title: Crystal structure and electron density of alpha-silicon nitride: experimental and theoretical evidence for the covalent bonding and charge transfer.
abstract: Crystal structure and electron-density distribution of alpha-silicon nitride (alpha-Si3N4, space group: P31c) have been investigated by a combined technique of the Rietveld method, the maximum-entropy method (MEM), and MEM-based pattern-fitting of high-resolution synchrotron powder diffraction data. In combination with density functional theory calculations, the present experimental electron-density distribution of the alpha-Si3N4 indicates covalent bonds between Si and N atoms and charge transfer from the Si to N atom. The triangular distribution around the N atoms, which is attributable to the nitrogen sp2 hybridization for the nearest silicon and nitrogen pairs, was found in both experimental and theoretical electron density distributions. The minimum electron density in an intralayer Si-N bond was a little lower than that in an interlayer bond, which would be responsible for the inequality between elastic constants, C33 > C11. The present work suggests that the high bulk modulus of the alpha-Si3N4 is attributable to the high minimum electron density of the Si-N bond.
authors: Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Nagatsuta-cho 4259, Midori-ku, Yokohama-shi, Kanagawa, 226-8502, Japan. yashima@materia.titech.ac.jp
authors: Masatomo
authors: M
authors: Yashima
authors: Yoshiaki
authors: Y
authors: Ando
authors: Yasunori
authors: Y
authors: Tabira
mesh_terms: D003460:Crystallization; D004583:Electrons; D008956:Models, Chemical; D015394:Molecular Structure; D033362:Powder Diffraction; D017655:Silicon Compounds / Q000737:chemistry*; D017356:Synchrotrons
chemical_list: D017655:Silicon Compounds; C032734:silicon nitride