transitions to lower symmetry phases, metal-insulator-metal transition, superconductivity with one of the highest elemental transition temperatures, and a maximum followed by a minimum in its melting line. The instability of the bcc structure of lithium is well established by the presence of a temperaturedriven martensitic phase transition The boundaries of this phase, have not been previously explored above 3 GPa. All higher pressure phase boundaries are either extrapolations or inferred based on indirect evidence. Like all other alkali metals, lithium undergoes a series of pressure-induced structural phase transitions to low symmetry phases, following an initial pressureinduced transition from bcc to fcc phase at room temperature. The low-temperature structure of lithium was initially identified to be hexagonal-close-packed[13] but later was shown to be samarium-type R-3 m H (166) structure (hR3) with 3 atoms per unit cell[1,14,15,16,17,18,19]
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