Defects

The Ideal

Square packing: Not most space efficient Hexagonal packing: Most space efficient

Unit Cells: the simplest repeating motif Can be different shapes and sizes The Rhomb Is the Unit cell Shape Of Hexagonal lattices

Packing: layers build up 3D solid

ABABABAB.... Packed up towards you Packing direction

ABABABAABABABA hcp Hexagonal Closest Packing: A B A B … Packing direction

ACBACBAACBACBA ccp Cubic Closest Packing: A B C A B C … Packing direction

Packing layers a more realistic view of how to build up structure sometimes not at all related to unit cell A B C A BC A Unit Cells: a conceptual way to build up structure sometimes resemble macroscopic crystalline solid assigned symmetry types, like P2 1 /c or P4mm called space groups used in X-ray crystallography

ccphcpbcc

Defects: creates useful materials 1.

Defects in metal structure Schottky Defect Frenkel Defect

More on Metals Cubic closest packing makes metals malleable: easily bendable Cu and Ag Work- hardening: creation of defects, loss of ccp lattice Work hardening, strain hardening, or cold work is the strengthening of a material by increasing the material's dislocation density. Wikipedia Alloys Sterling Silver = Ag (92.5%) + Cu (7.5%), a substitutional alloy Brass = Cu + Zn, a new structure, an intermetallic alloy Steel = Fe + C (~1%), carbide steel, an interstitial alloy Chrome = steel + Cr = Fe + C(~1%) + Cr(10%) Stainless steel = chrome steel, both interstitial and substitutional alloy “18/10” stainless is 18% Cr and 10% Ni Galvanized Steel = steel with Zn layer Molybdenum steel = Fe + C(<1%) + Cr(14%) + Ni(<2%) + Mo(1 %), “martensitic” steel: very strong and hard

Effect of added atoms and grains on metal structure. Smaller atom like C in iron Larger atom like P in iron Second crystal phases precipitated Defects and grain boundaries “pin” structure. All these inhibit sliding planes and harden the metal.

Ionic Solids as “Ideal structures” Build up Ionic Solids conceptually like this: assume Anions are larger than Cations, r- > r+ pack the Anions into a lattice: ccp, hcp or bcc add Cations to the interstitial spaces 2 x r- r- + r+

Consider red and blue balls the larger anions of A B packed layers; Where do the cations go? larger anions Smaller cations, r+/r- < 0.41 Larger cations, r+/r- > 0.41

Td cation holes are smaller than Oh holes 2x as many Td holes as Oh holes

Wurzite = Hexagonal ZnS hcp S 2- dianions (A B A packed) with Zn 2+ cations in 1/2 Td holes. Build it! See it! (as Chem3D)

Sphalerite or Zinc Blende = Cubic ZnS ccp S 2- dianions (A B C packed) with Zn 2+ cations in 1/2 Td holes. Build it! See it! (as Chem3D movie)

Fluorite = Cubic CaF 2 ccp Ca 2+ cations (A B C packed) with F 2- anions in all Td holes. Build it! See it! (as Chem3D movie)

Halite = NaCl ccp Cl anions (A B C packed) with Na cations in all Oh holes. Build it!See it in 3D!

Prototype Lattices 1:1 Ionic Solids NaCl (halite)packing type: ccp packing, all Oh sites filled cubicion sites: both anion and cation six coordinate, Oh ZnS (sphalerite)packing type: ccp packing, half Td sites filled cubicion sites: both anion and cation four coordinate, Td ZnS (wurzite)packing type: hcp packing, half Td sites filled hexagonalion sites: both anion and cation four coordinate, Td CsCl packing type: bcc packing cubic ion sites: both anion and cation eoght coordinate, Oh 2:1 Ionic Solids CaF2 (fluorite)packing type: ccp packing, all Td sites filled cubicion sites: anion four coordinate, Td and cation eight coordinate, Oh

Defects: creates useful materials 2.

Replace: -S with I -Zn with Hg (at vertices) -Zn with Cu (in middle) Replace: -S with I -Zn with Hg (at vertices) -Zn with Ag (in middle)

Thermochromic -turns black with heat, reverses to red on cooling Smallest cation, Cu+, moves into interstital sites, random order Thermochromic -turns red-orange with heat, reverses to yellow on cooling Smallest cation, Ag+, moves into interstital sites, random order

Other Structures are Described Based on Prototypes Example 1. Galena - PbS “has the NaCl lattice”. Note crystal morphology Example 2. pyrite - Fe(S 2 ) “has the NaCl lattice”, where (S 2 2- ) occupies Cl- site Note crystal morphologymorphology With more deviations: Example 3. CdI2: Layered Structure: I - forms hcp (ABA) layers and Cd 2+ occupies all Oh sites between alternate hcp (A B) layers Example 4. MoS2 : Layered Structure: S 2 2- forms (AA BB) layers and MoS2 Mo 4+ occupies all D3h sites between AA layers Note similarity to graphite. Used as lubricant.