Formation and Polymorphism of Semiconducting K<sub>2</sub>SiH<sub>6</sub> and Strategy for Metallization
Olga Yu. Vekilova, Doreen Beyer, Shrikant Bhat, Robert Farla, Volodymyr Baran, S. I. Simak, Holger Kohlmann, Ulrich Häußermann, Kristina Spektor
Abstract
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.