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 PbH6 is the first hydride to become stable, at just under 1 Mbar. For two previously studied stoichiometries, PbH4 and PbH8, energetically more favorable phases were identified to become stable around 2 Mbar. In all structures, the hydrogenic sublattices comprise negatively charged H-2(delta-) molecules. Competitive PbH4 and PbH6 structures are layered. PbH6 features H-2 molecules intercalated between hcp Pb layers, the stable phase of dense pure lead, thus offering a potentially straightforward route towards synthesis. In PbH8, the Pb lattice adapts a 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 PbH4 and PbH6 and 161-178 K for PbH8.