High-pressure stoichiometric evolution of the As-H system: Dimerization and decomposition

The discoveries that the unconventional stoichiometric hydrides ${\mathrm{SH}}_{3}$ and ${\mathrm{P}}_{4}{\mathrm{H}}_{6}$ become superconductors with critical temperatures (${T}_{c}$) of 203 and 103 K at high pressures have motivated extensive investigations of hydrogen-rich compounds. The As-H system, the hydride of the same main group, has also been predicted to have a variety of stoichiometric ratios with high ${T}_{c}$ under high pressure. However, its high-pressure stoichiometry evolution and superconductivity remain unknown. Here, we explored the stoichiometric evolution and thermodynamic process of the As-H system under high pressure. We observed that ${\mathrm{AsH}}_{3}$ underwent liquefaction and solidification at approximately 0.01 and 1.7 GPa, respectively. Upon further compression, ${\mathrm{AsH}}_{3}$ dimerizes into ${\mathrm{As}}_{2}{\mathrm{H}}_{4}$ at 7.0 GPa. ${\mathrm{As}}_{2}{\mathrm{H}}_{4}$ is unstable, decomposing into gray arsenic at pressures above 20.0 GPa or amorphous arsenic upon decompression. This comprehensive exploration of the high-pressure stoichiometric evolution of As-H is crucial for exploring superconductivity in the As-H system and understanding the internal arsenic cycling in planets like Jupiter and Saturn.