Formation and Polymorphism of Semiconducting K₂SiH₆ and Strategy for Metallization

K₂SiH₆, crystallizing in the cubic K₂PtCl₆ structure type (Fm3̅m), features unusual hypervalent SiH₆^2- complexes. Here, the formation of K₂SiH₆ at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH₃ as a precursor. At the investigated pressures, 8 and 13 GPa, K₂SiH₆ adopts the trigonal (NH₄)₂SiF₆ structure type (P3̅m1) upon formation. The trigonal polymorph is stable up to 725 °C at 13 GPa. At room temperature, the transition into an ambient pressure recoverable cubic form occurs below 6.7 GPa. Theory suggests the existence of an additional, hexagonal, variant in the pressure interval 3-5 GPa. According to density functional theory band structure calculations, K₂SiH₆ is a semiconductor with a band gap around 2 eV. Nonbonding H-dominated states are situated below and Si-H anti-bonding states are located above the Fermi level. Enthalpically feasible and dynamically stable metallic variants of K₂SiH₆ may be obtained when substituting Si partially by Al or P, thus inducing p- and n-type metallicity, respectively. Yet, electron-phonon coupling appears weak, and calculated superconducting transition temperatures are <1 K.