Synthesis of BaSiH6 Hydridosilicate at High Pressures─A Bridge to BaSiH8 Polyhydride

Abstract

Hydridosilicates featuring SiH₆ octahedral moieties represent a rather new class of compounds with potential properties relating to hydrogen storage and hydride ion conductivity. Here, we report on the new representative BaSiH₆ which was obtained from reacting the Zintl phase hydride BaSiH_∼1.8 with H₂ fluid at pressures above 4 GPa and subsequent decompression to ambient pressure. Its monoclinic crystal structure (C2/c, a = 8.5976(3) Å, b = 4.8548(2) Å, c = 8.7330(4) Å, β = 107.92(1)°, Z = 4) was characterized by a combination of synchrotron radiation powder X-ray diffraction, neutron powder diffraction, and DFT calculations. It consists of complex SiH₆^2– ions (d_Si–H ≈ 1.61 Å), which are octahedrally coordinated by Ba²⁺ counterions. The arrangement of Ba and Si atoms deviates only slightly from an ideal fcc NaCl structure with a ≈ 7 Å. IR and Raman spectroscopy showed SiH₆^2– bending and stretching modes in the ranges 800–1200 and 1400–1800 cm^–1, respectively, in agreement with a hypervalent Si–H bonding situation. BaSiH₆ is thermally stable up to 95 °C above which decomposition into BaH₂ and Si takes place. DFT calculations indicated a direct band gap of 2.5 eV and confirmed that at ambient pressure BaSiH₆ is a thermodynamically stable compound in the ternary Ba–Si–H system. The discovery of BaSiH₆ consolidates the compound class of hydridosilicates, accessible from hydrogenations of silicides at gigapascal pressures (<10 GPa). The structural properties of BaSiH₆ suggest that it presents an intermediate (or precursor) for further hydrogenation at considerably higher pressures to the predicted superconducting polyhydride BaSiH₈ [Lucrezi, R.; et al. npj Comput. Mater. 2022, 8, 119] whose structure is also based on a NaCl arrangement of Ba and Si atoms but with Si in a cubic environment of H.