Formation mechanism of chemically precompressed hydrogen clathrates in metal superhydrides

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.