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

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 ( P 4/ nmm -NpH 5 , Cmcm -NpH 7 , Fm 3̄ m -NpH 8 , P 6 3 / mmc -NpH 9 , and Fm 3̄ m -NpH 10 ) that are composed of unusual H cages with stoichiometries H 20 , H 24 , H 29 , and H 32 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.