L03C: Chapter 3 (Continued) Inorganic Compounds Reminder: Bonding ranges from ionic to covalent depending on relative electronegativity of constituents. For example: Ionic bonds are non-directional, while covalent bonds are highly directional. Anions (-) usually larger than cations (+). W.R. Wilcox, Clarkson University. Last revised July 31, 2013.

2 Factors that Determine Structure with Ionic Bonding 1. Relative sizes of ions – Formation of stable structures: -- maximize the number of oppositely charged ion neighbors.

Linear Triangular Tetrahedral The lower value for each corresponds to each anion being in contact both with neighboring anions and with neighboring cations Cubic

Example Computation of Minimum Ionic Ratio Determine minimum r cation /r anion for an octahedral site (C.N. = 6) a  2r anion

5 Rock Salt Structure Prototype is NaCl r Na = nm r Na /r Cl =  cations (Na + ) prefer octahedral sites r Cl = nm  VMSEVMSE MgO and FeO both have this structure.

Cesium Chloride Structure Since < < 1.0, cubic sites preferred Each Cs + has 8 neighbor Cl -  VMSEVMSE

Zinc Blende Structure Also called sphalerite. ZnS is the prototype Same as diamond structure, but with every other atom different. Face-centered cubic with one Zn and one S at each point. Tetrahedral sites. Many important semiconductor materials with this structure, for example: –SiC (almost as hard as diamond) –III-V compounds, e.g. GaAs –II-VI compounds, e.g. CdTe  VMSEVMSE ZnS also occurs in the wurtzite structure, which is hexagonal rather than cubic.

Fluorite structure Calcium fluoride (CaF 2 ) Have two anions for every cation. Cations are in every other cubic site. UO 2, ThO 2, ZrO 2, CeO 2 Antifluorite structure – positions of cations and anions reversed  VMSEVMSE

Perovskite Structure (CaTiO 3 ) BaTiO 3 important example  VMSEVMSE (112) shows that Ti doesn’t touch either Ba or O, so moves in electric field. Ferroelectric with many applications.

Unit cell may be large with many atoms For example, garnets such as Mg 3 Al 2 Si 3 O 12 :

Bond Hybridization Bond Hybridization is possible when there is significant covalent bonding –hybrid electron orbitals form –For example, for SiC with X Si = 1.8 and X C = 2.5 So ~ 89% covalent bonding Both Si and C prefer sp 3 hybridization Therefore, Si and C atoms occupy tetrahedral sites Same as diamond cubic, but with every other atom different Structure named after prototype, zincblende or sphalerite (ZnS). Many important semiconductors with this structure: III-V (GaAs, GaP, InSb, etc.); II-VI (CdTe, HgSe, etc.).  VMSEVMSE

12 Density Computations for Ceramics Number of formula units/unit cell Volume of unit cell Avogadro’s number = sum of atomic weights of all anions in formula unit = sum of atomic weights of all cations in formula unit