Abstract
Lithium monosilicide (LiSi) was formed at high pressures and high temperatures (1.0–2.5 GPa and 500–700°C) in a piston-cylinder apparatus. This compound was previously shown to have an unusual structure based on 3-fold coordinated silicon atoms arranged into interpenetrating sheets. In the present investigation, lowered synthesis pressures permitted recovery of large (150–200 mg) quantities of sample for structural studies via NMR spectroscopy ( 29Si and 7Li), Raman spectroscopy and electrical conductivity measurements. The 29Si chemical shift occurs at −106.5 ppm, intermediate between SiH 4 and Si(Si(CH 3) 3) 4, but lies off the trend established by the other alkali monosilicides (NaSi, KSi, RbSi, CsSi), that contain isolated Si 4 4− anions. Raman spectra show a strong peak at 508 cm −1 due to symmetric Si–Si stretching vibrations, at lower frequency than for tetrahedrally coordinated Si frameworks, due to the longer Si–Si bonds in the 3-coordinated silicide. Higher frequency vibrations occur due to asymmetric stretching. Electrical conductivity measurements indicate LiSi is a narrow-gap semiconductor ( E b∼0.057 eV). There is a rapid increase in conductivity above T=450 K, that might be due to the onset of Li + mobility.
Published Version
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