Hydrogenous Zintl Phase Ba₃Si₄H_<i>x</i> (<i>x</i> = 1–2): Transforming Si₄ “Butterfly” Anions into Tetrahedral Moieties

The hydride Ba₃Si₄H_<i>x</i> (<i>x</i> = 1–2) was prepared by sintering the Zintl phase Ba₃Si₄, which contains Si₄^6– butterfly-shaped polyanions, in a hydrogen atmosphere at pressures of 10–20 bar and temperatures of around 300 °C. Initial structural analysis using powder neutron and X-ray diffraction data suggested that Ba₃Si₄H_<i>x</i> adopts the Ba₃Ge₄C₂ type [space group <i>I</i>4/<i>mcm</i> (No. 140), <i>a</i> ≈ 8.44 Å, <i>c</i> ≈ 11.95 Å, <i>Z</i> = 8] where Ba atoms form a three-dimensional array of corner-condensed octahedra, which are centered by H atoms. Tetrahedron-shaped Si₄ polyanions complete a perovskite-like arrangement. Thus, hydride formation is accompanied by oxidation of the butterfly polyanion, but the model with the composition Ba₃Si₄H is not charge-balanced. First-principles computations revealed an alternative structural scenario for Ba₃Si₄H_<i>x</i>, which is based on filling pyramidal Ba₅ interstices in Ba₃Si₄. The limiting composition is <i>x</i> = 2 [space group <i>P</i>4₂/<i>mmm</i> (No. 136), <i>a</i> ≈ 8.4066 Å, <i>c</i> ≈ 12.9186 Å, <i>Z</i> = 8], and for <i>x</i> > 1, Si atoms also adopt tetrahedron-shaped polyanions. Transmission electron microscopy investigations showed that Ba₃Si₄H_<i>x</i> is heavily disordered in the <i>c</i> direction. Most plausible is to assume that Ba₃Si₄H_<i>x</i> has a variable H content (<i>x</i> = 1–2) and corresponds to a random intergrowth of <i>P</i>- and <i>I</i>-type structure blocks. In either form, Ba₃Si₄H_<i>x</i> is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive