Pressure-induced evolution of crystal and electronic structure of neptunium hydrides

Abstract

An extensive exploration of high-pressure phase diagrams of NpH_x (x = 1–10) compounds was performed by using swarm-intelligence-based CALYPSO structure searches. We propose five stable hydrogen-rich clathrate phases (P4/nmm-NpH₅, Cmcm-NpH₇, Fmm-NpH₈, P6₃/mmc-NpH₉, and Fmm-NpH₁₀) that are composed of unusual H cages with stoichiometries H₂₀, H₂₄, H₂₉, and H₃₂ in which the H atoms are weakly covalently bonded to one another, with neptunium atoms occupying centers of the cages. The electronic structure analyses show that these predicted hydrogen-rich structures are all metallic phases, and Np–H and H–H bonds are formed by ionic and covalent bond interactions, respectively. The charge transfer from the Np atom plays an important role in the stability of the proposed structures. All hydrogen-rich clathrate structures show superconductivity behavior in their respective stability pressure range. Our work is an important step in understanding the phase stability and bonding behavior of NpH_x under extreme conditions and provides a valuable reference for experimental synthesis and identification of cage-like neptunium hydrides.