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 (β) 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 β-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.
The epitaxial growth at 100 µm/h on on-axis 4H-SiC substrates is demonstrated in this study. Chloride-based CVD, which has been shown to be a reliable process to grow SiC epitaxial layers at rates above 100 µm/h on off-cut substrates, was combined with silane in-situ etching. A proper tuning of C/Si and Cl/Si ratios and the combination of different chlorinated precursors resulted in the homoepitaxial growth of 4H-SiC on Si-face substrates at high rates. Methyltrichlorosilane, added with silane, ethylene and hydrogen chloride were employed as precursors to perform epitaxial growths resulting in very low background doping concentration and high quality material, which could be employed for power devices structure on basal-plane-dislocation-free epitaxial layers.
The hydrogen storage materials ASiH3 (A = K and Rb) represent complex metal hydrides built from metal cations and pyramidal SiH3− ions. At room temperature, SiH3− moieties are randomly oriented because of dynamical disorder (α‐modifications). At temperatures below 200 K, ASiH3 exist as ordered low‐temperature (β) modifications. The vibrational properties of β‐ASiH3 were characterized by a combination of Raman spectroscopy and inelastic neutron scattering. Internal modes of SiH3− are observed in the spectral range 1800–1900 cm−1 (stretching modes) and 890–1000 cm−1 (bending modes). External modes are observed below 500 cm−1. Specifically, SiH3− librations are between 300–450 cm−1 and 270–400 cm−1 for A = K and Rb, respectively, SiH3− translations are between 95 and 160 cm−1, K+ translations are in the range 60–100 cm−1 and Rb+ translations in the range 50–70 cm−1. The red‐shift of libration modes for A = Rb is associated with a 15–30% reduction of the libration force constants of SiH3− ions in β‐RbSiH3. This correlates with a lower temperature for the β–α order–disorder phase transition (278 vs 298 K). Libration modes become significantly anharmonic with increasing temperature but are maintained up to at least 200 K. The vibrational properties of ASiH3 compare well to those of alkali metal borohydrides ABH4 (A = Na–Cs). Copyright © 2016 John Wiley & Sons, Ltd.
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