关注：

1) Hydrogen motifs的定义与判断

2) Hydrogen motifs与超导温度的关系

From Dr.Peng's paper

 (一） 超导电性起源

Two properties of metallic hydrides are particularly beneficial for promoting high-Tc superconductivity:

(i) a large H-derived electronic density of states at the Fermi level, and

(ii) large modifications of the electronic structure in response to the motion of the H atoms (electron-phonon coupling).

It appears to be important to satisfy both of these criteria in hydrides with high H content in order to achieve high Tc values.

(二） Hydrogen motifs定义：ELF、et al.

ELF

We examine the chemical bonding of the REH₆, REH₉, and REH₁₀ clathrate structures by calculating the electron localization functions (ELFs) [63] (Fig. S9 [38]).

The RE-H bonding is purely ionic in view of the absence of charge localization between RE and H【离子键】, while a weakly covalent H-H interaction is evident via the observation of charge localization between the nearest-neighbor H atoms【弱的共价相互作用】.

 距离判断：比单原子氢还要长

  Note that within the H24, H29, and H32 cages, the nearest H- distances are very similar and are equal to 1.24, 1.17, and 1.08 Å at 200 GPa, respectively, which are much longer than in the H2 gas molecule (0.74 Å) and slightly longer than the H-H distance (0.98 Å) [64] in monatomic H at

500 GPa.

COHP

Subsequent crystalline orbital Hamiltonian population calculations [65]were performed to confirm the H-H covalent bonding in these clathrate structures.

The results [Figs. S10(a) and S10(b) [38]] clearly reveal the occupancy of the H-H bonding states, which lends strong support to the existence of H-H covalent bonding in the structure.

Mulliken 分析

There is significant charge transfer from RE to H as derived from a Mulliken population analysis [66] [Fig. 3(a)]. The charges on the H atoms do not change much with increasing H content, but the RE charges increase significantly and provide additional electrons to the H atoms which stabilizes the clathrate structure.

The strong anion-cation interaction increases the Madelung energy of the ionic component of the RE-H bonding, which increases the stability of these clathrate structures.