Gold as a 6p-Element in Dense Lithium Aurides

The negative oxidation state of gold (Au) has drawn a great attention due to its unusual valence state that induces exotic properties in its compounds, including ferroelectricity and electronic polarization. Although monatomic anionic gold (Au^–) has been reported, a higher negative oxidation state of Au has not been observed yet. Here we propose that high pressure becomes a controllable method for preparing high negative oxidation state of Au through its reaction with lithium. First-principles calculations in combination with swarm structural searches disclosed chemical reactions between Au and Li at high pressure, where stable Li-rich aurides with unexpected stoichiometries (e.g., Li₄Au and Li₅Au) emerge. These compounds exhibit intriguing structural features like Au-centered polyhedrons and a graphene-like Li sublattice, where each Au gains more than one electron donated by Li and acts as a 6p-element. The high negative oxidation state of Au has also been achieved through its reactions with other alkali metals (e.g., Cs) under pressures. Our work provides a useful strategy for achieving diverse Au anions

Pressure-Induced Stable Li₅P for High-Performance Lithium-Ion Batteries

Black phosphorus, the result of white P under high pressure, has received much attention as a promising anode material for Li-ion batteries (LIBs). However, the final product of lithiation, <i>P</i>6₃/<i>mmc</i> Li₃P, is not satisfactory due to its poor conductivity. In this article we explore the high-pressure phase diagram of the Li–P system through first-principles swarm-intelligence structural search and present two hitherto unknown stable Li-rich compounds, <i>Fm</i>-3<i>m</i> Li₃P at 4.2 GPa and <i>P</i>6/<i>mmm</i> Li₅P at 10.3 GPa. Metallic Li₅P exhibits interesting structural features, including graphene-like Li layers and P-centered octadecahedrons, where P is 14-fold coordinated with Li. Interestingly, both compounds exhibit good dynamical and thermal stability properties at ambient pressure, and the theoretical capacity of <i>P</i>6/<i>mmm</i> Li₅P reaches 4326 mAhg^–1, the highest among the already known Li–P compounds. Additionally, their mechanical properties are also favorable for electrode materials. Our work represents a significant step toward the performance improvement of Li–P batteries and understanding Li–P compounds

Intrinsic Ferromagnetism in 2D Fe₂H with a High Curie Temperature

The rational design of ferromagnetic materials is crucial for the development of spintronic devices. Using first-principles structural search calculations, we have identified 73 two-dimensional transition metal hydrides. Some of them show interesting magnetic properties, even when combined with the characteristics of the electrides. In particular, the P3̅m1 Fe2H monolayer is stabilized in a 1T-MoS2-type structure with a local magnetic moment of 3 μB per Fe atom, whose robust ferromagnetism is attributed to the exchange interaction between neighboring Fe atoms within and between sublayers, leading to a remarkably high Curie temperature of 340 K. On the other hand, it has a large magnetic anisotropic energy and spin-polarization ratio. Interestingly, the above room-temperature ferromagnetism of the Fe2H monolayer is well preserved within a biaxial strain of 5%. The structure and electron property of surface-functionalized Fe2H are also explored. All of these interesting properties make the Fe2H monolayer an attractive candidate for spintronic nanodevices

Nonmetallic FeH₆ under High Pressure

High pressure induces unexpected chemical and physical properties in materials. For example, hydrogen-rich compounds under pressure have recently gained much attention as potential room-temperature superconductors, and iron hydrides have also gained significant interest as potential candidates for being the main constituents of the Earth’s core. It is well-known that pressure induces insulator-to-metal transitions, whereas pressure-induced metal-to-insulator transitions are rare, especially for transition metal hydrides. In this article, we have extensively explored the structural phase diagram of iron hydrides by using ab initio particle swarm optimization. We have found a new stable stoichiometry, FeH₆, above 213.7 GPa with <i>C</i>2/<i>c</i> symmetry. Interestingly, <i>C</i>2/<i>c</i> FeH₆ presents an unexpected nonmetallicity, and its band gap becomes larger with increasing pressure. This is in sharp contrast with <i>P</i>2₁/<i>m</i> FeH₄. The nonmetallicity of <i>C</i>2/<i>c</i> FeH₆ mainly originates from the pressure-induced hybridization between the Fe and H orbitals. This new compound shows a unique structure with a mixture of nonbonded hydrogen atoms in a helical iron framework. The strong Fe–Fe interaction and ionic Fe–H bonds are responsible for its structural stability. In addition, we have also found a more stable tetragonal FeH₂ structure with the same <i>I</i>4/<i>mmm</i> symmetry as the previously proposed one, the X-ray diffraction pattern of which perfectly agrees with that of the experiment