Stability and bonding nature of clathrate H cages in a near-room-temperature superconductor ${\mathrm{LaH}}_{10}$

Lanthanum hydride (${\mathrm{LaH}}_{10}$) with a sodalitelike clathrate structure was experimentally synthesized to exhibit a near-room-temperature superconductivity under megabar pressures. Based on first-principles density-functional theory calculations, we reveal that the metal framework of La atoms has excess electrons at interstitial regions. Such anionic electrons are easily captured to form a stable clathrate structure of H cages. We thus propose that the charge transfer from La to H atoms is mostly driven by the electride property of the La framework. Furthermore, the interaction between La atom and H cage induces a delocalization of La $5p$ semicore states to hybridize with the H $1s$ state. Consequently, the bonding nature of ${\mathrm{LaH}}_{10}$ is characterized as a mixture of ionic and covalent bonding between La atom and H cage. Our findings demonstrate that anionic and semicore electrons play important roles in stabilizing clathrate H cages in ${\mathrm{LaH}}_{10}$, which can be broadly applicable to other compressed rare-earth hydrides with clathrate structures.

Figure 1

(a) Optimized structure of ${\mathrm{LaH}}_{10}$ at 300 GPa, showing the ${\mathrm{H}}_{32}$ cage surrounding a La atom. There are two different types of H atoms, ${\mathrm{H}}_1$ and ${\mathrm{H}}_2$, depending on the surrounding bond directions. The separated La framework (${\mathrm{LaH}}_0$) and ${\mathrm{H}}_{32}$ cages (${\mathrm{La}}_0{\mathrm{H}}_{10}$) are displayed in (b) and (c), respectively. The $x$, $y$, and $z$ axes point along the [001], [010], and [001] directions, respectively. A (220) plane is drawn in (b). Calculated charge densities of ${\mathrm{LaH}}_0$ at 300 GPa, obtained with and without including semicore electrons, are plotted in (d) and (e), respectively. For comparison, the valence charge densities of ${\mathrm{LaH}}_0$ at 220 GPa and zero pressure, obtained without including semicore electrons, are displayed in Fig. S1 in the Supplemental Material [36]. The electron localization function (ELF) of ${\mathrm{LaH}}_0$, calculated at 300 GPa, is displayed in (f). In (e), ${\mathrm{A}}_1$ and ${\mathrm{A}}_2$ indicate the two anions in interstitial regions (see Fig. S2 in the Supplemental Material [36]), while the dashed circles represent the muffin-tin spheres around La and ${\mathrm{A}}_1$ (${\mathrm{A}}_2$) with the radii of 1.31 and 0.75 (0.75) Å, respectively. The charge densities are drawn on the (220) plane with a contour spacing of 0.1 $e$/Å${}^3$ in (d) and a contour spacing of 0.005 $e$/Å${}^3$ in (e). The ELF in (f) is drawn with a contour spacing of 0.05.

Figure 2

Calculated (a) band structure and (b) local DOS (LDOS) of ${\mathrm{LaH}}_0$ at 300 GPa. In (a), the projected bands onto the La $5d$, ${\mathrm{A}}_1$, and $A_{2}s$-like orbitals are represented by circles whose radii are proportional to the weights of the corresponding orbitals. The inset of (a) shows the Brillouin zone of the fcc primitive unit cell. In (b), the LDOS curves of La, ${\mathrm{A}}_1$, and ${\mathrm{A}}_2$ (pseudo)atoms are given. The energy zero represents $E_{\mathrm{F}}$.

Figure 3

Calculated total charge densities of (a) ${\mathrm{La}}_0{\mathrm{H}}_{10}$ and (b) ${\mathrm{LaH}}_{10}$ with the contour spacing of 0.1 $e$/Å${}^3$. In (b), the charge connection between La and ${\mathrm{H}}_1$ atoms around the point marked “$\times$” represents the covalent character of the $\mathrm{La}-{\mathrm{H}}_1$ bond. The ELF of ${\mathrm{LaH}}_{10}$ is also given in (c), with the contour spacing of 0.1.

Figure 4

Calculated (a) PDOS of ${\mathrm{LaH}}_{10}$ and (b) partial charge density within the energy range from $-15$ to $-5$ eV. In (b), the contour spacing is 0.05 $e$/Å${}^3$. (c) The Bader basin of the La atom in ${\mathrm{LaH}}_{10}$ is displayed in green.