Formation mechanism of chemically precompressed hydrogen clathrates in metal superhydrides

Abstract

Recently, the experimental discovery of high-T c superconductivity in compressed hydrides H 3 S and LaH 10 at megabar pressures has triggered searches for various superconducting superhydrides. It was experimentally observed that thorium hydrides, ThH 10 and ThH 9 , are stabilized at much lower pressures compared to LaH 10 . Based on first-principles density-functional theory calculations, we reveal that the isolated Th frameworks of ThH 10 and ThH 9 have relatively more excess electrons in interstitial regions than the La framework of LaH 10 . Such interstitial excess electrons easily participate in the formation of anionic H cage surrounding metal atom. The resulting Coulomb attraction between cationic Th atoms and anionic H cages is estimated to be stronger than the corresponding one of LaH 10 , thereby giving rise to larger chemical precompressions in ThH 10 and ThH 9 . Such a formation mechanism of H clathrates can also be applied to another experimentally synthesized superhydride CeH 9 , confirming the experimental evidence that the chemical precompression in CeH 9 is larger than that in LaH 10 . Our findings demonstrate that interstitial excess electrons in the isolated metal frameworks of high-pressure superhydrides play an important role in generating the chemical precompression of H clathrates.