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

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 (<i>C</i>2/<i>c</i>, <i>a</i> = 8.5976(3) Å, <i>b</i> = 4.8548(2) Å, <i>c</i> = 8.7330(4) Å, β = 107.92(1)°, <i>Z</i> = 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 (<i>d</i> _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 <i>a</i> ≈ 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. <i>npj Comput. Mater.</i> <b>2022</b>, <i>8</i>, 119] whose structure is also based on a NaCl arrangement of Ba and Si atoms but with Si in a cubic environment of H.