K2SiH6, crystallizing in the cubicK(2)PtCl(6) structure type (Fm3 ̅m), features unusual hypervalent SiH6 (2-) complexes. Here, the formation of K2SiH6 athigh 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)(2)SiF6 structure type (P3 ̅m1) upon formation. The trigonal polymorphis stable up to 725 & DEG;C at 13 GPa. At room temperature, the transitioninto an ambient pressure recoverable cubic form occurs below 6.7 GPa.Theory suggests the existence of an additional, hexagonal, variantin the pressure interval 3-5 GPa. According to density functionaltheory band structure calculations, K2SiH6 isa semiconductor with a band gap around 2 eV. Nonbonding H-dominatedstates are situated below and Si-H anti-bonding states arelocated above the Fermi level. Enthalpically feasible and dynamicallystable metallic variants of K2SiH6 may be obtainedwhen substituting Si partially by Al or P, thus inducing p- and n-typemetallicity, respectively. Yet, electron-phonon coupling appearsweak, and calculated superconducting transition temperatures are <1K. The formation of K2SiH6 at high pressuresstarting from KSiH3 or mixtures of KH and KSiH3 is investigated by in situ synchrotron diffraction experiments.Between 6.5 and 13 GPa, K2SiH6 adopts the trigonal(NH4)(2)SiF6 structure type (P3 ̅m1), which is stable up to 725 & DEG;C at 13 GPa. At room temperature, the transition into an ambientpressure-recoverable cubic form occurs below 6.5 GPa.