Synthesis of BaSiH Hydridosilicate at High Pressures-A Bridge to BaSiH Polyhydride.

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