SiS2 Research Articles

Electron Deficient Compounds. VI. The Structure of Beryllium Chloride

The simple compound, beryllium chloride, offers an opportunity to study halogen bridge bonding. Beryllium chloride is isomorphous with dimethylberyllium and silicon disulfide. The orthorhombic unit, a0=9.86, b0=5.36, c0=5.26A contains four beryllium chloride molecules. The structure consists of continuous chains:[Complex chemical formula]with an essentially tetrahedral configuration about beryllium. The Cl–Be–Cl bond angle within the four membered rings is 98.2° which is less than the tetrahedral angle, as is found in SiS2, instead of more, as in dimethylberyllium. The bond angles indicate that all bonds in beryllium chloride contain an electron pair, so that chlorine uses an unshared pair in forming bridge bonds. Hence, chlorides isomorphous with electron deficient metal alkyls are not to be classified as electron deficient compounds.

Optical Properties and Electronic Structure of Solid Silicon

The existence of an empty (conduction) band 1.5 or 2.0 volts above the highest filled band in metallic-looking semi-conductors such as silicon and certain sulfides is inferred from the optical properties of these substances. Reflectivity data of silicon are examined with the aid of the expression for the complex index of refraction as given by classical electromagnetic theory. Through use of this expression, which is checked for internal consistency, an oscillator strength of 1.6 electrons is calculated for the absorption, and a dielectric constant at low frequencies of 12.5 is computed. The Wigner-Seitz-Slater method of computing electronic energy bands in crystals is used to determine the band structure of silicon. A combination of one $s$, three $p$, and three $d$ functions for each of the two atoms in the unit cell is made, and through use of boundary conditions of continuity of value, of normal, and of tangential derivatives, a solution is obtained for the plane $x=y$. The resulting band structure closely resembles that obtained by Kimball for the diamond, except that silicon is more nearly metallic than is the diamond. The $3s$ level of the silicon atom splits into two bands, the $3p$ level into six bands, and the $3d$ level into six bands; the two $3s$ bands and the lowest two $3p$ bands are completely filled by electrons. At the observed half-distance between nearest neighbors, the gap between the uppermost filled band and the lowest empty band is much greater than that expected from consideration of the optical data. The form of the optical absorption as expected from band structure considerations is proposed. The width of the filled bands as observed in soft x-ray emission spectra is about equal to the width of the computed bands.