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.

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Factors that Determine Structure with Ionic Bonding1. 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 rcation/ranion for an octahedral site (C.N. = 6)

a 2ranion

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Rock Salt StructurePrototype is NaCl rNa = 0.102 nm

rNa/rCl = 0.564

cations (Na+) prefer octahedral sites

rCl = 0.181 nm

VMSE

MgO and FeO both have this structure.

Cesium Chloride Structure

Since 0.732 < 0.939 < 1.0, cubic sites preferred

Each Cs+ has 8 neighbor Cl-

VMSE

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 VMSE ZnS also occurs in the

wurtzite structure, which is hexagonal rather than cubic.

Fluorite structure

• Calcium fluoride (CaF2)

• Have two anions for every cation.• Cations are in every other cubic

site.

• UO2, ThO2, ZrO2, CeO2

• Antifluorite structure – positions of cations and anions reversed

VMSE

Perovskite Structure (CaTiO3)

BaTiO3 important example

VMSE (112) shows that Ti doesn’t touch

either Ba or O, so moves in electric field. Ferroelectric with many applications. http://en.wikipedia.org/wiki/Barium_titanate

Unit cell may be large with many atoms

• For example, garnets such as Mg3Al2Si3O12:

Bond HybridizationBond Hybridization is possible when there is significant

covalent bonding– hybrid electron orbitals form

– For example, for SiC with XSi = 1.8 and XC = 2.5

• So ~ 89% covalent bonding• Both Si and C prefer sp3 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.). VMSE

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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