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.

Among the family of transition metal dichalcogenides, ReS₂ occupies a special position, which crystalizes in a unique distorted low-symmetry structure at ambient conditions. The interlayer interaction in ReS₂ is rather weak, thus its bulk properties are similar to those of monolayer. However, how compression changes its structure and electronic properties is unknown so far. Here using ab initio crystal structure searching techniques, we explore the high-pressure phase transitions of ReS₂ extensively and predict two new high-pressure phases. The ambient pressure phase transforms to a “distorted-1T” structure at very low pressure and then to a tetragonal I4₁/amd structure at around 90 GPa. The “distorted-1T” structure undergoes a semiconductor–metal transition at around 70 GPa with a band overlap mechanism. Electron–phonon calculations suggest that the I4₁/amd structure is superconducting and has a critical superconducting temperature of about 2 K at 100 GPa. We further perform high-pressure electrical resistance measurements up to 102 GPa. Our experiments confirm the semiconductor–metal transition and the superconducting phase transition of ReS₂ under high pressure. These experimental results are in good agreement with our theoretical predictions.