Synthesis and Stability of Lanthanum Superhydrides

Abstract

Recent theoretical calculations predict that megabar pressure stabilizes very hydrogen-rich simple compounds having new clathrate-like structures and remarkable electronic properties including room-temperature superconductivity. X-ray diffraction and optical studies demonstrate that superhydrides of lanthanum can be synthesized with La atoms in an fcc lattice at 170 GPa upon heating to about 1000 K. The results match the predicted cubic metallic phase of LaH having cages of thirty-two hydrogen atoms surrounding each La atom. Upon decompression, the fcc-based structure undergoes a rhombohedral distortion of the La sublattice. The superhydride phases consist of an atomic hydrogen sublattice with H-H distances of about 1.1 Å, which are close to predictions for solid atomic metallic hydrogen at these pressures. With stability below 200 GPa, the superhydride is thus the closest analogue to solid atomic metallic hydrogen yet to be synthesized and characterized.

Quantum crystal structure in the 250-kelvin superconducting lanthanum hydride

“…This is completely consistent with the fcc arrangement of La atoms found experimentally [3]. However, Geballe et al [20] observed a rhombohedral distortion below ∼160 GPa, with an R-3m space group for the La sublattice and a rhombohedral angle of approximately 61.3°(c/a ∼ 2.38 in the hexagonal representation). Our calculations show that this distortion is compatible with slight anisotropic stress conditions in the DAC.…”

“…A sodalite type-structure with space group F m-3m and T c ∼280 K was suggested above ∼220 GPa (see Fig. 1), and a distorted version of it below with space group C2/m and a rhombohedral La sublattice [19,20]. By laser heating a lanthanum sample in a hydrogen-rich atmosphere within a diamond anvil cell (DAC), a lanthanum superhydride was synthesized right after [20].…”

“…1), and a distorted version of it below with space group C2/m and a rhombohedral La sublattice [19,20]. By laser heating a lanthanum sample in a hydrogen-rich atmosphere within a diamond anvil cell (DAC), a lanthanum superhydride was synthesized right after [20]. Based on the unit cell volume obtained by x-ray diffraction, the hydrogen to lanthanum ratio was estimated to be between 9 and 12.…”

Superconducting TaH5 at high pressure

“…Much theoretical effort has devoted to the investigation of the superconductivity in the compressed hydrides, however, not many of the predicted phases were confirmed by the later experiments (for example, NaH 3 and NaH 7 [14], LiH 2 and LiH 6 [15], FeH 5 [16], NbH 2.5 and NbH 3 [17], Ni 2 H 3 [18], TaH 3 [19] and CoH 2 [18]). Recently, the astonishing discoveries of superconductivity with T c above 200 K in compressed H 2 S [20,21] and LaH 10 [6,[22][23], both initiated by previous theoretical works, suggested the feasibility of searching for high T c in hydrogen-rich materials at high pressures. These observations have generated a flurry of searching for new hydrogen-dominating superconductors under pressures.…”

Structures and Superconducting Properties of Ultra‐Hydrogen‐Rich Selenium Hydride H6Se

“…In addition, other hydrogen‐rich compounds have been predicted to be superconductors, e.g., LiH 6 , KH 6 , MgH 6 , CaH 6 , PbH 4 (H 2 ) 2 , AsH 8 , SbH 4 , YH 6 , ScH 6 , YH 10 , and LaH 10 . In a very recent study, Geballe et al successfully synthesized the lanthanum superhydride LaH 10 at 170 GPa and 1000 K in an experiment, which had the same clathrate‐like structure that was predicted previously . This reignited interest in searching for hydrogen‐rich compounds or the so‐called superhydrides (MH n with n > 5), considering that increasing the content of hydrogen promotes the probability of those materials behaving like the pure solid hydrogen .…”