Polyhydride CeH9 with an atomic-like hydrogen clathrate structure

Abstract

Compression of hydrogen-rich hydrides has been proposed as an alternative way to attain the atomic metallic hydrogen state or high-temperature superconductors. However, it remains a challenge to get access to these states by synthesizing novel polyhydrides with unusually high hydrogen-to-metal ratios. Here we synthesize a series of cerium (Ce) polyhydrides by a direct reaction of Ce and H 2 at high pressures. We discover that cerium polyhydride CeH 9 , formed above 100 GPa, presents a three-dimensional hydrogen network composed of clathrate H 29 cages. The electron localization function together with band structure calculations elucidate the weak electron localization between H-H atoms and confirm its metallic character. By means of Ce atom doping, metallic hydrogen structure can be realized via the existence of CeH 9 . Particularly, Ce atoms play a positive role to stabilize the sublattice of hydrogen cages similar to the recently discovered near-room-temperature lanthanum hydride superconductors.

Praseodymium polyhydrides synthesized at high temperatures and pressures

“…Instead, in agreement with Ref. [16], we find that PrH 9 also adopts a P 6 3 /mmc structure, found experimentally in NdH 9 [36], ThH 9 [37] and for CeH 9 [22,38]. However, Ref.…”

“…NH 3 BH 3 has become a widely accepted method for in situ hydrogen generation to facilitate the synthesis of high pressure polyhydrides. [14][15][16]22] However, using pure H 2 instead of NH 3 BH 3 guarantees a higher density of hydrogen atoms available to synthesize high stoichiometry hydrides. Rhenium gaskets, indented to 9-18 µm thickness, were used to form the sample chamber in all experimental runs.…”

Second group of high-pressure high-temperature lanthanide polyhydride superconductors

“…While stability of the various RE polyhydrides had been studied by Peng et al [23] across the entire lanthanide series of elements, superconductivity properties have only been reported for the early RE hydrides of La, Ce, and Pr. Calculations of electronic and T c properties have accompanied recent experimental reports on syntheses of individual RE polyhydrides, CeH 9 [28,29], PrH 9 [30,31], NdH 9 [32], EuH 9 [33] and ThH 10 [24]. However, a systematic investigation of electronphonon coupling and superconducting T c across all RE hydrides is missing.…”

Origin of enhanced chemical precompression in cerium hydride $$\hbox {CeH}_{{9}}$$

“…Based on this covalent feature of the Ce-H bonds and the charge transfer from Ce to H atoms, we can say that the bonding nature between the Ce atoms to H 29 cages is characterized as a mixture of ionic and covalent. www.nature.com/scientificreports/ As mentioned above, the synthesis of CeH 9 requires much lower pressures 80-100 GPa 5,6 compared to that ( ∼170 GPa) for the synthesis of LaH 10 3,4 , indicating the variation of chemical precompression with respect to the occupation of f electrons. Here, we note that Ce atom with the electron configuration [Xe]4 f 1 5d 1 6s 2 has one 4f electron, while La atom with [Xe]5d 1 6s 2 represents no occupation of f orbitals.…”

“…(a) Calculated lattice constants of CeH 9 as a function of pressure using the f-valence scheme, in comparison with those obtained using the f-core scheme and experiment5 . The resulting c/a ratios as a function of pressure are also given in (b).…”