A comparative study of the structural, vibrational, electronic and thermoelastic properties of α−SiO2 and α−Si(NH)2 from first principles

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Abstract

We present a systematic comparison of the fundamental ground state structural, vibrational, and electronic properties of α-SiO2 (quartz) with those of the isoelectronic imide analog α-Si(NH)2 using first principles quantum-based simulations. Potential synthesis routes for crystalline α-Si(NH)2 are proposed using a thermochemistry approach that combines solid-state and molecular methods to treat heterogeneous reactions consistently. The α-quartz phase of Si(NH)2 is predicted to be thermodynamically stable with ΔGfo,298= -251 kJ/mol which is approximately a third of the value obtained for α-SiO2 (ΔGfo,298= -753 kJ/mol). The substitution of NH for O leads to a 7% increase in Si-X bond lengths, and an 8° reduction in Si-X-Si angle, and an overall molar volume increase of about 12% in the static lattice approximation. A detailed analysis of the compression behavior of the NH analog yields a 70% increase in bulk modulus and a systematic alignment of the NH groups parallel to the c-axis with increasing pressure. The electronic properties of the oxide and imide were obtained using the TB09 meta-GGA functional which predicts an indirect band gap of 8.2 eV for SiO2, in good agreement with observed values, and an indirect band gap of 5.5 eV in the NH analog. We also calculate the excitonic contributions to the dielectric function and show that the imide analog has an enhanced reflectivity in the 0–10 eV range compared to the oxide. Simulated Raman and infrared spectra of the oxide and imide indicate that symmetric Si-X vibrations involving rigid NH displacements have similar frequencies in the two systems while asymmetric modes that couple NH groups lead to splittings of key features in the imide.

Original languageEnglish (US)
Article number125564
JournalMaterials Chemistry and Physics
Volume278
DOIs
StatePublished - Feb 15 2022

Keywords

  • Compression
  • Density functional theory
  • Quartz
  • Silicon diimide
  • Silicon nitrogen oxygen

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics

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