ABSTRACT
The alkali metal silyl hydrides ASiH3 (A = K, Rb) and their deuteride analogues were prepared from the Zintl phases ASi. The crystal structures of ASiH3 consist of metal cations and pyramidal SiH3(-) ions. At room temperature SiH3(-) moieties are randomly oriented (α modifications). At temperatures below 200 K ASiH3 exist as ordered low-temperature (ß) modifications. Structural and vibrational properties of SiH3(-) in ASiH3 were characterized by a combination of neutron total scattering experiments, infrared and Raman spectroscopy, as well as density functional theory calculations. In disordered α-ASiH3 SiH3(-) ions relate closely to freely rotating moieties with C3v symmetry (Si-H bond length = 1.52 Å; H-Si-H angle 92.2 °). Observed stretches and bends are at 1909/1903 cm(-1) (ν1, A1), 1883/1872 cm(-1) (ν3, E), 988/986 cm(-1) (ν4, E), and 897/894 cm(-1) (ν2, A1) for A = K/Rb. In ordered ß-ASiH3 silyl anions are slightly distorted with respect to their ideal C3v symmetry. Compared to α-ASiH3 the molar volume is by about 15% smaller and the Si-H stretching force constant is reduced by 4%. These peculiarities are attributed to reorientational dynamics of SiH3(-) anions in α-ASiH3. Si-H stretching force constants for SiH3(-) moieties in various environments fall in a range from 1.9 to 2.05 N cm(-1). These values are considerably smaller compared to silane, SiH4 (2.77 N cm(-1)). The reason for the drastic reduction of bond strength in SiH3(-) remains to be explored.
ABSTRACT
The hydride Ba(3)Si(4)H(x) (x = 1-2) was prepared by sintering the Zintl phase Ba(3)Si(4), which contains Si(4)(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 Ba(3)Si(4)H(x) adopts the Ba(3)Ge(4)C(2) 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 Si(4) polyanions complete a perovskite-like arrangement. Thus, hydride formation is accompanied by oxidation of the butterfly polyanion, but the model with the composition Ba(3)Si(4)H is not charge-balanced. First-principles computations revealed an alternative structural scenario for Ba(3)Si(4)H(x), which is based on filling pyramidal Ba5 interstices in Ba(3)Si(4). The limiting composition is x = 2 [space group P4(2)/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 Ba(3)Si(4)H(x) is heavily disordered in the c direction. Most plausible is to assume that Ba(3)Si(4)H(x) has a variable H content (x = 1-2) and corresponds to a random intergrowth of P- and I-type structure blocks. In either form, Ba(3)Si(4)H(x) is classified as an interstitial hydride. Polyanionic hydrides in which H is covalently attached to Si remain elusive.
ABSTRACT
Silicon swallows up boron: The novel open tetrahedral framework structure (OTF) of the Zintl phase LiBSi2 was made by applying high pressure to a mixture of LiB and elemental silicon. The compound represents a new topology in the B-Si net (called tum), which hosts Li atoms in the channels (see picture). LiBSi2 is the first example where B and Si atoms form an ordered common framework structure with B engaged exclusively in heteronuclear B-Si contacts.
ABSTRACT
BaInGeH was synthesized by hydrogenating the intermetallic compound BaInGe. The crystal structure determination from the powder neutron diffraction data of BaInGeD [P3m1, Z = 1, a = 4.5354(3) A, c = 5.2795(6) A] reveals the presence of hydrogen in tetrahedral voids defined by three Ba atoms and one In atom.
