Phase stability and superconductivity of lead hydrides at high pressure

Density functional theory calculations and crystal structure predictions using the particle swarm optimization method have been combined to determine stable hydrides of lead under pressure. In contrast to other group-IVa hydrides, the stoichiometry ${\mathrm{PbH}}_{6}$ is the first hydride to become stable, at just under 1 Mbar. For two previously studied stoichiometries, ${\mathrm{PbH}}_{4}$ and ${\mathrm{PbH}}_{8}$, energetically more favorable phases were identified to become stable around 2 Mbar. In all structures, the hydrogenic sublattices comprise negatively charged ${{\mathrm{H}}_{2}}^{\ensuremath{\delta}\ensuremath{-}}$ molecules. Competitive ${\mathrm{PbH}}_{4}$ and ${\mathrm{PbH}}_{6}$ structures are layered. ${\mathrm{PbH}}_{6}$ features ${\mathrm{H}}_{2}$ molecules intercalated between hcp Pb layers, the stable phase of dense pure lead, thus offering a potentially straightforward route towards synthesis. In ${\mathrm{PbH}}_{8}$, the Pb lattice adapts a $\ensuremath{\beta}$-Sn structure, and hydrogen atoms form quasi-one-dimensional-chains. All structures were found to be metallic and to feature superconductivity in their respective stability range, with moderately high ${T}_{c}$ in the range 60--100 K for ${\mathrm{PbH}}_{4}$ and ${\mathrm{PbH}}_{6}$ and 161--178 K for ${\mathrm{PbH}}_{8}$.