High pressure properties of silicon subhydrides SiH, SiH2, and SiH3 are investigated and compared with those of SiH4 by using ab initio methods. Although the subhydrides are thermodynamically unstable at lower pressures, SiH, SiH2, and SiH3 are found to be stabilized above 486, 386, and 430 GPa, respectively. The predicted phases of SiH, SiH2, and SiH3 have Fm3¯m, I4/mmm, and I4/m symmetries, respectively, and are metallic. The density of states suggests that the electronic states at the Fermi energy are more delocalized in the subhydrides than in SiH4. This fact is well reflected in the superconducting transition temperature (Tc) estimated for SiH2 and SiH3. Indeed, Tc reaches 83 K in SiH2 at 400 GPa and 88 K in SiH3 at 450 GPa while that is 28 K in SiH4 at 400 GPa. Although Tc of SiH4 is expected to exceed 100 K above 503 GPa, the current capability of diamond anvil cells suggests that high-Tc silicon hydride is more likely to be attained experimentally in subhydrides.
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