Formation and Polymorphism of Semiconducting K 2 SiH 6 and Strategy for Metallization.

K 2 SiH 6 , crystallizing in the cubic K 2 PtCl 6 structure type ( Fm 3̅ m ), features unusual hypervalent SiH 6 2- complexes. Here, the formation of K 2 SiH 6 at high pressures is revisited by in situ synchrotron diffraction experiments, considering KSiH 3 as a precursor. At the investigated pressures, 8 and 13 GPa, K 2 SiH 6 adopts the trigonal (NH 4 ) 2 SiF 6 structure type ( P 3̅ m 1) 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, K 2 SiH 6 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 K 2 SiH 6 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.