Ceramic Crystal Structures

Interstitial sites = small holes between the lattice atoms where smaller atoms may be placed. The smaller atoms should be the exact size of the interstitial hole or slightly too large; they can’t be smaller than the space.

The structure of ionic ceramic materials depends on electrical neutrality (cations next to anions not other cations, no overall charge in the material) and size (cations are smaller than anions, anions close-pack and cations sit in the interstitial sites). (Remember basketballs and tennis balls)

Table 13.2 in the book shows common interstitial sites for ionic ceramic materials. The site occupied by the cation varies with size ratio of the cations to the anions (r C /r A ). The most important interstitial sites are the tetrahedral sites (Td) and the octahedral sites (Oh). See Example Problem 13.1; example in class

Ceramic Crystal Structures There are numerous different ceramic crystal structures. However, in this class we will only focus on five structures: Rock Salt [NaCl] [CsCl] Diamond Zinc blende Pervoskite

Rock Salt [NaCl] One way to think of this structure is as follows: Cl - ions form an FCC structure Na + ions fill all the Oh positions Examples: NaCl, MgO, FeO

[CsCl] One way to think of this structure is as follows: Cl - ions form a SC structure Cs + ions fill the body-centered positions Examples: CsCl, CsI, CsBr

Diamond Covlently bound ceramic One way to think about it: C forms an FCC lattice and then fills 1/2 of the Td positions Examples: C(diamond), Si, Ge

Top view of crystal shown on previous slide Bottom view of crystal shown on previous slide

Other C Structures: Graphite Covalent bonds within each plane van der Waals bonds between the planes Example of a layered ceramic structure

Other C Structures: Fullerenes Graphite-like structures that are wrapped up into a Sphere (C 60 or buckyball) Tube (carbon nanotubes)

Zinc Blende Covlently bound ceramic; similar (but not the same as) diamond One way to think about it: one atom forms an FCC lattice and the other fills 1/2 of the Td positions Examples: ZnS, GaAs, SiC

Pervoskite Ternary ceramic. One way to think about it: large cation makes SC structure, small cation fills body-centered site, anion fills face centered sites Examples: BaTiO 3, SrSnO 3

Silica Structures (SiO 2 ) This material likes to form tetrahedra that then share corners and/or sites with other tetrahedra to form a variety of crystalline (quartz, crystobalite or tridymite) and amorphous structures.

Silica Structures (SiO 2 ) Examples of crystalline and amorphous SiO 2 structures

Density of Ceramic Materials (examples) Same as calculating the density of metal crystal structures - only more complex  = n’ (  A C +  A A ) (V cell )(Avogodros No.)