posted on 2015-02-02, 00:00authored byVerina 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.