New cubic phase of Li3N: stability of the N3- ion to 200 GPa

X-ray Raman Scattering: A Hard X-ray Probe of Complex Organic Systems

This paper provides a review of the characterization of organic systems via X-ray Raman scattering (XRS) and a step-by-step guidance for its application. We present the fundamentals of XRS required to use the technique and discuss the main parameters of the experimental set-ups to optimize spectral and spatial resolution while maximizing signal-to-background ratio. We review applications that target the analysis of mixtures of organic compounds, the identification of minor spectral features, and the spatial discrimination in heterogeneous systems. We discuss the recent development of the direct tomography technique, which utilizes the XRS process as a contrast mechanism for assessing the three-dimensional spatially resolved carbon chemistry of complex organic materials. We conclude by exposing the current limitations and provide an outlook on how to overcome some of the existing challenges and advance future developments and applications of this powerful technique for complex organic systems.

Stability, Elastic Properties, and Deformation of LiBN2: A Potential High-Energy Material

Searching for high-energy-density materials is of great interest in scientific research and for industrial applications. Using an unbiased structure prediction method and first-principles calculations, we investigated the phase stability of LiBN₂ from 0 to100 GPa. Two new structures with space groups P4̅2₁ m and Pnma were discovered. The theoretical calculations revealed that Pnma LiBN₂ is stable with respect to a mixture of ¹/₃Li₃N, BN, and ¹/₃N₂ above 22 GPa. The electronic band structure revealed that Pnma LiBN₂ has an indirect band gap of 2.3 eV, which shows a nonmetallic feature. The Pnma phase has a high calculated bulk modulus and shear modulus, indicating its incompressible nature. The microscopic mechanism of the structural deformation was demonstrated by ideal tensile shear strength calculations. It is worth mentioning that Pnma LiBN₂ is dynamically stable under ambient conditions. The decomposition of this phase is exothermic, releasing an energy of approximately 1.23 kJ/g at the PBE level. The results provide new thoughts for designing and synthesizing novel high-energy compounds in ternary systems.

Novel lithium-nitrogen compounds at ambient and high pressures

Using ab initio evolutionary simulations, we predict the existence of five novel stable Li-N compounds at pressures from 0 to 100 GPa (Li₁₃N, Li₅N, Li₃N₂, LiN₂, and LiN₅). Structures of these compounds contain isolated N atoms, N₂ dimers, polyacetylene-like N chains and N₅ rings, respectively. The structure of Li₁₃N consists of Li atoms and Li₁₂N icosahedra (with N atom in the center of the Li₁₂ icosahedron) – such icosahedra are not described by Wade-Jemmis electron counting rules and are unique. Electronic structure of Li-N compounds is found to dramatically depend on composition and pressure, making this system ideal for studying metal-insulator transitions. For example, the sequence of lowest-enthalpy structures of LiN₃ shows peculiar electronic structure changes with increasing pressure: metal-insulator-metal-insulator. This work also resolves the previous controversies of theory and experiment on Li₂N₂.

Crystalline LiN5 Predicted from First-Principles as a Possible High-Energy Material

The search for stable polymeric nitrogen and polynitrogen compounds has attracted great attention due to their potential applications as high-energy-density materials. Here we report a theoretical prediction of an interesting LiN5 crystal through first-principles calculations and unbiased structure searching techniques. Theoretical calculations reveal that crystalline LiN5 is thermodynamically stable at pressures above 9.9 GPa, and remains metastable at ambient conditions. The metastability of LiN5 stems from the inherent stability of the N5(-) anions and strong anion-cation interactions. It is therefore possible to synthesize LiN5 by compressing solid LiN3 and N2 gas under high pressure and quench recover the product to ambient conditions. To the best of our knowledge, this is the first time that stable N5(-) anions are predicted in crystalline states. The weight ratio of nitrogen in LiN5 is nearly 91%, placing LiN5 as a promising high-energy material. The decomposition of LiN5 is expected to be highly exothermic, releasing an energy of approximately 2.72 kJ·g(-1). The present results open a new avenue to synthesize polynitrogen compounds and provide a key perspective toward the understanding of novel chemical bonding in nitrogen-rich compounds.

Doping-enhanced lithium diffusion in lithium-ion batteries

We disclose a distortion-assisted diffusion mechanism in Li3N and Li2.5Co0.5N by first-principles simulations. A B(2g) soft mode at the Γ point is found in α-Li3N, and a more stable α'-Li3N (P3m1) structure, which is 0.71 meV lower in energy, is further derived. The same soft mode is inherited into Li2.5Co0.5N and is enhanced due to Co doping. Consequently, unlike the usual Peierls spin instability along Co-N chains, large lithium-ion displacements on the Li-N plane are induced by a set of soft modes. Such a distortion is expected to offer Li atoms a route to bypass the high diffusion barrier and promote Li-ion conductivity. In addition, we further illustrate abnormal Born effective charges along Co-N chains which result from the competition between the motions of electrons and ion cores. Our results provide future opportunities in both fundamental understanding and structural modifications of Li-ion battery materials.