By adding small amounts of potassium to hydrogen, the energetically preferred compound KH 6 forms above 70 GPa and metalizes within 166 GPa. Two-layered C2/m and C2/c structures of KH 6 discovered here contain H 2 and H 3 units under high pressure. The metallic C2/c phase is found to be a one-dimensional conductor with a conducting hydrogen-bonded network and an insulating potassium chain. The layered hydrogen-dominant compound KH 6 is found to be a good example of pressure-induced metallization, which originates from a hydrogen sublattice at lower pressures than those in pure hydrogen system due to the "chemical precompression" from heavier element potassium.

Atomically thin 2D materials have drawn great attention due to their many potential applications. We herein report two novel structures of 2D C4N identified by first-principles calculations in combination with a swarm structure search. These two structures (with symmetry of Pm and P2/m) are almost degenerate in energy (with only 4 meV/atom difference) and exhibit quite similar structural topologies, both consisting of alternative arrays of C–N hexagon and arrays of C–N pentagon–octagon–pentagon. The Pm structure is semiconducting with a direct band gap of 90 meV at HSE. In contrast, the P2/m structure is a zero-band-gap semimetal and possesses the distorted Dirac cone, showing the direction-dependent Fermi velocity and electronic properties. Thus the predicted C4N monolayers are promising for applications in nanoelectronics.

In the framework of the ab initio random structure search method, we show that elemental Se and Te undergo pressure-induced structural transition from the bcc to fcc phase, in agreement with the theoretical results previously reported. By means of the pseudopotential plane-wave method based on density functional perturbation theory, the fcc structure for both elements is found to be another phonon-mediated superconducting phase of these materials. With a reasonable value for the Coulomb pseudopotential 𝜇 * = 0.12, the maximum superconducting transition temperature 𝑇c in the fcc phase of Se and Te is estimated to be about 5.7 K and 4.6 K, respectively. Furthermore, we show that in the entire fcc phase for Se and Te, the superconducting transition temperature decreases together with the increase in pressure, leading to the final suppression of the superconductivity. It is suggested that such behavior is mainly caused by the rapid increase in the mean-square phonon frequency ⟨𝜔 2 ⟩ with pressure. Finally, a very strong electron-phonon coupling value, for both Se and Te in the fcc phase, is found along the 𝐺-𝐾 high symmetry lines.

No abstract