In pursuit of new lithium-rich phases and potential electrides within the Li-N phase diagram, we explore theoretically the ground-state structures and electronic properties of LiN at P = 1 atm. Crystal structure exploration methods based on particle swarm optimization and evolutionary algorithms led to 25 distinct structures, including 23 dynamically stable structures, all quite close to each other in energy, but not in detailed structure. Several additional phases were obtained by following the imaginary phonon modes found in low-energy structures, as well as structures constructed to simulate segregation into Li and LiN. The candidate LiN structures all contain NLi polyhedra, with n = 6-9. They may be classified into three types, depending on their structural dimensionality: NLi extended polyhedral slabs joined by an elemental Li layer (type a), similar structures, but without the Li layer (type b), and three-dimensionally interconnected NLi polyhedra without any layering (type c). We investigate the electride nature of these structures using the electron localization function and partial charge density around the Fermi level. All of the structures can be characterized as electrides, but they differ in electronic dimensionality. Type-a and type-b structures may be classified as two-dimensional (2-D) electrides, while type-c structures emerge quite varied, as 0-D, 2-D, or 3-D. The calculated structural variety (as well as detailed models for amorphous and liquid LiN) points to potential amorphous character and likely ionic conductivity in the material.

We report low-frequency high-resolution Raman spectroscopy and ab-initio calculations on dense lithium from 40 to 200 GPa at low temperatures. Our experimental results reveal rich first-order Raman activity in the metallic and semiconducting phases of lithium. The computed Raman frequencies are in excellent agreement with the measurements. Free energy calculations provide a quantitative description and physical explanation of the experimental phase diagram only when vibrational effect are correctly treated. The study underlines the importance of zero-point energy in determining the phase stability of compressed lithium.

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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 ...