Lithium monosilicide (LiSi), a low-dimensional silicon-based material prepared by high pressure synthesis: NMR and vibrational spectroscopy and electrical properties characterization

Lithium monosilicide (LiSi) was formed at high pressures and high temperatures (1.0-2.5 GPa and 500-700degreesC) 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 (Si-29 and Li-7), Raman spectroscopy and electrical conductivity measurements. The Si-29 chemical shift occurs at - 106.5 ppm, intermediate between SiH4 and Si(Si(CH3)(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)similar to0.057eV). There is a rapid increase in conductivity above T = 450 K, that might be due to the onset of Li+ mobility. (C) 2003 Elsevier Science (USA). All rights reserved.