High-Pressure Structures of Disilane and Their Superconducting Properties

A systematic ab initio search for low-enthalpy phases of disilane (${\mathrm{Si}}_2{\mathrm{H}}_6$) at high pressures was performed based on the minima hopping method. We found a novel metallic phase of disilane with $Cmcm$ symmetry, which is enthalpically more favorable than the recently proposed structures of disilane up to 280 GPa, but revealing compositional instability below 190 GPa. The $Cmcm$ phase has a moderate electron-phonon coupling yielding a superconducting transition temperature $T_c$ of around 20 K at 100 GPa, decreasing to 13 K at 220 GPa. These values are significantly smaller than previously predicted $T_c$’s for disilane at equivalent pressure. This shows that similar but different crystalline structures of a material can result in dramatically different $T_c$’s and stresses the need for a systematic search for a crystalline ground state.

Figure 1

Enthalpy per formula unit of disilane as a function of pressure with respect to elements in their solid form $2\mathrm{Si}(s)+3{\mathrm{H}}_2(s)$. The decomposition enthalpies were computed from the predicted structures of hydrogen ($P{6}_3/m$, $C2/c$) [44], high-pressure phases of silicon ($P6/mmm$, $P{6}_3/mmc$, $Fm-3m$) [45, 46], and silane ($Fdd2$, $I{4}_1/a$, $Pbcn$) [10, 15]. The disilane structures with superscripts a and b are from this work and from Ref. 23, respectively. The dynamical instability towards the $Cmc{2}_1$ phase is indicated by the arrow.

Figure 2

The crystal structures of (a) the $Cmcm$ phase at 200 GPa and (b) the $P\mathrm{\text{−}}1$ phase at 300 GPa. The eigendisplacements which lead from the $Cmcm$ structure to the $Cmc{2}_1$ structure are visualized by arrows in (c).

Figure 3

Left: Phonon band dispersion of the $Cmcm$ structure at 200 GPa. Right: Calculated Eliashberg spectral function ${\alpha }^{2}F(\omega )$ (solid line) and phonon partial density of states (yellow, silicon; blue, hydrogen).

Figure 4

Fermi surface of the $Cmcm$ phase at several pressures: (a) 100, (b) 140, (c) 160, and (d) 220 GPa [47].