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Hydrogenous Zintl Phase Ba3Si4Hx (x = 1–2): Transforming Si4 “Butterfly” Anions into Tetrahedral Moieties

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posted on 2015-02-02, 00:00 authored by Verina F. Kranak, Daryn E. Benson, Lukas Wollmann, Milad Mesgar, Samrand Shafeie, Jekabs Grins, Ulrich Häussermann
The hydride Ba3Si4Hx (x = 1–2) was prepared by sintering the Zintl phase Ba3Si4, which contains Si46– 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 Ba3Si4Hx adopts the Ba3Ge4C2 type [space group I4/mcm (No. 140), a ≈ 8.44 Å, c ≈ 11.95 Å, Z = 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 Ba3Si4Hx, which is based on filling pyramidal Ba5 interstices in Ba3Si4. The limiting composition is x = 2 [space group P42/mmm (No. 136), a ≈ 8.4066 Å, c ≈ 12.9186 Å, Z = 8], and for x > 1, Si atoms also adopt tetrahedron-shaped polyanions. Transmission electron microscopy investigations showed that Ba3Si4Hx is heavily disordered in the c direction. Most plausible is to assume that Ba3Si4Hx has a variable H content (x = 1–2) and corresponds to a random intergrowth of P- and I-type structure blocks. In either form, Ba3Si4Hx is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive.

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