High-pressure structures of germane (${\mathrm{GeH}}_4$) are explored through ab initio evolutionary methodology to reveal a metallic monoclinic structure of $C2/c$ ($4\mathrm{\text{molecules}}/\mathrm{\text{cell}}$). The $C2/c$ structure consists of layerlike motifs containing novel “${\mathrm{H}}_2$” units. Enthalpy calculations suggest a remarkably wide decomposition ($\mathrm{Ge}+{\mathrm{H}}_2$) pressure range of 0–196 GPa, above which $C2/c$ structure is stable. Perturbative linear-response calculations for $C2/c$ ${\mathrm{GeH}}_4$ at 220 GPa predict a large electron-phonon coupling parameter $\lambda$ of 1.12 and the resulting superconducting critical temperature reaches 64 K.

A way to circumvent the high pressures needed to metallize hydrogen is to “precompress” it in hydrogen-rich molecules, a strategy probed theoretically for silane. We show that phases with tetrahedral ${\mathrm{SiH}}_4$ molecules should undergo phase transitions with sixfold- and eightfold-coordinate Si appearing above 25 GPa. The most stable structure found can be metallized at under a megabar and at a compression close to the prediction of Goldhammer-Herzfeld criterion. According to a BCS-like estimate, metallic silane should be a high-temperature superconductor.

The high-pressure phases and superconductivity of ${\mathrm{CaYH}}_{12}$ have been explored by using a particle swarm optimization structure prediction methodology in combination with first-principles calculations. Our results show that ${\mathrm{CaYH}}_{12}$ becomes stable with a cubic $Fd\overline{3}m$ structure above 170 GPa, where metal atoms form body-centered-cubic (bcc) lattices and hydrogens occupy all the tetrahedral interstices of these bcc lattices, completing sodalitelike cages. The electron-phonon coupling calculations indicate that the $Fd\overline{3}m$ structure is a potential high-temperature superconductor, with a calculated $T_c$ of 258 K at 200 GPa. Our current study provides a possibility for searching new high-$T_c$ superconductors in ternary hydrides.

There is great interest in the exploration of hydrogen-rich compounds upon strong compression where they can become superconductors. Stannane (SnH ₄ ) has been proposed to be a potential high-temperature superconductor under pressure, but its high-pressure crystal structures, fundamental for the understanding of superconductivity, remain unsolved. Using an ab initio evolutionary algorithm for crystal structure prediction, we propose the existence of two unique high-pressure metallic phases having space groups Ama 2 and P 6 ₃ / mmc , which both contain hexagonal layers of Sn atoms and semimolecular (perhydride) H ₂ units. Enthalpy calculations reveal that the Ama 2 and P 6 ₃ / mmc structures are stable at 96–180 GPa and above 180 GPa, respectively, while below 96 GPa SnH ₄ is unstable with respect to elemental decomposition. The application of the Allen-Dynes modified McMillan equation reveals high superconducting temperatures of 15–22 K for the Ama 2 phase at 120 GPa and 52–62 K for the P 6 ₃ / mmc phase at 200 GPa.

Electrides are unique compounds where most of the electrons reside at interstitial regions of the crystal behaving as anions, which strongly determines its physical properties. Interestingly, the magnitude and distribution of interstitial electrons can be effectively modified either by modulating its chemical composition or external conditions (e.g., pressure). Most of the electrides under high pressure are nonmetallic, and superconducting electrides are very rare. Here we report that a pressure-induced stable ${\mathrm{Li}}_6\mathrm{P}$ electride, identified by first-principles swarm structure calculations, becomes a superconductor with a predicted superconducting transition temperature $T_c$ of 39.3 K, which is the highest among the already known electrides. The interstitial electrons in ${\mathrm{Li}}_6\mathrm{P}$, with dumbbell-like connected electride states, play a dominant role in the superconducting transition. Other Li-rich phosphides, ${\mathrm{Li}}_5\mathrm{P}$, ${\mathrm{Li}}_{11}{\mathrm{P}}_2$, ${\mathrm{Li}}_{15}{\mathrm{P}}_2$, and ${\mathrm{Li}}_8\mathrm{P}$, are also predicted to be superconducting electrides, but with a lower $T_c$. Superconductivity in all these compounds can be attributed to a combination of a weak electronegativity of phosphorus (P) with a strong electropositivity of lithium (Li), and opens up the interest to explore high-temperature superconductivity in similar binary compounds.