Lithium monosilicide (LiSi), a low-dimensional silicon-based material prepared by high pressure synthesis: NMR and vibrational spectroscopy and electrical properties characterization |
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Authors: | Linda A. Stearns Jason Diefenbacher Paul F. McMillan |
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Affiliation: | a Department of Chemical & Materials Engineering, Arizona State University, Tempe, AZ 85287, USAb Department of Physical & Earth Sciences, Jacksonville State University, Jacksonville, AL 36265, USAc Center for Solid State Science, Arizona State University, Tempe, AZ 85287, USAd Department of Physics & Astronomy, Arizona State University, Tempe AZ 85287, USAe Davy Faraday Research Laboratories at the Royal Institution of Great Britain, London W1S 4BS, UKf Christopher Ingold Laboratory, Department of Chemistry, University College London, London WC1H 0AJ, UK |
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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 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 Si44− 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 (Eb∼0.057 eV). There is a rapid increase in conductivity above T=450 K, that might be due to the onset of Li+ mobility. |
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Keywords: | Lithium monosilicide Silicides High pressure synthesis Low-dimensional silicon Solid-state NMR Raman scattering Metastable phase transitions |
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